The Wildlife Society’s Invasive Species and the Forestry and Wildlife Working Groups are cosponsoring a workshop for the Lingering Hemlock Project. The workshop will be on Tuesday 28 April at 2:00 PM EDT.
The Lingering Hemlock Project is a subset of The Nature Conservancy’s “Tree Species in Peril” program. The project aims to locate and selectively breed eastern hemlocks with genetic resistance to the hemlock woolly adelgid (HWA).
Olivia Hall from the North Carolina Hemlock Restoration Initiative will share more about how natural areas can participate in the Lingering Hemlock Project. In the southeastern US, project partners can locate and record data on hemlocks that remain healthy despite HWA infestations. In the northeastern US, project partners can establish hemlock plots and monitor their health and decline annually.
Go here to learn more about the project & webinar, and find the link to join.
I have blogged about HWA often – although there is no simple method for finding the earlier blogs. In 2025 I posted 3 blogs – in March and one in August. If you need a reminder about HWA, visit TNC’s “don’t move firewood” website here.
Posted by Faith Campbell
We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.
For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at http://treeimprovement.utk.edu/FadingForests.htm
The Trump Administration proposes (again!) to end all funding for USFS Research and State, Private, and Tribal Forestry programs. The budget document claims that these cuts are necessary “to ensure fiscal responsibility w/ taxpayer dollars & appropriate alignment of resources w/ USFS’s responsibility to appropriately steward National Forest System lands.” Ending the SP&T programs is justified as “better balance[ing] the appropriate roles of federal & State governments. … and [restoring] federalism …] The document claims that the federal component of Forest Health Management [currently receiving $16 million] duplicates programs managed by the National Forest System; yet the actions listed under this second budget category all relate to water management, not insects or pathogens. The document says states should manage pests on non-federal lands [currently receives $42 million]. I think this approach ignores the need for coordinated management for each of hundreds of pest species, from detection to eradication or development of host resistance. Eliminating the Research program will deprive all forest managers of a scientific foundation for management efforts.
The Trump Administration’s proposed budget would hold funding for key APHIS programs steady. This is great news compared to the extreme cuts proposed for the Forest Service. The budget document says that it is essential to continue APHIS programs success; any stoppages or reductions would potentially cause catastrophic consequences for environmental health. Contrary to this statement, holding funding steady actually results in cuts due to continuing introductions of new pests and inflation.
Item
2024 Actual
2025 Actual
2026 Estimated
2027 Estimated
Field Crop & Rangeland Ecosystems Pests (incl cogongrass)……….
12,000
12,000
11,000
9,026
Pest Detection ………………………………………………..
29,000
29,000
29,000
29,000
Plant Protection Methods Development ………………….
21,500
21,500
21,500
21,500
Specialty Crop Pests …………………………………………
215,000
215,000
214,000
217,339
Tree & Wood Pests …………………………………………..
59,000
59,000
58,650
58,650
Subtotal, Plant Health …………………………………….
387,500
387,500
385,150
386,515
USDA Forest Service
Two USFS programs w/ vital roles in protecting resilience of the Nation’s forests in the face of invasions by non-native pests and plants: R&D program and FHM within SPT division
The many economic & ecological benefits from our forests are under growing threats from a variety of disturbances, ranging from fires & hurricanes to non-native pests. ~ 60% of forests owned by non-feds; USFS must address threats to forests outside NFS to achieve its mission of sustaining “health, diversity, & productivity of the nation’s forests & grasslands to meet the needs of present & future generations.”
Research & Development
The Continuing Resolution for FY26 funded Research at $308 million for the year. Ask Congress to maintain this level. + increase research on invasive species from the current level of 1% to 5%.
The area of our forests & woodlands that is threatened by alien pests is similar to that attributed to fire or western bark beetles. More than 41% of forest biomass in the “lower 48” states is at risk to established non-native pests.[1] If able, add reference to pests on Hawai`i or Caribbean islands. Since additional introductions almost guaranteed, even greater proportion of US’ forest resources at risk in future. If possible, name example, e.g., Phytophthora austrocedri.Forest managers cannot counter these threats without understanding how these P&P kill trees & what actions are effective counter measures. This knowledge is obtained by research.
At least 53 tree species in forests across America are already under attack by non-native pests and pathogens. Yet as of FY23, Research stations spent just 1% of appropriation studying a few of the dozens of NIS pests. Funding for alien pests has decreased 70% since FY2010 even as new pests enter our forests. This inadequate research effort means USFS cannot develop effective programs to prevent, suppress, & eradicate the majority of alien pests. One crucial strategy suffers particularly = efforts to breed trees able to thrive despite NIS pests. R&D currently supports only a few such projects.
Forest Health Management: Supporting the Full Continuum of Pest Management
The Continuing Resolution for FY26 funded State, Private, and Tribal forests program at $310.6 million. I have not found specifics for the FHM program. This was an increase over the $281 million level in FY25.
Non-native pests and pathogens arrive as contaminants or hitchhikers on imported goods, especially on wood packaging and plants. These imports usually arrive in cities or suburbs, so the pests establish there first. They immediately cause enormous damage to urban forests, forcing local governments and property owners to absorb high tree removal costs. They then spread to rural forests, including National forests. Examples include hemlock woolly adelgid, emerald ash borer, invasive shot hole borers, goldspotted oak borer, sudden oak death, and beech leaf disease.
The most effective approach is to kill the pests where they first appear – usually in those urban or semi-rural forests. This response is led by FHM Cooperative Lands subprogram. We urge maintain funding for this subprogram at the FY26 level (possibly $42 million) so that the agency’s experts can continue to assist the states and other partners in countering these pests. As these pests spread to rural areas – including to National forests, National parks, and other public lands, responsibility for their management involves FHM Federal Lands subprogram. So much maintain funding for this subprogram at FY26 levels.
A recent analysis[2] determined that the natural resource values of 92 National parks are threatened by forest pests. Western parks are threatened primarily by outbreaks of the native mountain pine beetle (Dendroctonus ponderosae). Those in the East face threats from more than a dozen species of non-native pests, including hemlock woolly adelgid, emerald ash borer, spongy moth, laurel wilt, and – most recently – beech leaf disease.
Again, combatting these pests requires understanding their life histories & traits – understanding gained through the research program mentioned above.
Funding reductions over the past decade have already shrunk the number of FHM projects & areas treated each year. While 53 tree species are threatened, only four [eastern oaks, loblolly & ponderosa pines, & hemlocks] are targeted by 95% of projects. To counter the threats to 50 additional tree taxa, FHM needs additional resources.[3]
Investing in urban forestry is key to addressing both parties’ priorities & advancing flexible & cost-effective solutions to a wide range of issues impacting American communities, businesses, & families. The USFS SPT division’s Urban & Community Forestry Program efficiently distributes funds to shovel-ready projects for improving communities by maintaining a healthy tree canopy. Federal “seed” money provides resources necessary to initiate & stabilize these local programs.
A surprisingly high proportion of the (inadequate) funding for breeding trees to mitigate the damage caused by non-native pests is from FHM or the NFS, rather than R&D. These programs should receive substantial increases. The model program is the Dorena Genetic Resource Center. It provides decades-long commitment, skilled staff, necessary facilities; these result in breeding successes, i.e., western white pines and Port-Orford cedar.
Invasive Plants
Invasions of forests by non-native plant species erode forest productivity & provision of the full range of ecosystem services, hinder forest uses, degrade biodiversity & habitat, and impose substantial financial costs. A recent analysis[4] documents that this threat is growing: the number of FIA inventory plots containing invasive plant species rose in 58.9% of surveyed counties. Furthermore, in 73.2% of the counties the plots experienced an increase in species richness of invading plants. Increases occurred in all regions, but were greater in the East: from 46% to 52.3%. In the Rocky Mountains, the proportion of invaded plots rose from 6% to 11%. In Hawai`i, this proportion grew from 70% to 83.2%. Again, USFS Research and FHM programs, working together, are key to making progress in countering these bioinvasions.
[1] Fei, S., R.S. Morin, C.M. Oswalt, and A.M. 2019. Biomass losses resulting from insect and disease invasions in United States forests. PNAS August 27, 2019. Vol. 116 No. 35 17371–17376
[2] Michalak, J.L., C.E. Littlefield, J.E. Gross, T.G. Mozelewski, J.J. Lawler. 2026. Relative Vulnerability of US National Parks to Cumulative & Transformational Climate Impacts. Conservation Letters, 2026 Vol 19, Issue 1; 19:e70020
[3] Coleman, T.W, A.D. Graves, B.W. Oblinger, R.W. Flowers, J.J. Jacobs, B.D. Moltzan, S.S. Stephens, R.J. Rabaglia. 2023. Evaluating a decade (2011–2020) of integrated forest pest management in the United States. Journal of Integrated Pest Management, (2023) 14(1): 23; 1–17
[4] Potter, K.M., B.V. Iannone III, K.H. Riitters, Q. Guo, K. Pandit, C.M. Oswalt. 2026. US Forests are Increasingly Invaded by Problematic NIS Plants. Forest Ecology & Management 599 (2026) 123281
USDA Animal and Plant Health Inspection Service
APHIS is responsible for preventing intro and spread of pests and invasive plants that harm agric, including forests. APHIS policy guides port inspections carried out by the DHS CBP. APHIS inspects imported live plants.
Introductions of pests and pathogens have continued to occur. APHIS funding has remained steady – which means it is not growing to match the rising threat. At minimum, maintain current levels.
FY2025 enacted FY26 House FY26 Senate
APHIS total $1,148 $1,146 $1,168
Plant health subtotal $387.5 $388.6
Agric. quarantine $35.5 $35.5 $35.5
Field crop and rangeland $12 $11 $11.5
Pest detection $29 $28.5 $29
Methods development $21.5 $21.5 $21.5
Specialty crops $206.5 $216.3 $208.5
Tree and wood pests $59 $59 $58.6
Emergency preparedness and response* $44.5 $44.5 $44.3
* this fund is apparently for both animal and plant emergencies
Rationale
Already introduced pests threaten the many forest products and services benefitting all Americans. Just 15 of the worst pests threaten 41% of forest biomass in the “lower 48” states – comparable to fire.[1] A significant proportion of the resulting costs are imposed on municipal governments and homeowners. Fifteen years ago, it was estimated[2] that the municipal governments were spending more than $1B / year, primarily on removing and replacing trees on public property killed by these non-native pests. Homeowners faced costs of $1B plus loss of another $1.5B in property value. A more recent study estimated that cities will have to spend $30M per year to remove and replace ~ 1.4M street trees by 2050. Additional trees in parks and on homeowners’ properties also die.[3]
A new pattern has appeared in recent years: more newly-introduced pests are being detected in the Pacific Coast states rather than in the East and Midwest. Two southern California counties are projected to pay $150M – $1B[4] to remove and replace trees killed by invasive shot hole borers. The emerald ash borer threatens 9,000 ash on the streets of Portland, Oregon and millions more in parks and the forested wetlands of Willamette Valley, including in Ankeny National Wildlife Refuge. The Mediterranean oak borer has already killed thousands of oak trees in the San Francisco Bay area; it also threatens urban forests and valued oak savannahs in Oregon.
Additional introductions of highly damaging wood-borers are likely because we continue to receive inadequately treated crates, pallets, and other forms of packaging made of wood. For 20 years, all countries shipping goods to North America must treat their wooden packaging per prescribed protocols. To address this risk, we urge a modest $1M increase in APHIS’ “Tree and Wood Pest” account. We also suggest that the Subcommittee inquire of APHIS what steps it will take to improve compliance with the treatment requirement. You should focus your inquiry on China; wood packaging from this country is three times more likely to harbor a tree-killing pest than the global average.[5]
Other pests—especially plant diseases and sap sucking insects—enter on imported plants. Pathogens introduced recently via this pathway include rapid ohia death in Hawai`i (threatening the species that constitutes 80% of the Islands’ forest biomass) and beech leaf disease (thin a dozen years has spread across much of the East).
All assessments of APHIS’ plant import programs’ effectiveness use data from 2009; at that time, plant imports were more than 100 times more likely to transport pests than was wood packaging.[6] APHIS has amended its regulations several times since 2009. We urge the Subcommittee to call for APHIS to facilitate independent analysis of the efficacy of its current phytosanitary programs in order to understand whether the updated regulations have reduced the risk of additional introductions.
Again, pests introduced via this pathway proliferate and spread – often facilitated by movement of firewood, plants, and outdoor household goods. APHIS’ programs have suffered severe failures to prevent such spread, for example in the cases of the emerald ash borer and sudden oak death. We suggest that the Subcommittee inquire of APHIS what steps it will take to improve containment efforts regarding damaging plant pests, including through collaboration with its state partners.
We ask for small increases to the Pest Detection and Methods Development programs. The first enables prompt detection of newly introduced pests … which is critical to successful pest eradication or containment. The second empowers APHIS to improve essential detection and eradication tools.
The current emergency fund of is far below the level needed to respond when a new pest is discovered. We thank both the House and the Senate for clearly recognizing that these appropriations are inadequate by including in their bills language reiterating the Agriculture Secretary’s power to access funds from other Departmental programs (usually the Commodity Credit Corporation) to respond to emergencies.
[1] Fei, S., R.S. Morin, C.M. Oswalt, and A.M. 2019. Biomass losses resulting from insect and disease invasions in United States forests. PNAS August 27, 2019. Vol. 116 No. 35 17371–17376
[2] Aukema, J.E., B. Leung, K. Kovacs, C. Chivers, K. O. Britton, J. Englin, S.J. Frankel, R. G. Haight, T. P. Holmes, A. Liebhold, D.G. McCullough, B. Von Holle.. 2011. Economic Impacts of Non-Native Forest Insects in the Continental United States PLoS One September 2011 (Volume 6 Issue 9)
[3] Hudgins, E.J., F.H. Koch, M.J. Ambrose, and B. Leung. 2022. Hotspots of pest-induced US urban tree death, 2020–2050. Journal of Applied Ecology
[4] Jetter, K. A. Hollander, B.E. Nobua-Behrmann, N. Love, S. Lynch, E. Teach, N. Van Dorne, J. Kabashima, and J. Thorne. 2022. Bioeconomic modeling of invasive species management in urban forests: final report.
[5] Haack RA, Hardin JA, Caton BP and Petrice TR (2022) Wood borer detection rates on wood packaging materials entering the United States during different phases of ISPM#15 implementation and regulatory changes. Front. For. Glob. Change 5:1069117. doi: 10.3389/ffgc.2022.1069117
[6] Liebhold, A.M., E.G. Brockerhoff, L.J. Garrett, J.L. Parke, and K.O. Britton. 2012. Live Plant Imports: the Major Pathway for Forest Insect and Pathogen Invasions of the US. www.frontiersinecology.org
Congressional Committees with Jurisdiction … & how to submit testimony
FUNDING APHIS
House Committee on Appropriations, Subcommittee on Agriculture, Rural Development, Food and Drug Administration, and Related Agencies
Chairman: Andy Harris (R-MD)
Members: Robert Aderholt, David Valadao, John Moolenaar, Dan Newhouse, Julia Letlow, Ben Cline, Ashley Hinson, Scott Franklin
Democrats à Sanford Bishop, Jr., Chellie Pingree, Lauren Underwood, Marie Gluesenkamp Perez, Marcy Kaptur, Debbie Wasserman Schultz
deadline: May 1; email to ag.approp@mail.house.gov
instructions: 5 pages, double-spaced in Times New Roman, 12 Point Font; single-sided; PDF attachment to your email. At top of 1st page, clearly indicate your name, title, & institutional affiliation (if any); In 1st paragraph, clearly state agency, program, & amount of funding in the request
Senate Committee on Appropriations, Subcommittee on Agriculture, Rural Development, Food and Drug Administration, and Related Agencies
Chairman: John Hoeven (R-ND)
Members: Republicans à Mitch McConnell, Susan Collins, Jerry Morn, Cindy Hyde-Smith, Deb Fischer, Mike Rounds
Democrats à Jeanne Shaheen, Jeff Merkley, Tammy Baldwin, Martin Heinrich, Gary Peter, Kirsten Gillibrand, Jon Ossof
deadline: not clear; might be 22 May; email to agri@appro.senate.gov
instructions: 4 pages.. At top of 1st page, clearly indicate your name, title, & institutional affiliation; state agency, program, & amount of funding in the request
FUNDING USFS
House Committee on Appropriations, Subcommittee on Interior, Environment and Related Agencies
Chairman: Mike Simpson (R-WY)
Members: Republicans à Mark Amodei, Guy Reschenthaler, Michael Cloud, Ryan Zinke, Jake Ellzey, Celeste Maloy
Democrats à Chellie Pingree (D-ME), Betty McCollum, Josh Harder, James E. Clyburn
deadline: 22 April; email to IN.Approp@mail.house.gov
instructions: 4 pages, single-spaced in 12 Point Font; single-sided; prefer PDF but other formats OK. At top of 1st page, clearly indicate your name, title, & institutional affiliation (if any); In 1st paragraph, clearly state agency, program, & amount of funding in the request
Senate Committee on Appropriations, Subcommittee on Interior, Environment and Related Agencies
Chairman: Lisa Murkowski (R- AK)
Members: Republicans à Mitch McConnell, Shelly Moore Capito, John Hoeven, Deb Fischer, Mike Rounds
Democrats à Jeff Merkley, Chris van Hollen, Martin Heinrich, Tammy Baldwin, Kirsetn Gillibrand, Jon Ossof
deadline: unclear; possibly mid-June; email to int@appro.senate.gov
instructions: 4 pages, single-spaced in Microsoft Word or Word Perfect; do NOT send PDF. At top of 1st page, clearly indicate your name, title, & institutional affiliation (if any); In 1st paragraph, clearly state agency, program, & amount of funding in the request
The US Department of Agriculture (USDA) and the North American Invasive Species Management Association (NAISMA) held the 34th annual forum on invasive species research at the end of February 2026. The agenda is available here. In this blog I summarize the presentations about invasive alien plants (IAS); a separate blog discusses findings on invasive plants. Formal proceedings will be available in some months.
The most important information from the meeting:
If NAISMA had not taken on the task of hosting the conference it would not have happened.
Government leaders allowed only 1 staffer per USDA Forest Service region to participate. Not allowed to come were people who had organized the whole meeting or individual sessions, and presenters discussing several topics, including preventing IAS plant spread, and progress on controlling cogongrass (major impediment to pine plantations, affecting harvests).
What do these decisions say about the genuineness of the USDA Secretary’s recent memorandum listing invasive species as one of four priority areas for the department’s research efforts?
The USFS International Program is one of the few sources of support for studying potential pests before they invade the US.
Early detection surveillance is undermined by reliance on deploying too few traps and in a too narrow, or the wrong, timeframe.
The Resistance Screening Center in Asheville, NC is no longer staffed, undermining breeding efforts in a region that reaches from Virginia to Texas.
A reminder to us all: Rebekah Wallace of the Center for Invasive Species and Ecosystem Health at the University of Georgia urged us all to provide citations for images used in informal materials – posters, presentations, outreach efforts, blogs, videos. Providing the citation increases our credibility and ensures that we avoid perpetuating mis-information!
an ash resistance breeding plot at the Holden Arboretum, Ohio
Summary of key research reports on tree-killing arthropods and pathogens
Jennifer Koch, researcher with the USFS, described the Trees in Peril program. TiP aims to increase the pace, scale and efficiency of resistance breeding programs for American beech; eastern hemlock; and green, white, and black ash. This includes integrating genomics with other approaches and strengthening partnerships. Partners are key to finding “lingering” trees, addressing some scientific questions, and possibly screening cuttings for resistance.
Koch first explained the value of resistance breeding for producing resistant stock for restoration and reducing habitat for pests. The goal is to develop resistance, which Koch defined as the ability of a tree to survive despite the pest. Full immunity is not required. TiP participants hope that by integrating breeding with other approaches, such as biocontrol, they can create a new ecological equilibrium in which the tree species will continue to play its ecological role. As Koch asserts, the public supports breeding more than some other approaches. Also, there is a record of success; she cites the USFS Dorena Genetic Resource Center, which has developed resistant seedlings for four five-needle pines and Port-Orford cedar.
The first step is to determine whether desired traits are inherited. Genomics and other tools can test cuttings while they are still young and small – a very important advance in efficiency. Still, once cuttings with the desired traits are identified, it often takes several rounds of breeding to raise resistance levels sufficiently high. Similar testing of immature clones later in the process also can accelerate creation of seed orchards.
Breeding programs also need to incorporate genetic diversity from across the species’ ranges. TiP partners are collecting genetic material from beech, hemlock, and ash trees across their extremely large ranges – much of eastern North America.
Finally, TiP is training additional people to contribute to these breeding efforts.
Progress on each taxon:
beech grafts in a breeding experiment at the Holden Arboretum
Beech – Breeders are dealing with two diseases. A decade ago they identified genetic markers associated with beech bark disease (BBD). Their efforts had led to orchards producing seedlings of which 50% are resistant. Then beech leaf disease (BLD) showed up! Early results of a pilot study suggest BLD symptom severity is under genetic control. Even better, some trees appear to be resistant to both diseases. Koch recommends that scientists first identify BBD-resistant trees, then test those trees for BLD resistance.
Ash – the emerald ash borer (EAB) is established in 40% of the range of ash species. (Note: I am not sure whether this statement includes Canada; I am fairly certain it does not include Mexico.) Nine of the 16 US species are vulnerable, five endangered – green, white, black, blue and pumpkin.
The process by which scientists determe that resistance traits are heritable and identifying promising genotypes is described in Mason et al. (2026). The effort to develop techniques to propagate rooted cuttings is described in Merkle et al. (2022).
Partners are helping to search for “lingering” ash. So far, 265 trees have been identified, and scion collected from 106 trees. Partners are also helping to plant cuttings for resistance testing.
The program has had to overcome several difficulties, including:
Black ash is dioecious, which complicates selection. Breeders are working on several approaches, but all are at early stages.
Many of the originally collected trees turned out to be unintended crosses of white and green ashes rather than pure species. This resulted in very low seed production.
Anticipating the introduction of ash dieback disease (caused by the fungus Hymenoscyphus fraxineus), TiP is collaborating with Europeans on searching for possible resistance to this threat as well.
Hemlock – the Hemlock woolly adelgid (HWA) causes mortality of 50 – 100% of overstory trees. TiP scientists are still trying to establish a test for heritability of HWA resistance. There are additional difficulties in propagating rooted cuttings. The University of Georgia, Holden Arboretum, and others are helping to resolve these issues.
Those who want to support this program by contributing funds, knowledge, facilities, or volunteer efforts should contact Dr. Rachel Kappler, Forest Health Collaborative Coordinator, Holden Forests & Garden.
One entity already actively helping the TiP program is the Ecological Research Institute through energizing citizen scientists. Radka Wildova described these efforts. The Monitoring and Managing Ash [MAMA] initiative has published detailed guidance on identifying “lingering” ash. For example, timing is crucial: searching too early points to trees that are not actually resistant. Searching too late means opportunities are missed (since “lingering” ash will die eventually because resistance is only partial) or a risk of confusing in-growth or regeneration for “lingering” trees.
The Institute could not create a similar action map for hemlocks because the adelgid has been present far longer. Recommends searching in sites where at least 80% of surrounding trees are dead or dying due to HWA or elongate hemlock scale. The program is also testing heritability of resistance among hemlocks on its own property, which was invaded 20-30 years ago.
[An unrelated initiative, the Hemlock Restoration Intiative, is pursuing protection and breeding efforts in the southern Appalachian mountains.]
Avalon Miller, Pennsylvania State University, discussed new techniques to detect American elm trees tolerant of this disease.
a healthy American elm in Fairfax County, Virginia; photo by F.T. Campbell
It is important to detect elm trees’ response to infection early in the infection process because the apparent mechanism of tolerance is some trees’ ability to limit growth and proliferation of the causal fungus Ophiostoma novo-ulmi in xylem vessels. Scientists sought to use spectral analysis to detect distal leaf stress as a signal of susceptible genotypes. The USFS has developed a small stem assay that is achieving 80% accuracy in identifying disease phenotype within two months of inoculation – before symptoms appear.
Future studies will focus on determining which metabolites vary in tolerant vs. susceptible trees, and whether that information suggests useful interventions. For example, it is thought that some trees respond too aggressively to the pathogen, thereby cutting off the flow of water and nutrients and killing themselves.
Meanwhile, continuing efforts to breed resistant elm are hampered by limited greenhouse space, the tree’s complex genetics, and vast geographic range, and great variation in trees’ responses.
Current USFS- and The Nature Conservancy-supported programs focus on the Northeast. I urge scientists in the Mid-Atlantic to engage; I have seen numerous healthy American elms in the Virginia and Maryland suburbs that could be included in a breeding program.
Courtney Johnson, North Carolina State University, described efforts to determine key aspects of the ALB invasion in South Carolina. First, the bad news: a second invaded site in the Charleston region was detected in 2025.
Because Charleston is much farther south than any other ALB infestation, questions have arisen about
its phenology (timing of development). Research has confirmed that the ALB in South Carolina has ~1 year development cycle, not multiple generations as some had feared. Beetle larvae stay in the phloem through the third instar. Adult flight season is from May – Sept; the peak is in July. Unlike earlier findings, adult beetles did not exhaust their natal tree before moving to a new tree to oviposit. (This is also true in the Massachusetts outbreak.)
Some of the beetles in South Carolina are larger. Outreach materials need to be amended to reflect this fact, e.g., much larger exit holes.
typical site of ALB infestation in Charleston South Carolina; arrows indicate infested red maple trees. Photo by David Coyle
Tree dissection and dendrology studies of the principal host, red maples, show that multi-stemmed trees and smaller branches are preferred. They also preferred vertical stems or bolts, although they did oviposit on horizontal bolts raised off the ground to mimic a tree branch. There was little oviposition on bolts on the ground. In practice this means managers can leave felled trees on the ground without prolonging the infestation. This is very helpful since swamps preclude using heavy equipment. picture
Chad Rigsby, Bartlett Tree Research Laboratory, described the results of testing the efficacy of several nematocides. A foliar spray, Bayer’s Broadform, has received emergency approval from many states. It suppresses nematode (Litylenchus crenatae mccannii ([LCM]) numbers when applied at very low rates. Trees can be treated as long as (green) leaves are present. Rigsby recommended not spraying until a tree displays symptoms.
Since foliar sprays cannot be applied in forests, near water, or on huge trees, scientists also sought a systemic injectable fungicide. Thiabendazole [TBZ] (commercial formula Arborjet 20-S) is available. Rigsby said applicators can avoid splitting of the bark by following protocols developed by the International Society of Arboriculture. Managers should inject a tree several times in the first year to get the disease under control; then they can apply less frequently.
injection of thiobenzadole into beech; photo by Matthew Borden of The Bartlett Tree Research Laboratories
Don Grosman of Arborjet believes mortality is the result of a disease complex, not just LCM. Any of three treatments containing phosphite greatly reduces nematode numbers and canopy symptoms. Low volumes of diluted product can be injected in a few minutes. However, Thiabendazole hypophosphite requires a high volume macro trunk injection. This is expensive and takes time
Testing shows potassium phosphite PHOSPHO-jet produced dramatic improvement in 1 year. There are early indications that one treatment might be effective for two years. Arborjet will test this finding again this year. The company is also testing another chemical – the name of which cannot yet be revealed.
Andrew Miles, Ohio State, described beech response to polyphosphate (PP30). This chemical is a biostimulant, not a treatment. It is used as a disease control agent in several crops, including woody species. Field observations indicate it does reduce disease severity. Scientists are trying to understand the mode of action. Experiments are under way in Cleveland MetroParks, where BLD was first detected. Miles called for experiments within buds as well as leaves, since LCM damages tissue while in the bud.
Scott Schlarbaum, University of Tennessee, collects butternuts; photo by F.T. Campbell
Anna Conrad, USFS, described ongoing efforts targetting this disease, which is present throughout the tree’s large range. A major challenge is distinguishing pure butternut from hybrids with Japanese walnut. Scientists have screened ~300 families from 22 states for possible resistance. At three sites in Indiana, the vast majority of highly resistant families are hybrids. Still, resistance was detected in up to 2.5% of pure butternuts; this level is sufficient to be enhanced through breeding. The program would benefit from genotyping across butternut’s range to identify lingering trees and confirm resistance.
Nicholas Dietschler, Cornell University, studies the relationship between western hemlocks and HWA in their shared native ranges in the Pacific Northwest. At all sites, lower numbers of HWA (of both PWN and Japanese lineages) survived on Western hemlock – in the absence of predators. Why? Dietschler believes western hemlock has better chemical defenses. For example, hemlocks exude pitch in response to adelgid herbivory. In eastern hemlocks, this induced resin might suppress the tree’s defenses. In addition, HWA also prompts greater suppression of phenolics in eastern hemlock. Dietschler concludes that bottom-up, tree-based defenses are a factor in the invasion and should be studied — while continuing efforts to find an effective combination of biocontrol agents.
Anne C.J. Peter, of Virginia Polytechnic Institute and State University, is comparing HWA chemical interaction with the most recent biocontrol agent, the silver fly Leucotaraxis argenticollis. (Scientists hope L. argenticollis will feed on summer populations of HWA; other biocontrol agents don’t suppress HWA at this stage.) The L. argenticollis population in the PNW feeds on HWA; however, its eastern North American relative L. rubidus feeds on pine adelgids, not the introduced HWA. It has been challenging to establish the PNW population in the East. One possibility is that the invasive HWA, which is from Japan, contain toxins that deter predators & parasitoids. Therefore, Peters is studying how both the western and eastern populations of Leucotaraxis deal with anthraquinones – compounds found in many plants and some insects, but not adelgids native to the eastern US.
Jian Duan, of the Agriculture Research Service Beneficial Insect Lab, summarized results of 15 years of biological control efforts. Over this period, four biocontrol agents have been introduced. I applaud APHIS’ rapid inclusion of this pest management approach; an egg parasitoid and two larval parasitoids were introduced before 2010, less than 8 years after the invasion was detected. Unfortunately, these agents proved less effective in northern parts of the EAB’s distribution. A fourth larval parasitoid was released in 2015. One or more of these biocontrol agents have been released in 479 counties in 34 U.S. states and three Canadian provinces.
To what degree have the wasps reduced EAB populations? Are those reductions resulting in regeneration?
Duan reported that at sites in Michigan, all four agents have spread rapidly. EAB populations crashed and recovered several times but overall numbers are lower. Ash saplings increased greatly after 2015; seedlings also increased. He concluded that the program has been successful but not spectacularly so.
Hannah Broadley, APHIS, described developments beginning with initial searches for possible agents in China in 2015 — just one year after the lanternfly was detected in Pennsylvania. The search has focused on agents that feed on SLF eggs and nymphs. Attention has narrowed to Dryinus sinicus. This wasp both preys on and parasitizes SLF nymphs – depending on the nymphal stage. Labs are developing a third colony and conducting host specificity testing. Scientist have begun drafting a petition for release; the review process will probably take more than one year. At the same time, scientists continue exploring other possible biocontrol agents – e.g., in Vietnam. The blizzard prevented this speaker from appearing.
Xingeng Wang, of the ARS Beneficial Insect Lab, described how Dryinus sinicus attacks SLF – with a graphic video! D. sinicus attacks on third instar are often unsuccessful. When it encounters a second instar nymph, however, D. sinicus switches from predation to parasitism: it lay an egg which then develops inside the SLF nymph. This parasitism kill seven times more nymphs than predation on older nymphs.
Individual D. sinicus wasps can live up to 60 days, lay an average of 175 eggs and parasitize ~137 nymphs! Since D. sinicus is most effective against just one instar, releases will need to be carefully timed.
Alex Wu, APHIS, discussed efforts to prevent establishment of four flighted spongy moth (Lymantria) species. APHIS seeks to improve the efficiency of trap analysis because states are submitting triple the number of trap contents of past years. The goal is to improve real-time qPCR efforts to distinguish the European species established in the East from the Asian flighted species, and to distinguish the several subspecies of latter taxon. Current qPCR results point to the wrong species ~ 5% of time. There are complexities: moths from Central Asia might be hybrids. Also Lymantria dispar japonica might be found in far southeastern corner of Korea – which is separated from Japan by a narrow strait.
Early Detection of Wood-Associated Beetles
Jiri Hulcr, University of Florida, discussed strengths and weaknesses of artificial intelligence (AI) in species identification of bark beetles. As he noted, differentiating a specific bark beetle species from among the more than 6,000 look-alike taxa is time-consuming. A properly trained AI program can help. Furthermore, no one can keep up with publications – in 2015 there were 432 discussing just bark beetles! AI can help researchers discover papers that they otherwise would miss and empower non-English speakers to search the literature.
Hulcr has created a website that now has 63,000 images of ambrosia and bark beetles to assist identification. This work has been funded by the USFS International Program – one of the few sources of support for studying potential pests before they invade. The website will be open source once it has been copyrighted to prevent “scraping” by bots. Hulcr invited participants to send more images to continue training the algorithm on more species.
In the discussion, Alain Roques noted that scientists in Europe and probably China are developing similar AI-assisted identification tools. He urged international coordination. Hulcr replied that scientists do coordinate – as long as funding is available. Jennifer Koch noted that historic collections have many taxonomic inaccuracies. She urged people to rely on genetics when trying to identify a species.
Hulcr says AI is much faster than people in completing some tasks. But managing bioinvasions continues to require trained people (taxonomists) to collect, detect and classify new species; and execute quality control. AI cannot do science, which Hulcr defined as generating new knowledge through observation, turning that information into data, and testing hypotheses, making assumptions based on that.
Hulcr says AI also cannot predict what the next damaging ambrosia beetle to enter the U.S. will be. He offered his predictions:
Euwallaceae destructans – from Indonesia – attacks live trees
Aggressive Platypdinae from Asia and South America (especially threatening to plantations where trees are stressed)
Cryphaus lipingensis (attacks pine seedlings)
Scolytus amygdali from the Meditteranean region – introduces pathogens during maturation feeding on living hosts; feeds on almonds and prunes – Rosaceae
Dryocoetes himalayensis – Asia and Europe; kills walnuts
Port of Marseille; via Wikimedia
Alain Roques, of Zoologie Forestiere in France, reported results of a beetle trapping study in France.
Since the European Union allows entry of species not listed as quarantine pests, it is vitally important to improve detection and analysis of the large percentage of detections that are “unknown” or “emerging”. Nearly 8,000 beetles have been trapped over five years; they belong to nearly 400 species, 35 non-native.
One approach is to develop more generic traps and lures. The EU is now using a blend combining 10 pheremones to trap Cerambycidae. Scientists are incorporating additional pheromones to the blend and to extend attractiveness longer than the current 10 days. There is still no generic lure for Buperstids.
Some species arrive regularly – is each detection a reintroduction? Or are these species established?
Roques asks whether we are trapping at the right sites. Half of Cerambycids are trapped only inside ports (of various types). Scolytids were trapped outside ports, at other “high-risk” locations– e.g., sawmills and recycling centers. In other words, they disperse more broadly. Roques wants to add the road network and to extend the survey to the entire European Union.
Davide Nardi, of the University of Padua, Italy, discussed results of his trapping program, which seeks to guide placement of traps. See Nardi et al. (2026) [full citation at end of this blog]. Important conclusions are:
Surveillance programs are probably under-sampling species. Halving the sampling effort (from 16 to 8 traps) resulted in failure to detect ~20% of the species at the site. Cutting the sampling effort to four traps resulted in missing ~ 40% of present species. This decline in catches is particularly severe in urban landscapes – the very places where insects are most likely to be introduced. Even when they deployed 16 traps per site almost 30% of total species richness was not detected, on average.
Urban landscapes might offer a higher diversity of potential tree hosts. They also have more barriers to insects’ spread, e.g., buildings. This means urban areas require a greater sampling effort.
Traps should be set near available forest patches or urban parks.
I was intrigued by Nardi’s suggestion that scientists use the data on native beetles included in the trap catches to alert countries receiving exports from these ports to which species might be transported to their shores.
Manoj Pandey, of Ohio State University, explored how environmental context shapes abundance and diversity of Scolytines caught in surveillance traps. His goal is to improve the efficacy of the USFS’ two- decade-old Early Detection Rapid Response trapping program, which targets bark and ambrosia beetles at high-risk sites. These include transit sites, destination sites, and wood waste treatment sites. Pandey analyzed program catches from 2010 through 2019.
He found that among native species bark beetles dominated catches; ambrosia beetles dominated non-native captures. Climate [minimum/maximum temperature and precipitation] was the most important factor determining which species were caught. Overall, both Scolytines and ambrosia beetles are governed more by ecological requirements than by human population levels. Among Scolytines, native species (which are adapted to stressed trees) are affected by precipitation; non-native species are favored by warmer temperatures. Ambrosia beetles – both native and non-native – are more affected by precipitation levels than bark beetles, probably because of the formers’ symbiotic relationship with fungi. Ambrosia beetles are also more likely to be generalists and to be attracted by deciduous forests.
The other influential criterion was landscape – whether forests are evergreen, deciduous, or diverse. Deciduous forests attract both types of beetles, but the influence is stronger for non-natives. Conifer (evergreen) forests attracted native species. Higher human population density was associated with higher trap catches. Propagule pressure – measured via human population density and per capita income – was less important, perhaps because the traps are always placed near population centers.
Xyleborus monographus; photo by U. Schmidt
I am concerned because this trapping program did not detect the Mediterranean oak borer (Xyleborus monographus) before it was detected in California and Oregon. The project also did not find the greater shot hole borer Euwallaceae interjectus on the West Coast before it was detected in Santa Cruz, California. This ambrosia beetle has been established in the Southeast for years (M. Pandey, pers. comm. 12 March 2026).
Other Pests and Pathogens
Thomas G. Paul, at Ohio State, explored whether understanding the temperature regime during transit can provide early warning of which wood-associated pests might arrive. He obtained ocean surface temperature data along shipping routes from China to the U.S. West Coast and across the Atlantic. He then related those temperatures to degree-days needed for development by Xylosandrus germanus (from Asia) and Ips typographus (from Europe). At present there is still lots of uncertainty, including how to factor in the insect’s stage at the time of departure, the relationship between ocean air temperature and temperature inside a container, and possible effects of a container left to sit for several days in the port of import.
Eliana Torres Bedoya, also of Ohio State, provided an update on spore trapping for improved detection of pathogens across large landscapes. In 2024 the project developed standardized protocols for surveillance. To learn what is going on in the region, one should sample many sites across the area of interest. To find a particular pathogen, officials also need to know which season to sample. Torres Bedoya notes that few states sample in the autumn, which probably results in biased results.
In 2025, the program was expanded to 10 states. Species searched for are chosen by participating states. They include the causal agents of oak wilt, thousand cankers disease, laurel wilt, annosum root disease, and the beech leaf disease nematode (Litylenchus crenatae mccannii). Participants – including state phytosanitary officials — are now asking how to respond to a detection. For example, DNA from Bretziella fagacearum, the cause of oak wilt, was detected in several states where no disease has yet been identified (New Hampshire, Massachusetts, West Virginia, and Ohio). DNA of Geosmithia morbida, the causal agent of thousand cankers disease of walnut, was detected in New Hampshire, Massachusetts, and Maryland. What should managers do in response to these findings?
Torres Bedoya explained that her team is now working to make the spore-trapping process more user-friendly. I noted that my poster previous blog discussed using these techniques at the interface of forests and agricultural land uses.
During other discussions, I learned that Jason Smith of the University of Mount Union is trapping for DNA from LCM in order to track the spread of BLD.
Brown spot needle blight
Several speakers addressed this disease, which is of increasing concern to pine timber interests in the American South and around the world. New Zealand is exploring resistance breeding of Pinus radiata in advance of introduction
The disease has long been known in long-needle pine – at the “grass” stage (early seedlings). In recent years needle blight has begun damaging loblolly and other pine species – in both plantations and natural forests. Jason Smith, from the University of Mount Union, was asked for help by the industry in 2016. He found that one factor is increasing reliance on herbicides instead of fire to control ground-level vegetation. The large doses of inoculum remains in the litter, rather than being killed by periodic fire – as in the past. Smith thinks it is also possible that the pines suffer subtle damage from herbicides. Other possible factors are the widespread planting of genetically identical monocultures and climate change.
Colton Meinecke at the University of Georgia reported that Lecanostica acidola has been confirmed as the disease agent at these sites by Koch’s postulates. Scientist at the University of Georgia, University of Mississippi, and other entities are collaborating on development of a predictive model. Work includes sampling needles from both the litter and canopy, tracking tree condition, destructive sampling of dead trees, and spore trapping.
In the discussion, Smith warned that dying pines are not being detected by aerial forest health surveys because they are conducted too late in the season. This is because the surveys focus on one specific pest, the southern pine beetle. He called for a more comprehensive survey program.
Meinecke reported that the disease is more severe in western parts of the Gulf Coast regions. It is also causing problems in Christmas tree plantations, especially Scots pine.
He has found evidence of some genetic resistance. He is trying to develop a rapid test of a tree’s vulnerability using spectral wave length. Meinecke is also experimenting with stand management approaches. He praised the close cooperation with experts from around globe and New Zealand’s pro-active preparation for combatting the disease before it arrives.
Kier Klepzig, of the Jones Center at Ichauway in Georgia, described establishment of a Pine Pandemic Preparedness Plan, stimulated by awareness that a non-native pest might be introduced that attacks loblolly pine (Pinus taeda) – the foundation of the southeastern “woodbasket”. [Of course, Sirex noctiliois already established in the eastern United States. Although it is a severe pest of loblolly growing in plantations in the Southern Hemisphere, industry and federal and state agencies have dismissed concerns in North America.] The Pine Pandemic Preparedness Plan has four components: communication, detection and diagnosis, delimitation and assessment, response.
As concern about brown spot needle blight grew, the Southern Group of State Foresters ask the “P4” team to engage. Klepzig and Kamal Gandhi pulled together a working group which has the goal of developing guidance for managing the disease within two to five years. The task force is developing a website for data-sharing. The task force is also studying genetics of the host and pathogen, fungicides, the role of fire, resistance screening, and spore trapping. Industrial concerns about coordinating with competitors cause challenges.
Ashley Schulz, of Mississippi State University, has reviewed experience with biocontrol for clues on species’ traits important for facilitating invasion. She analyzed information on 394 insects introduced to North America for biocontrol of invasive plant species (see other blog) and 87 agents targeting 325 insect pests. For each species, data was recorded on whether it established, level of impact, the insect’s feeding guild, climate matching, host specialization, and evolutionary history. For the 87 entomophagous insects, she also recorded host feeding guild and host specialization.
Schulz found that entomophagous insects introduced as biocontrol agents were more likely to establish if they are a specialist. Higher impact was also associated with specialization. Parasitoids had higher impacts than predators. What does this indicate re: invasive species? Schulz said that insects which can hide or defend themselves, i.e., specialists, are likely to be more successful invaders.
Schulz recommends more analysis of what can be learned from experience with biocontrol agents. However, such studies are challenged by poor records, lack of empirical evidence and quantitative data, the lower number of biocontrol agents introduced recently, and funding shortages that preclude post-release monitoring.
Schulz also mentioned that she worries that a proposal to drop the word “harm” from definition of invasiveness could result in biocontrol agents being lumped with invasive species. This would further hamper implementation of biocontrol. She considered this loss to have particularly bad affects at a time when there are growing restrictions on pesticide use.
SOURCES
Mason, M.E., Carey, D.W., Romero-Severson, J. et al. Select genotypes of white and green ash show heritable, elevated resistance to emerald ash borer. New Forests57, 12 (2026). https://doi.org/10.1007/s11056-025-10158-x
Merkle, S.A., J.L. Koch, A.R. Tull, J.E. Dassow, D.W. Carey, B.F. Barnes, M.W.M. Richins, P.M. Montello, K.R. Eidle, L.T. House, D.A. Herms, K.J.K. Gandhi. 2022. Application of somatic embryogenesis for development of emerald ash borer-resistant white ash and green ash varietals. New Forests https://doi.org/10.1007/s11056-022-09903-3
Nardi, D., D. Rassati, A. Battisti, M. Branco, C. Courtin, M. Faccoli, N. Feddern, et al. 2026. “Integrating Landscape Ecology into Generic Surveillance Plans for Bark- and Wood-Boring Beetles.” Ecological Applications 36(2): e70194. https://doi.org/10.1002/eap.70194
Posted by Faith Campbell
We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.
For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at http://treeimprovement.utk.edu/FadingForests.htm
America elms on a street in Southeast District of Columbia; photo by F.T. Campbell
I applaud recent developments regarding one of the most devastating and widespread non-native tree-killing diseases, “Dutch” elm disease (DED). Brief descriptions of ecological importance of elms, the disease’s impact in North American and Europe, and difficulties in managing the rapidly evolving causal pathogens here. (See also a review of the ecological value of American elm here.)
Restoring America elm would be wonderful, so I rejoice at steps forward.
One task is to improve detection of the disease in forests. Currently detection is tardy because it relies on observation of visual symptoms followed by molecular confirmation. This process demands considerable time and labor; it is also error-prone. Earlier molecular detection methods also are labor intensive, costly, & have operational limitations.
A group of scientists led by Jian Jin and Songlin Fei are testing whether new spectral imaging & artificial intelligence can improve early detection. (See the publication by Wei et al.; full citation at the end of this blog.) Their goal is to detect subtle changes associated w/ disease developments before visual symptoms appear. The new technology — high-precision leaf spectral imagers — is already in use for agr crops. The devices needed are inexpensive and hand-held/portable. Can collect hyper- or multi-spectral images of a whole leaf in the field. This systems is also non-destructive & rapid.
To test applicability of this technology, the scientists inoculated the fungus responsible for DED into trees with known – and varying — disease susceptibilities. Then they collected spectral images of leaves from those trees to test accuracy of analyses conducted via both traditional machine learning & state-of-art deep learning models. These collections were made at three different times: 96 hours after inoculation / before visual symptoms; 4 weeks after inoculation / during visual symptom development; 15 weeks / foliar symptoms noticeable. They recorded the declining status of the using the traditional visual symptoms – wilting, yellowing, browning of leaves.
While detection accuracy varied by time of specimen collection and genetic heritage of the particular tree, machine-learning-based spectral & spatial analysis of high-resolution hyper & multi-spectral leaf images did detect DED symptoms. This advance would help detect pockets of disease in the forest and might be useful in screening elm genotypes for susceptibility to the pathogen. This latter ability would support resistance breeding programs.
However, further study is needed to determine whether light conditions, seasonal variations, or interactions with other pathogens might influence leaves’ spectral signature. Furthermore, scientists should test application of the process to additional elm genotypes. As Enrico Bonello and others have pointed out, however, the ideal would be to detect infection before even the start of symptoms – in other words, to detect even more subtle changes.
A second task is to breed American elms that can survive – even thrive – despite the continuing presence of the disease-causing pathogens. I rejoice here, too. So far, scientists have found varying levels of resistance in large “lingering” elms. This resistance appears to be heritable. Scientists are preparing reports of this progress for publication.
The USFS Northern Research Station is leading efforts of multiple partners to find and screen resistance of large elms across several regions. In New England, the principal partner is The Nature Conservancy; in the upper Midwest partners include the Army Corps of Engineers and Wisconsin Department of Natural Resources, In the lower Midwest the USFS is working with Metroparks Toledo, University of Illinois, Urbana Champaign, Appalachia Ohio Alliance and others. The Great Lakes Basin Forest Health Collaborative is helping to coordinate these efforts.
American elm has a huge range – covering much of the United States east of the Great Plains. Map Restoring the species to that range requires efforts throughout that range – so as to capture the genetic variability within the species and perpetuate its adaptations to the wide range of ecological conditions.
While restoring this magnificent and ecologically important tree species is worthwhile per se, a second motivation has emerged: using elms to restore riparian and wetland ecosystems now being harmed by loss of ash trees to the emerald ash borer.
Knight et al. (full citation at the end of this blog) note that these efforts’ success will depend not only on developing elms that can survive DED. It is also necessary to determine restoration strategies and silvicultural treatments that will promote the young trees’ ability to flourish despite challenges by storms, floods, competition with other plants, and wildlife feeding.
This team of USFS researchers describe ongoing tests of reintroduction strategies & silvicultural requirements in the Service’ Region 9. They note that reintroduction focuses on a single species. The goal of ecosystem restoration requires considering a broader range of factors. Both are important components for the success.
Testing Elm Reintroduction Factors
Research projects they describe include testing results of planting both bare-root seedlings and containerized stock. The latter approach is more labor-intensive but appears to provide better survival. When competing vegetation was removed & then controlled to prevent regrowth, large containerized trees had excellent survival & rapid growth. They also documented the value of caging trees to prevent deer browsing.
Other research projects explore elm seedlings’ ability to tolerate cold, floods, and shade. Scientists in New England and Wisconsin are observing how well progeny from various crosses between DED-tolerant American elms & local survivor trees are enduring the regions’ winters. One test is deploying progeny from paternal lines that are from different plant cold hardiness zones. It will be important to identify and plant trees that are adapted to local environmental conditions on top of being resistant to the DED pathogen.
Another group of tests investigates flood tolerance. Even minor dips or rises on floodplains lead to very different flooding intensities. Some of these experiments also consider shade tolerance. This is because managers hope can establish understory trees poised to grow rapidly by planting elm seedlings before harvest or mortality of canopy trees (e.g., ash). In one experiment in floodplain forests in Ohio, so far many elm seedlings have survived extensive spring & fall flooding. The seedlings are thriving across a range of microsite light environments. Even competition from invasive herbaceous plants does not appear to have impeded the elms’ survival.
DED has two methods of infecting nearby elms: that pathogen is either vectored by beetles that burrow below the tree’s bark, or through direct fungal contact via grafting of roots. Scientist do not yet know whether trees that tolerate DED infections caused by beetle attacks can withstand infection via root grafts. An experiment using paired elms was initiated in 2011. At the time of their writing, the trees had not yet grown sufficiently large to form root grafts – necessary before scientist could begin the experimental inoculations.
Finally, these many plantings have revealed some “unknown unknowns” — factors not previously identified. Knight et al. describe two studies:
1) Under the National Elm Trials, scientists are studying growth, stress and pest resistance, and horticultural performance of DED-tolerant American elm cultivars & other elm species and hybrids in 16 states. (See details here.)
2) A system of sentinel restoration sites has been established. Multiple DED-tolerant American elm selections have been planted in eight locations in four states to be an “early warning” system to identify additional pathogens of concern. Knight cites detection of a wood wasp at one site in Ohio and competition of thick grass and feeding by rodent on their roots in Minnesota.
Testing Restoration Strategies
As Knight et al. remind us, Eastern forests experience many forms of disturbance, including non-native pests and plants, increases in deer populations, land clearing, grazing, & climate change. Foresters want to know whether DED-resistant American elms might be used in restoration plantings in response to these natural and anthropogenic disturbance? They value elm for its ability to thrive in a wide variety of conditions. Furthermore, the species supports a diverse array of insect herbivores, which then support higher trophic levels, e.g., birds (Tallamy 2009). Another factor, not mentioned by Knight et al., is that even vulnerable elms can grow to some size before they are killed by DED. Knight et al. say multiple studies are testing use of American elm as one of several native species to be plant in ash ecosystems devastated by EAB. In northern Minnesota, the experiment is occurring in wet forest ecosystems formerly dominated by black ash. In Ohio researchers are observing elms planted in riparian systems where green ash forests used to be found. They report that early data indicate good initial survival of American elm in both studies.
The Nature Conservancy’s Connecticut River Program planted over 1900 disease-tolerant American elm cultivars at 76 sites in four New England states over the decade 2010 – 2021. Several DED-tolerant selections and their progeny were planted. Survival has varied considerably, they think depending on site factors, e.g., ice flows, height and density of competing vegetation, climate, damage from voles, deer browsing, others.
More recently, the partners have moved away from crossing survivor elms with cultivars because that results in too many related progeny, insufficient genetic diversity. In addition, the trees would not be adapted to the planting site because one parent was not local).
The Nature Conservancy’s participation has been funded by a grant of ~$2.4 million from a private foundation. TNC is helping to identify “lingering” or “survivor” American elm and restore them to floodplains and urban forests across New England. TNC has also funded groundbreaking research at the USFS to accelerate the breeding program and develop best practices for American elm reintroduction.
The Vermont chapter has been particularly active. Since 2014 it has been managing experimental elm trees plantings at 10 TNC natural areas and 26 partner-owned sites across the state. This effort has yielded ~7,000 trees that represent 142 novel crosses between 23 survivor elms identified by TNC in New England & several varieties identified by USFS from other parts of the country. Scientists plan to inoculate these trees in spring 2026. The trees’ vulnerability to the pathogen will then be evaluated over two years.
Knight et al. expect that in a decade or less these and other research projects will contribute needed understanding of various American elm propagules’ cold tolerance, flood tolerance, shade tolerance, response to competing vegetation, & root grafting. This information will allow managers to maximize survival of planted elm trees. It will also demonstrate how to usefully employ elms in ecosystem restoration. They caution that guidelines will probably vary to fit specific situations & site characteristics e.g., forest type, competing species, local hydrology, etc.
Knight et al. also identify topics that require additional research. The first factor mentioned are social & ecological contexts of restoration strategies. Social context will guide the formulation of a more strategic approach — setting goals, addressing such questions as the public perception & value of American elm in urban & forest areas, forest manager goals for incorporation of American elm, & municipal requirements for urban trees. It is essential to determine the long-term durability of resistance. Also need to explore how best to promote spread of DED-tolerant genes given the high numbers of local, non-resistant elms across the landscape. scale strategies.
Knight et al. note that need experimental plantings in additional parts of the species’ enormous range to identify potential problems, test performance on different soil types and in different climates. Need experiments to identify interactions among elm genetics and abiotic & biotic environ variables to guide silvicultural, site preparation, and planting strategies. I have observed apparently thriving American elms along roadways in the Washington, D.C. metropolitan area.
thriving American elm at the Fairfax County landfill in Lorton; photo by F.T. Campbell
I believe no one is protecting them. Certainly other elms in the area have died. So far I have not found people trying to find “lingering” or “survival” elms here. I seek people who want to work with these trees!
dead American elm on National Mall in Washington, D.C., close to the Lincoln Memorial; photo by F.T. Campbell; other nearby elms are also dead
The USDA Forest Service is not the only entity engaged in breeding American elms. The University of Minnesota is supporting an American elm breeding program through its Minnesota Invasive Terrestrial Plants and Pest Center (MITPPC) (see Bernardt citation at end of blog). Scientists are identifying DED-resistant elms in the wild, cloning and testing them, and replanting the strongest candidates across urban and natural landscapes. Their goal is to reintroduce the more resilient clones across Minnesota’s urban and natural landscapes, restoring lost canopy and biodiversity while preparing forests for a future stressed by climate change.
Bernardt describes the usual four essential steps: identification of trees that appear to be resistant; propagation of clones from those trees; growing sufficient numbers of these; and testing them for resistance. The Minnesota program – like many similar programs for breeding the many tree species being killed by non-native pests – ask the public to help in searching for “survivor” trees—American elms that appear to be withstanding Dutch elm disease even as others around them succumb.
The article summarizes the next steps and challenges. It notes, for example, that using clones rather than seedlings is essential because resistance is not reliably passed on during sexual reproduction. (However, Cornelia Pinchot Wilson has told me that colleagues should soon publish articles demonstrating that resistance is heritable.) Furthermore, the clones must be grown for several years—often five or more—until the trees are large enough to be tested for resistance. The article does not indicate whether the earlier step of propagating elm clones is challenging or easy. It has been difficult for other species, e.g., chestnuts, whitebark pine, ash, and koa.
To confirm whether a specific tree is resistant, the team typically tests each tree twice, since environmental factors like location and weather can influence outcomes. Trees that pass these tests move on to the next stage: reintroduction plantings in natural areas and parks. These field-growing trees serve two important roles. First, they contribute to restoring elm populations in natural and urban landscapes. Second, these trees can be observed over the long term to confirm whether they exhibit persistent resistance and are adapted to local environmental conditions.
The project’s success, the researchers say, hinges on collaboration. State agencies, local governments, and community members all play critical roles. Among those helping have been the Minnesota Department of Natural Resources, the Izaak Walton League, and Three Rivers Park District.
The article reminds us that resistance breeding is a long-term process. As noted above, clones must be grown for years before they can be tested.
Also, resistant trees aren’t immune to the pathogen. Instead, they survive despite the disease in sufficient numbers to restore the species to some of its former range and ecological role.
Finally, the trees must also survive the every-day challenges of life as a tree: storms, animal feeding, and other pests and diseases – native and non-native. The article mentions elm yellows disease but not elm zig-zag sawfly which has been moving West (it was detected in Ohio in 2023 and Wisconsin in 2024). Nor does it mention the fungus Plenodomus tracheiphilus, which is killing American elms in Alberta. Breeding program staff can help – for example, the Minnesota program now uses larger protective tubes to better shield the young trees from wildlife.
The Minnesota program plans to establish seed orchards. They hope that by planting trees confirmed to be resistant near to each other, they will cross-pollinate and produce seeds that are more likely to carry resistance, possibly even combining different resistance genes. Trees in these orchards would capture a broad range of resistance traits, helping future generations of elms stand strong against Dutch elm disease.
Program leaders Ryan Murphy and Ben Held also hope new technologies for studying genes will enable discovery of the genetic basis for resistance to DED. Identifying resistance genes or markers would make producing resistant trees a lot easier. It would also enable breeders to build up genetic diversity more deliberately.
SOURCES
Bernhardt, C. 2025. “Reviving a Giant” July 2025. University of Minnesota. Minnesota Invasive Terrestrial Plants and Pest Center. Website?
Knight, K.S., L.M. Haugen, C.C. Pinchot, P.G. Schaberg, & J.M. Slavicek. Undated. American elm (Ulmus americana) in restoration plantings: a review.
Wei, X.; Zhang, J.; Conrad, A.O.; Flower, C.E.; Pinchot, C.C.; Hayes-Plazolles, N.; Chen, Z.; Song, Z.; Fei, S.; Jin, J. Machine Learning-based Spectral and Spatial Analysis of Hyper-and multi-spectral Leaf Images for Dutch Elm Disease Detection and Resistance Screening. Artif. Intell. Agric. 2023, 10, 26–34. https://doi.org/10.1016/j.aiia.2023.09.003.
Posted by Faith Campbell
We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.
For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at http://treeimprovement.utk.edu/FadingForests.htm
The Trump Administration’s budget for Fiscal Year 2026 [which begins at the end of September 2025] proposes to eliminate funding for nearly all USFS research & Forest Health Protection.
Proposed Cuts to USFS Research: Timber the Sole Aim
In a letter from Office of Management and Budget (OMB) to Senate Appropriations Committee Chair Susan Collins (R-Maine, Director Russell Vought says the Administration wants to manage National forests “for their intended purpose of producing timber” and that the research and development program “is out of step with the practical needs of forest management for timber production.” The Administration proposes to eliminate funding for USFS research projects other than the small portion covering Forest Inventory and Analysis.
I understand that the USFS Chief told various NGOs that his job is to run the National Forest System, increase timber production by 40%, and do nothing else.
This single aim conflicts with the 1897 legislation founding and authorizing the USFS. It also violates provisions of subsequent legislation such as the Multiple-Use Sustained-Yield Act of 1960 and the National Forest Management Act of 1976. It also departs from long-standing US Forest Service policy – which is the intention.
The “intended purpose” of establishing “forest reserves” [which were later renamed National forests] has never been solely for timber production. The “Organic Act” of 1897 provided that any new forest reserves would have to meet the criteria of forest protection, watershed protection, and timber production.
Specifically, theORGANIC ACT OF 1897 [PUBLIC–No.2.] says:
“[All public lands heretofore designated and reserved by the President of the US under the provisions of the Act [of] March 3rd 1891, the orders for which shall be and remains in full force and effect, unsuspended and unrevoked, and all public lands that may hereafter be set aside as public forest reserves under said act, [these were the “forest reserves,”predecessors of “National Forests]” shall be as far as practicable controlled and administered in accordance with the following provisions:
“No public forest reservation shall be established, except to improve and protect the forest within the reservation, or for the purpose of securing favorable conditions of water flows, and to furnish a continuous supply of timber for the use and necessities of [US] citizens; but it is not the purpose or intent of these provisions, or of the Act providing for such reservations, to authorize the inclusion therein of lands more valuable for the mineral therein, or for agricultural purposes, than for forest purposes.”
The Department of the Interior, which then managed these forest reserves, promptly issued implementing regulations. The regulations stated that the “object” of forest reservations was:
“2. Public forest reservations are established to protect and improve the forests for the purpose of securing a permanent supply of timber for the people and insuring conditions favorable to continuous water flow.”
Therefore, I think the Administration has exaggerated the emphasis on timber production by calling it “the” intended purpose of the original establishment of National forests. The Administration has also chosen to ignore subsequent legislation, such as the Multiple-Use Sustained-Yield Act of 1960 and the National Forest Management Act of 1976.
Sec. 13 of the NFMA limits the sale of timber from each national forest to a quantity equal to or less than a quantity which can be removed from such forest annually in perpetuity on a sustained-yield basis. This limit might be exceeded under certain circumstances, but such excess must still be consistent with the multiple-use management objectives of the land management plan. Further, Sec. 14 requires public input into any decision to raise timber allowances.
During his period as Chief (1905 – 1910), Gifford Pinchot invented and applied the concept of “conservation” of natural resources. As a result “wise use” became accepted as the national goal.
Culminating more than a century of legislation and informed policy, the mission of the USDA Forest Service is to “sustain the health, diversity, and productivity of the nation’s forests and grasslands to meet the needs of present and future generations.”
Proposed Cuts to State, Private, and Tribal Forests
The budget also cuts $303 million from the State, Private, and Tribal Forests program. (I understand this zeroes out the entire program). The OMB Director alleges that the program has been “plagued by oversight issues, including allegation of impropriety by both the Agency and State governments.” I understand that this would eliminate the cooperative projects managed by the Forest Health Protection program, too.
Implications for Non-native Insects and Pathogens
Remember that USFS’s research and development program is intended to improve forest managers’ understanding of ecosystems, including human interactions and influences, thereby enabling improvements to the health and use of our Nation’s forests and grasslands. Most importantly to me, this program provides foundational knowledge needed to develop effective programs to prevent, suppress, mitigate, and eradicate the approximately 500 non-native insects and pathogens that are killing America’s trees.
The Forest Health Program provides technical and financial assistance to the states and other forest-management partners to carry out projects (designed based on the above research) intended to prevent, suppress, mitigate, and eradicate those non-native insects and pathogens. The program’s work on non-federal lands is crucial because introduced pests usually start their incursions near cities that receive imports (often transported in crates, pallets, or imported plants).
Eliminating either or both programs will allow these pests to cause even more damage to forest resources – including timber.
Both supporting research and on-the-ground management must address pest threats across all U.S. forests, including the more than 69% that are located on lands managed by others than the USFS. Already, the 15 most damaging of these pests threaten destruction of 41% of forest biomass in the “lower 48” states. This is a rate similar in magnitude to that attributed to fire (Fei et al. 2019). It is ironic that the Administration considers the fire threat to be so severe that it has proposed restructuring the government’s fire management structure.
I remind you that the existing USFS R&D budget allocates less than 1% of the total appropriation to studying a few of the dozens of highly damaging non-native pests. I have argued that this program should be expanded, not eliminated. Adequate funding might allow the USFS to design successful pest-management programs for additional pests (as suggested by Coleman et al.).
As a new international report (FAO 2025) notes, genetic resources underpin forests’ resilience, adaptability, and productivity. Funding shortfalls already undercut efforts to breed trees able to thrive despite introduced pests and climate change (the latter threat is still real, although the Administration disregards it). I have frequently urged the Congress to increase funding for USFS programs – which are sponsored primarily by the National Forest System and State, Private, and Tribal, although some are under the R&D program.
Please ask your Member of Congress and Senators to oppose these proposed cuts. Ask them to support continued funding for both USFS R&D and its State, Private, and Tribal Programs targetting non-native insects and pathogens. America’s forests provide resources to all Americans – well beyond only timber production and they deserve protection.
Contacting your Representative and Senators is particularly important if they serve on the Appropriations committees.
House Appropriations Committee members:
Republicans: AL: Robert Aderholt, Dale Strong; AR: Steve Womack; AZ: Juan Ciscomani; CA: Ken Calvert, David Valadao, Norma Torres; FL: Mario Diaz-Balart, John Rutherford, Scott Franklin; GA: Andrew Clyde; ID: Michael Simpson; IA: Ashley Hinson; KY: Harold Rogers; LA: Julia Letlow; MD: Andy Harris; MI: John Moolenaar; MO: Mark Alford; MS: Michael Guest; MT: Ryan Zinke; NC: Chuck Edwards; NV: Mark Amodei; NY: Nick LaLota; OH: David Joyce; OK: Tom Cole, Stephanie Bice; PA: Guy Reschenthaler TX: John Carter, Chuck Fleishmann, Tony Gonzales, Michael Cloud, Jake Ellzey; UT: Celeste Maloy; VA: Ben Cline; WA: Dan Newhouse; WV: Riley Moore
Democrats: CA: Pete Aguilar, Josh Harder, Mike Levin; CT: Rosa DeLauro; FL: Debbie Wasserman Schultz, Lois Frankel; GA: Sanford Bishop; HI: Ed Case IL: Mike Quigley, Lauren Underwood; IN: Frank Mrvan; MD: Steny Hoyer, Glenn Ivey; ME: Chellie Pingree; MN: Betty McCollum; NJ: Bonnie Watson Coleman NY: Grace Meng, Adriano Espaillat, Joseph Morelle; NV: Susie Lee; OH: Marcy Kaptur; PA: Madeleine Dean; SC: James Clyburn; TX: Henry Cuellar, Veronica Escobar; WA: Marie Gluesenkamp Perez; WI: Mark Pocan
Senate Appropriations Committee members:
Republicans: AK: Lisa Murkowski; AL: Katie Britt; AR: John Boozman (AR); KS: Jerry Moran; KY: Mitch McConnell; LA: John Kennedy; ME: Susan Collins; MS: Cindy Hyde-Smith; ND: John Hoeven; NE: Deb Fischer; OK: Markwayne Mullin; SC: Lindsey Graham; SD: Mike Rounds TN: Bill Hagerty; WV: Shelley Moore Capito;
Democrats: CT: Chris Murphy; DE: Chris Coons; GA: Jon Ossof; HI: Brian Schatz; IL: Richard Durbin; MD: Chris van Hollen; MI: Gary Peters; NH: Jeanne Shaheen; NM: Martin Heinrich; NY: Kirsten Gillibrand; OR: Jeff Merkley; RI: Jack Reed; WA: Patty Murray; WI: Tammy Baldwin
SOURCES
Coleman, T.W, A.D. Graves, B.W. Oblinger, R.W. Flowers, J.J. Jacobs, B.D. Moltzan, S.S. Stephens, R.J. Rabaglia. 2023. Evaluating a decade (2011–2020) of integrated forest pest management in the United States. Journal of Integrated Pest Management, (2023) 14(1): 23; 1–17
FAO. 2025. The Second Report on the State of the World’s Forest Genetic Resources. FAO Commission on Genetic Resources for Food and Agriculture Assessments, 2025. Rome.
Fei, S., R.S. Morin, C.M. Oswalt, and A.M. 2019. Biomass losses resulting from insect and disease invasions in United States forests. PNAS August 27, 2019. Vol. 116 No. 35 17371–17376
Posted by Faith Campbell
We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.
For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at https://treeimprovement.tennessee.edu/
ʻŌhiʻa trees killed by ROD; photo by Richard Sniezko, USFS
Several Hawaiian tree species are at risk due to introduced forest pests. Two of the Islands’ most widespread species are among the at-risk taxa. Their continuing loss would expose watersheds on which human life and agriculture depend. Habitats for hundreds of other species – many endemic and already endangered – would lose their foundations. These trees also are of the greatest cultural importance to Native Hawaiians.
I am pleased to report that Hawaiian scientists and conservationists are trying to protect and restore them.
Other tree species enjoy less recognition … and efforts to protect them have struggled to obtain support.
1) koa (Acacia koa)
Koa is both a dominant canopy tree and the second-most abundant native tree species in Hawai`i in terms of areas covered. The species is endemic to the Hawaiian archipelago. Koa forests provide habitat for 30 of the islands’ remaining 35 native bird species, many of which are listed under the U.S. Endangered Species Act. Also dependent on koa forests are native plant and invertebrate species and the Islands’ only native terrestrial mammal, the Hawaiian hoary bat. Finally, koa forests protect watersheds, add nitrogen to degraded soils, and store carbon [Inman-Narahari et al.]
Koa forests once ranged from near sea level to above 7000 ft (2100 m) on both the wet and dry sides of all the large Hawaiian Islands. Conversion of forests to livestock grazing and row-crop agriculture has reduced koa’s range. Significant koa forests are now found on four islands – Hawai’i, Maui, O‘ahu, and Kauaʻi. More than 90% of the remaining koa forests occur on Hawai`i Island (the “Big Island) [Inman-Narahari et al.]
In addition to its fundamental environmental role, koa has immense cultural importance. Koa represents strength and the warrior spirit. The wood was used traditionally to make sea-going canoes. Now Koa is widely used for making musical instruments, especially guitars and ukuleles; furniture, surfboards, ornaments, and art [Inman-Narahari et al.]
Koa timber has the highest monetary value of any wood harvested on the Islands. However, supplies of commercial-quality trees are very limited (Dudley et al. 2020). Harvesting is entirely from old-growth forests on private land. [Inman-Narahari et al.]
Koa forests are under threat by a vascular wilt disease caused by Fusarium oxysporum f. sp. koae (FOXY). This disease can kill up to 90% of young trees and – sometimes — mature trees in native forests. The fungus is a soil-dwelling organism that spreads in soil and infects susceptible plants through the root system (Dudley et al. 2020).
Conservation and commercial considerations have converged to prompt efforts to breed koa resistant to FOXY. Conservationists hope to restore native forests on large areas where agriculture has declined. The forestry industry seeks to enhance supplies of the Islands’ most valuable wood. Finally, science indicated that a breeding program would probably be successful. Field trials in the 1990s demonstrated great differences in wilt-disease mortality among seed sources (the proportion of seedlings surviving inoculation ranged from 4% to 91.6%) [Sniezko 2003; Dudley et al. 2009].
In 2003, Dudley and Sniezko outlined a long-term strategy for exploring and utilizing genetic resistance in koa. Since then, a team of scientists and foresters has implemented different phases of the strategy and refined it further (Dudley et al. 2012, 2015, 2017; Sniezko et al. 2016]
First, scientists determined that the wilt disease is established on the four main islands. Having obtained more than 500 isolates of the pathogen from 386 trees sampled at 46 sites, scientists tested more than 700 koa families from 11 ecoregions for resistance against ten of the most highly virulent isolates (Dudley et al. 2020).
The Hawaiian Agricultural Research Center (HARC), supported by public and private partners, has converted the field-testing facilities on Hawai`i, Maui, and Oahu into seed orchards. The best-performing tree families are being grown to maturity to produce seeds for planting. It is essential that the seedlings be not just resistant to FOXY but also adapted to the ecological conditions of the specific site where they are to be planted [Dudley et al. 2020; Inman-Narahari et al. ] Locally adapted, wilt-resistant seed has been planted on Kauaʻi and Hawai`i. Preparations are being made to plant seed on Maui and O‘ahu also. Scientists are also exploring methods to scale up planting in both restoration and commercial forests [R. Hauff pers. comm.].
koa; photo by David Eickhoff via Flickr
Restoration of koa on the approximately half of lands in the species’ former range that are privately owned will require that the trees provide superior timber. Private landowners might also need financial incentives since the rotation time for a koa plantation is thought to be 30-80 years. [Inman-Narahari et al.]
Plantings on both private and public lands will need to be protected from grazing by feral ungulates and encroachment by competing plants. These management actions are intensive, expensive, and must be maintained for years.
Some additional challenges are scientific: uncertainties about appropriate seed zones, efficacy of silvicultural approaches to managing the disease, and whether koa can be managed for sustainable harvests. Human considerations are also important: Hawai`i lacks sufficient professional tree improvement or silvicultural personnel, a functioning seed distribution and banking network — and supporting resources. Finally, some segments of the public oppose ungulate control programs. Inman-Narahari et al.
Finally, scientists must monitor seed orchards and field plantings for any signs of maladaptation to climate change. (Dudley et al. 2020).
2) ʻŌhiʻa Metrosideros polymorpha)
ʻŌhiʻa lehua is the most widespread tree on the Islands. It dominates approximately 80% the biomass of Hawaii’s remaining native forest, in both wet and dry habitats. ʻŌhiʻa illustrates adaptive radiation and appears to be undergoing incipient speciation. The multitude of ecological niches and their isolation on the separate islands has resulted in five recognized species in the genus Metrosideros. Even the species found throughout the state, Metrosideros polymorpha, has eight recognized varieties (Luiz et al. (2023) (some authorities say there are more).
Loss of this iconic species could result in significant changes to the structure, composition, and potentially, the function, of forests on a landscape level. High elevation ‘ohi‘a forests protect watersheds across the state. ʻŌhiʻa forests shelter the Islands’ one native terrestrial mammal (Hawaiian hoary bat), 30 species of forest birds, and more than 500 endemic arthropod species. Many species in all these taxa are endangered or threatened (Luiz et al. 2023). The increased light penetrating interior forests following canopy dieback facilitates invasion by light-loving non-native plant species, of which Hawai`i has dozens. There is perhaps no other species in the United States that supports more endangered taxa or that plays such a geographical dominant ecological keystone role [Luiz et al. 2023]
For many Native Hawaiians, ‘ōhi‘a is a physical manifestation of multiple Hawaiian deities and the subject of many Hawaiian proverbs, chants, and stories; and foundational to the scared practice of many hula. The wood has numerous uses. Flowers, shoots, and aerial roots are used medicinally and for making lei. The importance of the biocultural link between ‘ōhi‘a and the people of Hawai`i is described by Loope and LaRosa (2008) and Luiz et al. (2023).
In 2010 scientists detected rapid mortality affecting ‘ōhi‘a on Hawai‘i Island. Scientists determined that the disease is caused by two recently-described pathogenic fungi, Ceratocystis lukuohia and Ceratocystis huliohia. The two diseases, Ceratocystis wilt and Ceratocystis canker of ʻōhiʻa, are jointly called “rapid ‘ōhi‘a death”, or ROD. The more virulent species, C. lukuohia, has since spread across Hawai`i Island and been detected on Kaua‘i. The less virulent C. huliohia is established on Hawai`i and Kaua‘i and in about a dozen trees on O‘ahu. One tree on Maui was infected; it was destroyed, and no new infection has been detected [M. Hughes pers. comm.] As of 2023, significant mortality has occurred on more than one third of the vulnerable forest on Hawai`i Island, although mortality is patchy.
[ʻŌhiʻa is also facing a separate disease called myrtle rust caused by the fungus Austropuccinia psidii; to date this rust has caused less virulent infections on ‘ōhi‘a.]
rust-killed ‘ōhi‘a in 2016; photo by J.B. Friday
Because of the ecological importance of ‘ōhi‘a and the rapid spread of these lethal diseases, research into possible resistance to the more virulent pathogen, C. lukiohia began fairly quickly, in 2016. Some ‘ōhi‘a survive in forests on the Big Island in the presence of ROD, raising hopes that some trees might possess natural resistance. Scientists are collecting germplasm from these lightly impacted stands near high-mortality stands (Luiz et al. 2023). Five seedlings representing four varieties of M. polymorpha that survived several years’ exposure to the disease are being used to produce rooted cuttings and seeds for further evaluation of these genotypes.
ʻŌhiʻa flowers
Encouraged by these developments, and recognizing the scope of additional work needed, in 2018 stakeholders created a collaborative partnership that includes state, federal, and non-profit agencies and entities, ʻŌhiʻa Disease Resistance Program (‘ODRP) (Luiz et al. 2023). The partnership seeks to provide baseline information on genetic resistance present in all Hawaiian taxa in the genus Metrosideros. It aims further to develop sources of ROD-resistant germplasm for restoration intended to serve several purposes: cultural plantings, landscaping, and ecological restoration. ‘ODRP is pursuing screenings of seedlings and rooted cuttings sampled from native Metrosideros throughout Hawai`i while trying to improve screening and growing methods. Progress will depend on expanding these efforts to include field trials; research into environmental and genetic drivers of susceptibility and resistance; developing remote sensing and molecular methods to rapidly detect ROD-resistant individuals; and support already ongoing Metrosideros conservation. If levels of resistance in wild populations prove to be insufficient, the program will also undertake breeding (Luiz et al. 2023).
To be successful, ‘ODRP must surmount several challenges (Luiz et al. 2022):
increase capacity to screen seedlings from several hundred plants per year to several thousand;
optimize artificial inoculation methodologies;
determine the effects of temperature and season on infection rates and disease progression;
find ways to speed up seedlings’ attaining sufficient size for testing;
develop improved ways to propagate ʻōhiʻa from seed and rooted cuttings;
establish sites for field testing of putatively resistant trees across a wide range of climatic and edaphic conditions;
establish seed orchard, preferably on several islands;
establish systems for seed collection from the wide variety of subspecies/varieties;
if breeding to enhance resistance is appropriate, it will be useful to develop high-throughput phenotyping of the seed orchard plantings.
Developing ROD-resistant ‘ōhi‘a is only one part of a holistic conservation program. Luiz et al. (2023) reiterate the importance of quarantines and education to curtail movement of infected material and countering activities that injure the trees. Fencing to protect these forests from grazing by feral animals can drastically reduce the amount of disease. Finally, scientists must overcome the factors there caused the almost complete lack of natural regeneration of ‘ōhi‘a in lower elevation forests. Most important are competition by invasive plants, predation by feral ungulates, and the presence of other diseases, e.g., Austropuccinia psidii.
Hawaii’s dryland forests are highly endangered: more than 90% of dry forests are already lost due to habitat destruction and the spread of invasive plant and animal species. Two tree species are the focus of species-specific programs aimed at restoring them to remaining dryland forests. However, support for both programs seems precarious and requires stable long-term funding; disease resistance programs often necessitate decades-long endeavors.
naio in bloom; photo by Forrest & Kim Starr via Creative Commons
1) naio (Myoporum sandwicense)
Naio grows on all of the main Hawaiian Islands at elevations ranging from sea level to 3000 m. While it occurs in the full range of forest types from dry to wet, naio is one of two tree species that dominate upland dry forests. The other species is mamane, Sophora chrysophylla. Naio is a key forage tree for two endangered honeycreepers, palila (Loxioides bailleui) and `akiapola`au (Hemignathus munroi). The tree is also an important host of many species of native yellow-face bees (Hylaeus spp). Finally, loss of a native tree species in priority watersheds might lead to invasions by non-native plants that consume more water or increase runoff.
The invasive non-native Myoporum thrips, Klambothrips myopori, was detected on Hawai‘i Island in December 2008 (L. Kaufman website). In 2018 the thrips was found also on Oahu (work plan). The Myoporum thrips feeds on and causes galls on plants’ terminal growth. This can eventually lead to death of the plant.
Aware of thrips-caused death of plants in the Myoporum genus in California, the Hawaii Department of Lands and Natural Resources Division of Forestry and Wildlife and the University of Hawai‘i began efforts to determine the insect’s distribution and infestation rates, as well as the overall health of naio populations on the Big Island. This initiative began in September 2010, nearly two years after the thrips’ detection. Scientists monitored nine protected natural habitats for four years. This monitoring program was supported by the USFS Forest Health Protection program. This program is described by Kaufman.
naio monitoring sites from L. Kaufman article
The monitoring program determined that by 2013, the thrips has spread across most of Hawi`i Island, on its own and aided by human movement of landscaping plants. More than 60% of trees being monitored had died. Infestation and dieback levels had both increased, especially at medium elevation sites. The authors feared that mortality at high elevations would increase in the future. They found no evidence that natural enemies are effective controlling naio thrips populations on Hawai`i Island.
Kaufman was skeptical that biological control would be effective. She suggested, instead, a breeding program, including hybridizing M. sandwicensis with non-Hawaiian Myoporum species that appear to be resistant to thrips. Kaufman also called for additional programs: active monitoring to prevent thrips from establishing on neighboring islands; and collection and storage of naio seeds.
Ten years later, in February 2024, DLNR Division of Forestry and Wildlife adopted a draft work plan for exploring possible resistance to the Myoporum thrips. Early steps include establishing a database to record data needed to track parent trees, associated propagules, and the results of tests. These data are crucial to keeping track of which trees show the most promise. Other actions will aim to hone methods and processes. Among practical questions to be answered are a) whether scientists can grow even-aged stands of naio seedlings; b) identifying the most efficient resistance screening techniques; and c) whether K. myopori thrips are naturally present in sufficient numbers to be used in tests, or – alternatively – whether they must be augmented. [Plan]
Meanwhile, scientists have begun collecting seed from unaffected or lightly affected naio in hotspots where mortality is high. They have focused on the dry and mesic forests of the western side of Hawai`i (“Big”) Island, where the largest number of naio populations still occur and are at high risk. Unfortunately, these “lingering” trees remain vulnerable to other threats, such as browsing by feral ungulates, competition with invasive plants, drought, and reduced fecundity & regeneration.
Hawai`i DLNR has secured initial funding from the Department of Defense’s REPI program to begin a pest resistance project and is seeking a partnership with University of Hawai`i to carry out tests “challenging” different naio families’ resistance to the thrips [R. Hauff pers. comm.]
wiliwili; photo by Forrest & Kim Starr
2) wiliwili (Erythrina sandwicensis)
Efforts to protect the wiliwili have focused on biological control. The introduced Erythrina gall wasp, Quadrastichus erythrinae (EGW) was detected on the islands in 2005. It immediately caused considerable damage to the native tree and cultivated nonnative coral trees.
A parasitic wasp, Eurytoma erythrinae, was approved for release in November 2008 – only 3 ½ years after EGW was detected on O‘ahu. The parasitic wasp quickly suppressed the gall wasp’s impacts to both wiliwili trees and non-native Erythrina. By 2024, managers are once again planting the tree in restoration projects.
However, both the gall wasp and a second insect pest – a bruchid, Specularius impressithorax – can cause loss of more than 75% of the seed crop. This damage means that the tree cannot regenerate. By 2019, Hawaiian authorities began seeking permission to release a second biocontrol gent, Aprostocitus nites.Unfortunately, the Hawai’i Department of Agriculture still has not approved the release permit despite five years having passed. Once they have this approval, the scientists will then need to ask USDA Animal and Plant Health Inspection Service (APHIS) for its approval [R. Hauff, pers. comm.]
SOURCES
www.RapidOhiaDeath.org
Dudley, N., R. James, R. Sniezko, P. Cannon, A. Yeh, T. Jones, & Michael Kaufmann. 2009? Operational Disease Screening Program for Resistance to Wilt in Acacia koa in Hawai`i. Hawai`i Forestry Association Newsletter August 29 2009
Dudley, N., T. Jones, K. Gerber, A.L. Ross-Davis, R.A. Sniezko, P. Cannon & J. Dobbs. 2020. Establishment of a Genetically Diverse, Disease-Resistant Acacia koa Seed Orchard in Kokee, Kauai: Early Growth, Form, & Survival. Forests 2020, 11, 1276; doi:10.3390/f11121276 www.mdpi.com/journal/forests
Friday, J. B., L. Keith, and F. Hughes. 2015. Rapid ʻŌhiʻa Death (Ceratocystis Wilt of ʻŌhiʻa). PD-107, College of Tropical Agriculture and Human Resources, University of Hawai‘i, Honolulu, HI. URL: https://www.ctahr.HI.edu/oc/freepubs/pdf/PD-107.pdf Accessed April 3, 2018.
Friday, J.B. 2018. Rapid ??hi?a Death Symposium -West Hawai`i (“West Side Symposium”) March 3rd 2018, https://vimeo.com/258704469 Accessed April 4, 2018 (see also full video archive at https://vimeo.com/user10051674)
Inman-Narahari, F., R. Hauff, S.S. Mann, I. Sprecher, & L. Hadway. Koa Action Plan: Management & research priorities for Acacia koa forestry in Hawai`i. State of Hawai`i Department of Land & Natural Resources Division of Forestry & Wildlife no date
Kaufman, L.V, J. Yalemar, M.G. Wright. In press. Classical biological control of the erythrina gall wasp, Quadrastichus erythrinae, in Hawaii: Conserving an endangered habitat. Biological Control. Vol. 142, March 2020
Loope, L. and A.M. LaRosa. 2008. ‘Ohi’a Rust (Eucalyptus Rust) (Puccinia psidii Winter) Risk Assessment for Hawai‘i.
Luiz, B.C. 2017. Understanding Ceratocystis. sp A: Growth, morphology, and host resistance. MS thesis, University of Hawai‘i at Hilo.
Luiz, B.C., C.P. Giardina, L.M. Keith, D.F. Jacobs, R.A. Sniezko, M.A. Hughes, J.B. Friday, P. Cannon, R. Hauff, K. Francisco, M.M. Chau, N. Dudley, A. Yeh, G. Asner, R.E. Martin, R. Perroy, B.J. Tucker, A. Evangelista, V. Fernandez, C. Martins-Keli’iho.omalu, K. Santos, R. Ohara. 2023. A framework for establishlishing a rapid ‘Ohi‘a death resistance program New Forests 54, 637–660. https://doi.org/10.1007/s11056-021-09896-5
Sniezko, R.A., N. Dudley, T. Jones, & P. Cannon. 2016. Koa wilt resistance & koa genetics – key to successful restoration & reforestation of koa (Acacia koa). Acacia koa in Hawai‘i: Facing the Future. Proceedings of the 2016 Symposium, Hilo, HI: www.TropHTIRC.org , www.ctahr.HI.edu/forestry
Posted by Faith Campbell
We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.
For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at https://treeimprovement.tennessee.edu/
The number of introduced forest pathogens are increasing – creating a crisis that is recognized by more scientists. These experts say tree diseases are reshaping both native and planted forests around the globe. The diseases are threatening biodiversity, ecosystem services, provision of products, and related human wellbeing. Some suggest that bioinvasions might threaten forests as much as climate change, while also undermining forests’ role in carbon sequestration.
Unfortunately, I see little willingness within the plant health regulatory community to tackle improving programs to slow introductions. Even when the scientists documenting the damage work for the U.S. Department of Agriculture – usually the U.S. Forest Service — USDA policy-makers don’t act on their findings. [I tried to spur a conversation with USDA 2 years ago. So far, no response.]
counties where beech leaf disease has been detected
What the scientists say about these pests’ impacts
Andrew Gougherty (2023) – one of the researchers employed by the USDA Forest Service – says that emerging infectious tree diseases are reshaping forests around the globe. Furthermore, new diseases are likely to continue appearing in the future and threaten native and planted forests worldwide. [Full references are provided at the end of the blog.] Haoran Wu (2023/24) – a Master’s Degree student at Oxford University – agrees that arrival of previously unknown pathogens are likely to alter the structure and composition of forests worldwide. Weed, Ayers, and Hicke (2013) [academics] note that forest pests — native and introduced — are the dominant sources of disturbance to North American forests. They suggest that, globally, bioinvasions might be at least as important as climate change as threats to the sustainability of forest ecosystems. They are concerned that recurrent forest disturbances caused by pests might counteract carbon mitigation strategies.
Scientists have proclaimed these warnings for years. Five years ago, Fei et al. (2019) reported that the 15 most damaging pests introduced to the United States — cumulatively — had already caused tree mortality to exceed background levels by 5.53 teragrams of carbon per year. As these 15 pests spread and invasions intensify, they threaten 41.1% of the total live forest biomass in the 48 coterminous states. Poland et al. (2019) (again – written by USFS employees) document the damage to America’s forest ecosystems caused by the full range of invasive species, terrestrial and aquatic.
Fei et al. and Weed, Ayers, and Hicke (2013) also support the finding that old, large trees are the most important trees with regard to carbon storage. This understanding leads them to conclude that the most damaging non-native pests are the emerald ash borer, Dutch elm disease fungi, beech bark disease, and hemlock woolly adelgid. As I pointed out in earlier blogs, other large trees, e.g., American chestnut and several of the white pines, were virtually eliminated from much of their historical ranges by non-native pathogens decades ago. These same large, old, trees also maintain important aspects of biological diversity.
It is true that not all tree species are killed by any particular pest. Some tree genera or species decrease while others thrive, thus altering the species composition of the affected stands (Weed, Ayers, and Hicke). This mode of protection is being undermined by the proliferation of insects and pathogens that cumulatively attack ever more tree taxa. And while it is true that some of the carbon storage capacity lost to pest attack will be restored by compensatory growth in unaffected trees, this faster growth is delayed by as much as two or more decades after pest invasions begin (Fei et al.).
ash forest after EAB infestation; Photo by Nate Siegert, USFS
Still, despite the rapid rise of destructive tree pests and disease outbreaks, scientists cannot yet resolve critical aspects of pathogens’ ecological impacts or relationship to climate change. Gougherty notes that numerous tree diseases have been linked to climate change or are predicted to be impacted by future changes in the climate. However, various studies’ findings on the effects of changes in moisture and precipitation are contradictory. Wu reports that his study of ash decline in a forest in Oxfordshire found that climate change will have a very small positive impact on disease severity through increased pathogen virulence. Weed, Ayers, and Hicke go farther, making the general statement that despite scientists’ broad knowledge of climate effects on insect and pathogen demography, they still lack the capacity to predict pest outbreaks under climate change. As a result, responses intended to maintain ecosystem productivity under changing climates are plagued by uncertainty.
Clarifying how disease systems are likely to interact with predicted changes in specific characteristics of climate is important — because maintaining carbon storage levels is important. Quirion et al. (2021) estimate that, nation-wide, native and non-native pests have decreased carbon sequestration by live forest trees by at least 12.83 teragrams carbon per year. This equals approximately 9% of the contiguous states’ total annual forest carbon sequestration and is equivalent to the CO2 emissions from more than 10 million passenger vehicles driven for one year. Continuing introductions of new pests, along with worsening effects of native pests associated with climate change, could cause about 30% less carbon sequestration in living trees. These impacts — combined with more frequent and severe fires and other forest disturbances — are likely to negate any efforts to improve forests’ capacity for storing carbon.
Understanding pathogens’ interaction with their hosts is intrinsically complicated. There are multiple biological and environmental factors. What’s more, each taxon adapts individually to the several environmental factors. Wu says there is no general agreement on the relative importance of the various environmental factors. The fact that most forest diseases are not detected until years after their introduction also complicates efforts to understand factors affecting infection and colonization.
The fungal-caused ash decline in Europe is a particularly alarming example of the possible extent of such delays. According to Wu, when the disease was first detected – in Poland in 1992 – it had already been present perhaps 30 years, since the 1960s. Even then, the causal agent was not isolated until 2006 – or about 40 years after introduction. The disease had already spread through about half the European continent before plant health officials could even name the organism. The pathogen’s arrival in the United Kingdom was not detected until perhaps five years after its introduction – despite the country possessing some of the world’s premier forest pathologists who by then (2012) knew what they to look for.
Clearly, improving scientific understanding of forest pathogens will be difficult. In addition, effective policy depends on understanding the social and economic drivers of trade, development, and political decisions are primary drivers of the movement of pathogens. Wu calls for collaboration of ecologists, geneticists, earth scientists, and social scientists to understand the complexity of the host-pathogen-surrounding system. Bringing about this new way of working and obtaining needed resources will take time – time that forests cannot afford.
However, Earth’s forests are under severe threat now. Preventing their collapse depends on plant health officials integrating recognition of these difficulties into their policy formulation. It is time to be realistic: develop and implement policies that reflect the true level of threat and limits of current science.
Background: Rising Numbers of Introductions
Gougherty’s analysis of rising detections of emerging tree diseases found little evidence of saturation globally – in accord with the findings of Seebens et al. (2017) regarding all taxa. Relying on data for 24 tree genera, nearly all native to the Northern Hemisphere, Gougherty found that the number of new pests attacking these tree genera are doubling on average every 11.2 years. Disease accumulation is increasing rapidly in both regions where hosts are native and where they are introduced, but more rapidly in trees’ native ranges.This finding is consistent with most new diseases arise from introductions of pathogens to naïve hosts.
Gougherty says his estimates are almost certainly underestimates for a number of reasons. Countries differ in scientific resources and their scientists’ facility with English. Scientists are more likely to notice and report high-impact pathogens and those in high-visibility locations. Where national borders are closer, e.g., in Europe, a minor pest expansion can be reported as “new” in several countries. New pathogens in North America appear to occur more slowly, possibly because the United States and Canada are very large. He suggests that another possible factor is the U.S. (I would add Canada) have adopted pest-prevention regulations that might be more effective than those in place in other regions. (See my blogs and the Fading Forest reports linked to below for my view of these measures’ effectiveness.)
ash dieback in the UK
Wu notes that reports of tree pathogens in Europe began rising suddenly after the 1980s. He cites the findings by Santini et al. (2012) that not only were twice as many pathogens detected in the period after 1950 than in the previous 40 years, the region of origin also changed. During the earlier period, two-thirds of the introduced pathogens came from temperate North America. After 1950, about one-third of previously unknown disease agents were from temperate North America. Another one-third was from Asia. By 2012, more than half of plant infectious diseases were caused by introduction of previously unknown pathogens.
What is to be done?
Most emerging disease agents do not have the same dramatic effects as chestnut blight in North America, ash dieback in Europe, or Jarrah dieback in Australia. Nevertheless, as Gougherty notes, their continued emergence in naïve biomes increases the likelihood of especially damaging diseases emerging and changing forest community composition.
Gougherty calls for policies intended to address both the agents being introduced through trade, etc., and those that emerge from shifts in virulence or host range of native pathogens or changing environmental conditions. In his view, stronger phytosanitary programs are not sufficient.
Wu recommends enhanced monitoring of key patterns of biodiversity and ecosystem functioning, He says these studies should focus on the net outcome of complex interactions. Wu also calls for increasing understanding of key “spillover” effects – outcomes that cannot be currently assessed but might impact the predicted outcome. He lists several examples:
the effects of drought–disease interactions on tree health in southern Europe,
interaction between host density and pathogen virulence,
reproductive performance of trees experiencing disease,
effect of secondary infections,
potential for pathogens to gain increased virulence through hybridization.
potential for breeding resistant trees to create a population buffer for saving biological diversity. Wu says his study of ash decline in Oxfordshire demonstrates that maintaining a small proportion of resistant trees could help tree population recovery.
Quirion et al. provide separate recommendations with regard to native and introduced pests. To minimize damage from the former, they call for improved forest management – tailored to the target species and the environmental context. When confronting introduced pests, however, thinning is not effective. Instead, they recommend specific steps to minimize introductions via two principal pathways, wood packaging and imports of living plants. In addition, since even the most stringent prevention and enforcement will not eliminate all risk, Quirion et al. advocate increased funding for and research into improved strategies for inspection, early detection of new outbreaks, and strategic rapid response to newly detected incursions. Finally, to reduce impacts of established pests, they recommend providing increased and more stable funding for classical biocontrol, research into technologies such as sterile-insect release and gene drive, and host resistance breeding.
USDA HQ
Remember: reducing forest pest impacts can simultaneously serve several goals—carbon sequestration, biodiversity conservation, and perpetuating the myriad economic and societal benefits of forests. See Poland et al. and the recent IUCN report on threatened tree species.
SOURCES
Barrett, T.M. and G.C. Robertson, Editors. 2021. Disturbance and Sustainability in Forests of the Western United States. USDA Forest Service Pacific Northwest Research Station. General Technical Report PNW-GTR-992. March 2021
Clark, P.W. and A.W. D’Amato. 2021. Long-term development of transition hardwood and Pinus strobus – Quercus mixedwood forests with implications for future adaptation and mitigation potential. Forest Ecology and Management 501 (2021) 119654
Fei, S., R.S. Morin, C.M. Oswalt, and A.M. 2019. Biomass losses resulting from insect and disease invasions in United States forests. Proceedings of the National Academy of Sciences. www.pnas.org/cgi/doi/10.1073/pnas.1820601116
Gougherty AV (2023) Emerging tree diseases are accumulating rapidly in the native and non-native ranges of Holarctic trees. NeoBiota 87: 143–160. https://doi.org/10.3897/neobiota.87.103525
Lovett, G.M., C.D. Canham, M.A. Arthur, K.C. Weathers, and R.D. Fitzhugh. 2006. Forest Ecosystem Responses to Exotic Pests and Pathogens in Eastern North America. BioScience Vol. 56 No. 5 May 2006
Lovett, G.M., M. Weiss, A.M. Liebhold, T.P. Holmes, B. Leung, K.F. Lambert, D.A. Orwig, F.T. Campbell, J. Rosenthal, D.G. MCCullough, R. Wildova, M.P. Ayres, C.D. Canham, D.R. Foster, S.L. Ladeau, and T. Weldy. 2016. Nonnative forest insects and pathogens in the United States: Impacts and policy options. Ecological Applications, 26(5), 2016, pp. 1437-1455
Poland, T.M., Patel-Weynand, T., Finch, D., Miniat, C. F., and Lopez, V. (Eds) (2019), Invasive Species in Forests and Grasslands of the United States: A Comprehensive Science Synthesis for the United States Forest Sector. Springer Verlag.
Quirion, B.R., G.M. Domke, B.F. Walters, G.M. Lovett, J.E. Fargione, L. Greenwood, K. Serbesoff-King, J.M. Randall, and S. Fei. 2021 Insect and Disease Disturbance Correlate With Reduced Carbon Sequestration in Forests of the Contiguous US. Front. For. Glob. Change 4:716582. [Volume 4 | Article 716582] doi: 10.3389/ffgc.2021.716582
Weed, A.S., M.P. Ayers, and J.A. Hicke. 2013. Consequences of climate change for biotic disturbances in North American forests. Ecological Monographs, 83(4), 2013, pp. 441–470
Wu, H. 2023/24. Modelling Tree Mortality Caused by Ash Dieback in a Changing World: A Complexity-based Approach MSc/MPhil Dissertation Submitted August 12, 2024. School of Geography and the Environment, Oxford University.
Posted by Faith Campbell
We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.
For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at https://treeimprovement.tennessee.edu/
Oregon ash dominate wetlands of Ankeny NWR; photo by Wyatt Williams, Oregon Department of Forestry
One of these insects is the emerald ash borer (EAB). We easterners have “been there & done that”. However, programs aimed at conserving wetlands and riparian areas of the Western states – and the associated species — are at least as vulnerable to loss of ash. Worse, other tree taxa, specifically oaks, and the open woodlands they inhabit — are also under threat. The ecological tragedies continue to affect ever more forests.
|Emerald Ash Borer in Oregon and British Columbia
The emerald ash borer (EAB; Agrilus planipennis) was detected in Oregon in June 2022. Officials had been expecting an introduction and had begun preparations. Unsurprisingly, the infestation is more widespread than known at first: detections in two new locations, fairly close to the original in Forest Grove, mean the infested area now occupies three neighboring counties — Washington, Yamhill, and Marion counties.
Oregon officials are trying to slow spread of EAB by removing infested trees. Surveys in Washington County had identified 190 infested ash trees; 80 were removed in April 2024. They treated healthy ash trees in Washington County with injections of the systemic insecticide emamectin benzoate. The effort was already a daunting task: the survey had disclosed 6,500 ash trees in the vicinity. The city of Portland – only 25 miles away – has 94,000 ash trees (Profita 2024).
In May, 2024 EAB was detected in the city of Vancouver in British Columbia. This detection in the sixth Canadian province adds to the threat to the ecosystems of the region. The Canadian Food Inspection Agency (CFIA) now regulates the movement of all ash material such as logs, branches, and woodchips, and all species of firewood, from the affected sites.
The CFIA is also conducting surveillance activities to determine where EAB might be present, and is collaborating with the City of Vancouver, the Vancouver Board of Parks and Recreation, the Province of British Columbia, and other stakeholders to respond to the detections and slow the spread of this pest.
Importance of Oregon ash (Fraxinus latifolia)
The Oregon ash is the only ash species native to the Pacific Northwest. Its range stretches from southern British Columbia to so California, where it has hybridized with velvet ash (F. velutina). It is highly susceptible to EAB attack; there is a high probability that Oregon ash could be rendered functionally extinct (Maze, Bond and Mattsson 2024). This vulnerability prompted the International Union for Conservation of Nature (IUCN) to classify Oregon ash as “near threatened” as long ago as 2017 (Melton et al. 2024).
Oregon ash typically grows in moist, bottomland habitats. There it is a late-successional climax species. In Oregon’s Willamette Valley and Washington’s Puget Trough, the tree improves streams’ water quality by providing shade, bank stabilization, and filtration of pollutants and excess nutrients. Maintaining these ecological services is particularly important because these streams are crucial to salmonids (salmon and trout) and other native aquatic species (Maze, Bond and Mattsson 2024).
So it is not surprising that one component of Oregonians’ pre-detection preparations was an analysis of the likely impact of widespread ash mortality on populations of salmon, trout, and other aquatic species. I summarize the key findings of Maze, Bond and Mattsson here.
According to this study, salmonids and other cold-water aquatic species suffer population declines and health effects when stream water temperatures are too warm. A critical factor in maintaining stream temperatures is shade – usually created by trees. In the Pacific Northwest many streams’ temperatures already exceed levels needed to protect sensitive aquatic species. A key driver of increased stream temperatures – at least in the Willamette Basin – is clearing of forests to allow agriculture.
Decreasing streams’ temperatures is not only a good thing to do; it is legally required by the Endangered Species Act because several salmon and steelhead trout species are listed. In one response, the Oregon Department of Environmental Quality recommends restoration and protection of riparian vegetation as the primary methods for increasing stream shading and mitigating increased stream temperatures in the lower Willamette Basin.
The forests shading many low-elevation forested wetlands and tributaries of the Willamette and lower Columbia rivers are often composed exclusively of Oregon ash. Loss of these trees’ shade will affect not just the immediate streams but also increase the temperature of mainstem waterways downstream.
Oregon ash – EAB detection site; photo by Wyatt Williams, Oregon Department of Forestry
Replacements for Oregon Ash?
The magnitude of the ecological impacts of ash mortality in the many forested wetlands in the Willamette Valley will largely be determined by what plant associations establish after the ash die. Oregon ash is uniquely able to tolerate soils inundated for extended periods. No native tree species can fill the void when the ash die. Oregon white oak (Quercus garryana), black cottonwood (Populus trichocarpa), and the alders (Alnus rubra and A. rhombifolia), are shade intolerant and unlikely to persist in later seral stages in some settings.
If non-native species fill the gaps, they will provide inferior levels of ecosystem services – I would think particularly regarding wildlife habitat and invertebrate forage. Maze, Bond and Mattsson expect loss of ash to trigger significant physical and chemical changes. These will directly impact water quality and alter native plant and animal communities’ composition and successional trajectories.
The authors cite expectations of scientists studying loss of black ash (F. nigra) from upper Midwestern wetlands. There, research indicates loss of ash from these systems is likely to result in higher water tables and a conversion from forested to graminoid- or shrub-dominated systems. Significant changes follow: to food webs, to habitat structure, and, potentially, to nitrogen cycling.
Maze, Bond and Mattsson expect similar impacts in Willamette Valley wetlands and floodplains, especially those with the longest inundation periods and highest water tables. That is, there will probably be a broad disruption of successional dynamics and, at many sites, a conversion to open, shrub-dominated systems or to wetlands invaded by exotic reed canary grass (Phalaris arundinacea), with occasional sedge-dominated (Carex obnupta) wetlands. They think this change is especially likely under canopies composed of Oregon white oak (see below). The authors admit some uncertainty regarding the trajectories of succession because 90 years of water-control projects has almost eliminated the possibility of high-intensity floods.
Steelhead trout
Oregon Ash and Salmonids
Maze, Bond and Mattsson point out that all salmonids that spawn in the Willamette basin and the nearly 250,000 square mile extent of the Columbia basin upstream of Portland pass through the two wooded waterways in the Portland area that they studied. Applying a model to simulate disappearance of ash from these forests, the authors found that the reduced shade would raise the “solar load” on one waterway, which is wide and slow-moving, by 1.8%. On the second, much narrower, creek (mean channel width of 7 m), solar load was increased by of 23.7%.
Maze, Bond and Mattsson argue that even small changes can be important. Both waterbodies already regularly exceed Oregon’s target water temperature throughout the summer. Any increase in solar loading and water temperatures will have implications for the fish – and for entities seeking to comply with Endangered Species Act requirements. These include federal, state, and local governments, as well as private persons.
The Willamette and lower Columbia Rivers, and their tributaries, traverse a range of elevations. Ash trees comprise a larger proportion of the trees in the low elevation riparian and wetland forests. Consequently, Maze, Bond and Mattsson expect that EAB-induced loss of Oregon ash will have significant impacts on these rivers’ water quality and aquatic habitats. The higher water temperatures will affect aquatic organisms at multiple trophic levels.
They conclude that the EAB invasion West of the Cascade Mountain range constitutes an example of the worst-case forest pest scenario: the loss of a dominant and largely functionally irreplaceable tree species that provides critical habitat for both ESA-listed and other species, along with degradation of ecosystem services that protect water quality.
Breeding Oregon Ash … Challenges to be Overcome
According to Melton et al. (2024), Oregon ash does not begin to reproduce until it is 30 years old. Such an extended reproductive cycle could complicate breeding efforts unless scientists are able to accelerate flowering or use grafting techniques to speed up reproduction – as suggested by Richard Sniezko, USFS expert on tree breeding.
Melton et al. (2024) note that the IUCN has recently highlighted the importance of maintaining a species’ genetic variation in order to maintain its evolutionary potential. Consequently, they examined genetic variation in Oregon ash in order to identify the species’ ability to adjust to both the EAB threat and climate change. The authors sequenced the genomes of 1,083 individual ash trees from 61 populations. These spanned the species’ range from Vancouver Island to southern California. The genetic analysis detected four genetic clusters:
British Columbia;
Washington to central Oregon – including the Columbia River and its principal tributaries;
Southwest Oregon and Northwest California — the Klamath-Siskiyou ecoregion; and
all other California populations.
Connectivity between populations (that is, the potential corridors of movement for pollen and seeds and hence, genetic flow) was greatest in the riparian areas of the Columbia River and its tributaries in the center to the species’ range. Despite this evidence of connectivity, nucleotide diversity and effective population size were low across all populations. This suggests that the patchy distribution of Oregon ash populations might reduce its long-term evolutionary potential. As average temperatures rise, the regional populations will become more distinct genetically. The species’ ability to adjust to future climate projections is most constrained in populations on Vancouver Island and in smaller river valleys at the eastern and western edges of the range. Populations in southern California might be “pre-adapted” to warmer temperatures.
The resulting lower effective population size might exacerbate risks associated with EAB. The authors warned that although seeds from more than 350 maternal parent trees have been preserved since 2019, these collections do not cover the full genomic variation across Oregon ash’s range. Some genomic variation that represents adaptive variation critical to the species’ long-term evolution might be missing. They advocate using the genetic data from their study to identify regions where additional collections of germplasm are needed for both progeny trials and for long-term conservation.
Oregon white oak with symptoms of Mediterranean oak borer infestation; photo by Christine Buhl, Oregon Department of Forestry
Oregon White Oak (Quercus garryana) and the Mediterranean Oak Borer
The U.S. Department of Interior has been working with regional partners for 10 years to protect oak and prairie habitat for five ESA-listed species, two candidate species, and numerous other plant and animal species of concern. In August 2025 the Department announced creation of the Willamette Valley Conservation Area. It becomes part of the Willamette Valley National Wildlife Refuge Complex. These units are managed predominantly to maintain winter habitat for dusky geese (a separate population of Canada geese). Other units in the Complex are William L. Finley National Wildlife Refuge, Ankeny National Wildlife Refuge, and Baskett Slough National Wildlife Refuge.
These goals too face threats from non-native forest pests. First, the forested swamps of Ankeny NWR are composed nearly 100% of ash.
Second, Oregon white oak now confronts its own non-native pest – the Mediterranean oak borer (Xyleborus monographus). This Eurasian ambrosia beetle has been introduced to the northern end of the Willamette Valley (near Troutville, Oregon). It is likely that infestations are more widespread. Authorities are surveying areas near Salem. A separate introduction has become established in California, north of San Francisco Bay plus in Sacramento County in the Central Valley. Oregon white oak is vulnerable to at least one of the fungi vectored by this borer – Raffaelea montety. https://www.dontmovefirewood.org/pest_pathogen/mediterranean-oak-borer/
SOURCES
Maze, D., J. Bond and M. Mattsson. 2024. Modelling impacts to water quality in salmonid-bearing waterways following the introduction of emerald ash borer in the Pacific Northwest, USA. Biol Invasions (2024) 26:2691–2705 https://doi.org/10.1007/s10530-024-03340-3
Melton, A.E., T.M. Faske, R.A. Sniezko, T. Thibault, W. Williams, T. Parchman, and J.A. Hamilton. 2024. Genomics-driven monitoring of Fraxinus latifolia (Oregon Ash) for conservation and emerald ash borer resistance breeding. https://link.springer.com/article/10.1007/s10530-024-03340-3
We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.
For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at https://treeimprovement.tennessee.edu/
The House and Senate Agriculture committees are edging toward adopting the next Farm Bill, which is a year past due. Farm bills set policy, funding levels, and more, for 5 years. Each covers a wide range of subjects, including crop subsidies and insurance; food stamps; rural development (including wifi access); forestry policy; and research.
As you might remember, CISP aims to improve USDA’s programs — not only to prevent introductions of non-native tree killing pests and pathogens but also to better respond to those that enter the US and become established. I summarize here what the Senate and House bills have in common and how they differ on these issues.
I understand that the minorities, that is, House Democrats and Senate Republicans, have not accepted all aspects of the majorities’ drafts. So let’s take the opportunity to ask for better bills.
Both the House and Senate bills would “simplify” the USDA Forest Service’s obligations to prepare environmental assessments under the National Environmental Policy Act (NEPA). I have not analyzed which bill weakens NEPA more.
The Senate Bill: The Rural Prosperity and Food Security Act of 2024
The Senate bill addresses forest pest species in several places: Title II — Conservation, Title VII — Research, and Title VIII — Forestry. Here, I describe relevant sections, beginning with the section that partially addresses CISP’s proposal.
Title VIII — Forestry. Section 8214 requires the USDA Secretary to establish a national policy to counter threats posed by invasive species to tree species and forest ecosystems and identify areas for interagency cooperation.
This mandate falls far short of what we sought in a previous bill (S. 1238). However, depending on the exact wording of the bill and accompanying report, perhaps we can succeed in building a stronger program.
It is most important to obtain funding for applied, directed research into resistance breeding strategies, “bulking up,” and planting seedlings that show promise. Please contact your senators and ask them to work with the sponsors – Peter Welch [D-VT], Maggie Hassan [D-NH], and Mike Braun [R-IN] – to try to incorporate more of S. 1238 in the final bill.
The Senate bill contains other provisions that might be helpful for invasive species management – although not part of what CISP and our partners asked for.
‘ōhi‘a trees killed by rapid ‘ōhi‘a death; photo by Richard sniezko, USFS
Title VIII — Forestry. In Section 8506, the Senate bill would require that the US Departments of Agriculture and Interior continue working with Hawai`i to address the pathogen that causes rapid ‘ōhi‘a death. The section authorizes $5 million for each of the coming five fiscal years to do this work. Unfortunately, authorization does not equal funding. Only the Senate and House Appropriations Committees can make this funding available. Hawai`i’s endemic ‘ōhi‘a trees certainly face a dire threat. CISP is already advocating for funding to support resistance breeding and other necessary work.
Title VIII — Forestry. Sections 8247 and 8248 support USDA Forest Service’s nursery and tree establishment programs. My hesitation in fully supporting these provisions is that I fear the urge to plant lots of trees in a hurry will divert attention for the need to learn how to propagate many of the hardwood tree species that have been decimated by non-native pests. However, I agree that the U.S. lacks sufficient nursery capacity to provide anything close to the number of seedlings sought. Perhaps this program can be adjusted to assist the “planting out” component of our request.
Title VII — Research. Section 7208 designates several high-priority research initiatives. On this list are spotted lanternfly, and “invasive species”. A number of forest corporations have been urging Members of Congress to upgrade research on this broad category, which I believe might focus more on invasive plants than the insects and pathogens on which CISP focuses. How the two ideas are integrated will be very important.
Another high-priority initiative concerns the perceived crisis in failed white oak regeneration.
Title VII — Research.Section 7213 mandates creation of four new Centers of Excellence at 1890 Institutions. These are historically Black universities that are also land-grant institutions]. These centers will focus on: 1) climate change, 2) forestry resilience and conservation; 3) food safety, bioprocessing, and value-added agriculture; and, 3) food and agricultural sciences and the social sciences.
Title II — Conservation. Section 2407 provides mandatory funding (which is not subject to annual appropriations) of $75 million per year to the national feral swine eradication/control program (run by USDA APHIS’ Wildlife Service Division). I discuss this program in a separate blog.
The Senate bill also mandates use of several conservation and other programs to address the causes and impacts of climate change. This requirement is directly countered by the House Agriculture Committee’s bill (see below).
Title VIII — Forestry. This section contains none of the provisions CISP’ sought to USDA’s management of tree-killing non-native insects and diseases.
Instead, the House bill calls on the USFS to establish a comprehensive approach to addressing the demise of the giant sequoia trees.
Title VII — Research The House bill, like the Senate’s, lists the invasive species and white oak research initiatives as high priority. The House, unlike the Senate, does not include spotted lanternfly.
Title II — Conservation. As I noted above, the House bill explicitly rescinds all unobligated conservation funding from the Inflation Reduction Act. It reallocates these funds to the traditional conservation programs, e.g., the Environmental Quality Incentive Program and Watershed Protection and Flood Prevention. The bill would use these funds to support “orphan” programs – naming specifically the national feral swine eradication/control program. The House bill provides $150 million – apparently across the five years covered by the Farm Bill, so $30 million per year. Finally, the House allocates 60% of the hog management funds to APHIS, 40% to the Natural Resources Conservation Service.
spotted lanternfly – target of at least 11 projects funded through APHIS’ the Plant Pest and Disease Management and Disaster Prevention Program in FY24. Photo by Holly Raguza, Pennsylvania Department of Agriuculture
Title X —Horticulture, Marketing, and Regulatory Reform. The House’s summary says it is taking steps to protect plant health. It does this by increasing funding for the grant program under the Plant Pest and Disease Management and Disaster Prevention Program – §7721 of the last (2018) Farm Bill. The increase would raise the amount of money available each year from the current level of $70 million to $90 million. These funds are mandatory; they are not subject to annual appropriations. Research, development, and outreach projects funded by this program have certainly added to our understanding of plant pests, hence to their effective management. However, they are usually short-term projects. Therefore they are not suitable for the long-term commitment required for resistance breeding programs. See here and here.
Title III — Trade. Here, the House bill exacerbates the current imbalance between trade promotion and phytosanitary protection. The bill doubles the authorized funding for USDA’s Market Access and Foreign Market Development programs. I concede that this measure probably does reflect a bipartisan consensus in the Congress to support robust programs for promoting agricultural exports.
Also under this Title, the House bill requires the USDA Secretary to conduct regular assessments to identify risks to critical infrastructure that supports food and agriculture sector. This might be helpful – although it is not clear that this assessment would include to threats to forest or urban trees not used commercially (e.g., for timber).
At a recent forum on biological control sponsored by the National Association of State Foresters (NASF), it was reported that participants noted several problems: insufficient funding, significant delays in refilling positions, inadequate research capacity, lack of brick-and-mortar infrastructure, and declining college enrollments in biocontrol-related studies. The NASF Forest Science Health Committee is developing a “Statement of Needs” document that NASF and others can use to lobby for funding to fill these gaps. I hope you will join them in doing so!
salt cedar (Tamarix sp.) attacked by biocontrol agent; photo by J.N. Stuart via Flickr
However, as I note above, empowering resistance breeding programs requires a long-term commitment, that is, a comprehensive alteration of policies and infrastructure – beyond annual appropriations.
Posted by Faith Campbell
We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.
For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at http://treeimprovement.utk.edu/FadingForests.htm
TACF back-crossed American-Chinese chestnut; photo by F.T. Campbell
I have advocated for considerably expanding efforts to breed trees resistant to non-native pests (including pathogens) for a decade. Again and again, I and others have pointed out the dire consequences for our forests if we Americans do not rise to the challenge.
In 2014, Scott Schlarbaum – coauthor of Fading Forests III – American Forests: What Choice Will We Make? warned that without restoration becoming an integral part of a strategy addressing non-native plant pests, American ecosystems are doomed to continuing transformation. Once established, a non-native pest is never eliminated, but its impact can be reduced through a combination of measures – as long as support is made available. Scott advised initiating a germplasm conservation strategy when invasion is imminent or once the pest is likely to become a resident pest. (See Chapter 6).
I have posted seven blogs since August 2021 describing the current status of various efforts and urging the U.S. Government and conservation organizations to step up. [To view these blogs, go to www.nivemnic.us, scroll below Archives to “Categories” and click on “resistance breeding.”
More, and Recent, Voices: Implications of Not Acting
More recently, several USDA Forest Service (USFS) experts, including Richard Sniezko, C. Dana Nelson, and Jennifer Koch, have published articles making the same point. These scientists note that many of the decimated species were formerly among the most common trees in our forests. Therefore, the cumulative effect of their disappearance on forest species composition and function is multiplied.
One blog, posted in 2022, is particularly pertinent. It summarizes a special issue of the journal Plants, People, Planet devoted to resistance breeding. The opening essay, by R.J.A. Buggs, concisely reviews six major reasons why so many believe that resistance breeding is a failed strategy.
Port-Orford cedar – one of the trees for which resistance breeding has been successful; photo courtesy of Richard Sniezko, USFS
Others say there have been successes – all through application of classic tree improvement measures, not “genetic engineering.” Pike, Koch and Nelson (2021) list as successes Port-Orford-cedar (Chamaecyparis lawsoniana), the western five-needle pine species, koa (Acacia koa), and resistance to fusiform rust (Cronartium quercuum f. sp. fusiforme) in the commercially-important loblolly (Pinus taeda) and slash (P. elliottii) pines. They also cite encouraging progress by The American Chestnut Foundation (TACF) through backcross breeding of America and Asian chestnuts and a USFS/private foundation effort to expand the genetic base of American elms (Ulmus americana). I regret to say this, but some of these efforts seem to me to be still in experimental stages or — at best — early in widespread – ‘though still experimental — plantings.
Participants in a 2021 Purdue University workshop have again called for greatly expanding breeding. See the special issue of New Forests, Vol. 54 Issue 4. Once again, experts reiterate the urgency of acting, then outline the opportunities and challenges.
In one of the articles (Jacobs et al.) several people – including me! – note that several keystone tree species or genera in North America and Europe have been driven to functional extinction by non-native pests. By this we mean they are no longer sufficiently abundant and/or of adequate size to reproduce sexually or perform their ecological function. Examples include – on both continents – ashes (Fraxinus) and elms; and on North America – American chestnut (Castanea dentata), butternut (Juglans cinerea), and whitebark pine (Pinus albicaulis).If these threats are left unchecked, these at-risk tree species might develop truncated ranges, lose genetic diversity, and face becoming threatened, endangered, or extinct.
In another article, Nelson says the question that should be asked about applying genetic engineering (GE) techniques to tree breeding is whether we should let a species be reduced to a marginal role — or disappear — when GE provides a solution to saving and restoring the species. His case study is a detailed history of TACF’s development of a transgenic American chestnut (called “Darling 58”). He points out that decades of breeding efforts were based on the hope of developing blight resistance within the native gene pool or to obtain resistance from related species through hybridization. However, those efforts have not yet provided trees suitable for restoring the “king of the Appalachian forest” to native landscapes. Nelson wrote his description before TACF discovered flaws in the GE trees they had been working with and decided to pursue different GE “lines” (see below).
Barriers
The overall strategy is clear. Schlarbaum, Sniezko, and Dana Nelson all describe essentially the same steps, built on the same kinds of expertise and facilities.
Of course, each species will require years of input by a range of experts. These challenges are not trivial. However, the experts named above agree that the principal barrier is the absence of sustained, long-term commitment of resources and facilities. With sufficient resources, many of the scientific challenge can be overcome for at least some of the species at risk.
So, what are the scientific challenges? First, scientists must assess whether the tree species contains sufficient genetic variation in resistance. This involves locating candidate resistant trees; developing and applying short-term assay(s) to screen hundreds or thousands of candidate trees; and determining the levels of resistance present. Second, scientists must develop resistant planting stock for use in restoration. This stage includes scaling up the screening protocol; selecting the resistant candidates or progeny to be used; breeding to increase resistance; establishing seed orchards or other methods to deliver large numbers of resistant stock for planting; and additional field trials to further validate and delineate resistance. Sniezko and Koch (2017) and Sniezko and Nelson (2022) discuss the challenges and describe successes.
facilities at Dorena Genetic Resource Center; photos courtesy of Richard Sniezko, USFS
Complicating the restoration phase is the fact that the resistant tree must be able to thrive and compete in an ecosystem that has changed greatly from that in which it formerly resided. Causes of these changes include repercussions from the absence of the tree species – and possibly associated species; the possible presence of other biotic stresses (pests); and climate change. This is discussed by Nelson (2022). See also my blog.
Successfully completing these steps requires a long-term commitment, which includes significant funding and strong supportive infrastructure. Schlarbaum pointed out that the public and politicians don’t understand the complexity of the restoration challenge and the resources required. He documented the shrinking tree improvement infrastructure as of 2014. At that time, funding for all USFS regional breeding programs was just $6 million. State and land grant university breeding programs were fragmented and seriously underfunded. Only 28 states still had some type of tree improvement activity – and some of these programs were only seed orchards, not active breeding and testing programs. Members of university-industrial cooperatives focus on a small number of commercial species – which are not the species threatened by non-native pests. I believe these resources have shrunk even farther in the decade since 2014.
A separate source of funds for resistance breeding is the Forest Health Protection program, which is under the Deputy Chief for State, Private, and Tribal Forestry rather than the Deputy Chief for Research and Development. While nation-wide data on seed or scion collection or screening to identify and evaluate genetic resistance are poorly reported, Coleman et al. indicate that the USFS Dorena Genetic Resource Center screens unspecified “hundreds” of seed lots for resistance to pathogens annually. The Center also participates in seed, cone, and scion collections, especially of white pines vulnerable to white pine blister rust (WPBR). Supplemental Table S3 lists projects funded over the two decades analyzed by Coleman et al. (2011 – 2020). These included efforts to identify and evaluate possible genetic bases for resistance to, e.g., hemlock woolly adelgid, balsam woolly adelgid, laurel wilt, emerald ash borer, butternut canker, rapid ʻōhiʻa death; and gene conservation for eastern hemlock, ashes, chestnut, in addition to the five-needle pines. Currently, FHP allocates $1.2 million annually to support the group of activities called Genetic Conservation, Resistance and Restoration (R. Cooksey, pers. comm.).
American beech grafts to be tested for resistance to beech bark disease; at USFS center in Delaware, Ohio; photo courtesy of Jennifer Koch, USFS
USFS scientists involved in these projects describe challenges arising from efforts to cobble together funding from these many sources to support coherent programs. Overall funding levels still fall short of the need, and failure to obtain funding for one component of a program stymies the entire endeavor.
However, some developments are encouraging. The number of private foundations devoted to tree breeding has increased in the last decade. The American Chestnut Foundation (TACF) and American Chestnut Cooperators Foundation (ACCF) have been joined by the White Pine Ecosystem Foundation, the Great Lakes Basin Forest Health Collaborative, Forest Restoration Alliance, ‘Ohi‘a Disease Resistance Program … These organizations raise awareness, coordinate efforts by multiple parties, and provide opportunities for individuals to contribute funds and volunteer work.
In Hawai`i, disease resistance programs with both koa (Dudley et al.) and ʻōhiʻa ((Metrosideros polymorpha) (Luiz et al.) are active. Work with ash species to find and develop resistance to emerald ash borer is under way but limited due to lack of funds.
Finally, we can persuade Congress to incorporate the provisions of two bills, H.R. 3174 and S. 1238, into the next Farm Bill. The bills would, inter alia, create two grant program. One would fund research addressing specific questions impeding the recovery of native tree species that have suffered severe levels of mortality caused by non-native plant pests. The second would fund implementation of projects to restore these pest-decimated tree species to the forest.
Funded projects would be required to be part of a forest restoration strategy that incorporates a majority of the following components:
(1) Collection and conservation of native tree genetic material;
(2) Production of propagules of the target tree species in numbers sufficient for landscape-scale restoration;
(3) Preparation of planting sites in the target tree species’ former habitats;
Facilities needed to support successful breeding programs
Sniezko and Nelson identified these needs as follows:
(a) growing space (e.g., greenhouses);
(b) seed handling and cold storage capacity;
(c) inoculation infrastructure;
(d) field sites for testing;
(e) database capability for collecting, maintaining, and analyzing data;
(f) areas for seed orchard development;
(g) skilled personnel (tree breeders, data managers, technicians, administrative support personnel, and access to expertise in pathology and entomology).
There are very few facilities dedicated primarily to development of populations of trees with resistance to non-native pests; the most notable is the Dorena Genetic Resource Center. Even the existing programs require significant funding increases to accelerate current programs or expand to address additional species. Sniezko and Nelson stress further that a resistance breeding program has different objectives, magnitude and focus than most research projects. It is applied science, that is, an action-oriented effort that is solution-minded—countering the impact of a major disturbance caused by a pest (in our case, a non-native pest).
Schlarbaum provides a shorter but similar list of facilities needed:
production of propagules (seed or clones);
mass propagation in growing facilities, e.g., bare-root seedling nursery or greenhouses;
site preparation of former habitat and planting; and
post-planting maintenance.
Schlarbaum emphasized that each of these activities requires different skill sets, equipment, facilities, and infrastructure.
Genetic Engineering as a Specific Tool
There is considerable interest in the potential role of genetic engineering in pest resistance breeding. None of the successful programs world-wide has yet used genetic engineering (Sniezko and Koch 2017). While incorporating it into holistic breeding programs might result in greater efficiency for certain processes, it raises legal and social acceptability issues. Jacobs et al. discuss the type of education and outreach program needed to generate widespread public support this approach to tree species “rescues.” They call for USDA Forest Service to lead this education effort.
The focus of the 2021 workshop hosted by Purdue University was to explore the pros and cons of using biotechnology in restoring pest-threatened forest tree species. The special issue of New Forests contains several participants’ analyses.
The overall conclusions are that:
“Genetic engineering” – defined as “any technique that uses recombinant, synthesized, or amplified nucleic acids to modify a genome” – is only one type of biotechnology applicable to tree breeding. Other biotechnologies include tissue culture-based propagation, molecular-based genetic markers, gene cloning and sequencing, and genome mapping and sequencing.
These new technologies can increase the efficiency of more traditional breeding techniques, However, biotechnologies cannot substitute for holistic programs that incorporate all helpful methods. Careful consideration goes into selecting which techniques are appropriate for a particular host-pest system.
Each tree species has unique needs regarding seed or scion collection; seedling propagation in nurseries; site preparation and planting techniques; and management of regeneration after its re-introduction into forests. Scientists don’t yet understand these various needs of many threatened species.
In the eastern U.S., the tree-breeding infrastructure is based in the Southeast and focused on a few pine species grown commercially. The facilities do not match the greatest need. That is, many of the at-risk species are hardwoods native to the Northeast.
Current resources are inadequate to support the sustained, long-term commitment of resources and facilities necessary to be successful.
Dana Nelson addressed the role of genetic engineering (GE) in detail. He emphasized repeatedly that GE is not a short-cut to tree improvement. Incorporating a GE component does not avoid the other steps. It can, though, provide new possibilities to address problems. Nelson says the crucial, initial question is – can GE solve the specific forest conservation or management problem more effectively and efficiently than existing methods? There are some important subtleties to consider. First, success does not require achieving immunity (100% resistance); the level of resistance needs to be only sufficient to allow the tree species to survive, reproduce and co-evolve with the pest. Second, “efficiency” is an important consideration. We cannot afford delay because during those years or decades the wild tree loses genetic variability as more trees die. Also, changes in the environment continues to change, and the decimated tree species is not adapting.
If genetic engineering promises to contribute meaningfully, then the breeders must answer several follow-up questions before proceeding to develop a specific plan. Nelson also stresses that the planned activities must be integrated with an ongoing tree breeding program to ensure project success.
Nelson provides a lengthy description of the process of integrating genetic engineering into tree breeding programs.
GE in Chestnut Breeding – Setback
The most prominent breeding effort incorporating genetic engineering in the U.S. has been The American Chestnut Foundation’s (TACF) program to restore American chestnut (Castanea dentata). For decades, TACF has pursued development of trees resistant to the fungus which causes chestnut blight (Cryphonectria parasitica). Over the past decade, hopes have centered on a genetically engineered line into which was inserted a gene from wheat (oxalate oxidase; OxO). The OxO gene detoxifies the oxalic acid produced by the chestnut blight fungus and thus prevents the cankers from killing the tree.
Years of tests have shown the gene to be effective and to cause no environmental harm. In 2023, when trees in outside test plots grew larger, scientists observed disappointing results. Trees’ blight tolerance varied greatly. Worse, resistant trees grew more slowly and exhibited lower overall fitness. [For a full discussion of the issues, visit TACF’s website] Prompted by these disappointments, scientists carried out further molecular analyses. They found that the OxO gene was on a different chromosome than expected.
TACF researchers now suspect that the trees’ variable performance stems primarily from the placement of the OxO gene and the fact that the gene is always “switched on”. That constant expression appears to result in high metabolic costs for the trees. Since all the genetic lines developed to date have this defect, TACF is no longer pursuing research efforts with any of the GE trees developed to date. The Foundation believes it would be irresponsible to continue efforts – by itself and by partners – focused on a genetic line that looks unable to compete successfully when introduced to the forest.
Instead, TACF has begun investigating other transgenic lines that use a “wound inducible” promoter that switches on the OxO gene only in cells where the plant is wounded. Researchers at both the State University of New York College of Environmental Science and Forestry (SUNY-ESF) and the University of Georgia are working with a variety of inducible promoters. TACF is also testing whether inducible OxO expression can be “stacked”onto genes for blight resistance present in the backcross hybrids. Finally, TACF and Virginia Tech are also exploring whether resistance can be enhanced by insertion of genes from Chinese chestnut directly into American chestnut using methods similar to OxO insertion.
It will be years before we know if these approaches provide sufficient levels of resistance. TACF will undertake more extensive testing for efficacy through the tree’s full life cycle – in the lab, greenhouse, and field – before submitting a new GE organism to regulators for review. Meanwhile, it will continue rigorous testing for plant health and environmental risks and will strengthen the cooperative structure to facilitate sharing of intellectual property and provide full transparency.
The Darling GE line was the most important transgenic hybrid chestnut line TACF had invested in. So this is a major setback – and comes when regulatory approval seemed near.
Let’s keep this in perspective, however. As a colleague has said, based on his years of teaching science to middle school students, “There are no failures in science, just reductions in the unknown; Edison failed a thousand times before getting the light bulb right, etc….” The technology is ready when it is ready. In addition, he praised TACF for choosing to explain its decision frankly: “nothing builds credibility like early failures openly admitted.”
Meanwhile, TACF continues to make gains in blight resistance with its traditional American chestnut backcross hybrid breeding program. They have established a genetically diverse, reproducing population of thousands of trees representing hundreds of breeding lines. These trees are planted in TACF’s expansive network of germplasm conservation orchards and regional breeding and backcross orchards. They have substantially increased resistance to both the blight and Phytophthora cinnanomi in these populations. The future inclusion of transgenic and/or gene-edited trees will further increase those gains.
Another Approach
Meantime, the American Chestnut Cooperators Foundation (ACCF), which breeds from persistent pure American chestnut, now has some trees that are nearly 50 years old. The program has bred five generations of pure American chestnuts that show durable blight resistance. Many trees are 60 feet tall or higher; they produce nuts. Vice President Jenny Abla (pers. comm.) reports that they show excellent canker response (swollen and superficial). The picture shows one of their most notable stands, which is in the Jefferson National Forest. Dr. Sniezko is exploring whether this program shows sufficient promise to justify increased support from the USFS.
ACCF chestnut trees; photo courtesy of Jenna Abla
Improving Coordination – will funds follow?
In July 2023, representatives from essentially all the forest tree resistance breeding programs in the U.S. met at Dorena Genetic Resource Center in Oregon to discuss their current successes and how to fast-track all programs. This is the first such meeting since 1982 (Richard Sniezko, pers. comm.). I encourage us all to study the report when it emerges and encourage USFS leadership to support the more unified enterprise.
Status of Efforts to Conserve Other Tree Species
The special issue of New Forests (Vol. 54 Issue 4) included several articles exploring the specifics of breeding elms, ashes, and ʻōhiʻa. These describe difficult challenges … and scientists determined to make progress on overcoming them.
“survival” American elm at Longwood Gardens; photo by F.T. Campbell
Elms (Ulmus spp.) (see article by Martin et al.)
Let’s not forget that elms were keystone species in Europe and North America until attacked by two epidemics of “Dutch” elm disease during the 20th Century. While hybrid elms are available for urban plantings, many consider them not appropriate for planting in natural forests because these genotypes are not native.
Martin et al. describe a bewildering conglomeration of complexities and possibilities arising from biotic and abiotic factors. Initiation and especially intensity of the disease in a particular tree depend on
the species or strain of the tree, vectoring beetle, and pathogen;
timing of the attack; and
adequacy of water supplies at that time.
Possible targets for manipulation include the pathogen, its beetle vector, and the tree’s response — either in its bark or xylem. Martin et al. suggest that a combination of resistance to the pathogen within the xylem, resistance to beetles’ feeding wounds, and lowering tree clues that attract the beetles could considerably enhance longer-term overall resistance in the field.
However, verifying which approaches produce the best result will be complicated by the trees’ sensitivity to environmental factors such as season and water supply. Apparent resistance might actually be tied to, for example, low water supplies during the spring when the attack occurred.
Restoration strategies, including resistance to pests, must accommodate the diverse ecological conditions in the species’ large range, the rapid evolution of the Ophiostoma pathogens; and other pests and pathogens that attack elms. Nor do scientists know appropriate planting strategies.
Martin et al. believe Dutch elm disease is unlikely to be spread by movement of living elm plants, although other pests could be (and have been).
ash trees to be tested for resistance to emerald ash borer; photo courtesy of Jennifer Koch, USFS
Ashes (Fraxinus spp.)
While a USFS team led by Jennifer Koch link are conducting much of the on-the-ground efforts to breed ash trees resistant to the emerald ash borer (EAB; Agrilus plannipennis), Stanley et al. note that scientists cannot simply cross most North American ash species with the Asian ash, F. mandshurica, because the two groups are sexually incompatible. Scientists have instead focused on trying to enhance the resistance to EAB that is apparently present in a small proportion of ash trees, called “lingering ash.” Scientists funded by USDA Forest Service have already devoted over 14 years to finding such lingering ash to be tested for resistance.
Testing these trees is not simple (see Stanley et al.). But scientists are overcoming some of the obstacles. They have shown that the capability of a few green ash (Fraxinus pennsylvanica) (less than 1%) to defend themselves from EAB attack is genetic. Genes determine the relative abundance of specific metabolites manufactured by the tree; high levels kill more beetle larvae. These trees’ tolerance is not immunity but it might be sufficient to allow the tree to survive and grow. The level of metabolites synthesized by succeeding generations of the tree can probably also be enhanced by breeding.
To restore ash it is necessary to propagate large numbers of clones and to root the resulting embryos. This has been challenging. Merkle et al. describe five years of efforts to develop techniques that allow in vitro propagation to speed up selection and breeding. These techniques will facilitate establishment of numerous groups of propagules with the genetic differences needed to accommodate the large geographic range of several ash trees. For example, the green ash range covers more than half the continental U.S. plus multiple Canadian provinces.
ʻōhiʻa on lava field, Hawaii Volcanoes National Park
‘Ōhi‘a (Metrosideros polymorpha)
‘Ohi‘a is the most widespread tree species on the Hawaiian Islands. It provides vitally important habitat for conservation of countless taxa of endemic birds, insects, and plants. It is also of great cultural importance for Native Hawaiians.
Luiz et al. review the tree species’ importance, the many threats to native Hawaiian forests, and a coalition’s efforts to counter the most recent – and alarming – threat, rapid ʻōhiʻa death (ROD).
Rapid ʻōhiʻa death is caused by two introduced species of in the genus Ceratocystis. C. lukuohia colonizes the tree’s sapwood and kills the tree quickly. This disease is present on two islands, Hawai`i and Kaua‘i. It has the potential to devastate ‘ohi‘a forests across the state. The other pathogen, C. huliohia, invades the phloem, cambium, and outer xylem, resulting in a well-defined area of necrotic tissue and slower mortality. This disease is on Hawai`i and Kaua‘i, plus Maui and O‘ahu. The two pathogens have different origins. C. lukuohia belongs to a genetic line that is based in Latin America, C. huliohia to a genetic line based in Asia and Australia.
Conservationists formed a coalition and developed a strategy to guide the process of identifying and developing disease resistance in M. polymorpha and, if possible, other Metrosideros species on the Islands. Luiz et al. describe the coalition’s many activities. The challenges are familiar ones:
obtaining sufficient facilities to screen large numbers of seedlings;
developing techniques for inoculation, propagation, and speeding up growth of seedlings;
improving techniques for detecting individual infected and healthy trees across difficult terrain;
testing trees native to all parts of the tree’s range, which is not large in area, but covers a great variety of elevations and climates); and
needing to develop trees resistant to both C. lukuohia and C. huliohia.
Luiz et al. reiterate the necessity to manage all threats to healthy ʻōhiʻa stands, for example, by
curtailing human spead of infected wood, using both quarantines and supportive public education;
testing repellants to reduce beetle attack.
reducing injuries to trees by fencing forests and removing feral ungulates. link to website?
SOURCES
Buggs, R.J.A. 2020. Changing perceptions of tree resistance research. Plants, People, Planet. 2020;2:2–4. https://doi.org/10.1002/ppp3.10089
Coleman, T.W., A.D. Graves, B.W. Oblinger, R.W. Flowers, J.J. Jacobs, B.D. Moltzan, S.S. Stephens, R.J. Rabaglia. 2023. Evaluating a decade (2011–2020) of integrated forest pest management in the United States. Journal of Integrated Pest Management. (2023) 14(1): 23; 1–17
Dudley, N.; Jones, T.; Gerber, K.; Ross-Davis, A.L.; Sniezko, R.A.; Cannon, P.; Dobbs, J. 2020. Establishment of a Genetically Diverse, Disease-Resistant Acacia koa A. Gray Seed Orchard in Kokee, Kauai: Early Growth, Form, and Survival. Forests 2020, 11, 1276 https://doi.org/10.3390/f11121276
Jacobs, D.F., R. Kasten Dumroese, A.N. Brennan, F.T. Campbell, A.O. Conrad, J.A. Delborne, et al. 2023. Reintroduction of at-risk forest tree species using biotech depends on regulatory policy, informed
by science and with public support. New Forests (2023) 54:587–604
https://doi.org/10.1007/s11056-023-09980-y
Luiz, B.C., C.P. Giardina, L.M. Keith, D.F. Jacobs, R.A. Sniezko, M.A. Hughes, J.B. Friday, P. Cannon, R. Hauff, K. Francisco, M.M. Chau, N. Dudley, A. Yeh, G. Asner, R.E. Martin, R. Perroy, B.J. Tucker, A. Evangelista, V. Fernandez, C. Martins-Keli.iho.omalu, K. Santos, R. Ohara. 2023. A framework for establishing a rapid ‘Ohi‘a death resistance program. New Forests https://doi.org/10.1007/s11056-021-09896-5
Martín, J.A., J. Domínguez, A. Solla, C.M. Brasier, J.F. Webber, A. Santini, C. Martínez-Arias, L. Bernier, L. Gil1. 2023. Complexities underlying the breeding and deployment of Dutch elm disease resistant elms. New Forests https://doi.org/10.1007/s11056-021-09865-y
Merkle, S.A., J.L. Koch, A.R. Tull, J.E. Dassow, D.W. Carey, B.F. Barnes, M.W.M. Richins, P.M. Montello, K.R. Eidle, L.T. House, D.A. Herms and K.J.K. Gandhi. 2023. Application of somatic embryogenesis for development of emerald ash borer-resistant white ash and green ash varietals. New Forests https://doi.org/10.1007/s11056-022-09903-2
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Pike, C.C., J. Koch, C.D. Nelson. 2021. Breeding for Resistance to Tree Pests: Successes, Challenges, and a Guide to the Future. Journal of Forestry, Volume 119, Issue 1, January 2021, Pages 96–105, https://doi.org/10.1093/jofore/fvaa049
Sniezko, R.A., J. Koch, J-J. Liu and J. Romero-Severson. 2023. Will Genomic Info Facilitate Forest Tree Breeding for Disease and Pest Resistance? Forests 2023, 14, 2382.
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Sniezko, R.A. and C.D. Nelson. 2022. Chapter 10, Resistance breeding against tree pathogens. In Asiegbu and Kovalchuk, editors. Forest Microbiology Volume 2: Forest Tree Health; 1st Edition. Elsevier
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Posted by Faith Campbell
We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.
For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at http://treeimprovement.utk.edu/FadingForests.htm
