Asian giant hornet (Vespa mandarinia); photo by University of Florida Dept. of Entomology
Washington State’s “Giant Hornet – Hornet Herald” for June asks people to help with detecting this pest by monitoring paper wasp nests (hornets attack them). Hornet visits last 5 – 10 minutes while the hornet removes paper wasp larvae. How to help:
Locate paper wasp nests that you have access to and can monitor through October. Then log the nest locations using the form here
Visit the nests each week, observe them, and then log your nest activity on a different form – here. Please monitor the nests for at least 5 minutes during the day once per week, but you can check the nests for as long and as often as you would like.
If you would like guidance on how to become a citizen-science monitor or trapper of Asian giant hornets – or presumably other bioinvaders – go here
Meanwhile, Washington State Department of Agriculture entomologists are in South Korea testing several hornet attractants and studying hornet foraging behavior. The goal is to improve Washington’s trapping and tracking techniques.
Of course, 2022 is only half over, but so far neither Washington nor British Columbia has confirmed any detections.
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
loblolly pine – tree species showing highest value in this study; via Flickr
More scientists are examining the importance of American forests in providing ecosystem services – and the threat to those values raised by non-native pests and other factors. This is a broader perspective than used in the past – and it includes climatechange. also here
Jeannine Cavender-Bareu and colleagues (full citation at the end of this blog) found that changes in the abundance and composition of US trees have the potential to undermine the benefits and societal values derived from those forests now. They examined threats associated with increasing invasive pests and pathogens, greater frequency of major fires, and climate change. Together, these constitute a complex set of global change drivers – and the impact of each is accelerating.
The authors tried to measure the impact of these forces on forests’ ability to provide five key ecosystem services. Two are “regulating” services—regulation of climate and air quality. The other three are “provisioning” services—production of wood products, food crops, and Christmas trees.
Unfortunately, they could not find sufficient data to analyze five other ecosystem services, which are equally or more important. They include both regulatory and provisioning services: water management, such as erosion control, flood and storm surge regulation; urban heat island regulation and energy savings; providing habitats for species (biodiversity); recreation; or ornamental, spiritual, and aesthetic values.
Cavender-Bareu and colleagues concluded that the value of the five analyzed services provided by 400 tree species across the contiguous United States over the years 2010-2012 is $114 billion per year. The non-market “regulatory” values far exceeds their current commercial value.
Climate regulation via carbon storage in tree biomass provides 51% of this net annual value;
Human health improvements linked to trees’ filtering of air pollution provide an additional 37% of the annual net value.
Provisioning services, such as wood products, fruit and nut crops, and Christmas trees, provide only 12% of the net annual value. (By my calculation, wood products constituate almost three-quarters of this sum.)
The authors then tried to identify which tree lineages, e.g., taxonomic families, genera, or species, provide the greatest proportion of each of these ecosystem services. They also identified threats to these lineages. Together, this knowledge allows managers to target forestry management practices to the specific lineages within a landscape where ecosystem service are most at risk.
Table 1 in the article ranks 10 tree genera by the aggregate net value they provide: pine, oak, maple, Douglas-fir, hemlock, cherry/almond, spruce, hickories, yellow or tulip poplar, and ash. The table also provides separate dollar values for each of the five benefits.
Two lineages—pines and oaks — provide 42% of the value of these services (annually, pines = $25.4 billion; oaks = $22.3 billion). They note that these high values result from the large number of pine and oak species occupying diverse ecological niches. Oaks have the highest annual values for climate moderation or carbon storage ($10.7 billion) and air quality regulation ($11 billion). Oaks’ air quality regulation value reflects three factors: the genus’ abundance, the trees’ size, and the large numbers planted in cities and suburbs, that is, near human populations affected by pollution. Other than this issue of location, closely related tree species tend to have similar air quality regulation values.
Many lineages provide wood products, but the amounts vary widely among related species. Pines dominate annual net revenues from wood products at $7.4 billion, due in part to their high volume and higher than average price. The most valuable species in the context of this study’s set of ecosystem services are loblolly pine (Pinus taeda) and Douglas-fir (Pseudotsuga menziesii).
Edible fruits are concentrated in two lineages — family Rosaceae, especially genera Prunus and Malus; and family Rutaceae, genus Citrus. This category demonstrates the impact of disease: annual net returns from citrus products were actually negative during the 2010 – 2012 period due to abnormally low market prices and the prevalence of citrus greening disease in Florida, Arizona and California.
northern red oak – high value for timber & carbon sequestration; photo by dcrjsr via Wikimedia
Trees at Risk
As climate change progresses, the mix of tree species that provide critical ecosystem services will be altered—with unknown consequences. There could be increases in some services but also widely-expected losses in ecosystem benefits and human well-being.
An estimated 81% of tree species are projected to have at least 10% of their biomass exposed to climates outside their current climate envelope, impacting nearly 40% of total tree biomass in the contiguous U.S. An estimated 40% of species are projected to face increasing fire frequency. In both cases, individual species’ vulnerability depends more on where that species grows than on its genetic lineage. This analysis demonstrates a threatening interaction between these two disturbance agents: the species most valuable for carbon storage are also the most at risk from the increasing fire threat.
Known (established) pests threaten 16% of tree species and potentially affect up to 40% of total tree biomass. At greatest risk are the oak and pine genera (due to mountain pine beetle and oak wilt) plus most of the crop species. The authors cite chestnut blight and Dutch elm disease as examples of pests decimating once-dominant tree species — ones provided many services. In contrast to climate and fire risks, genetic relationships explain much of the risk of pest damage because most pests attack individual species, genera, or families. (There are exceptions – sudden oak death and the Fusarium fungi vectored by invasive shot hole borers attack species across a wide range of families.)
Cavender-Bareu and colleagues conclude that major losses to pest attack of dominant species and lineages that currently provide high-value ecosystem services would undermine forest capacity to provide important benefits—at least for decades. They note that pest threats appear to be increasing partially as a consequence of climate change, demonstrating that multiple threats can interact and exacerbate outcomes. They say policy interventions aimed at slowing pests’ spread will probably be necessary to preserve the ecosystem service of climate and air quality regulation.
The high diversity of tree taxa in U.S. forests might buffer losses of ecosystem service if the most valuable lineages (oaks and pines) are compromised. However, other species will be needed to fill the voids their loss creates. Ensuring this possibility will require intentional management of forests and trees in the face of myriad and simultaneous threats.
The authors also show how tree-provided ecosystem services are distributed across the U.S. depending largely on the locations of forests, tree plantations, and orchards. Climate and air quality regulation occurs everywhere forests grow. Timber production is concentrated in a subset of the regions that also produce high climate regulation and air pollution removal, including the Southeast, Pacific Northwest, Northeast, and Upper Midwest.
The most valuable tree crops are grown on the coasts, often where forests do not grow—e.g., California; and in several Southwestern, Southern, and Eastern states.
Cavender-Bareu and colleagues found that climate change threatens species in all parts of the continent. Wildfires are expected to increase especially in California and the Intermountain West, which they say coincides with high annual storage of carbon. (This finding is opposite from those of Quirion et al. (2021) which pointed to the slow growth of pines in this region as reducing carbon storage potential.)
Cavender-Bareu and colleagues found that pest threats are highest in the Southwest and Southeast. These pests (native and non-native) are predicted to disproportionally affect species that generate high annual net values for climate regulation, air quality regulation, and wood products – e.g., pines and oaks. As noted above, these values are driven by their abundance. They note that mountain pine beetle and oak wilt have not yet reached areas with high wood product production in Northeast and Southeast.
Other studies (see Aukema et al. 2010) and here & here show that the greatest threats from non-native pests are to the Northeast/Midwest, and the Pacific coast – and Hawai`i & here.
Rock Creek Park, Washington, D.C. – an urban forest! photo by Bonnachaven
Cavender-Bareu and colleagues’ analysis advances our understanding of the threat several change drivers pose to benefits Americans receive from our forests. However, we must remember that some of the most important ecosystem services were not included because of insufficient data. Missing services:
1) most urban ecosystems. Inclusion of urban trees in the analysis would significantly increase the value of avoided health damage due to tree-based removal of air pollution. Urban trees also help regulate climate change (Nowak et al. estimate 643 M Mg of carbon are stored in urban areas, at a value of $2.31 billion annually).
2) many other regulating ecosystem services, such as erosion control, flood regulation, storm surge regulation, urban heat island regulation, energy savings due to shade, and species habitat / biodiversity.
3) recreation, ornamental, spiritual, and aesthetic values.
A complete accounting would also require estimates of the damage trees cause and the cost of their maintenance. For example, the full cost of irrigating almond trees; allergies and irritations due to tree pollen and sap; injuries to people and property caused by falling trees and limbs; trees’ role in spreading fires; trees’ contribution to volatile organic compounds (a pollutant).
The estimated annual values of the climate and air quality regulation have large uncertainty. These arise from uncertainty re: the social cost of carbon, the value of a statistical life, and uncertainty in the air pollution dose–mortality response function. The estimated annual values of the provisioning services are more precise because they are calculated from the market price for the per unit value of tree crops, wood products, and Christmas trees, as well as reliable data on production volume.
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
Cavender-Bareu, J.M., E. Nelson, J.E. Meireles, J.R. Lasky, D.A. Miteva, D.J.Nowak, W.D. Pearse, M.R. Helmus, A.E. Zanne, W.F. Fagan, C. Mihiar, N.Z. Muller, N.J.B. Kraft, S. Polasky. 2022. The hidden value of trees — Quantifying the ecosystem services of tree lineages and their major threats across the contiguous. PLOS Sustainability and Transformation April 5, 2022.
Quirion BR, Domke GM, Walters BF, Lovett GM, Fargione JE, Greenwood L, Serbesoff-King K, Randall JM & Fei S (2021) Insect and Disease Disturbances Correlate With Reduced Carbon Sequestration in Forests of the Contiguous United States. Front. For. Glob. Change 4:716582. Volume 4 Article 716582 doi: 10.3389/ffgc.2021.716582
This blog asks YOU!!! to support funding for key USDA programs. Each is essential for protecting the resilience of the Nation’s forests in the face of invasive pests. Please help by contacting your members of the House and Senate Appropriations Committees. I provide a list of members – by state – at the end of this blog.
While the two key federal programs overlap, they are separately managed: USDA’s Animal and Plant Health Inspection Service (APHIS) and USDA’s Forest Service (USFS). These two agencies are funded by different subcommittees of the House and Senate’s Appropriations committees. APHIS is funded by the Subcommittees on Agriculture and Related Agencies. USFS is funded by the Subcommittees on Interior.
Your letter or email need be no more than a couple paragraphs. To make the case for greater funding, feel free to pick-and-choose from the information that follows. Your greatest impact comes from speaking specifically about what you know and where you live.
These are the specific dollar things we’d like you to ask for. The rationale for each is below.
Appropriations for APHIS programs (in $millions)
Program
FY 2021
FY 2022 CR
FY 2023 Pres’ request
Please ask
Tree & Wood Pest
$60.456
$61.217
$63
$70
Specialty Crops
$196.553
$209.553
$219
$219
Pest Detection
$27.733
$28.218
$29
$30
Methods Development
$20.844
$21.217
$22
$23
Appropriations for USFS programs (in $millions)
Program
FY 2021
FY 2022 CR
FY 2023 Pres’ request
Please ask
Forest Health Protection Coop Lands
$30.747
$30.747
$36,747
$51
FHP Federal Lands
$15.485
$15.485
$22.485
$32
Research & Development
$258.760
$258.760
$317.773
$317.733
% for forest invaders
~1%?
?
0
$16 M
Background on the Threat
I’m sure you are familiar with the many ecosystem services provided by America’s forests and woodlands – wildland, rural, and urban. (Besides – maybe you just love trees!) I assume you also know that these forests are under threat from a growing number of non-native insects and pathogens.
For a quick review, see earlier blogs re: 1) an estimate that 41% of forest biomass in the “lower 48” states is at risk to mortality caused by the most damaging 15 species; black ash swamps of the upper Midwest; unique forest ecosystems of Hawai`i; riparian forests in the far West; stream canyons of the Appalachian range and; high-elevation forests of the West; and unique forests of Southwest Oregon. Also, see the thorough discussion of these pests’ impacts in Invasive Species in Forests and Grasslands of the United States: A Comprehensive Science Synthesis for the United States Forest Sector – blog; link available here]
Meanwhile, newly-discovered pests continue to appear and require research and management. The most troubling current example is beech leaf disease. It’s killing beech trees from Ohio to Maine and south to Virginia.
These introduced pests usually first appear in cities or suburbs because they arrive on imported goods shipped to population centers. The immediate result is enormous damage to urban forests. A recently published article (“Hotspots of pest-induced US urban tree death, 2020–2050”), projects that, by 2050, 1.4 million street trees in urban areas and communities will be killed by introduced insect pests. Removing and replacing these trees is projected to cost cities $30 million per year. Additional urban trees – in parks, other plantings, on homeowners’ properties, and in urban woodlands – are also expected to die.
As we know, newly-arrived pests don’t stay in those cities. Some spread on their own. Others are carried far and wide on firewood, plants, patio furniture, even storage pods. And so they proliferate in rural and wildland forests, including US National Forests.
As we know too well, many pests—especially the highly damaging wood-borers—arrive in inadequately treated crates, pallets, and other forms of packaging made of wood. Other pests—e.g., spotted lanternfly —take shelter, or lay their eggs, in or on virtually any exposed hard surface, such as steel or decorative stone.
Imports from Asia have historically transported the most damaging pests. Unfortunately, imports from Asia have reached unprecedented volume – currently they’re running at a rate of 20 million shipping containers per year. Research findings lead to an estimate that at least 7,500 of these containers are carrying a tree-killing pest. The “Hotspots” authors found that if a new woodborer that attacks maples or oaks is introduced, 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 to the South – and southern ports are receiving more direct shipments from Asia!
Some types of pests—especially plant diseases and sap sucking insects —come on imported plants. A principle example is sudden oak death (SOD; and which attacks more than 100 species of trees and shrubs). Other examples are the rapid ʻōhiʻa death pathogen that threatens Hawai`i’s most widespread tree, ʻōhiʻa lehua; and beech leaf disease, a newly discovered threat that is killing beech trees in a band stretching from Ohio to Maine.
Background on Specific USDA Funding Requests
APHIS
To reduce the risk of new pest introductions and strengthen response to many important pests, please ask your member of Congress and Senators to support appropriations that support key APHIS programs in the table above. (I assume you know that APHIS is responsible for preventing introduction and spread of invasive pests. While most port inspections are carried out by the Department of Homeland Security’s Bureau of Customs and Border Protection, APHIS sets the policy guidance. APHIS also inspects imports of living plants.)
Thank your member for the incremental increases in funding for these programs in FY22 but suggest that a more substantial investment is warranted.
The Tree and Wood Pests account supports eradication and control efforts targeting principally the Asian longhorned beetle (ALB) and spongy (formerly gypsy) moth. Eradicating the ALB normally receives about two-thirds of the funds. The programs in Massachusetts, New York, Ohio, and South Carolina must continue until eradication succeeds.
The Tree and Wood Pests account formerly also funded APHIS’ emerald ash borer (EAB) regulatory program. APHIS terminated this program in January 2021. The probable result is that EAB will spread more rapidly to the mountain and Pacific Coast states. Indeed, the “Hotspots” article identified Seattle and Takoma as likely to lose thousands of ash trees in coming decades. This result shows what happens when APHIS programs are inadequately funded.
Re: the plant diseases and sap sucking insects that enter the country on imported plants, APHIS’ management is through its Specialty Crops program. Repeatedly, SOD-infected plantsand have been shipped from nurseries in the Pacific Coast states to vulnerable states across the East and South. Clearly this program needs re-assessment and – perhaps – additional funding.
The Specialty Crops program also is home to APHIS’ efforts to counter the spotted lanternfly, which has spread from Pennsylvania to Maryland, Delaware, New Jersey, Virginia, West Virginia, Ohio, even Indiana. This pest threatens both native trees and agricultural crops – including hops, grapes, apples, and more. California has adopted a state quarantine in hopes of preventing its introduction to that state. Still, APHIS has not established a quarantine.
Please ask the Congress to support the Administration’s request for $219 million for the Specialty Crops program. However, urge them to adopt report language to ensure that APHIS allots adequate funding under this budget line to management of both sudden oak death and spotted lanternfly.
Two additional APHIS programs are the foundation for effective pest prevention. First, the Pest Detection program is key to the prompt detection of newly introduced pests that is critical to successful pest eradication or containment. Please ask the Congress to fund Pest Detection at $30 million. Second, the “Methods Development” program enables APHIS to improve development of essential detection and eradication tools. Please ask the Congress to fund Methods Development at $23 million.
Please ask your member of Congress to support the Administration’s request for a $50.794 million fund for management of emergencies threatening America’s agricultural and natural resources. This program includes a $6 million increase for work with the Climate Conservation Corps specifically targetting invasive species. Although the details are not yet clear, the program’s focus will be to improve surveillance and mitigation methods.
US Forest Service
The USFS has two programs critical to managing non-native tree-killing pests – Forest Health Management (or Protection; FHP) and Research and Development (R&D). FHP provides technical and financial assistance to USFS units (e.g., National forests and regions), other federal agencies, states, municipalities, and other partners to detect and manage introduced pests – including several that APHIS regulates and dozens that it does not. R&D funds efforts to understand non-native insects, diseases, and plants – which are usually scientific mysteries when they first are detected. Of course, this knowledge is crucial to effective programs to prevent, suppress, and eradicate the bioinvader. See the table at the beginning of the blog for specific funding requests for each program.
The Forest Health Management Program (FHP) has two funding streams: Federal Lands and Cooperative Lands (all forests under non-federal management, e.g., state and private forests, urban forests). Both subprograms must be funded in order to ensure continuity of protection efforts – which is the only way they can be effective. Some members of Congress prefer to focus federal funding on National forests. However, allowing pests to proliferate until they reach a federal forest border will only expose those forests to exacerbated threats. Examples of tree-killing pests that have spread from urban areas to National forests include the hemlock woolly adelgid, emerald ash borer, polyphagous and Kuroshio shot hole borers, sudden oak death, and laurel wilt disease. [All profiled here]
Adequate funding for FHP is vital to realizing the Administration’s goals of ensuring healthy forests and functional landscapes; supporting rural economies and underserved communities; enhancing climate change adaptation and resilience; and protecting biological diversity.
Please ask your Member of Congress and Senators to provide $51 million for work on non-federal cooperative lands. This level would partially restore capacity lost over the last decade. Since Fiscal Year (FY) 2010, spending to combat 11 specified non-native insects and pathogens fell by about 50%. Meanwhile, the pests have spread. Also, please ask your Member and Senators to support a $32 million appropriation for the Federal Lands subprogram for FY23 which is allocated to pests threatening our National forests directly.
A vital component of the FHP program is its leadership on breeding pest-resistant trees to restore forests decimated by pests. FHP’s Dorena Genetic Resource Center, in Oregon, has developed Port-Orford cedar seedlings resistant to the fatal root-rot disease. and blog. These seedlings are now being planted by National forests, the Bureau of Land Management, and others. In addition, pines with some resistance to white pine blister rust are also under development. The Dorena Center offers expert advice to various partners engaged in resistance-breeding for Oregon’s ash trees and two tree species in Hawai`i, koa and ʻōhiʻa. and blog.
The USFS research program is well funded at $317 million. Unfortunately, only a tiny percentage of this research budget has been allocated to improving managers’ understanding of specific invasive species and, more generally, of the factors contributing to bioinvasions. Funding for research conducted by USFS Research stations on ten non-native pests decreased from $10 million in Fiscal Year 2010 to just $2.5 million in Fiscal Year 2020 – less than 1% of the total research budget. This cut of more than 70% has crippled the USFS’ ability to develop effective tools to manage the growing number of pests.
To ensure the future health of America’s forests, please ask your Member of Congress and Senators to request the Subcommittee to include in its report instructions that USFS increase the funding for this vital research area to 5% of the total research budget. The $16 million would fund research necessary to improving managers’ understanding of invasive forest insects’ and pathogens’ invasion pathways and impacts, as well as to developing effective management strategies. Addressing these threats is vital to supporting the Administration’s priorities of increasing adaptation and resilience to climate change and implementing nature-based solutions.
The USFS Research and Development program should expand its contribution to efforts to breed trees resistant to non-native pests; programs deserving additional funding include hemlocks resistant to hemlock woolly adelgid; ashes resistant to emerald ash borer; beech resistant to both beech bark disease and beech leaf disease; link to DMF and elms resistant to Dutch elm disease. The Research program also continues studies to understand the epidemiology of laurel wilt disease, which has spread to sassafras trees in Kentucky and Virginia.
Members of House Appropriations Committee
STATE
MEMBER
APHIS APPROP
USFS APPROP
AL
Robert Aderholt
X
Calif
Barbara Lee David Valadao Josh Harder
X X
X
FL
Debbie Wasserman Scultz
X
GA
Sanford Bishop
X
ID
Mike Simpson
X
IL
Lauren Underwood
X
MD
Andy Harris
X
ME
Chellie Pingree
X
X
MI
John Moolenaar
X
MN
Betty McCollum
X
X
NV
Susie Lee Mark Amodei
X X
NY
Grace Meng
X
OH
Marcy Kaptur David Joyce
X X
PA
Matt Cartwright
X
TX
Henry Cuellar
X
UT
Chris Stewart
X
WA
Dan Newhouse Derek Kilmer
X
X
WI
Mark Pocan
X
Members of Senate Appropriations Committee
STATE
MEMBER
APHIS APPROP
USFS APPROP
AK
Lisa Murkowski
X
Calif
Diane Feinstein
X
X
FL
Marco Rubio
X
HI
Brian Schatz
X
IN
Mike Braun
X
KS
Jerry Moran
X
KY
Mitch McConnell
X
X
MD
Chris Van Hollen
X
ME
Susan Collins
X
MS
Cindy Hyde-Smith
X
X
MO
Roy Blunt
X
X
MT
Jon Tester
X
X
ND
John Hoeven
X
NM
Martin Heinrich
X
X
OR
Jeff Merkley
X
X
RI
Jack Reed
X
TN
Bill Hagerty
X
VT
Patrick Leahy
X
X
WV
Shelly Moore Capito
X
WI
Tammy Baldwin
X
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
monarch butterfly on swamp milkweed; photo by Jim Hudgins, USFWS
I have been impressed recently by two groups of scientists who are trying to broaden understanding of the impacts of invasive plants by examining the interactions of those plants with insects. As they note, herbivorous insects are key players in terrestrial food webs; they transfer energy captured by plants through photosynthesis to other trophic levels. This importance has been recognized since Elton first established the basic premises of food webs (1927) [Burghardt et al.; full citation at end of blog] Arthropods comprise significant members of nearly every trophic level and are especially important as pollinators. If introduced plants cause changes to herbivore communities, there will probably be effects on predators, parasites, and other wildlife through multitrophic interactions [Lalk et al.; Tallamy, Narango and Mitchell].
[I briefly summarize the findings of a third group of scientists at the end of this blog. The third group looks at the interaction between agriculture – that is, planting of non-native plants! – and climate change.]
One approach to studying this issue, taken by Douglas Tallamy of the University of Delaware and colleagues, is to look at the response of herbivorous insects to NIS woody plants fairly generally. They integrate their studies with growing concern about the global decline in insect populations and diversity. They note that scientists have focused on light pollution, development, industrial agriculture, and pesticides as causes of these declines. They decry the lack of attention to disruption of specialized evolutionary relationships between insect herbivores and their native host plants due to widespread domination by non-indigenous plants [Richard, Tallamy and Mitchell].
In their studies, Tallamy and colleagues consider not just invasive plants, but also non-native plants deliberately planted as crops or ornamentals, or in forestry. They point out that such introduced plants have completely transformed the composition of plant communities in both natural and human-dominated ecosystems around the globe. At least 25% of the world’s planted forests are composed of tree species not native to their locale. At least one-sixth of the globe is highly vulnerable to plant invasions, including biodiversity hotspots [Richard, Tallamy and Mitchell].
A different approach, taken by Lalk and colleagues, is more closely linked to concern about impacts of the plants themselves. They have chosen to pursue knowledge about relationships between individual species of invasive woody plants and the full range of arthropod feeding guilds – pollinators, herbivores, twig and stem borers, leaf litter and soil organisms. In so doing, they decry the general absence of data.
Both teams agree that:
Invasive plants are altering ecosystems across broad swaths of North America and the impacts are insufficiently understood.
The invasive plant problem will get worse because non-native species continue to be imported and planted. (Reminder: the Tallamy team considers impacts of deliberate planting as well as bioinvasion.)
Plant-insect interactions are the foundation of food webs, so changes to them will have repercussions throughout ecosystems.
Tallamy team
Non-native plants have replaced native plant communities to a greater or lesser extent in every North American biome – including anthropogenic landscapes [Burghardt]. The first trophic level in suburban and urban ecosystems throughout the U.S. is dominated by plant species that evolved in Southeast Asia, Europe, and South America [Tallamy and Shropshire]. Abundant non-native plants not only dominate plant biomass; they also reduce native plant taxonomic, functional and phylogenetic diversity and heterogeneity. Thus, they indirectly alter the abundance of native insects [Burghardt; Richard, Tallamy and Mitchell].
I think these articles might actually underestimate the extent of these impacts. While Richard, Tallamy and Mitchell report that more than 3,300 species of non-native plants are established in continental U.S., years ago Rod Randall said that more than 9,700 non-native plant species were naturalized in the U.S. (probably includes Hawai` i. The Tallamy team cites USDA Forest Service data showing 9% of forests in the southeast are invaded by just 33 common invasive plant species [Richard, Tallamy and Mitchell], I have cited that and other sources showing even greater extents of plant invasion in the east and here; other regions and here.
The Tallamy team has conducted several field experiments that demonstrate that the presence of non-native plants suppress numbers and diversity of native lepidopteran caterpillars. These non-native woody plants have not replaced the ecological functions of the native plants that used to support insect populations. This is true whether or not the non-native plants are deliberately planted or are invading various ecosystems on their own. [Richard, Tallamy and Mitchell]. (Of course, they expect plant invasions to grow; they note that some of the many ornamental species that are not yet invasive will become so.)
The result is disruption of the ecological services delivered by native plant communities, including the focus of their studies: plants’ most fundamental contribution to ecosystem function: generation of food for other organisms [Burghardt].
They note that plants’ relationship to insects differs depending on the insects’ feeding guilds — folivores, wood eaters, detritivores, pollinators, frugivores, and seed-eaters; and among herbivores with different mouthparts — chewing or sucking; and as host plant specialists or generalists. They decry studies that fail to recognize these differences [Tallamy, Narango, and Mitchell].
The Tallamy team explores why insect populations decline among non-native plants. That is,
1) Do insects directly requiring plant resources have lower fitness when using non-native plants; do they not recognize them as viable host plants; or do they avoid them altogether?
2) Are reductions in numbers of specialist herbivores mitigated by generalists? A decade of research shows that both specialists and generalists decline.
The team’s studies focus on lepidopteran larvae (caterpillars). Insect herbivores are both the largest taxon of primary consumers and extremely important in transferring energy captured by plants through photosynthesis to other trophic levels [Burghardt]. In addition, insects with chewing mouthparts are typically more susceptible to defensive secondary metabolites contained in leaves than are insects with sucking mouthparts that tap into poorly defended xylem or phloem fluids [Tallamy, Narango and Mitchell].
A study by Burghardt et al. found that 75% of all lepidopteran species and 93% of specialist species were found exclusively on native plant species. Non-native plants that were in the same genus as a native plant often supports a lepidopteran community that is a similar but depauperate subset of the community found on its native congener. In fact, the insect abundance and species richness supported by non-native congeners of native species was reduced by 68%.
A meta-analysis of 76 studies by other scientists found that, with few exceptions, caterpillars had higher survival and were larger when reared on native host plants. Plant communities invaded by non-native species had significantly fewer Lepidoptera and less species richness. In three of eight cases examined, non-native plants functioned as ecological traps, inducing females to lay eggs on plants that did not support successful larval development. Richard, Tallamy and Mitchell cite as an example the target of many conservation efforts, monarch butterflies (Danaus plexxipus), which fail to reproduce when they use nonnative swallowworts (Vincetoxicum species.) instead of related milkweeds (Asclepias species.).
Tallamy and Shropshire ranked 1,385 plant genera that occur in the mid-Atlantic region by their ability to support lepidopteran species richness. They found that introduced ornamentals are not the ecological equivalents of native ornamentals. This means that solar energy harnessed by introduced plants is largely unavailable to native specialist insect herbivores.
Tallamy, Narango, and Mitchell describe the following patterns:
1) Insects with chewing mouthparts are typically more susceptible to defensive secondary metabolites contained in leaves than are insects with sucking mouthparts that tap into poorly defended xylem or phloem fluids. As a result, sucking insects find novel non-indigenous plants to be acceptable hosts more often. However, there are more than 4.5 times as many chewing (mandibulate) insect herbivores than sucking (haustellate) species. It follows that the largest guild of insect herbivores is also the most vulnerable to non-native plants as well as being the most valuable to insectivores.
native azalea Rhododendron periclymenoides; photo by F.T. Campbell
2) Woody native species, on average, support more species of phytophagous insects than herbaceous species.
3) Although insects are more likely to accept non-native congeners or con-familial species as novel hosts, non-native congeners still reduced insect abundance and species richness by 68%.
4) Host plant specialists are less likely to develop on evolutionarily novel non-indigenous plants than are insects with a broader diet. There are far more specialist species than generalists, so generalists will not prevent serious declines in species richness and abundance when native plants are replaced by non-indigenous plants. In addition, non-native plants cause significant reductions in species richness and abundance even of generalists. In fact, generalists are often locally specialized on particular plant lineages and thus may function more like specialists than expected.
5) Any reduction in the abundance and diversity of insect herbivores will probably cause a concomitant reduction in the insect predators and parasitoids of those herbivores – although few studies have attempted to measure this impact beyond spiders, which are abundant generalists. The vast majority of parasitoids are highly specialized on particular host lineages.
6) Studies comparing native to non-native plants must avoid using native species that support very few phytophagous insects as their baseline, e.g., in the mid-Atlantic region tulip poplar trees (Liriodendron tulipifera) and Yellowwood (Cladrastus kentuckea).
7) Insects that feed on well-defended living tissues such as leaves, buds, and seeds are less likely to be able to include non-native plants in their diets than are insects that develop on undefended tissues like wood, fruits, and nectar. Although this hypothesis has never been formally tested, they note the ease with which introduced wood borers – emerald ash borer, Asian longhorned beetle, polyphagous and Kuroshio shot-hole borers, redbay ambrosia beetle, Sirex woodwasp (all described in profiles posted here — have become established in the US.
palamedes swallowtail Papilio palamedes; photo by Vincent P. Lucas; this butterfly depends on redbay, a tree decimated by laurel wilt disease vectored by the redbay ambrosia beetle
Lalk and Colleagues
As noted, Lalk and colleagues have a different frame; they focus on individual introduced plant species rather than starting from insects. They also limit their study to invasive plants. The authors say there is considerable knowledge about interactions between invasive herbaceous plants and arthropod communities, but less re: complex interactions between invasive woody plants and arthropod communities, including mutualists (e.g., pollinators), herbivores, twig- and stem-borers, leaf-litter and soil-dwelling arthropods, and other arthropod groups.
They ask why this knowledge gap persists when invasive shrubs and trees are so widespread and causing considerable ecological damage. They suggest the answer is that woody invaders rarely encroach on high-value agricultural systems and some are perceived as contributing ecosystem services, including supporting some pollinators and wildlife.
Lalk and colleagues seek to jump-start additional research by summarizing what is currently known about invasive woody plants’ interactions with insects. They found sufficient data about 11 species – although even these data are minimal. They note that all have been cultivated and sold in the U.S. for more than 100 years. All but one (mimosa) are listed as a noxious weed by at least one state; two states (Rhode Island and Georgia) don’t have a noxious weed list. None of the 11 is listed under the federal noxious weed statute.
Ailanthus altissima
Illustrations of how minimal the existing information is:
Tree-of-heaven (Ailanthus altissima) is noted to be supporting expanded populations of the Ailanthus webworm moth (Atteva aurea), which is native to Central America; and to be the principal reproductive host for SLF (Lycorma delicatua) https://www.dontmovefirewood.org/pest_pathogen/spotted-lanternfly-html/
Chinese tallow (Triadica sebifera) is thought to benefit both native generalist bee species and non-indigenous European honeybees (Apis mellifera).
Chinese privet (Ligustrum sinense) appears to suppress populations of butterflies, bees, and beetles.
Lalk and colleagues then review what is known about interactions between individual invasive plant species in various feeding guilds. They point out that existing data on these relationships are scarce and sometimes contradictory.
They believe this is because interactions vary depending on phylogenetic relationships, trophic guild, and behavior (e.g., specialized v. generalist pollinator). Arthropods can be “passengers” of a plant invasion. That is, they can be affected by that invasion, with follow-on effects to other arthropods in the community. Also, arthropods can be “drivers” of invasion, increasing the success of the invasive plants.
They then summarize the available information on various interactions. For example, they note that introduced plants can compete with native plants in attracting pollinators, causing cascading effects. Or they can increase pollination services to native plants by attracting additional pollinators.
They note that herbivore pressure on invasive plants can have important impacts on growth, spread, and placement within food webs. They note that these cases support the “enemy release hypothesis”, although they think there are probably additional driving mechanisms.
Lalk and colleagues note that most native twig- and stem-borers (Coleoptera: Buprestidae, Curculionidae, Cerambycidae; Hymenoptera: Siricidae) are not considered primary pests but that some of our most damaging insect species are wood borers (see above).
Some of these borers are decomposers; in that role, they are critical in nutrient cycling.
Arthropods in leaf litter and soil also serve important roles in the decomposition and cycling of nutrients, which affects soil biota, pH, soil nutrients, and soil moisture. They act as a trophic base in many ecosystems. Lalk and colleagues suggest these arthropod communities probably change with plant species due to differences in leaf phytochemistry. They cite one study that found litter community composition differed significantly between litter beneath tree-of-heaven, honeysuckle (Lonicera maackii), and buckthorn (Rhamnus cathartica) compared to litter underneath surrounding native trees.
Recommendations
Both the Tallamy and Lalk teams call for ending widespread planting of non-native plants. Lalk and colleagues discuss briefly the roles of
The nursery industry (including retailers); they produce what sells.
Scientists and educators have not sufficiently informed home and land owners about which species are invasive or about native alternatives.
Private citizens buy and plant what their neighbors have, what they consider aesthetically pleasing, or what is being promoted.
States have not prohibited sale of most invasive woody plants. Regulatory actions are not a straightforward matter; they require considerable time, supporting information, and compromise.
Tallamy team calls for restoration ecologists in the eastern U.S. to consider the number of Lepidopterans hosted by a plant species when deciding what to plant. For example, oaks (Quercus), willows (Salix), native cherries (Prunus)and birches (Betula) host orders of magnitude more lepidopteran species in the mid-Atlantic region than tulip poplar.(Those lepidopteran in turn support breeding birds and other insectivorous organisms.) [Tallamy & Shropshire]
Lalk and colleagues focused on identifying several key knowledge gaps:
How invasive woody plants affect biodiversity and ecosystem functioning
How they themselves function in different habitats.
Do non-native plants drive shifts in insect community composition, and if so, what is that shift, and how does it affect other trophic levels?
How do IAS woody plants affect pollinators?
The authors do not minimize the difficulty of separating such possible plant impacts from other factors, including climate change and urbanization.
Outhwaite et al. (full citation at end of this blog) note that past studies have shown that insect biodiversity changes are driven primarily by land-use change (which is another way of saying planting of non-native species – as Dr. Tallamy and colleagues describe it) and increasingly by climate change. They south to examine whether these drivers interact. They found that the combination of climate warming and intensive agriculture is associated with reductions of almost 50% in the abundance and 27% in the number of species within insect assemblages relative to levels in less-disturbed habitats with lower rates of historical climate warming. These patterns were particularly clear in the tropics (perhaps partially because of the longer history of intensive agriculture in temperate zones). They found that high availability of nearby natural habitat (that is, native plants) can mitigate these reductions — but only in low-intensity agricultural systems.
Outhwaite et al. reiterate the importance of insect species in ecosystem functioning, citing pollination, pest control, soil quality regulation & decomposition. To prevent loss of these important ecosystem services, they call for strong efforts to mitigate climate change and implementation of land-management strategies that increase the availability of natural habitats.
SOURCES
Burghardt, K. T., D. W. Tallamy, C. Philips, and K. J. Shropshire. 2010. Non-native plants reduce abundance, richness, and host specialization in lepidopteran communities. Ecosphere 1(5):art11. doi:10.1890/ES10-00032.
Lalk, S. J. Hartshorn, and D.R. Coyle. 2021. IAS Woody Plants and Their Effects on Arthropods in the US: Challenges and Opportunities. Annals of the Entomological Society of America, 114(2), 2021, 192–205 doi: 10.1093/aesa/saaa054
Richard, M. D.W. Tallamy and A.B. Mitchell. 2019. Introduced plants reduce species interactions. Biol Invasions
Tallamy, D.W., D.L. Narango and A.B. Mitchell. 2020. Ecological Entomology (2020), DOI: 10.1111/een.12973 Do Non-native plants contribute to insect declines?
Tallamy, D.W. and K.J. Shropshire. 2009. Ranking Lepidopteran Use of Native Versus Introduced Plants Conservation Biology, Volume 23, No. 4, 941–947 2009 Society for Conservation Biology DOI: 10.1111/j.1523-1739.2009.01202.x
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
tanoak killed by SOD; photo by Joseph O’Brien, via Bugwood
I am belatedly catching up with developments regarding sudden oak death (SOD; Phytophthora ramorum). The situation is worsening, with three of the four existing strains now established in U.S. forests. Nursery outbreaks remain disturbingly frequent.
This information comes primarily from the California Oak Mortality Task Force’s (COMTF) newsletters posted since October; dates of specific newsletters are shown in brackets.
Alarming presence of variants & hybridization
The long-feared risk of hybridization among strains has occurred. Canadian authorities carrying out inspections of a British Columbia nursery found a hybrid of European (EU1) and North American (NA2) clonal lineages. These hybrids are viable, can infect plants and produce spores for not only long-term survival but also propagation. So far the hybrid has been found in a single nursery; it has not spread to natural forests. The pathogen is considered eradicated in that nursery, so it is hoped it cannot reproduce further. [December 2021 newsletter, summarizing research by R. Hamelin et al.]
Noted British forest pathologist Clive Brasier warned in 2008 about the risk of hybrids evolving in nurseries which harbor multiple strains of related pathogens. (See full citation at end of the blog.)
The threat is clear: three of the four known variants are already established in forests of the Pacific Northwest – NA1, NA2, and EU1. (For an explanation of P. ramorum strains and mating types, go here.)
In Oregon, the EU1 strain was detected in a dying tanoak (Notholithocarpus densiflorus) tree in the forests of Curry County in 2015. Genetic analysis revealed that the forest EU1 isolates were nearly identical to EU1 isolates collected in 2012 from a nearby nursery during routine monitoring. This detection was considered to be evidence that multiple distinct P. ramorum introductions had occurred. The scientists expressed concern that the presence of this strain – which is of the A1 mating type while the widely established NA1 population of the pathogen in the forest is of the A2 mating type — makes the potential for sexual recombination more likely. Therefore, the state prioritized eradication of the EU1 forest infestation [Grünwald et al. 2016]. (For an explanation of P. ramorum strains and mating types, go here.)
The NA2 strain was detected in 2021, 33 km north of the closest known P. ramorum infestation. Because Oregonians genotype all detections on the leading front of the infection, they completed Koch’s postulates and found this surprising result [February 2022]. NA2 is thought to be more aggressive than the NA1 lineage [February 2022]. Surveys and sampling quickly determined that the outbreak is well established — 154 positive detections [February 2022] across more than 500 acres [October 2021]. Oregon Department of Forestry immediately began treatments; the goal is to prevent overlap with existing NA1 and EU1 populations. [April 2022; summarizing research by Peterson et al.] Given the number of infected trees and the new variant, Oregon pathologists believe this to be a separate introduction to Oregon forests that has been spreading in the area for at least four years [February 2022].
Scientists [April 2022; summarizing research by Peterson et al.] again note evidence of repeated introductions of novel lineages into the western US native plant communities; this region is highly vulnerable to Phytophthora establishment, justifying continued monitoring for P. ramorum not only in nurseries but also in forests.
SOD in Oregon; photo by Oregon Department of Forestry
The EU1 strain is also present in northern California, specifically in Del Norte County. It was detected there in 2020. Despite removal of infected and nearby host trees (tanoaks) and treatment with herbicide to prevent resprouting, the EU1 strain was again detected on tanoaks in 2021. The detected strain is genetically consistent with the EU1 outbreak in Oregon forests. Oddly, the usual strain found in North American forests, the NA1 strain, was not detected in Del Norte Co. in 2021 [February 2022].
One encouraging research finding [April 2022; summarizing research by Daniels, Navarro, and LeBoldus] is that established treatment measures have had significant impact on both the NA1 & EU1 lineages. They found on average 33% fewer positive samples at treated sites where NA1 is established; 43% reduction in P. ramorum prevalence at EU1 sites. Prevalence of P. ramorum in soil was not affected by treatment.
SOD Spread in Forests
In California, the incidence of new Phytophthora ramorum infections fell in 2021 to a historic low – estimated 97,000 dead trees across 16,000 acres, compared to ~885,000 dead trees across 92,000 acres in 2019 [April 2022]. It is agreed that the reason is the wave of mortality sparked by the very wet 2016-2017 winter has subsided and has been followed by several years of drought [February 2022].
data showing decline in new SOD detections in California in 2021 (no data collected in 2020)
In Oregon, however, SOD continues to spread. In 2010, the OR SOD Program had conceded that eradication was no longer feasible. Instead, authorities created a Generally Infested Area (GIA) where removal of infested tanoaks was now optional (not mandated) on private and state-owned lands. Since then, SOD has continued to spread and intensify within the designated zone. The GIA has been expanded eight times since its establishment in 2012; it now it covers 123 sq. mi. There has also been an immediate increase in tanoak mortality [December 2021].
In 2021, two new infestations were detected outside the GIA. One outbreak is on the Rogue River-Siskiyou National Forest along the Rogue River, 6 miles north of any previously known infestation. The second is just outside Port Orford [February 2022], 33 km north of the closest known infestation. This second infestation is composed of the NA2 variant [see above]. The Oregon Department of Agriculture (ODA) established emergency quarantines at these sites and began eradication efforts at both sites. The Oregon legislature appropriated $1.7 million to Oregon Department of Forestry to carry out an integrated pest management program to slow spread of the disease [February 2022].
Scientific research indicates that this situation might get worse. While it has long been recognized that California bay laurel (= Oregon myrtle) (Umbellularia californica) and tanoak are the principal hosts supporting sporulation and spread, it has now been determined that many other native species in the forest can support sporulation. Chlamydospore production was highest on bigleaf maple (Acer macrophyllum)and hairyCeanothus (Ceanothus oliganthus). All the other hosts produced significantly fewer spores than tanoak and myrtle [October 2021; summarizing research by Rosenthal, Fajardo, and Rizzo]
Furthermore, studies that aggregate observations of disease on all hosts, not paying attention to their varying levels of susceptibility, might lead scientists to misinterpret whether the botanic diversity slows spread of the pathogen [October 2021 summarizing research by Rosenthal, Simler-Williamson, and Rizzo].
Monitoring to detect any possible spread to the East
SOD risk map based on climate & presence of host species; USFS
The USDA Forest Service continues its Cooperative Sudden Oak Death Early Detection Stream Survey in the East. In 2021, 12 states participated – Alabama, Florida, Georgia, Illinois, Maryland, Mississippi, North Carolina, Pennsylvania, South Carolina, Texas, West Virginia, and Wisconsin. Samples were collected from 79 streams in the spring. Two streams were positive, both in Alabama. Both are associated with nurseries that were positive for P. ramorum more than a decade ago [October 2021].
Continued infestations in the nurseries
USDA Animal and Plant Health Inspection Service (APHIS) reported that in 2021, the agency supported compliance activities, diagnostics, and surveys in nurseries in 22 states. P. ramorum was detected at 17 establishments. Eight were new; nine had been positive previously. These included seven nurseries that ship intrastate – all had been positive previously. Six were already under compliance agreements. Also positive were three big box stores – none previously infected; and six nurseries that sell only within one state – five new. Infections at the big box outlets and half the intrastate nurseries were detected as a result of trace-forwards from other nurseries.
P. ramorum was detected in 300 samples in 2021 – 144 from plants in the genus Viburnum; 106 from Rhodendron (including azalea); and much lower numbers from other genera.
APHIS funds states for annual nursery surveys, compliance activities, and diagnostics through the: Plant Protection Act Section 7721 and the Cooperative Agricultural Pest Survey (CAPS) program. Table 4 lists states receiving survey funds. APHIS also supported compliance and diagnostic activities in California, Louisiana, Oklahoma, Oregon, Pennsylvania, Washington, and several states through Florida.
APHIS’ report – which provides few additional details about the nursery detections – can be found here.
California:
The California Department of Food and Agriculture (CDFA) reported that three of the eight nurseries regulated under either the federal or state sudden oak death program tested positive in 2021. This was down from five positive nurseries in 2020 [February 2022]. (In the past, numbers of nurseries testing positive have declined during droughts, risen during wet years.) At one interstate-shipping nursery 145 positive Viburnum tinus plants were detected by regulators in December 2021. Apparently the detection efforts were prompted by a trace-back from a nursery in an (unnamed) other state [April 2022].
Oregon:
Oregon continues to struggle with the presence of Phytopththora ramorum in the state’s nurseries. Early in 2021 the situation looked good. Three of eight interstate shippers and two intrastate shippers “passed” their sixth consecutive inspection with no P. ramorum detected so they were released from state and federal program inspection requirements. A fourth interstate-shipping nursery had ceased operating. By the end of the year, however, circumstances had deteriorated. One of the four interstate shippers still under regulatory scrutiny appeared to be badly infested. After routine autumn monitoring detected an infected plant, subsequent delimitation samplings detected 30 additional positive foliar samples and a large number (24) of samples were inconclusive. By spring 2022 six nurseries had to be inspected following trace-forwards from out-of-state nurseries. No P. ramorum was detected in five of these nurseries; the sixth had one positive foliar sample, so it is now under more stringent regulatory supervision [April 2022].
Washington:
Washington has only one interstate shipping nursery that is regulated under APHIS’ program; it tested negative in autumn 2021 [December 2021]. Meanwhile, USDA & Washington Department of Agriculture (WSDA) decided to deregulate the Kitsap County Botanical Garden where P. ramorum had been detected in 2015. Since then, more than 5,000 samples have been collected; 99.1% have tested negative. The last positive plant sample was collected in February 2016. Under a compliance agreement, the botanical garden will continue the best management practices deemed successful in eradicating the pathogen [December 2021]. However, water at the site continues to test positive [February 2022]. These water detections – in Washington and Alabama (above) – raise troubling questions.
Meanwhile, in late winter [April 2022], WSDA had to conduct two trace-forward investigations on plants that shipped from (unnamed) out-of-state nurseries. As of the April newsletter, 13 samples from four locations were all negative.
A stubborn problem has been the persistence of SOD infections in nurseries after the Confirmed Nursery Protocol has been carried out. Research indicates the reason might be that the pathogen is still there in the form of soilborne inoculum in buried, infested leaf debris [December 2021 newsletter; summarizing research by Peterson, Grünwald, and Parke].
Another native tree identified as host
photo by Miguel Vieira; via Wikimedia
Dieback on golden chinquapin, Chrysolepis chrysophylla, a slow growing, evergreen tree native to the U.S. west coast has been confirmed as caused by Phytophthora ramorum. The detection was in a part of Marin County, California heavily infested by P. ramorum since early in the epidemic. Affected trees were large overstory trees. Unlike other hosts in the Fagaceae, there were no external bole cankers [April 2022 newsletter; summarizing research by Rooney-Latham, Blomquist, Soriano, and Pastalka].
SOURCES
Brasier, C.M. 2008. The biosecurity threat to the UK and global environment from international trade in plants. Plant Pathology (2008) 57, 792-808
Grunwald, N.J., M.M. Larsen, Z.N. Kamvar, P.W. Reeser, A. Kanaskie, J. Laine and R. Wiese. 2016. First Report of the EU1 Clonal Lineage of Phytophthora ramorum on Tanoak in an Oregon Forest. Disease Notes. May 2016, Vol. 100, No. 5, p. 1024
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
black berry eating hawthorn berries; photo by Paul D. Vitucci
Articles by Evan Fricke and colleagues remind us to look more broadly at bioinvasion to consider the impact on ecosystem function and evolution. They focus on animal interactions with plants in the shared environment, especially animals’ role as seed dispersers.
The authors also remind us that natural barriers explain why there are different species in different areas and thus how evolution and speciation follow different paths in different places. Think of Galapagos finches evolving in isolation from a few ancestors that somehow made it over the ocean from mainland South America.
These points are made in two recent articles.
In the first, Fricke and Svenning 2020 (full citation at end of this blog) note that about half of all plant species depend on animals to disperse their seeds. Animal seed dispersal is influenced by several drivers of global change, including local or generalized extinction (= defaunation); bioinvasion; and habitat fragmentation. The decline of large vertebrates has a particularly important role in these interactions.
Their study focused on fleshy-fruited plants that are dispersed by animals. (The study does not include nuts, e.g., acorns, which are presumably subject to some of the same pressures.) They expect evolution of the affected plants and animals to proceed differently as a result of the new partnerships, but they did not study any such interactions.
Their study covered animal seed-dispersal interactions with plants at 410 locations. The data encompassed 24,455 unique animal-plant pairs involving 1,631 animal and 3,208 plant species. Three quarters of the animals were birds; most of the rest were mammals, primarily bats and primates. Only 1% were in other animal groups – lizards, tortoises, or fish.
fruit bats on Luzon, Philippines; photo by Francesco Vernonesi; Flickr.com
They found that introduced plants and animals are twice as likely as native species to interact with introduced partners. The resulting interactions are likely to amplify biotic homogenization in future ecosystems. Already, introduced species have largely replaced missing native frugivore species in some places. In fact, mutualisms in which either or both the plant and animal is an introduced species are now about seven times higher than decades ago.
These mutual-benefit interactions of introduced species are even more prevalent in areas where human modification of the environment is greater. The proportion of introduced species and of novel interactions caused by introduced plant or animal species was higher for oceanic island systems than for continental bioregions. This finding adds a new dimension to the already recognized heightened susceptibility of remote islands to invasion and their loss of native species. Continental bioregions’ networks typically had few introduced animals and a greater prevalence of intro plants than animals.
Fricke and colleagues think plant-frugivore networks are likely to increasingly favor a relatively few introduced generalists over many native species, reducing the uniqueness of future biotas. The result might be to reduce resilience of terrestrial ecosystems by, first, allowing perturbations to propagate more quickly; and, second, by exposing disparate ecosystems to similar drivers. They called for giving higher priority to managing increasing ecological homogenization.
In the second article, Fricke, Ordonez, Rogers, and Svenning (2022) note that climate change requires many plant species to shift their populations hundreds of meters to tens of kilometers per year to track their climatic niche. Earth is also experiencing the formation of novel communities as species introductions and shifting ranges result in co-occurrence of species that do not share co-evolutionary history. They conclude that the novel mutualistic interaction networks will influence whether certain plant species persist and spread.
These authors examined four scenarios to assess how current long-distance dispersal has been affected by past defaunation and invasion and how it is threatened by species endangerment. These scenarios are as follows:
1st scenario (current scenario) = natural and introduced ranges of extant species today.
2nd scenario (natural scenario) = mammal and bird ranges as they would be if unaffected by extinctions, range contractions, or introductions.
3rd scenario (extinction scenario) = those bird and mammal species listed as vulnerable or endangered by the IUCN go extinct.
4th scenario (extirpation of introduced species scenario) = introduced species are extirpated.
Fricke and colleagues estimate that extinction of at least local populations of seed-dispersing mammals and birds has already reduced the capacity of plants to track climate change by 60% globally. The effect is strongest in temperate regions and regions with little topographic complexity. Two examples are eastern North America and Europe. These regions face a double threat: rapid climate change and loss of large mammals that provided long-distance dispersal.
The extinction scenario is most evident in Southeast Asia and Madagascar. The remaining animal seed dispersers are already threatened or endangered. Fricke and colleagues project that future loss of vulnerable and endangered species from their current ranges would result in a further reduction of 15% in the capacity of plants to track climate change.
The contrary situation is found on islands which have few native mammals. Introduced species are now important long-distance seed dispersers. In some cases, the introduced animals are dispersing invasive plant seeds, e.g., on Hawai`i feral hogs are spreading the invasive plant strawberry guava (Psidium cattleianum).
strawberry guava on Maui; photo by Forest and Kim Starr
People’s actions have resulted in ecoregions disproportionately losing the species that provide long-distance seed dispersal function, i.e., large mammals. In other words, human activities have caused not only rapid climate change—requiring broad-scale range shifts by plants—but also defaunation of the birds and mammals needed by plants to do so. Habitat fragmentation and other land-use changes will likely amplify existing constraints on plant range shifts.
Fricke and colleagues say their findings emphasize the importance of not only promoting habitat connectivity to maximize the functional potential of current seed dispersers but also restoring biotic connectivity through the recovery of large-bodied animals to increase the resilience of vegetation communities under climate change.
SOURCES
Fricke, E. C., & Svenning, J. C. (2020). Accelerating homogenization of the global plant–frugivore meta-network. Nature, 585(7823), 74-78. https://www.nature.com/articles/s41586-020-2640-y
Fricke, E. C., Ordonez, A., Rogers, H. S., & Svenning, J. C. (2022). The effects of defaunation on plants’ capacity to track climate change. Science, 375(6577), 210-214. https://www.science.org/doi/full/10.1126/science.abk3510
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
brown tree snake Boiga irregularis; via Wikimedia; one of the species on which the most money is spent on preventive efforts
In recent years a group of scientists have attempted to determine how much invasive species are costing worldwide. See Daigne et al. 2020 here.
Some of these scientists have now gone further in evaluating these data. Cuthbert et al. (2022) [full citation at end of blog] see management of steadily increasing numbers of invasive, alien species as a major societal challenge for the 21st Century. They undertook their study of invasive species-related costs and expenditures because rising numbers and impacts of bioinvasions are placing growing pressure on the management of ecological and economic systems and they expect this burden to continue to rise (citing Seebens et al., 2021; full citation at end of blog).
They relied on a database of economic costs (InvaCost; see “methods” section of Cuthbert et al.) It is the best there is but Cuthbert et al. note several gaps:
Only 83 countries reported management costs; of those, only 24 reported costs specifically associated with pre-invasion (prevention) efforts.
Data comparing regional costs do not incorporate consideration of varying purchasing power of the reporting countries’ currencies.
Data available are patchy so global management costs are probably substantially underestimated. For example, forest insects and pathogens account for less than 1% of the records in the InvaCost database, but constitute 25% of total annual costs ($43.4 billion) (Williams et al., in prep.) .
Still, their findings fit widespread expectations.
These data point to a total cost associated with invasive species – including both realized damage and management costs – of about $1.5 trillion since 1960. North America and Oceania spent by far the greatest amount of all global money countering bioinvasions. North America spent 54% of the total expenditure of $95.3 billion; Oceania spent 30%. The remaining regions each spent less than $5 billion.
Cuthbert et al. set out to compare management expenditures to losses/damage; to compare management expenditures pre-invasion (prevention) to post-invasion (control); and to determine potential savings if management had been more timely.
Economic Data Show Global Efforts Could Be – But Aren’t — Cost-Effective
The authors conclude that countries are making insufficient investments in invasive species management — particularly preventive management. This failure is demonstrated by the fact thatreported management expenditures ($95.3 billion) are only 8% of total damage costs from invasions ($1.13 trillion). While both cost or losses and management expenditures have risen over time, even in recent decades, losses were more than ten times larger than reported management expenditures. This discrepancy was true across all regions except the Antarctic-Subantarctic. The discrepancy was especially noteworthy in Asia, where damages were 77-times higher than management expenditures.
Furthermore, only a tiny fraction of overall management spending goes to prevention. Of the $95.3 billion in total spending on management, only $2.8 billion – less than 3% – has been spent on pre-invasion management. Again, this pattern is true for all geographic regions except the Antarctic-Subantarctic. The divergence is greatest in Africa, where post-introduction control is funded at more than 1400 times preventive efforts. It is also significant for Asia and South America.
Even in North America – where preventative actions were most generously funded – post-introduction management is funded at 16 times that of prevention.
Cuthbert et al. worry particularly about the low level of funding for prevention in the Global South. They note that these conservation managers operate under severe budgetary constraints. At least some of the bioinvasion-caused losses suffered by resources under their stewardship could have been avoided if the invaders’ introduction and establishment had been successfully prevented.
While in the body of the article Cuthbert et al. seem uncertain about why funding for preventive actions is so low, in their conclusions they offer a convincing (to me) explanation. They note that people are intrinsically inclined to react when impact becomes apparent. It is therefore difficult to motivate proactive investment when impacts are seemingly absent in the short-term, incurred by other sectors, or in different regions, and when other demands on limited funds may seem more pressing. Plus efficient proactive management will prevent any impact, paradoxically undermining evidence of the value of this action!
Aedes aegypti mosquito; one of the species on which the most money is spent for post-introduction control; photo by James Gathany; via Flickr
Delay Costs Money
The reports contained in the InvaCost database indicate that management is delayed an average of 11 years after damage was first been reported. Cuthbert et al. estimate that these delays have caused an additional cost of about $1.2 trillion worldwide. Each $1 of management was estimated to reduce damage by $53.5 in this study. This finding, they argue, supports the value of timely invasive species management.
They point out that the Supplementary Materials contain many examples of bioinvasions that entail large and sustained late-stage expenditures that would have been avoided had management interventions begun earlier.
Although Cuthbert et al. are not as clear as I would wish, they seem to recognize also that stakeholders’ varying perceptions of whether an introduced species is causing a detrimental “impact” might also complicate reporting – not just whether any management action is taken
Cuthbert et al. are encouraged by two recent trends: growing investments in preventative actions and research, and shrinking delays in initiating management. However, these hopeful trends are unequal among the geographic regions.
Which Taxonomic Groups Get the Most Money?
About 42% of management costs ($39.9 billion) were spent on diverse or unspecified taxonomic groups. Of the costs that were taxonomically defined, 58% ($32.1 billion) was spent on invertebrates [see above re: forest pests]; 27% ($14.8 billion) on plants; 12% ($6.7 billion) on vertebrates; and 3% ($1.8 billion) on “other” taxa, i.e. fungi, chromists, and pathogens. For all of these defined taxonomic groups, post-invasion management dominated over pre-invasion management.
When considering the invaded habitats, 69% of overall management spending was on terrestrial species ($66.1 billion); 7% on semi-aquatic species ($6.7 billion); 2% on aquatic species ($2.0 billion); the remainder was “diverse/unspecified”. For pre-invasion management (prevention programs), terrestrial species were still highest ($840.4 million). However, a relatively large share of investments was allocated to aquatic invaders ($624.2 million).
Considering costs attributed to individual species, the top 10 targetted for preventive efforts were four insects, three mammals, two reptiles, and one alga. Top expenditures for post-invasion investments went to eight insects [including Asian longhorned beetle], one mammal, and one bird.
Asian longhorned beetle
Just two of the costliest species were in both categories: insects red imported fire ant(Solenopsis invicta) and Mediterranean fruitfly (Ceratitis capitate). None of the species with the highest pre-invasion investment was among the top 10 costliest invaders in terms of damages. However, note the lack of data on fungi, chromists, and pathogens. (I wrote about this problem in an earlier blog.)
Discussion and Recommendations
Cuthbert et al. conclude that damage costs and post-invasion spending are probably growing substantially faster than pre-invasion investment. Therefore, they call for a stronger commitment to enhancing biosecurity and for more reliance on regional efforts rather than ones by individual countries. Their examples of opportunities come from Europe.
Drawing parallels to climate action, the authors also call for greater emphasis on during decision-making to act collectively and proactively to solve a growing global and inter-generational problem.
Cuthbert et al. focus many of their recommendations on improving reporting. One point I found particularly interesting: given the uneven and rapidly changing nature of invasive species data, they think it likely that future invasions could involve a new suite of geographic origins, pathways or vectors, taxonomic groups, and habitats. These could require different management approaches than those in use today.
As regards data and reporting, Cuthbert et al. recommend:
1) reducing bias in cost data by increasing funding for reporting of underreported taxa and regions;
2) addressing ambiguities in data by adopting a harmonized framework for reporting expenditures. For example, agriculture and public health officials refer to “pest species” without differentiating introduced from native species. (An earlier blog also discussed the challenge arising from these fields’ different purposes and cultures.)
3) urging colleagues to try harder to collect and integrate cost information, especially across sectors;
4) urging countries to report separately costs and expenditures associated with different categories, i.e., prevention separately from post-invasion management; damage separately from management efforts; and.
5) creating a formal repository for information about the efficacy of management expenditures.
While the InvaCost database is incomplete (a result of poor accounting by the countries, not lack of effort by the compilers!), analysis of these data points to some obvious ways to improve global efforts to contain bioinvasion. I hope countries will adjust their efforts based on these findings.
Seebens, H. S. Bacher, T.M. Blackburn, C. Capinha, W. Dawson, S. Dullinger, P. Genovesi, P.E. Hulme, M.van Kleunen, I. Kühn, J.M. Jeschke, B. Lenzner, A.M. Liebhold, Z. Pattison, J. Perg, P. Pyšek, M. Winter, F. Essl. 2021. Projecting the continental accumulation of alien species through to 2050. Glob Change Biol. 2021;27:970-982.
Williams, G.M., M.D. Ginzel, Z. Ma, D.C. Adams, F.T. Campbell, G.M. Lovett, M. Belén Pildain, K.F. Raffa, K.J.K. Gandhi, A. Santini, R.A. Sniezko, M.J. Wingfield, and P. Bonello 2022. The Global Forest Health Crisis: A Public Good Social Dilemma in Need of International Collective Action. submitted
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
Hakalau Forest, Hawai“i; nearly 90% of Hawaiian flora is unique to the Islands
A recent article by Yang et al. 2021 (full citation at the end of this blog) seeks to determine the extent to which introduced plants reduce the uniqueness of regional floras. They analyzed data from 658 regions covering about 65.7% of the Earth’s ice-free land surface and about 62.3% of the planet’s known plant species.
They found strong homogenization of plant species’ taxonomic and phylogenetic diversity results from introductions of plant species to ecosystems beyond their native range. Homogenization caused by regional extinctions of native floral species occurs much less frequently.
There are two aspects of a region’s floral uniqueness. One is the number of species that it shares with other regions. This is taxonomic uniqueness. The other is the distinctiveness of the evolutionary history of the region. When several species are endemic to a region’s flora, and lack close relatives in other regions, that equals phylogenetic uniqueness.
The effect of a species introduction differs depending on which of these aspects one focuses on. Thus, naturalization of a species closely related to native species (e.g., a congeneric species) will have less impact on the phylogenetic floristic uniqueness of the region than naturalization by a distantly related species. Taxonomic uniqueness, however, will be affected to the same degree, irrespective of the phylogenetic distance between the naturalized and native species.
Yang et al. found strong homogenization of plant diversity. They found that species introductions increased the taxonomic similarity in 90.7% of all regional pairs and phylogenetic similarity in 77.2% of all region pairs. Most homogenization results from introductions of plant species to ecosystems beyond their native range. Homogenization caused by regional extinctions of native floral species occurs much less frequently.
This loss of regional biotic uniqueness or distinctiveness changes biotic interactions and species assemblages. These, in turn, have ecological and evolutionary consequences at larger scales and higher levels.
The degree of homogenization between regions’ floras depends on three factors:
1) The distance between the donor and recipient regions. Since nearby regions share more species, an introduction from a more distant origin is more likely to be a novel species and so contribute to homogenization of “donor” and “receiving” floras.
2) Climatic similarity, especially temperature. A plant species introduced from a climatically similar but geographically distant place is more likely to establish than a species from a different climatic zone. As a result, the recipient area’s flora is changed to more closely resemble the flora of the donor region with which it shares climatic conditions – regardless of the distance between them.
3) The level of exchange of goods and people between two regions. The higher the rate of exchange between two regions, the greater the chance that a species will be introduced and become established. Yang et al. used the existence of current or past administrative relationships (e.g., colonial relationship) between two regions as a proxy for intensity of trade and transport between donor and recipient regions. They found that floras of regions with current or past administrative links have taxonomically become more similar to each other than the floras of regions with no such links.
flora of the Cape Floral Kingdom – South Africa; photo from Michael Wingfield
Establishment of introduced species can increase floristic similarity of the donor and recipient regions (= floristic homogenization) when the species is native to one of the two regions and naturalizes in the other, or when it is not native to both regions and naturalizes in both. On the other hand, a species introduction can decrease the floristic similarity of the two regions (i.e., enhance floristic differentiation) when the species is not native to both regions but naturalized in only one.
Homogenization hotspots differed slightly depending on whether one focused on taxonomic or phylogenetic aspects.
The regions with the greatest average increase in taxonomic similarity with other regions due to naturalized alien species were New Zealand, portions of Australia, and many oceanic islands. The Australasian situation probably reflects its long biogeographic isolation from other parts of the globe and its highly unique native flora. As a result, nearly all non-native plants introduced to Australasia strongly increase levels of its floristic similarity to the rest of the world. Oceanic islands have species-poor floras with large proportions of unique endemics. They have also received high numbers of naturalized alien plants.
Hotspots of phylogenetic homogenization on continents are the same as those for taxonomic homogenization, but this is not true for islands. Yang et al. think this is because islands’ native floras were established by natural colonization from nearby continental floras so – despite subsequent speciation – they retain their phylogenetic relationship to the donor areas’ floras.
Yang et al. concede that they lacked high-quality data on native and naturalized alien species lists for a third of Earth’s ice-free terrestrial surface, especially Africa, Eastern Europe, and tropical Asia. They believe, however, that data from these regions are unlikely to change the overall finding. (Scientists are beginning to compile lists of forest pests in Africa). link to blog
Yang et al. note that introduction and naturalization of alien species are likely to increase in the future, thusaccelerating floristic homogenization. The ecological, evolutionary and socioeconomic consequences are largely unknown.They call for stronger biosecurity regulations of trade and transport and other measures to protect native vegetation.
SOURCE
Yang, Q., P. Weigelt, T.S. Fristoe, Z. Zhang, H. Kreft, A. Stein, H. Seebens, W. Dawson, F. Essl, C. König, B. Lenzner, J. Pergl, R. Pouteau, P. Pyšek, M. Winter, A.L. Ebel, N. Fuentes, E.L.H. Giehl, J. Kartesz, P. Krestov, T. Kukk, M. Nishino, A. Kupriyanov, J.L. Villaseñor, J.J. Wieringa, A. Zeddam, E. Zykova and M. van Kleunen. 2021. The global loss of floristic uniqueness. NATURE COMMUNICATIONS (2021) 12:7290. https://doi.org/10.1038/s41467-021-27603-y
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
western white pine in Idaho; photo by Chris Schnepf, #1171053 Bugwood
In this blog I will use one site-specific study to demonstrate what forest resources we are losing as a result of non-native pest introductions – in this case, the pathogen causing white pine blister rust.
The study was carried out nearly a decade ago by two eminent USFS pathologists working in the forests of southwest Oregon (Coos, Curry, Douglas, Jackson, Josephine, and Lane counties). Ellen and Don Goheen analyzed the current and past presence of two giants of western forests, sugar pine (Pinus lambertiana) and western white pine (P. monticola), changes in their status, and causes of mortality.
Southwest Oregon is a region of high climatic, geologic, and floristic diversity. Its forests contain 26 species of conifers including three species of five-needle pines: sugar pine, western white pine, and whitebark pine (P. albicaulis). Of these, sugar pine is widely distributed in mixed conifer forests on a variety of sites but primarily at lower elevations or otherwise with warmer climates. Western white pine is more widely distributed, including at higher elevations and on ultramafic soils (defined here) in the Siskiyou Mountains. Whitebark pine is limited to the highest elevations on the Cascade crest and in scattered island populations in the Siskiyou Mountains.
Sugar and western white pines have great aesthetic, ecological, and economic value. They are large: 50% of the live sugar pines and 18% of the western white pines sampled in the study are 30 inches dbh or greater. They can reach heights for 200 feet. In the study area, sugar pines constituted just 5% of the live trees, but 17% of the basal area. These large trees provide important nesting cavities for wildlife.
All three five-needle pines are vulnerable to white pine blister rust (WPBR), which is caused by the introduced pathogen Cronartium ribicola. They are also vulnerable to lethal levels of infestation by the native mountain pine beetle (MPB; Dendroctonus ponderosae). What have been the combined impacts of these major pests?
As of the first decade of the 21st Century, WPBR and MPB are causing substantial mortality in all size classes, from saplings to large trees. Half of the total basal area of western white pine, 30% of the total basal area of sugar pines is comprised dead trees. The impact of MPB has been exacerbated by substantial increases in tree densities arising from decades of fire exclusion.
sugar pine in the Sierra Nevada; photo by S. Rae, via Flickr
Status Now
Looking at all forests in Oregon and Washington, sugar, western white, and whitebark pines, combined, were reported on 14% of plots (a total of 2,128 plots) included in the Forest Inventory and Analysis (FIA) monitoring program. On these plots, western white was found on a little more than half (58%); sugar pine on one-third; and whitebark pine on only 16%.
Dead pines were found on a quarter of these 2,128 plots. Three quarters of the dead pines showed symptoms of WPBR, while 86% showed evidence of mountain pine beetle infestation. Among living pines, 32% were infected with WPBR, 10% had bark beetle attacks.
The intensive study of five-needle pines in southwest Oregon was based on both the FIA plots and other plots laid out as part of a separate Continuous Vegetation Survey. (See the methods section of the source.) Thus, the total for this study was 2,749 plots. In this study area, five-needle pines were more common than in the wider region. The three species grew on 31% of the 2,749 permanent plots examined — twice as high as the average for all of Oregon and Washington. Sugar pine grew on 64% of the five-needle pine plots; western white pine on 53%; whitebark on only 0.5%.
Agents of Mortality in Southwest Oregon
WPBR was ubiquitous – in more than 93% of pine stands surveyed. Already, 13% of the sugar pines and 17% of western white pines were dead. This proportion is far higher than the 5% of trees of all tree species in the same stands that were dead. In both hosts, 80 – 90% of dead seedlings and saplings had been killed by WPBR. Additional losses are probable: most of the surviving pole-sized and smaller trees had cankers near their boles, so the scientists thought they would probably soon succumb.
The mountain pine beetle’s impact is even worse, especially on larger trees. Trees killed by MPB attacks were encountered in 84% of surveyed stands. MPB had infested 73% of dead large sugar pines (> 20 cm (8 in) dbh), 69% of dead large western white pines.
Other agents, including root diseases, dwarf mistletoes, and pine engraver beetles influence five-needle pine health in southwest Oregon to a much lesser extent than WPBR or MPB. The exception is the Siskiyou Mountains, where the ultramafic soils provide suboptimal growing conditions. These agents might weaken trees to some extent, thus predisposing them to MPB infestation. WPBR infections might have similar effects by killing tops and numerous branches of large trees.
Specifics
1. Mountain pine beetle is native to southwest Oregon. Levels of infestation have varied over the decades since measurements began in the 1950s. Infestations have probably increased substantially in recent decades, linked to the cooler, shaded conditions found in dense stands that have resulted from fire suppression. In addition to the infestations on western white and sugar pines described above, MPBs have caused significant mortality in mature whitebark pines. There is evidence of infestation on 31% of all dead whitebark pines.
In southwest Oregon, MPB have killed five-needle pines in most years; here, they are less closely tied to drought than in other parts of the West.
2. White pine blister rust probably reached southwest Oregon in the 1920s. Its presence and intensity is greatly influenced by climate and environmental conditions. Southwest Oregon has a Mediterranean climate that is less favorable to rust spread — yet, the disease is widespread and devastating. The combination of microsites supporting cooler and moister conditions – perhaps especially where fogs linger – mean that disease is most prevalent on flat or gently sloping areas and northern aspects, at higher elevations.
Blister rust requires an alternate host, usually gooseberry (Ribes spp), to complete its life cycle. Perhaps surprisingly, in southwest Oregon it is not necessary for Ribes to be close to the pines for the trees to become infected. One reason is probably the presence of other alternate hosts in the Castilleja (paintbrushes) and Pedicularis (louseworts) genera. The other likely explanation is transport by fog banks of spores from Ribes in canyons and valleys to the higher-elevation slopes.
Despite the high levels of mortality caused by WPBR and MPB, there is substantial regeneration of both western white and sugar pines. However, the numerous seedlings are unlikely to grow into dominant trees unless released from the competition found in overstocked, dense stands. Therefore, even in the absence of WPBR, the Goheens consider the seedlings’ futures to be tenuous if they are not eventually exposed to more sunlight through management or natural disturbance.
These Threats Have Been Present for Decades
The Goheens compared their findings to those of several past studies; the results confirm that five-needle pines have suffered high levels of mortality since the 1950s due to WPBR and other factors. All the western white pines had disappeared from two of four sites. Significant declines were observed at the two other sites in the Umpqua and Rogue River National forests.
Forest stands in 10 “Areas of Special Interest” that in 1825 were open, park-like stands with widely spaced trees had become dense dominated by Douglas-fir, true firs, and incense-cedar.
Sugar pines, which in 1825 had made up as much as a third of the trees in the low elevation stands had been reduced to very low numbers.
The Goheens note that all these threats are directly caused or greatly influenced by human activities. Noting that sugar and western white pines provide many values in the forests of southwest Oregon, they called for management using appropriate, integrated, silvicultural prescriptions to ensure the future of western white and sugar pines in southwest Oregon.
SOURCE
Goheen, E.M. and D.J. Goheen. 2014. Status of Sugar and Western White Pines on Federal Forest Lands in SW OR: Inventory Query and Natural Stand Survey Results. USDA Forest Service Pacific Northwest Region. SWOFIDSC-14-01 January 2014
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
symptomatic feeding by EZM larva; photo by Gyorgy Csoka via Bugwood
The elm zigzag sawfly (EZM; Aproceros leucopoda) was reported in the Western Hemisphere for the first time in Quebec in July 2020.
In 2021, only a year later, the sawfly was confirmed in northern Virginia [David Gianino, State Plant Regulatory Official (SPRO) of Virginia, pers. comm.]
There is 700 miles between Quebec and Virginia.
In September 2022, the sawfly was detected in St. Lawrence County, New York — just across the St. Lawrence River from Canada, where the insect has been known for two years. There is no information yet on impacts. [Brynda, S. “New pest affecting elm trees in St. Lawrence County.” October 3, 2022.
Impact in Europe
Elm zigzag sawfly is native to Eastern Asia — Japan and China for certain and, possibly Far Eastern Russia. There it is considered a minor pest. Serious localized defoliation, though, has been reported at least once, on the island of Hokkaido (Blank et al. 2021).
The sawfly was first detected outside its native range in Hungary and Poland in 2003. By 2010, the outbreak was revealed to be present over an area of 1,700 km, from eastern Ukraine to Austria. Other countries reporting the sawfly were Hungary, Poland, Romania, and Slovakia (Blank et al. 2010). Spread continued. By 2013 or 2014 elm zigzag sawfly was also reported in Belgium, Netherlands, and Germany — apparently the result of separate instances of human-assisted transport. German scientists calculated a natural spread rate of 45–90 km/yr. By 2018 the insect had reached the United Kingdom.
Severe localized defoliation by the species has been recorded on elms in a variety of situations across Europe. In some countries, defoliation has reached 74% or higher, even 100%. However, in other countries, such as Bulgaria, defoliation rates appear to be much lower (1-2%). Aproceros leucopoda showed no preference for host trees of a particular age. Heavily defoliated trees in Hungary did not seem to be dying (Blank et al. 2010).
The fear – in Europe and North America – is that elms already severely depleted by Dutch elm disease will be unable to sustain any decline in vigor caused by defoliation (Blank et al. 2010)
Probable Hosts
On the European continent, the sawfly has fed on several elms, including Ulmus minor, U. pumila and U. pumila var. arborea,U. glabra, and possibly. U. laevis (Blank et al. 2010). In the United Kingdom, it has fed on English elm (Ulmus procera), wych elm (U. glabra) and field elm (U. minor).
In Japan, collaborators in the Blank et al. (2010) study collected sawfly larvae on U. japonica and U. pumila.
In Virginia, larvae were collected from Chinese elm (U. parvifola). However, all species of elm trees native to North America are considered at risk. Also threatened are the native elm-browsing insects which might be out-competed by elm zigzag sawfly.
How the Sawfly Is Moved
Some have suggested that the EZS is transported on plants for planting, but they have not reported observations. Because elms are usually moved while dormant, it is more likely that the cryptic wintering cocoons are transported in leaf litter accompanying the trees rather than on the trees themselves.
American elms in Arlington County, Va; photo by F.T. Campbell
Worrying Traits
The elm zigzag sawfly matures very rapidly. The total time from oviposition to emergence of mature individuals is 24–29 days (Blank et al. 2010). They can produce up to six or seven generations per year. The sawfly is also parthenogenic, so it can reproduce in the absence of males. As a result, populations can build up rapidly. No specific predators are known. The impact of generalist native parasitoids in Europe has not yet been studied.
Also, EZS tolerates a wide range of climates. Conditions on Hokkaido are similar to those in Central Europe. However, Hokkaido’s winters are usually colder, summers warmer, and annual precipitation higher. Blank et al. (2010) did not know limiting temperature and humidity but thought it probable that this species could spread into northern and south-western Europe wherever elms grow. In North America, the Canadian Food Inspection Agency expressed concern that EZS would be able to withstand temperatures as low as –30 °C which includes much of Canada.
While the elm zigzag sawfly was on the Alert List on the European and Mediterranean Plant Protection Organization (EPPO), in 2015 it was removed since no EPPO member country had requested international action (Blank et al. 2010).
SOURCES
Blank, S.M., H. Hara, J. Mikulas, G. Csoka, C. Ciornei, R. constantineanu, I. Constantineanu, L. Roller, E. Altemhofer, T. Huflejt, G. Vetek. 2010. Aproceros leucopoda (Hymenoptera: Argidae): An East Asian pest of elms (Ulmus spp.) invading Europe. European Journal of Entomology · March 2010
DOI: 10.14411/eje.2010.045
Blank, S.M., T. Köhler, T. Pfannenstill, N. Neuenfeldt, B. Zimmer, E. Jansen, A. Taeger, A.D. Liston. Zig-zagging across Central Europe: recent range extension, dispersal speed and larval hosts of Aproceros leucopoda (Hymenoptera, Argidae) in Germany. https://jhr.pensoft.net/articles.php?id=4395
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
