A recent study (Hudgins, Koch, Ambrose & Leung 2022; full citation at end of blog) projects that, by 2050, 1.4 million street trees in urban areas and communities will be killed by introduced insect pests. This represents 2.1- 2.5% of all urban street trees. Nearly all of this mortality will occur in a quarter of the 30,000 communities evaluated. Additional urban trees – in parks, other plantings, on homeowners’ properties, and in urban woodlands – are also expected to die.
Loss of these trees will undercut all the ecosystem services provided by urban trees.
The principal cause of mortality will be the emerald ash borer (EAB). Already, an estimated 230,000 ash trees have been killed by EAB. The authors predict that 6,747 communities not yet affected by the EAB will suffer the highest losses between now and 2060. Most of these communities are in a 350,000 square mile area of the northeast and central states. However, the risk is far wider, reaching as far as Seattle.
In the top ‘hotspot cities’ projected mortality is in the range of 5,000–25,000 street trees. These include Milwaukee; the Chicago area (Chicago / Aurora / Naperville / Arlington Heights); Cleveland; and Indianapolis. As in previous studies, the highest insect impacts are in the Northeast. Pests impacting this region – in addition to the emerald ash borer – include the spongy moth (formerly called gypsy moth) and hemlock woolly adelgid.
Because insect-killed trees must be treated or removed to minimize the risk to human life and property, the pest risk represents an economic as well as ecological threat. Removing and replacing just the street trees is projected to cost cities $30 million per year. Considering the cities I mentioned above, Milwaukee faces costs estimated at $13 million; Warwick, RI $2.5 million; Baltimore $1.7; Richmond and Virginia Beach $7.3 million and $700,000 respectively; and three New Jersey cities (Jersey City, Elizabeth City, and Patterson) $1.6 million combined.
USDA APHIS ended the federal quarantine for EAB in 2021. Therefore these cities and states are on their own to protect themselves from not only this and other damaging insects but also their extraordinarily high economic costs.
The study evaluated the risk to 48 genera of trees in about 30,000 communities. The most widely planted genera are maples (Acer spp.) and oaks (Quercus spp.). Consequently, they will die in largest numbers. An estimated 26.5 million maples and 5.9 million oaks are at risk, primarily in the East. As noted above, EAB is expected to kill 99% of ash trees in 6,747 communities across the country. In the Southwest, there are 3.4 million pines (Pinus spp.); the threat to them is not woodborers, but scale insects (San Jose scale [Quadraspidiotus perniciosus] and calico scale [Eulecanium cerasorum]).
As we know, urban forests are easily invaded because they are close to ports of entry and are often composed primarily of highly susceptible species. Hudgins, Koch, Ambrose and Leung analyzed the potential risk associated with introduction of a new woodboring insect from Asia – which they point out is the source of most imported goods. They determined that if such an introduced pest were to attack maples or oaks, it could kill 6.1 million trees and cost American cities $4.9 billion over 30 years. The risk would be highest if this pest were introduced via a port in the South.
In an earlier blog I reported that the U.S. is currently importing about 20 million shipping containers filled with goods from Asia per year. I have often blogged about the pest risk associated with wood packaging accompanying these imports. The number of containers from Asia entering Southeastern ports rose by more than 10% from December to January.
Hudgins, Koch, Ambrose & Leung combined four sources of information to produce these estimates:
- a model of spread for 57 species of introduced insect pests already determined to cause significant damage to trees;
- the distribution of genera of urban street trees across 30,000 US communities;
- a model of host mortality in response to each insect-host combination; and
- the cost of removing and replacing dead trees, linked to tree size (dbh).
They excluded several categories of pests. One of the most damaging, Asian longhorned beetle, was excluded because scientists have already developed control methods to limit its spread. Also excluded were species present in the U.S. for less than five years; species with no known economic impacts; and species for which no hosts in natural North American forests have been identified. Also excluded – although the authors do not mention this – are species that did not qualify for inclusion in the Aukema et al. study (see reference at end of this blog) because they have been introduced from nearby portions of North America, e.g., goldspotted oak borer. Finally, the study does not include pathogens. Some pathogens have caused huge losses of urban trees in the past, e.g., Dutch elm disease; some are causing losses now, e.g., sudden oak death. The authors do mention the Fusarium disease vectored by polyphagous (and Kuroshio) shot hole borers in southern California.
Consequently, the study’s estimate of 1.4 million street trees dead and costs of $30 million per year are underestimates.
The study has generated considerable media interest, including in the Washington Post.
SOURCES
Aukema, J.E., D.G. McCullough, B. Von Holle, A.M. Liebhold, K. Britton, & S.J. Frankel. 2010. Historical Accumulation of Nonindigenous Forest Pests in the Continental United States. Bioscience. December 2010 / Vol. 60 No. 11
Hudgins, E.J., F.H. Koch, M.J. Ambrose, B. Leung. 2022. Hotspots of pest-induced US urban tree death, 2020–2050. Journal of Applied Ecology 2022;00:1-11 DOI: 10.1111/1365-2664.14141
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