New study evaluates “candidate pool” from which invasive species might come

Campanula latifolia – one of the species detected as an “emerging” invasive species in the database relied upon by the authors of the study

The authors of a new study note that officials managing invasive species programs rely largely on knowledge of a species’ previous invasion history to predict its level of threat in the geographic area under their responsibility. This approach does not work with the many introduced species that have no history of a previous detected invasion. Hanno Seebens and 49 coauthors – including tree-pest experts Eckehard G. Brockerhoff, Marc Kenis, Andrew M. Liebhold, and Alain Roques — have sought to figure out how great a handicap that lack of data is. See “Global rise in emerging alien species results from increased accessibility of new source.” The study is available for $10 here. Figures, tables, and references are available without charge.

The study used a database of 45,984 first records of establishment of 16,019 species belonging to the following major taxonomic groups: vascular plants, mammals, birds, fishes, insects, crustaceans, mollusks, and other invertebrates.

Last year, many of the same scientists, relying on the same database, found that the rate of new introductions of alien species has risen rapidly since about 1800 – and shows no sign of slowing down. The adoption of national and international biosecurity measures during the 20th century have slowed introductions – but they are not sufficiently effective, especially regarding those plants and animals that are introduced primarily accidentally as stowaways on transport vectors or contaminants of commodities (e.g., algae, insects, crustaceans, mollusks and other invertebrates). The 2017 study found a strong correlation between these “accidental” alien species’ spread and the market value of goods imported into the region of interest. For that study, go here.  I blogged about the findings on 1 March 2017 – here.

In the new 2018 article, the scientists found that even after many centuries of invasions the rate of emerging alien species is still high. Across all taxonomic groups, one out of four detections during 2000 – 2005 was of a species that had not been previously recorded anywhere as alien. Detections of “new” or “emerging” aliens is occurring at an even higher rate for some taxonomic groups. But new detections of insects fit the average – every fourth detection during 2000 – 2005 was of a species not previously recorded outside its native range.

The authors conclude that the continuing high proportion of “emerging” alien species is best explained by the interplay of 1) the incorporation into the pool of potential alien species of species native to regions formerly not accessible to traders; 2) increases in introduction rates due to higher import volumes; and 3) probably rising establishment rates as a consequence of land degradation that facilitates establishment in recipient regions. This process compensates for the decrease of new invaders from historically important source regions – from which potentially invasive species have presumably already taken advantage of pathways and been recorded as introduced somewhere.

emerald ash borer Agrilus planipennis – one of the species in the database of “emerging” invasive species

 

The number of insect species in the database candidate species pool is 20,611 species – an admittedly small fraction of all insects (for example, there are more than 350,000 beetle species worldwide). Twenty-four percent of these insect species have already been established somewhere outside their native ranges. However, the authors note that data gaps – which are larger for some taxonomic groups and geographic regions – mean that the number of actual “first” introductions is probably larger than records indicate, and consequently the estimated size of the candidate species pools may also be higher. Indeed, the paper does not attempt to estimate the actual size of the invasive species “pool” for insects.

The authors analyzed the importance of eight factors – temperature, relative humidity, import values, three land-use categories, number of botanical gardens, and human population size – in explaining the continued high number of “emerging” invaders detected in recent years. While these factors were explanatory for some taxonomic groups, they had a very low predictive value for insects.

For vascular plants, every third record of an introduction in 2000 – 2005 was of an “emerging” alien  species. Interestingly, the number of botanical gardens in a country was a significant predictor for emerging alien vascular plants. However, as the authors of the article point out, reliance on this factor ignores the probable importance of other contributors such as the number of species planted in the receiving country; similarities between source and receiving environments; and introductions by acclimatization societies, European explorers or settlers, and plant hunters.

Acer ginnala –one of the species detected as an “emerging” invasive species in the database; photo by J. Weisenhorn, University of Minnesota extension

In any case, lots of previously undetected alien species are detected each year. In this database, 58% of the species had a single record; 86% of all species have no more than two first records in countries on the same continent. The large number of species with only one or two records led the authors to conclude that most species will not spread widely. I question that conclusion because species often require some time to spread to new locations – either local or distant. The authors do admit that they are unable to determine which species have a high potential for spread.

ash trees at the St. Louis arch – before arrival of emerald ash borer

 

The continued high rate of introduction of new species leads the authors to estimate that between 1% and 16% of all species on Earth – depending on the taxonomic group – qualify as potential invasive alien species. The authors did not attempt to estimate the true candidate pool or percentage of invasive species for insects. For vascular plants, the authors estimated the candidate pool at 47,000 species (out of a total of 368,000 species on Earth), or 13%.

Like its predecessor, this study’s importance arises from its broad perspective – covering the entire globe and a wide range of taxonomic groups. Its major conclusion that invasions will continue on a large scale serves as a warning to all stakeholders. These include officials charged with protecting agriculture and the broader economy, or the natural environment; conservationists; and those engaged in the economic activities that promote invasion.

However, the authors found that the data did not support more specific advice. First, as noted above, they were unable to determine which of the “emerging” invasive species in all taxonomic groups have a high potential to spread.

For those of us focused on invasive species that threaten native plants, data gaps limit the predictive value of the study the most. The database is too scant even to estimate the invasive species “pool” of potential insect pests. Plant pathogens are not included in the analysis.

 

 

Posted by Faith Campbell and Phyllis Windle

 

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.

 

 

Bad News & Good News – current situation

American beech; FT Campbell

 

I recently attended USDA’s annual Interagency Research Forum on Invasive Species in Annapolis, MD,  and have good and bad news to report about forest pests – mostly about insects but also a little on weeds.

Bad News

New pest: The European leaf-mining weevil is killing American beech in Nova Scotia. Jon Sweeney of Natural Resources Canada thinks it could spread throughout the tree species’ range. (I alerted you to another new pest of beech – beech leaf disease – at the beginning of December.  Beech is already hard-hit by beech bark disease.)

Other bad news concerns the spread of already-established pests:

  1. Hemlock woolly adelgid has been detected in Nova Scotia – where it has probably been present for years.
  2. Emerald ash borer has been detected in Winnipeg, Manitoba – home to an estimated 350,000 ash trees. Winnipeg is 1,300 km (870 miles) from Saulte Ste. Marie, the closest Canadian outbreak. The closest U.S. outbreak is in Duluth, Minnesota — 378 miles.
  3. Despite strenuous efforts by Pennsylvania (supported, but not adequately, by APHIS), (see my blog from last February ), spotted lanternfly has been detected in Delaware, New York, and Virginia. A map showing locations of apple orchards in the Winchester, Virginia area is available here.
  4. There is continued lack of clarity about biology and impact of velvet longhorned beetle (see my blog from last February.) The Utah population appears to be growing. APHIS is funding efforts to develop trapping tools to monitor the species.
  5. Alerted at the Forum, I investigated a disease on oak trees caused by the pathogen Diplodia corticola. Already recorded in Florida, California, Massachusetts and Maine, last year the disease was also detected in West Virginia. Forest pathologists Danielle Martin and Matt Kasson don’t expect this disease to cause widespread mortality. However, they do expect it to weaken oaks and increase their vulnerability to other threats.

spread of laurel wilt disease

Laurel wilt disease is one of the worst of the established non-native pests. Two speakers at the Forum described its ecological impacts.

Dr. John Riggins of the University of Mississippi reported that 24 native herbivorous insects are highly dependent on plants vulnerable to the laurel wilt insect-pathogen complex. One of these, the Palamedes swallowtail butterfly (Papilio palamedes) has suffered a three-fold to seven-fold decline in populations at study sites after the death of redbay caused by laurel wilt.

Dr. Frank Koch of the USDA Forest Service expects that the disease will spread throughout most of the range of another host, sassafras. (See a map of the plant’s range). With the climate changing, the insect is unlikely to suffer winter cold mortality in the heart of the tree’s range in Kentucky, West Virginia, and Virginia.

Apparently many birds depend on spicebush, a shrub in the Lauraceae family, but there is no easily available data on any changes to its distribution or health.

 

Good News

Other speakers at the Forum provided encouraging information.

Scientists described progress on breeding American elm trees resistant to or tolerant of the introduced Dutch elm disease (DED). USFS scientists led by James Slavicek and Kathleen Knight are trying to improve the genetic diversity and form of disease-tolerant American elms and to develop strategies for restoring them to the forest.

More than 70 seedlings planted in an orchard are being inoculated with the DED pathogen to test the trees’ tolerance. The project continues to collect seeds or cuttings from apparently resistant or tolerant trees. If you are aware of a large surviving elm in a natural setting (not urban planting), please contact the program via its website.

The project is also experimenting with methods for restoring trees in the forest. In one such experiment, elms, sycamores, and pin oaks have been planted at sites in Ohio where openings had been created by the death of ash attacked by emerald ash borer. Survival of the elm seedlings has been promising.

 

Also, there is cause to be optimistic re:

  1. Walnut / thousand cankers disease

In the East, walnut trees appear to recover from thousand cankers disease. One factor, according to Matt Ginzel of Purdue University, is that the thousand canker disease fungus, Geosmithia morbida, is a weak annual canker that would not cause branch or tree mortality in the absence of mass attack by the walnut twig beetle. Another factor is the greater reliability of precipitation in the East. Dr. Ginzel is now studying whether mass attack by the beetle is sufficient – alone – to kill walnut trees.

 

  1. b) Sirex noctilio

In Ontario, Laurel Haavik, U.S. Forest Service, finds both low impacts (so far) and evidence of resistance in some pine trees.

 

Also, scientists are making progress in developing tools for detecting and combatting highly damaging pests.

  1. Richard Stouthammer of U.C. Riverside has detected an effective chemical attractant for use in monitoring polyphagous and Kuroshio shot hole borers.  He is testing other pheromones that could improve the attractant’s efficacy. He has also detected some chemicals that apparently repel the beetles. His colleague, pathologist Akiv Eskalen, is testing endophytes that attack the beetles’ Fusarium fungus.
  2. Several scientists are identifying improved techniques for surveillance trapping for wood-boring beetles. These include Jon Sweeney of Natural Resources Canada and Jeremy Allison of the Great Lakes Forestry Centre.

 

Progress has also been made in biocontrol programs targetting non-native forest pests.

  1. Winter moth

Joseph Elkington of the University of Massachusetts reports success following 12 years of releases of the Cyzenis moth – a classical biocontrol agent that co-evolved with the winter moth in Europe. The picture is complex since the moths are eaten by native species of insects and small mammals and parasitized by a native wasp. However, native predators didn’t control the winter moth when it first entered Massachusetts.

2) Emerald ash borer

Jian Duan of the Agriculture Research Service reported that biocontrol agents targeting the  are having an impact on beetle densities in Michigan, where several parasitoids were released in 2007 to 2010. The larval parasitoid Tetrasrticus planipennisi appears to be having the greatest impact. A survey of ash saplings at these sites in 2015 found that more than 70% lacked fresh EAB galleries. In other trees, larval density was very low – a level of attack that Duan thinks the trees can survive.

However, Tetrasrticus has a short ovipositor so it is unlikely to be able to reach EAB larvae in larger trees with thicker bark. Furthermore, most of the biocontrol agents were collected at about 40o North latitude. It is unclear whether they will be as successful in controlling EAB outbreaks farther South.

Consequently, Duan noted the need to expand the rearing and release of a second, larger braconid wasp Spathius galinae, continue exploration in the southern and western edges of the EAB native range for new parasitoids; and continue work to determine the role of the egg parasitoids.

A brochure describing the U.S. EAB biocontrol program is available here

Canada began its EAB biocontrol program in 2013, using parasitoids raised by USDA APHIS. While evaluating the efficacy of these releases, Canada is also testing whether biocontrol can protect street trees.

3) Hemlock woolly adelgid

Scientists have been searching for a suite of biocontrol agents to control HWA for 25 years. Scientists believe that they need two sets of agents – those that will feed on the adelgid during spring/summer and those that will feed on HWA during winter/spring.

The first agent, Sasajiscymnus tsugae, was released in large numbers beginning in 1995. It is easy to rear. However, there are questions regarding its establishment and impact.

Laricobius nigrinus – a winter/spring feeder from the Pacific Northwest – was released beginning in 2003. It is widely established, especially in warmer areas. A related beetle, L. osakensis, was discovered in a part of Japan where eastern North American populations of HWA originated. Releases started in 2012. Scientists are hopeful that this beetle will prove more effective than some of the other biocontrol agents.

Winter cold snaps in the Northeast have killed HWA. While HWA populations often rebound quickly, predatory insects might suffer longer-term mortality. This risk intensifies the importance of finding agents that attack HWA during the spring or summer. Two new agents – the silver flies Leucopis artenticollis and L. piniperda – may be able to fill this niche. Both are from the Pacific Northwest. Initial releases have established populations.

 

4) USDA scientists are at earlier stages of actively seeking and testing possible biocontrol agents targetting Asian longhorned beetle and spotted lanternfly.

 

5) Invasive Plant Management

A study in New York City shows that invasive plant removal can have lasting effects. Lea Johnson  of the University of Maryland studied vegetation dynamics in urban forest patches in New York City. Her publications are available here.

In the 1980s New York undertook large scale restoration of its parks, including removal of invasive plants – especially multiflora rose, porcelainberry (Ampelopsis) and oriental bittersweet (Celastris). The goal was to establish self-sustaining forest with regeneration of native species. In 2006, Dr. Johnson was asked to evaluate the parks’ vegetation. She compared restored sites and similar sites without restoration.

I find it promising that Dr. Johnson found persistent differences in forest structure and composition as much as 15 or 20 years after restoration was undertaken. Treated sites had significantly lower invasive species abundance, a more complex forest structure, and greater native tree recruitment.

Still, shade intolerant species were abundant on all sites. The native shade tolerant species that had been planted did not do as well because gaps in the canopy persist.

 

CONCLUSIONS

As always, the annual Interagency Research Forum on Invasive Species provides an excellent opportunity to get an overview of non-native pest threats to America’s forests and the ever-wider range of scientists’ efforts to combat those threats. Presenters from universities as well as USDA, Canadian, and state agencies describe the status of host tree and pest species, advance promising technologies for detection, monitoring and control, and – increasingly – strategies for predicting potential pests’ likely impact. The networking opportunities are unparalleled.

 

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.

 

You Might Be Surprised By Who is Authorized to Manage Wildlife on Federal Lands

mountain goats – introduced onto USFS-managed lands in the Columbia River Gorge at state initiative; in Utah, the state introduced mountain goats on lands adjacent to a USFS Research Natural Area

 

The journal Environmental Law has just published a 135-page article that debunks a common myth of wildlife management – a piece that the U.S. Forest Service tried to quash. The authors’ analysis could affect the introduction of potentially invasive non-native species – and the reintroduction of native ones – on federal lands.

Nie, M., C. Barns, J. Haber, J. Joly, K. Pitt & S. Zellmer. 2017. Fish and Wildlife Management on Federal Lands; debunking state supremacy. Environmental Law, Vol. 47, no. 4 (2017).

The article reviews the legal authority of federal and state governments to manage wildlife on federal lands.  The authors examined wildlife-related provisions within the National Park System, National Wildlife Refuge System, National Forest System, Bureau of Land Management, the special case of Alaska, the National Wilderness Preservation System, and the Endangered Species Act. They also reviewed cases where federal and state agencies came into conflict over wildlife management on federal lands.

Citing the U.S. Constitution, federal land laws, and relevant case law, the authors assert that federal agencies have an obligation, not just the discretion, to manage and conserve fish and wildlife on lands and waters under their management. They say that the often-cited statement that “the states manage wildlife and federal land agencies only manage wildlife habitat” is wrong from a legal standpoint. This is the myth that the article debunks.

Furthermore, the authors find that federal agencies frequently apply their powers in an inconsistent and sometimes even unlawful fashion. Due to political pressures, they may back down when confronted by states wanting to manage wildlife to achieve their own goals – even when the state’s goals conflict with the legally-mandated purposes of the federal land under question. Such goals might include ensuring maximum populations of “game” animals or introduction of species to new habitats – regardless of the potential impact on native plants and animals.

The authors note that federal land and wildlife laws provide ample opportunities for constructive intergovernmental cooperation in wildlife management. They call for truly mutual collaboration by federal, state, and tribal authorities in managing wildlife. However, such cooperation is blocked in part by states choosing to challenge the constitutional powers, federal land laws, and U.S. government supremacy. In addition, the authors contend, most states have not put together programs that address their own conservation obligations. These obligations are inherent in the widely recognized doctrine of wildlife being a public trust to be managed for the present and future benefit of the people, not the government or private individuals.

According to the website of the Forest Service Employees for Environmental Ethics,  posting of a draft of this article on the University of Montana website (where lead author Martin Nie teaches) led the U.S. Forest Service to pressure the university to withdraw the article. The university refused, and the Forest Service ended its contract with Nie and his research center.

The paper can be downloaded here. 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.

Posted by Faith Campbell

New Woodborer Detected – Importance of Surveillance By-Catch

 

Agrilus smaragdifrons – photo by Ryan Rieder, New Jersey Department of Agriculture

 

At least 11 non-native metallic wood-boring beetles in the genus Agrilus  have been introduced to either the United States or Canada – or both. The most recent detection is Agrilus smaragdifrons Ganglbauer, which feeds on the invasive plant tree of heaven (Ailanthus altissima). This information comes largely from an important new paper by noted entomologist E. Richard Hoebeke at the University of Georgia and others (see the reference Hoebeke et al. 2017 at the end of this blog).

 

Two more Agrilus species that are native to Mexico and – in one case, also Arizona – have been introduced to separate parts of the U.S. and are killing naïve hosts there. These are A. prionus (which attacks soapberry trees in Texas) and A. auroguttatus (the goldspotted oak borer, which attacks several oak trees in California). Both species are described here

 

The genus Agrilus is considered to be the largest genus of the entire Animal Kingdom; it has over 3,000 valid species (Hoebeke et al. 2017).

 

Most of the Agrilus introduced to North America do not attack trees. Several attack crops such as grapes, currants and gooseberries, and rasberries (Hoebeke et al. 2017; (Jendek and Grebennikov 2009; reference at the end of the blog). Others attack horticultural plants including roses, wisteria, and mimosa (Jendek and Grebennikov 2009).

 

Still others attack plants that are invasive, such as honeysuckles (Lonicera spp). One, A. hyperici Creutzer, was deliberately introduced as a biocontrol agent targeting St. John’s wort (Hypericum perforatum L.) (Jendek and Grebennikov 2009).

 

However, Agrilus sulcicollis attacks oaks, beech, chestnut and other trees in the Fagaceae family in its native Europe. The beetle was detected in Ontario in 2006 (Jendek and Grebennikov 2009).

 

The most recently detected East Asian “jewel” beetle, Agrilus smaragdifrons, was discovered by analysis of Agrilus species caught in surveillance programs targeting other species – usually emerald ash borer (EAB) (A. planipennis). The beetle was first identified in traps deployed by the New Jersey Department of Agriculture. Unlike in many trapping programs, New Jersey screened the trap catches for all beetles in the family Buprestidae (which includes EAB). In 2015, two samples from separate trapping sites in the state contained a distinct but unrecognized species. These were identified by Dr. Hoebeke as the East Asian A. smaragdifrons (Hoebeke et al. 2017).

 

Alerted to the new species, scientists conferred and found additional detections of the species. An EAB biosurveillance program in New England utilizing the native ground-nesting wasp Cerceris fumipennis also detected the A. smaragdifrons in at least one location in central Connecticut in 2015. (The wasps capture beetles in the Buprestid family to feed to their young. By observing which species of beetles are brought to their nests by the wasps, scientists can learn which species are present in an area.)

 

Pennsylvania has collected A. smaragdifrons in surveillance programs targeting either EAB or spotted lantern fly (Lycorma delicatula (White))(Hoebeke et al. 2017).

locations where A. smaragdifrons has been detected; map from Hoebeke et al. 2017

It turned out that A. smaragdifrons has been in the U.S. for several years. One scientist photographed the beetle – without knowing what it was – in 2011 in New Jersey and posted the image at BugGuide (http://bugguide.net/node/view/1139674/bgimage ; accessed by Hoebeke and colleagues on 1 May 2017).

 

Recent field observations in China and the U.S. have observed both adults and larvae feeding on tree of heaven. In Beijing, many Ailanthus trees in gardens or along roadsides have succumbed to attack by this wood-borer. Other tree species on the grounds of Beijing Forestry University have not been attacked by A. smaragdifrons (Hoebeke et al. 2017). Still, no proper host-specificity test has yet been conducted on the beetle.

 

Of course, Ailanthus is widespread across North America, from southern Canada to Florida, and even along river courses in the arid Southwest. According to the USDA Forest Service (see the third on-line reference at the end of the blog), Ailanthus is known to be present in 42 states. It is most abundant in the Mid-Atlantic and Northeastern states. For example, 18% of the forest plots inventoried by the USDA Forest Service Forest Inventory Analysis program in West Virginia had Ailanthus present. Efforts are under way to try to find biocontrol agents (Hoebeke et al. 2017).

 

 

Importance of analyzing by-catch in insect detection surveys.

 

While most managers of pest surveys ignore the non-target species caught in their traps (“by-catch”), this detection shows that examining the by-catch can sometimes result in discovering previously unknown species. (Other examples of such detections include the pine pest Sirex noctilio in New York in 2004 and the oak-feeding Agrilus sulcicollis in Ontario and later Michigan.

 

Hoebeke and his colleagues strongly recommend that scientists pay attention to non-target insects captured in their surveys, especially those insects that show up in any abundance for the first time.

 

SOURCES

 

Hoebeke, E.R., E. Jendek, J.E. Zablotny, R. Rieder, R. Yoo, V.V. Grebennikov and L. Ren. 2017. First North American Records of the East Asian Metallic Wood-Boring Beetle Agrilus smaragdifrons Ganglbauer (Coleoptera: Buprestidae: Agrilinae), a Specialist on Tree of Heaven (Ailanthus altissima, Simaroubaceae) Proceedings of the Entomological Society of Washington, 119(3):408-422.

 

This article demonstrates how to distinguish the Ailanthus beetle from other Agrilus species.

 

Jendek, E. and V.V. Grebennikov. 2009. Agrilus sulcicollis (Coleoptera: Buprestidae), a new alien species in North America. Canadian Entomologist 141: 236–245.

Maryland has declared A. smaragdifrons its “invasive species of the month” for December 2017. Visit http://mdinvasivesp.org/invader_of_the_month.html

Information about Ailanthus as an invasive plant is available at

https://www.invasivespeciesinfo.gov/plants/treeheaven.shtml ; https://www.nps.gov/plants/alien/pubs/midatlantic/midatlantic.pdf

https://www.nrs.fs.fed.us/pubs/43136

Biological Control Approved for Invasive Black and Pale Swallow-wort!

black swallow-wort; photo by Leslie J. Mehrhoff, University of Connecticut

Help is on the way!

With funding support through the Northeast IPM Partnership, University of Rhode Island entomologist Richard Casagrande has been leading a team to find biological control agents for two invasive plant species. The target species, black swallow-wort (Vincetoxicum nigrum) and pale swallow-wort (Vincetoxicum rossicum), are native to Europe and members of the milkweed family Apocynaceae (previously Asclepiadaceae). In the U.S., their vigorous growth overtakes and smothers small trees, shrubs and other native plants and threatens the survival of the monarch butterfly whose larvae rely on milkweed for their development. They are currently found in the northeastern and mid-Atlantic states but could spread much farther.

(See Faith’s earlier blog about USDA speeding up approvals of biocontrols for invasive plants here.

U.S. native swallow-wort species belong to the genus Cynanchum and include a dozen or so rare and endangered plant species. It was essential to consider these native species in the investigations. Feeding tests would need to show definitively that the potential biocontrol species would not attack native swallow-worts or other native members of the milkweed family. And, Jennifer Dacey, Casagrande’s graduate student, wanted to find out how well the exotic swallow-worts might provide for monarch butterflies. The results were alarming.  All of the monarch larvae died when hatching on black swallow-wort.  “They stopped eating after a single bite,” says Casagrande.

pale swallow-wort; photo by Leslie J. Mehrhoff, University of Connecticut

Why biological control?

Small infestations of invasive swallow-wort, seedlings and young plants can be pulled up by hand, mature plants can be dug up, and frequent mowing can suppress populations in fields. However, most infestations are too extensive to control by hand. Systemic herbicides – those that are carried through the plant to the roots — can be used to control large infestations, using foliar sprays. Several years of treatment will likely be needed due to the persistence of swallow-wort seeds. These efforts can be part of an overall Integrated Pest Management strategy but the best long-range solution is biological control. Biocontrol relies on finding herbivores that have coevolved to feed on specific invasive plants in their native range that will not have a significant impact on non-target species. Graduate student Aaron Weed worked with Swiss scientists to identify a handful of specialist plant herbivores, mainly beetles and moths that evolved with black swallow-wort and pale swallow-wort in their native ranges in Europe and were highly unlikely to feed on other plant species.

Approval process.

All biological control agents must be approved for release by the U.S. Department of Agriculture, Animal and Plant Health Inspection Service (APHIS). APHIS sets up a Technical Advisory Group, or “TAG”, to review the research on feeding tests conducted by the researchers, called “no-choice” tests.  Potential biocontrol agents are tested for feeding on an extensive selection of native plant species and their relatives to ensure the agents are specific to the target species and won’t pose a threat to agriculture or to rare, threatened or endangered species or to other native species. The TAG list includes, naturally, most native milkweed relatives and even species more distantly related.

“Luckily, none of our native plants is closely related to the [invasive]swallow-worts,” says Casagrande. “That makes [swallow-wort] a great candidate for classical biological control.  The Tag list also includes a suite of Eurasian plants you might expect these specialists to nibble at now and then, and even plants that could host these specialists’ relatives. The bar is high for these no-choice tests: biocontrols must prove they’ll die before they switch.”

Casagrande’s team examined five possible biocontrol specialists in their quarantine lab, including two European moth species (Hypena opulenta and Abrostola asclepiadis) that feed on swallow-wort leaves in their native range. The researchers wanted to be sure these insects wouldn’t jump to non-target plants on the TAG list, since the last thing anyone wants is a new pest dominating the landscape, threatening agriculture, native ecosystems, and rare plants.

Results?

Both leaf-eating moths “passed the acid test,” says Casagrande. However, scientists have only petitioned for and received approval for Hypena opulenta, which was approved by the USDA in September 2017. They may seek approval for Abrostela in the future but for now are focused on rearing, releasing, and studying the effectiveness of Hypena.  Releases in Canada started in 2013 when Hypena was approved there. Since then, it has established and spread but it is too soon to evaluate its effectiveness.

Releases in the U.S.

Hypena opulenta was released on Naushon Island, Massachusetts, in early September 2017 – the only release in the United States – where both black and pale swallow-wort occur. The field release was carried out by placing about 400 larvae in each of 4 large cages containing both swallow-wort species in sun and shade locations. The larvae will be allowed to grow and develop in the cages for a little while before the cages are opened to allow the larvae to escape and start establishing on the island.

Next steps?

Funding will be sought to support rearing of Hypena at University of Rhode Island and other locations in the U.S. Dr. Lisa Tewksbury, Manager of Biological Control at URI, is running the program. It will take a few years to get to the point of having sufficient moths to distribute widely.  Best practices for releasing and monitoring will be developed.

Thanks to the Northeast IPM Partnership and the interest and dedicated efforts of Casagrande and his research team, we now have the most effective tool to use against two highly invasive plant species that will also protect our native species and natural ecosystems.

 

Posted by Jil Swearingen

Jil recently retired from the federal government and works as an invasive species consultant. She has 28 years of experience working on invasive species at the county, regional and national level in areas of education, outreach and management. Jil initiated and co-founded the Mid Atlantic Invasive Plant Council and serves on the board. Jil serves as the Coordinator for the Mid Atlantic Early Detection Network, a project she initiated and co-developed, and she continues to serve as Chair for the Plant Conservation Alliance’s Alien Plant Working Group and manager of the Weeds Gone Wild website. Jil is lead author of the book, Plant Invaders of Mid-Atlantic Natural Areas. She was recently elected to serve on the Board of Directors of the Maryland Native Plant Society

 

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.