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Forest Pests: What’s Improved, What’s Still to Do

sassafras – vulnerable to the rapidly spreading laurel wilt disease; photo by F.T. Campbell

In summer 2019 I posted several blogs summarizing my analysis of forest pest issues after 30 years’ engagement. I reported the continuing introductions of tree-killing insects and pathogens; their relentless spread and exacerbated impacts. I noted the continued low priority given these issues in agencies tasked with preventing and solving these problems. Also, Congress provides not only insufficiently protective policies but also way too little funding. I decried the impediments created by several Administrations; anti-regulatory ideology and USDA’s emphasis on “collaborating” with “clients” rather than imposing requirements.

In my blogs, I called for renewed effort to find more effective strategies – as I had earlier advocated in my “Fading Forests” reports (link provided at the end of this blog), previous blogs, and Lovett et al. 2016

Areas of Progress

Now two years have passed. I see five areas of progress – which give me some hope.

1) Important Activities Are Better Funded than I had realized

a) The US Forest Service is putting significant effort into breeding trees resistant to the relevant pests, more than I had realized. Examples include elms and several conifer species in the West – here and here.

b) USDA has provided at least $110 million since FY2009 to fund forest pest research, control, and outreach under the auspices of the Plant Pest and Disease Disaster Prevention Program (§10201 of the Farm Bill). This total does not include additional funding for the spotted lanternfly. Funded projects, inter alia: explored biocontrol agents for Asian longhorned beetle and emerald ash borer; supported research at NORS-DUC on sudden oak death; monitored and managed red palm weevil and coconut rhinoceros beetle; and detected Asian defoliators. Clearly, many of these projects have increased scientific understanding and promoted public compliance and assistance in pest detection and management.

This section of the Farm Bill also provided $3.9 million to counter cactus pests – $2.7 million over 10 years targetting the Cactoblastis moth & here and $1.2 million over four years targetting the Harissia cactus mealybug and here.

flat-padded *Opuntia* cactus – vulnerable to the *Cactoblastis* moth; National Park Service photo

2) Additional publications have documented pests’ impacts – although I remain doubtful that they have increased decision-makers’ willingness to prioritize forest pests. Among these publications are the huge overview of invasive species published last spring (Poland et al.) and the regional overview of pests and invasive plants in the West (Barrett et al.).

3) There have been new efforts to improve prediction of various pests’ probable virulence (see recent blogs and here.

4) Attention is growing to the importance of protecting forest health as a vital tool in combatting climate change — see Fei et al., Quirion et al., and IUCN. We will have to wait to see whether this approach will succeed in raising the priority given to non-native pests by decision-makers and influential stakeholders.

5) Some politicians are responding to forest pest crises – In the US House, Peter Welch (D-VT) is the lead sponsor of H.R. 1389. He has been joined – so far – by eight cosponsors — Rep. Kuster (D-NH), Pappas (D-NH), Stefanik (R-NY), Fitzpatrick (R-PA), Thompson (D-CA), Ross (D-NC), Pingree (D-ME), and Delgado (D-NY). This bill would fund research into, and application of, host resistance! Also, it would make APHIS’ access to emergency funds easier. Furthermore, it calls for a study of ways to raise forest pests’ priority – thus partially responding to the proposal by me and others (Bonello et al. 2020; full reference at end of blog) to create federal Centers for Forest Pest Control and Prevention.

This year the Congress will begin work on the next Farm Bill – might these ideas be incorporated into that legislation?

What Else Must Be Done

My work is guided by three premises:

1) Robust federal leadership is crucial:

The Constitution gives primacy to federal agencies in managing imports and interstate trade.
Only a consistent approach can protect trees (and other plants) from non-native pests that spread across state lines.
Federal agencies have more resources than state agencies individually or in likely collective efforts – even after decades of budget and staffing cuts.

2) Success depends on a continuing, long-term effort founded on institutional and financial commitments commensurate with the scale of the threat. This requires stable funding; guidance by research and expert staff; and engagement by non-governmental players and stakeholders. Unfortunately, as I discuss below, funding has been neither adequate nor stable.

3) Programs’ effectiveness needs to be measured. Measurement must focus on outcomes, not just effort (see National Environmental Coalition on Invasive Species’ vision document).

Preventing New Introductions – Challenges and Solutions

We cannot prevent damaging new introductions without addressing two specific challenges.

1) Wood packaging continues to pose a threat despite past international and national efforts. As documented in my recent blogs, the numbers of shipping containers – presumably with wood packaging – are rising. Since 2010, CBP has detected nearly 33,000 shipments in violation of ISPM#15. The numbers of violations are down in the most recent years. However, a high proportion of pest-infested wood continues to bear the ISPM#15 mark. So, ISPM#15 is not as effective as it needs to be.

We at CISP hope that by mid-2022, a new analysis of the current proportion of wood packaging harboring pests will be available. Plus there are at least two collaborative efforts aimed at increasing industry efforts to find solutions – The Nature Conservancy with the National Wooden Pallet and Container Association; and the Cary Institute with an informal consortium of importers using wooden dunnage.

2) Imports of living plants (“plants for planting”) are less well studied so the situation is difficult to assess. However, we know this is a pathway that has often spread pests into and within the US. There have been significant declines in overall numbers of incoming shipments, but available information doesn’t tell us which types of plants – woody vs. herbaceous, plant vs. tissue culture, etc. – have decreased.

APHIS said, in a report to Congress (reference at end of blog), that introductions have been curbed – but neither that report nor other data shows me that is true.

Scientists are making efforts to improve risk assessments by reducing the number of organisms for which no information is available on their probable impacts (the “unknown unknowns”).

Solving Issues of Prevention

While I have repeatedly proposed radical revisions to the international phytosanitary agreements (WTO SPS & IPPC) that preclude prevention of unknown unknowns (see Fading Forests II and blog), I have also endorsed measures aimed at achieving incremental improvements in preventing introductions, curtailing spread, and promoting recovery of the affected host species.

citrus longhorned beetle exit hole in bonsai tree; USDA APHIS photo

The more radical suggestions focus on: 1) revising the US Plant Protection Act to give higher priority to preventing pests introductions than to facilitating free trade (FF II Chapter 3); 2) APHIS explicitly stating that its goal is to achieve a specific, high level of protection (FF II Chapter 3); 3) APHIS using its authority under the NAPPRA program to prohibit imports of all plants belonging to the 150 genera of “woody” plants that North America shares with Europe or Asia; 4) APHIS prohibiting use of packaging made from solid wood by countries and exporters that have a record of frequent violations of ISPM#15 in the 16 years since its implementation.

Another action leading to stronger programs would be for APHIS to facilitate outside analysis of its programs and policies to ensure the agency is applying the most effective strategies (Lovett et al. 2016). The pending Haack report is an encouraging example.

I have also suggested that APHIS broaden its risk assessments so that they cover wider categories of risk, such as all pests that might be associated with bare-root woody plants from a particular region. Such an approach could speed up analyses of the many pathways of introduction and prompt their regulation.

Also, APHIS could use certain existing programs more aggressively. I have in mind pre-clearance partnerships and Critical Control Point integrated pest management programs. APHIS should also clarify the extent to which these programs are being applied to the shipments most likely to transport pests that threaten our mainland forests, i.e. imports of woody plants belonging to genera from temperate climates. APHIS should promote more sentinel plant programs. Regarding wood packaging, APHIS could follow the lead of CBP by penalizing importers for each shipment containing noncompliant SWPM.

Getting APHIS to prioritize pest prevention over free trade in general, or in current trade agreements, is a heavy lift. At the very least, the agency should ensure that the U.S. prioritize invasive species prevention in negotiations with trading partners and in developing international trade-related agreements. I borrow here from the recent report on Canadian invasive species efforts. (I complained about APHIS’ failure to even raise invasive species issues during negotiation of a recent agricultural trade agreement with China.)

Solving Issues of Spreading Pests

The absence of an effective system to prevent a pest’s spread within the U.S. is the most glaring gap in the so-called federal “safeguarding system”. Yet this gap is rarely discussed by anyone – officials or stakeholders. APHIS quarantines are the best answer – although they are not always as efficacious as needed – witness the spread of EAB and persistence of nursery outbreaks of the SOD pathogen.

APHIS and the states continue to avoid establishing official programs targetting bioinvaders expected to be difficult to control or that don’t affect agricultural interests. Example include laurel wilt, and two boring beetles in southern California – goldspotted oak borer, Kuroshio shot hole borer and polyphagous shot hole borer and their associated fungi.

One step toward limiting pests’ spread would come from strengthening APHIS’ mandate in legislation, as suggested above. A second, complementary action would be for states to adopt quarantines and regulations more aggressively. For this to happen, APHIS would need to revise its policies on the “special needs exemption” [7 U.S.C. 7756]. Then states could adopt more stringent regulations to prevent introduction of APHIS-designated quarantine pests (Fading Forests III Chapt 3).

Finally, APHIS should not drop regulating difficult-to-control species – e.g., EAB. There are repercussions.

APHIS’ dropping EAB has not only reduced efforts to prevent the beetle’s spread to vulnerable parts of the West. It has also left states to come up with a coherent approach to regulating firewood; they are struggling to do so.

Considering interstate movement of pests via the nursery trade, the Systems Approach to Nursery Certification (SANC) program) is voluntary and was never intended to include all nurseries. Twenty-five nurseries were listed on the program’s website as of March 2020. It is not clear how many nurseries are participating now. The program ended its “pilot” phase and “went live” in January 2021. Furthermore, the program has been more than 20 years in development, so it cannot be considered a rapid response to a pressing problem.

Solving Issues of Recovery and Restoration via Resistance Breeding

I endorse the findings of two USFS scientists, Sniezko and Koch citations. They have documented the success of breeding programs when they are supported by expert staff and reliable funding, and have access to appropriate facilities. The principle example of such a facility is the Dorena Genetic Resource Center in Oregon. Regional consortia, e.g., Great Lakes Basin Forest Health Collaborative and Whitebark Pine Ecosystem Foundation are trying to overcome gaps in the system. I applaud the growing engagement of stakeholders, academic experts, and consortia. Questions remain, though, about how to ensure that these programs’ approaches and results are integrated into government programs.

resistant and vulnerable ash seedlings; photo courtesy of Jennifer Koch, USFS

In Bonello et al., I and others call for initiating resistance breeding programs early in an invasion. Often other management approaches, e.g., targetting the damaging pest or manipulating the environment, will not succeed. Therefore the most promising point of intervention is often with by breeding new or better resistance in the host. This proposal differs slightly from my suggestion in the “30 years – solutions” blog, when I suggested that USFS convene a workshop to develop consensus on breeding program’s priorities and structure early after a pest’s introduction.

Funding Shortfalls

I have complained regularly in my publications (Fading Forests reports) and blogs about inadequate funding for APHIS Plant Protection program and USFS Forest Health Protection and Research programs. Clearly the USDA Plant Pest and Disease Management and Disaster Program has supported much useful work. However, its short-term grants cannot substitute for stable, long-term funding. In recent years, APHIS has held back $14 – $15 million each year from this program to respond to plant health emergencies. (See APHIS program reports for FYs 20 and 21.) This decision might be the best solution we are likely to get to resolve APHIS’ need for emergency funds. If we think it is, we might drop §2 of H.R. 1389.

Expanding Engagement of Stakeholders

Americans expect a broad set of actors to protect our forests. However, these groups have not pressed decision-makers to fix the widely acknowledged problems: inadequate resources – especially for long-term solutions — and weak and tardy phytosanitary measures. Employees of federal and state agencies understand these issues but are restricted from outright advocacy. Where are the professional and scientific associations, representatives of the wood products industry, forest landowners, environmental NGOs and their funders, plus urban tree advocates – who could each play an important role? The Entomological Society’s new “Challenge” is a welcome development and one that others could copy.

SOURCES

Bonello, P., Campbell, F.T., Cipollini, D., Conrad, A.O., Farinas, C., Gandhi, K.J.K., Hain, F.P., Parry, D., Showalter, D.N, Villari, C. and Wallin, K.F. (2020) Invasive Tree Pests Devastate Ecosystems—A Proposed New Response Framework. Front. For. Glob. Change 3:2. doi: 10.3389/ffgc.2020.00002

Green, S., D.E.L. Cooke, M. Dunn, L. Barwell, B. Purse, D.S. Chapman, G. Valatin, A. Schlenzig, J. Barbrook, T. Pettitt, C. Price, A. Pérez-Sierra, D. Frederickson-Matika, L. Pritchard, P. Thorpe, P.J.A. Cock, E. Randall, B. Keillor and M. Marzano. 2021. PHYTO-THREATS: Addressing Threats to UK Forests and Woodlands from Phytophthora; Identifying Risks of Spread in Trade and Methods for Mitigation. Forests 2021, 12, 1617 https://doi.org/10.3390/f12121617ý

Krishnankutty, S., H. Nadel, A.M. Taylor, M.C. Wiemann, Y. Wu, S.W. Lingafelter, S.W. Myers, and A.M. Ray. 2020. Identification of Tree Genera Used in the Construction of Solid Wood-Packaging Materials That Arrived at U.S. Ports Infested With Live Wood-Boring Insects. Journal of Economic Entomology 2020, 1 – 12

Liebhold, A.M., E.G. Brockerhoff, L.J. Garrett, J.L. Parke, and K.O. Britton. 2012. Live plant imports: the major pathway for forest insect and pathogen invasions of the US. Front. Ecol. Environ. 2012; 10(3):135-143

Lovett, G.M., M. Weiss, A.M. Liebhold, T.P. Holmes, B. Leung, K.F. Lambert, D.A. Orwig, F.T. Campbell, J. Rosenthal, D.G. McCullough, R. Wildova, M.P. Ayres, C.D. Canham, D.R. Foster, SL. Ladeau, and T. Weldy. 2016. NIS forest insects and pathogens in the US: Impacts and policy options. Ecological Applications, 26(5), 2016, pp. 1437–1455

Mech, A.M., K.A. Thomas, T.D. Marsico, D.A. Herms, C.R. Allen, M.P. Ayres, K.J. K. Gandhi, J. Gurevitch, N.P. Havill, R.A. Hufbauer, A.M. Liebhold, K.F. Raffa, A.N. Schulz, D.R. Uden, & P.C. Tobin. 2019. Evolutionary history predicts high-impact invasions by herbivorous insects. Ecol Evol. 2019 Nov; 9(21): 12216–12230.

Poland, T.M., Patel-Weynand, T., Finch, D., Miniat, C. F., and Lopez, V. (Eds) (2019), Invasive Spp in Forests and Grasslands of the US: A Comprehensive Science Synthesis for the US Forest Sector. Springer Verlag. (in press).

Roy, B.A., H.M Alexander, J. Davidson, F.T Campbell, J.J Burdon, R. Sniezko, and C. Brasier. 2014. Increasing forest loss worldwide from invasive pests requires new trade regulations. Front Ecol Environ 2014; 12(8): 457–465

Schulz, A.N., A.M. Mech, M.P. Ayres, K. J. K. Gandhi, N.P. Havill, D.A. Herms, A.M. Hoover, R.A. Hufbauer, A.M. Liebhold, T.D. Marsico, K.F. Raffa, P.C. Tobin, D.R. Uden, K.A. Thomas. 2021. Predicting non-native insect impact: focusing on the trees to see the forest. Biological Invasions.

United States Department of Agriculture Animal and Plant Health Inspection Service. Report on the Arrival in the US of Forest Pests Through Restrictions on the Importation of Certain Plants for Planting. https://www.caryinstitute.org/sites/default/files/public/downloads/usda_forest_pest_report_2021.pdf

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

Wood packaging – Progress? or Paralysis by Analysis?

This February marks 16 years since APHIS began full implementation of ISPM#15. I have blogged often about the failure of ISPM#15 to curtail the risk associated with wood packaging; scroll below the chronological list of blogs to the “categories”, click on “wood packaging”. The best summary of the issues is found in a blog posted in September 2017.

As I have reported in many previous blogs, U.S. imports – especially those from Asia – have been rising since August 2020. Thus, January through October 2021, U.S. imports from Asia are 10.5% higher than the same period in 2020 (Mongelluzzo Dec. 9, 2021). Port officials expect import volumes from Asia to remain high in the first half of 2022, with perhaps a pause in February linked to Asian New Year celebrations (Mongelluzzo Dec. 15 2021). Shipping tonnage devoted to carrying goods from Asia to North America rose by 17% when one compares 2020 to 2021 (Lynch and Wadekar 2021).

These increases are important because of the history of pest introductions in wood packaging from Asia.

This increase is seen most acutely at the ports of Los Angeles and Long Beach, which handle about 50% of all U.S. imports from Asia. Such imports during January – November 2021 were 19.4% higher than the same period in 2020; 21.2% higher than during the same period in 2019 (Mongelluzzo Dec. 15 2021).

The rise in imports – and associated pest risk — is not limited to southern California. At the largest port on the East coast – New York/New Jersey – import volumes through October were 20% higher than the same period a year ago. The port is also receiving a higher number of large ships – those carrying 9,000 or more containers (Angell Dec. 22, 2021).

We do not know how many of these containers hold the heaviest commodities most often associated with wood packaging infested by insects — machinery (including electronics); metals; tile and decorative stone (such as marble or granite counter tops). I see many potential links to the COVID-prompted “home improvement” boom. I wonder whether furniture should be included here …

wood packaging associated with stone; photo courtesy of Canadian Food Inspection Agency

1. 2021 Data on Violations

A recent webinar sponsored by The Nature Conservancy’s Continental Dialogue on Non-Native Forest Insects and Diseases and the Entomological Society of America revealed important new information on the pest risk associated with these imports. (Presentations have been posted on the Dialogue’s website). Several of the presentation have particularly significant implications for protecting the US from pests.

Jared Franklin, acting director for agriculture enforcement for DHS’s Customs and Border Protection (CBP), reported that pest detections and shipper violations in Fiscal Year (FY) 2021 follow patterns set earlier. There is, however, an interesting decline in numbers of violations despite enhanced inspection intensity. When the number of incoming air passengers crashed because of COVID-19, CBP assigned inspectors to cargo instead.

Type of violation	FY2018	FY2019	FY2020	FY2021
Lack ISPM#15 mark	1,662	1,825	1,662	1,459
Live quarantine pest found	756	747	509	548
TOTAL VIOLATIONS	2,418	2,572	2,171	2,007

Unfortunately, in FY2016 CBP stopped recording whether pests were detected on marked or unmarked SWPM.

As usual, most of the pests were detected in wood packaging accompanying miscellaneous cargo. Also, as usual, the most commonly detected pests are Cerambycid beetles. During a discussion of why Cerambycids outnumber Scolytids, Bob Haack pointed out that most bark beetles are eliminated by the debarking required by ISPM#15.

2. Updating a Key Study of the Wood Packaging Pathway

Bob Haack revealed that he has received permission to update his earlier landmark study aimed at determining the arrival rate of pests in wood packaging (see Haack et al., 2014). I have long advocated for an update. All my comment about the wood packaging risk have – perforce – relied on this now outdated report. Bob hopes to have results in a few months.

This time, he will work with Toby Petrice (USFS) and Jesse Hardin and Barney Caton (APHIS). While the 2014 study focused on changes in approach rates resulting from U.S.’ implementation of ISPM#15, the new study will presumably uncover current levels of compliance. The authors will use more than 73,000 new port inspection records to detect trends from 2010 through 2020, as well as the original database of about 35,000 inspections made during 2004-2009.

Bob notes that there have been significant changes in ISPM#15 since 2009. These include: a) a requirement that wood be debarked before treatment; b) approval of new treatments (dielectric heat in 2013 and sulphuryl fluoride in 2018); and c) new official definitions of “reuse,” “repair,” and “remanufacturer”.

Besides discovering overall levels of compliance, Bob and colleagues will probably select some aspects of the wood packaging pathway for specific analysis. For example, Dialogue participants want to know whether dunnage has a higher interception rate than pallets. Also, the earlier study included only wood packaging that bore the ISPM mark. This new research might compare live pest interception rates on marked versus unmarked wood.

3) A Study to Improve ISPM#15

Erin Cadwalader reported on the Entomological Society’s Grand Challenge, particularly the request from APHIS that the Society provide guidance on improving ISPM#15. This request was made in 2019; subsequent efforts to conduct a broad scoping process have been complicated and delayed by COVID-19. The goal is to determine what area of effort would lead to either 1) the highest reduction in pest incidence; or 2) the best ISPM#15 compliance.

ESA’s preliminary proposal aims to evaluate the risk associated with various types of wood packaging by analyzing data from five ports over a period of five years. Webinar participants discussed the proposal, especially trying to determine why data already collected by APHIS and CBP – specifically via Agriculture Quarantine Inspection Monitoring (AQIM) – are not adequate to support the study. Another question is whether it is useful for ESA investigators to attempt to rear insects from wood packaging rather than rely on APHIS’ identifications using molecular techniques. Erin noted that some insects – probably particularly small wood borers – might escape detection by inspectors but show up when the wood is placed in rearing chambers.

There will be further discussion of the study’s scope and methodology at the Society’s annual meeting in Autumn 2023 near Washington, D.C. (The 2022 meeting will be in Vancouver; USDA officials rarely get permission to travel to meetings outside the U.S.) ESA estimates that the study will take five years and be completed in 2028.

I am concerned that APHIS might not act on the basis of Bob Haack’s findings as soon as they are available. If they wait for completion of the ESA study, it could be at least six years from now before action is even proposed. I hope that if Haack and colleagues uncover persistent inadequacies in ISPM#15 implementation, APHIS will act unilaterally to address the problem – at least as regards the threat to the U.S. The ESA study might then become the foundation for revising the overall standard per se, that is, the entire world trading system.

Also, APHIS has already carried out a focused study of pests in wood packaging. How can their findings be incorporated into APHIS’ decisions so as to expedite action?

Wu et al. (2017) proved the efficacy of DNA identification tools and that serious pest species continued (at that time) to be present in wood packaging. Krishnankutty et al. (2020) found that 84% of interceptions occurred in wood belonging to only three families: pine, spruce, and poplar. Shipments with coniferous wood came about equally from Europe, Asia, and Mexico. Wood packaging made from poplars came primarily from China. Most of the pests in hardwood were polyphagous, and were considered to pose a higher risk. Pests in softwood samples were mostly oligophagous (feed on two or more genera in the same family). I presume that these findings prompted the studies by Mech et al. and Schulz et al.

As has been true in most studies, pest detections were often associated with shipments of heavy items, such as stone, ceramics, and terracotta; vehicles and vehicle parts; machinery, tools, and hardware; and metal. A high proportion (87%) of the wood packaging bore the ISPM15 mark, also as usual. (Data provided by CBP in past Dialogue meetings showed an even higher proportion of pest-infested wood to be marked.)

Conclusion

Clearly, programs aimed at curtaining the pest risk associated with wood packaging have not been sufficiently effective. I hope APHIS’ approval of Bob Haack’s study and agreement with the Entomological Society indicates a new willingness to understand why and take actions to fix the problems.

SOURCES

Haack, R.A., K.O. Britton, E.G. Brockerhoff, J.F. Cavey, L.J. Garrett. 2014. Effectiveness of the International Phytosanitary Standard ISPM No. 15 on Reducing Wood Borer Infestation Rates in Wood Packaging Material Entering the United States. PLoS ONE 9(5): e96611. doi:10.1371/journal.pone.0096611

Lynch, D.J. and N. Wadekar. 2021. “Africa left with fallout of US supply chain crisis”. The Washington Post. December 17, 2021.

Mongelluzzo, B. Dec 09, 2021. New long-dwell container fee bearing fruit in Oakland https://www.joc.com/port-news/terminal-operators/new-long-dwell-container-fee-bearing-fruit-oakland_20211209.html?utm_campaign=CL_JOC%20Ports%2012%2F15%2F21%20%20_PC00000_e-production_E-121985_TF_1215_0900&utm_medium=email&utm_source=Eloqua

Mongelluzzo, B. Dec. 15 2021. LA port expects imports to surge further in Q2https://www.joc.com/port-news/us-ports/la-port-expects-imports-surge-further-q2_20211215.html?utm_source=Eloqua&utm_medium=email&utm_campaign=CL_JOC%20Daily%2012/16/21_PC00000_e-production_E-122356_KB_1216_0617

Wu,Y., N.F. Trepanowski, J.J. Molongoski, P.F. Reagel, S.W. Lingafelter, H. Nadel1, S.W. Myers & A.M. Ray. 2017. Identification of wood-boring beetles (Cerambycidae and Buprestidae) intercepted in trade-associated solid wood packaging material using DNA barcoding and morphology. Scientific Reports 7:40316

Posted by Faith Campbell

Living Plant Imports: Scientists Try to Counter Longstanding Problems

American chestnut – nearly eradicated by a disease introduced on imported plants

Shipments of living plants (called by phytosanitary agencies “plants for planting”) have long been recognized as the most “effective” pathway for transporting pests. To those of us concerned about forest ecosystems, the focus is on woody plants. I have no reason to think herbaceous plant imports are any less risky.

International Rules Impede Prevention Efforts

Efforts to prevent pest introductions via shipments of plants for planting suffered a severe setback when the World Trade Organization Agreement on the Application of Sanitary and Phytosanitary Standards (SPS Agreement) came into force in 1995. Two years later the International Plant Protection Convention (IPPC) was amended to conform to those new trade rules.

David McNamara, then Assistant Director of the European and Mediterranean Plant Protection Organization, identified the ramifications of the new regime: phytosanitary agency officials “have come to realize that our work has changed from ‘preventing introduction of pests while not interfering unduly with trade’ to ‘facilitating trade while doing our utmost to prevent pest introduction.’” [See Chapter 3 of Fading Forests II (2003), available here where I detail how the SPS Agreement and IPPC rules changed phytosanitary policy.]

I was not alone in raising the alarm about the ramifications of the new regime: that phytosanitary regulations target only pests known to cause damage; that commodities from all sources be treated as if they posed equal pest risks, which is not true; that phytosanitary rules impose the lowest level of restriction on trade required to achieve the chosen level of protection.

Scientists Try to Reverse the Damaging Requirements

For example, world-renowned UK pathologist Clive Brasier (2008; full reference at end of the blog) criticized the requirement that pests be identified before they can be regulated. Dr. Brasier estimated that 90% of plant pathogens might be unknown to science, and thus not eligible for regulation under the WTO/IPPC regime. This means that damaging pests are frequently regulated only after they have been introduced and initiated the essentially permanent alteration of the receiving (naïve) environment. He called for an approach based on Darwinian evolutionary theory: maintenance of the geographic barriers that separate species.

A growing number of scientists have reiterated the criticisms in hopes of persuading regulators to reverse the flaws identified in the international trade rules. More than 70 scientists affiliated with the International Union of Forest Research Organizations signed the Montesclaros Declaration in 2011. Circa 2015 – 20 years after the SPS Agreement came into force – several publications reiterated these criticisms and provided scientific support for changing the rules: Roy et al. 2014; Eschen, Roques and Santini 2015; Jung et al. 2015; Klapwijk et al. 2016; and now Barwell et al. 2021. Summaries follow.

Roy et al. (2014) said the WTO SPS rules have been largely ineffective at protecting forests and other ecosystems (natural or managed) for two main reasons: (1) their primary aim is to promote international trade rather than protect the environment and (2) they require that a species be identified as a pest before it can be regulated, even though invading organisms are often either “new” (i.e. scientifically unknown) species or not troublesome within their native ranges.

Eschen, Roques and Santini (2015) found that regulators’ focus on known pests meant that 90% of the exotic insect pests detected in Europe 1995–2004 had not been designated for regulation before they became established on the continent.

Klapwijk et al. (2016) concluded that the European Union phytosanitary rules have provided insufficient protection because often harmful organisms that enter the EU were unknown, and therefore unregulated, before establishment. A pending amendment would still not provide for precautionary assessments of high-risk commodities or provide for restrictions on the highest-risk commodities, such as imports of large plants or plants in soil. Green et al. (2021) call the international system “fallible” in the face of huge volumes of imports, including large, semi-mature trees. As Jung et al. 2018 point out, the scientific community has repeatedly urged regulators to require the use of preventative system approaches for producing Phytophthora-free nursery stock.

Scott Schlarbaum, University of Tennessee, and I reiterated these issues and cited additional examples in Chapter 7 of Fading Forests III. Since 2015 I have blogged numerous times about the risks associated with imported plants for planting and detection of numerous previously unknown Phytophthora species in Vietnam. [On the website, scroll to the bottom of the monthly listing of blogs, find the “categories” section, click on “plants as pest vectors”.]

Billions of Plant on the Move

Shipment of plants among America, Europe and Asia put all three continents at risk. First, North America, Europe and Asia share more than 100 genera of tree species (USDA 2000), so introduced insects and microbes are likely to find suitable hosts in their new home.

Second, North America and Europe import high volumes of plants. The U.S. imported an estimated 3.2 billion plant “units” (cuttings, rooted plants, tissue culture, etc.) in 2007 (Liebhold et al. 2012). By 2020, imports had declined to 1.8 B plant units plus nearly 723,000 kilograms of woody plant seeds (USDA 2021). Epanchin-Niell (pers. comm.) found that in the period FY2010-FY2012, the U.S. imported an average of about 300 million woody plant units per year (in 16,700 shipments). The plants included representatives of 175 woody plant genera. Europe imports even more plants; just 10 continental countries imported 4.3 billion living plants from overseas in 2010; 20.8% were woody plants (Jung 2015). The United Kingdom, home to famously enthusiastic gardeners, imported £1.3 billion worth of plants in 2018 (Green et al. 2021). Eschen, Roques and Santini (2015) document the rising number of invertebrate pests and pathogens associated with these imports. Green et al. (2021) note the risk to social values, especially tree plantings to sequester carbon, posed by rising introductions of tree-killing pathogens.

In response to the obvious failings of the international phytosanitary system, non-governmental experts have sought strict limits on imports of plant taxa and types posing the highest risk. Campbell and Schlarbaum (2003 and 2014) and Roy et al. (2014) advocate allowing entry of woody plants only in the form of seed and tissue cultures. Lovett et al. (2016) calls for applying APHIS’ NAPPRA authority to prohibit imports of woody plants in the 150 genera that North America shares with Europe and Asia. (I have criticized how NAPPRA is applied in earlier blogs – here and here.) Eschen, Roques and Santini (2015) suggest requiring that most imported plants be subjected to post-entry quarantine.

illustration of poor management practices that facilitate infection by *Phytopthora ramorum*; from nursery education material circulated by Washington State University

Yet, I see no evidence that either American or European governments are willing to consider substantial alteration of the international system – even in order to curb the highest risk. The current WTO/IPPC system at least contemplates another solution: requiring that imported plants be produced under clean stock or critical control point production programs. See ISPM#36 and RSPM#24 and USDA APHIS’ revision of the Q-37 regulation. Use of critical control point approaches has been suggested by Campbell and Schlarbaum (2014). It is also part of the comprehensive program called for by Jung et al. (2015). Jung et al. (2015) note the need for rigorous enforcement as well as campaigns to develop consumer awareness, creating an incentive for the nursery industry to distribute only clean stock. However, the non-governmental authors advocate application of critical control point programs to far more plant taxa than the phytosanitary officials have envisioned, so apparent agreement between advocates and officials is illusory. Attempts to create such a program are more advance domestically, for example see Swiecki, et al, 2021.

New Ways to Fix the System?

Unwilling to challenge the WTO/IPPC system directly, national phytosanitary officials are instead adopting approaches and technologies aimed at reducing the number of species that remain “unknown”. New molecular identification techniques are facilitating rapid identification of difficult-to-distinguish microbes at ports or as part of screening or monitoring programs. This advance is cheered by scientists [e.g., Eschen, Roques and Santini (2015); Jung et al. (2015)] as well as phytosanitary officials.

Authorities are also attempting to improve inspection at the border by targetting shipments thought to be of high risk.

Both these actions have limited efficacy, however. Eschen, Roques and Santini (2015) still say that given the difficulty of reliably identifying fungi and fungal-like organisms, authorities should reject any consignment with disease symptoms. Furthermore, greater certainty in identifying organisms does not overcome information gaps about their invasibility or possible virulence.

Targetting based on past interceptions, a mainstay of inspection programs, is increasingly considered unreliable – scientists warn about the “bridgehead effect”. That is, when non-native pests establish in new countries and then are transported from there [see Bertelsmeier and Ollier (2021); although this article concerns ants].

Others are exploring strategies to improve authorities’ ability to evaluate poorly known species’ possible impacts. There is enthusiastic endorsement of the concept of “sentinel” plantings. These are a tool to detect pests that attack tree species growing outside the host tree’s natural range. Others are trying to identify species traits or other factors that can be used to predict impacts, as explored below.

Scientists’ Efforts in North America

loblolly pine (*Pinus taeda*) — one of the pines tested by Li *et al*. photo by Dcrjsr, via Wikimedia

One team assessed 111 fungi associated with 55 Asian and European scolytine beetle species. None was found to be virulent pathogens on two pine species and two oak species native to the Southeastern U.S. (defined as having an impact similar to Dutch elm disease or laurel wilt). Twenty-two fungal species were minor pathogens (Li et al. 2021).

Mech et al. (2019) are trying to rank threats by non-native insects pose to North American tree species. (They did not evaluate pathogens). They evaluated the probability of a non-native insect causing high impact on a novel North American host as a function of the following: (a) evolutionary divergence time between native and novel hosts; (b) life history traits of the novel host; (c) evolutionary relationship of the non-native insect to native insects that have coevolved with the shared North American host; and (d) the life history traits of the non-native insect. The team has published its analyses of insects that specialize on conifers and hardwoods; they will publish on generalist insect pests in the near future. The insects evaluated were those identified in studies by Aukema et al. (2010) and Yamanaka et al. (2015).

Regarding conifers, the factors driving impacts were found to be:

1) The time (in millions of years) since a North American host tree species diverged from a coevolved host of the insect in its native range.

2) The tree host species’ shade and drought tolerance.

3) The presence or absence of a closely related native herbivore in North America.

None of the insect life history traits examined, singly or in combination, had predictive value.

There are interesting differences when considering hardwoods. Schultz et al. (2021) find that the most important predictive factor is an insect trait: being a scolytine beetle. Two tree-related factors are moderately predictive: moderate density of the wood, and divergence time between native and novel hardwood hosts.While this last factor is shared with the analysis of insects on conifers, the divergence period itself differs. For hardwood trees there is no predictive value tied to whether a related native insect attacks the North American host.

[For details, see also the blogs posted here and here.]

In a report issued earlier this year, in response to §10110 of the Agriculture Improvement Act (Farm Bill) of 2018 (USDA 2021), APHIS claims that recent changes to managing plant imports has cut interceptions via the plants for planting pathway to 2% of total forest pest interceptions during the period 2013 – 2018. The contributing agency actions are listed as

• Developing an offshore greenhouse certification program that gives U.S. producers a more reliable supply chain of healthy plant cuttings;

• Implementing risk-based sampling to focus port inspections on higher-risk shipments [but note questions about this approach raised by Eschen, Roques and Santini (2015)].

• Began using of molecular diagnostics at ports to detect high-risk pests that physical inspection would miss;

• Restricting imports of some plants under authority of the NAPPRA program; and

• Increasingly applying standardized systems approaches.

APHIS says its preclearance programs span 23 countries and cover 68 different types of commodities. In addition, APHIS has certified 25 offshore facilities in 12 countries. However, the report does not say how many of these agreements cover production of woody plants – those most likely to transport forest pests.

APHIS has had a greenhouse certification program with Canada since 1996. A high proportion of U.S. woody plant imports comes from Canada. The recent report (USDA 2021) lists source countries for the highest numbers of pest interceptions for plants for planting – although not in order of detections. Canada is listed – in bold type. The meaning of this highlight is not explained. (China is also listed in bold.) More disturbing, the report makes no mention of the suspicion that at least some of the plants infested by Phytophthora ramorum that were shipped to 18 states in spring 2019 originated in a British Columbia nursery.

Scientists’ Efforts in Europe

The focus in Europe appears to be on pathogens, specifically the Phytophthora genus. Europeans are responding to several recently-introduced highly damaging diseases caused by species in the genus that were unknown to science before introduction. Barwell and colleagues (full reference at end of the blog) sought to explain the species’ impact as measured by traits such as number of countries invaded, latitudinal limits, and host range. They evaluated factors they thought would be easily discerned, such as species’ traits, phylogeny and time since description (as a proxy for extent of scientific understanding of the species’ behavior). The most predictive traits were thermal minima, oospore wall index and growth rate at optimum temperature. They found that root-attacking species of Phytophthora were reported in more countries and on more host families than foliar-attacking species.

Japanese larch plantation in Britain killed by *Phytophthora ramorum*; photo from UK Forest Research

Progress – but Still Incomplete Solution to the SPS/IPPC Conundrum

Perhaps these efforts to close information gaps earlier in the invasion process will be accepted by the phytosanitary agencies and the findings will be incorporated into their decision-making. If this happens, scientists’ efforts might contribute substantially to overcoming the challenges created by the SPS/IPPC system. Presumably acting on scientific findings is more acceptable than the more radical approach that I and others have suggested. Still, there remain the “unknown unknowns” – and the SPS/IPPC system continues to hinder measures that might be effective in preventing their introduction.

Meanwhile, the British are pursuing both a nursery certification/accreditation program and a coordinated strategy for early detection of Phytophthora pathogens in the nursery trade. Green et al. (2021) found that nursery owners could not justify the cost of adopting best management practices if they were aimed at preventing the presence of Phytophthora alone. They could if the program sought to curtail the presence and spread of numerous plant pathogens. A decade ago in the U.S., The Nature Conservancy explored a possible structure combining a clean stock system with insurance. The latter would reimburse participating nurseries for inventory lost to pests as long as the nursery used prescribed pest-avoidance strategies. The SANC program attempts to incentivize adoption of clean stock systems by the American nursery industry. However, it does not include the insurance concept.

Another helpful step would be to change the pest risk assessment process by assessing the risks more broadly. Perhaps the analysis could evaluate the risks associated with – and determine effective measures to counter – certain organisms, i.e.:

(a) pests associated with any bare-root woody plants from a particular region, for example East Asia; (b) pests associated with roots or stems, without limiting the study to particular kinds of plants or geographic regions of origin; or

(c) single types of pests, such as a fungal pathogen without regard to its species, on any imported plant (regardless of taxon or country of origin), especially learning how to prevent their presence.

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

Barwell, L.J., A. Perez-Sierra, B. Henricot, A. Harris, T.I. Burgess, G. Hardy, P. Scott, N. Williams, D.E. L. Cooke, S. Green, D.S. Chapman, B.V. Purse. 2021. Evolutionary trait-based approaches for predicting future global impacts of plant pathogens in the genus Phytophthora. Journal of Applied Ecology 2021; 58:718-730

Bertelsmeier, C. and S. Ollier. 2021. Bridgehead effects distort global flows of alien species. Diversity and Distributions https://onlinelibrary.wiley.com/doi/full/10.1111/ddi.13388

Brasier C.M. 2008. The biosecurity threat to the UK and global environment from international trade in plants. Plant Pathology 57: 792–808.

Eschen, R., A. Roques and A. Santini. 2015. Taxonomic dissimilarity in patterns of interception and establishment of alien arthropods, nematodes and pathogens affecting woody plants in Europe. Journal of Conservation Biogeography Diversity and Distributions (Diversity Distrib.) (2015) 21, 36–45

Jung, T., et al. 2015. Widespread Phytophthora infestations in European nurseries put forest, semi-natural and horticultural ecosystems at high risk of Phytophthora diseases. Forest Pathology. November 2015.

Jung, T., A. Pérez-Sierra, A. Durán, M. Horta Jung, Y. Balci, B. Scanu. 2018. Canker and decline diseases caused by soil- and airborne Phytophthora species in forests and woodlands. Persoonia 40, 2018: 182–220

Klapwijk, M.J., A.J. M. Hopkins, L. Eriksson, M. Pettersson, M. Schroeder, A. Lindelo¨w, J. Ro¨nnberg, E.C.H. Keskitalo, M. Kenis. 2016. Reducing the risk of invasive forest pests and pathogens: Combining legislation, targeted management and public awareness. Ambio 2016, 45(Suppl. 2):S223–S234 DOI 10.1007/s13280-015-0748-3

Li, Y., C. Bateman, J. Skelton, B. Wang, A. Black, Y. Huang, A. Gonzalez, M.A. Jusino, Z.J. Nolen, S. Freeman, Z. Mendel, C. Chen, H. Li, M. Kolařík, M. Knížek, J. Park, W. Sittichaya, P.H. Thai, S. Ito, M. Torii, L. Gao, A.J. Johnson, M. Lu, J. Sun, Z. Zhang, D.C. Adams, J. Hulcr. 2021. Pre-invasion assessment of exotic bark beetle-vectored fungi to detect tree-killing pathogens. Phytopathology. https://doi.org/10.1094/PHYTO-01-21-0041-R

Roy, B.A., H.M Alexander, J. Davidson, F.T. Campbell, J.J. Burdon, R. Sniezko, and C. Brasier. 2014. Increasing forest loss worldwide from invasive pests requires new trade regulations. Frontiers in Ecology and the Environment 12(8), 457-465

Swiecki, T. J., Bernhardt, E. A., Frankel, S. J., Benner, D., & Hillman, J. (2021). An accreditation program to produce native plant nursery stock free of Phytophthora for use in habitat restoration. Plant Health Progress, PHP-02. https://apsjournals.apsnet.org/doi/abs/10.1094/PHP-02-21-0025-FI

United States Department of Agriculture Animal and Plant Health Inspection Service and Forest Service. 2000. Pest Risk assessment for Importation of Solid Wood Packing Materials into the United States.

Yamanaka, T., Morimoto, N., Nishida, G. M., Kiritani, K. , Moriya, S. , & Liebhold, A. M. (2015). Comparison of insect invasions in North America, Japan and their Islands. Biological Invasions, 17, 3049–3061. 10.1007/s10530-015-0935-y

Posted by Faith Campbell

A Case Study Documents Forest Losses due to White Pine Blister Rust

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 21^st 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

New Asian Defoliator – a Threat to Elms?

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

Sinon, S. First confirmed sighting of a new invasive in North America: elm zigzag sawfly – Invasive Species Centre. https://www.invasivespeciescentre.ca/first-confirmed-sighting-of-a-new-invasive-in-north-america-elm-zigzag-sawfly/

(United Kingdom) Forest Research Elm zigzag sawfly (Aproceros leucopoda) https://www.forestresearch.gov.uk/tools-and-resources/fthr/pest-and-disease-resources/elm-zigzag-sawfly/

Posted by Faith Campbell

In the News: Big, Colorful Joro Spider

Joro spider; photo by Dorothy Kozlowski, University of Georgia

Lately there has been lots of media attention to an introduced spider which has attracted attention because it is large and showy – and very numerous in 2021. The Joro spider, (Trichonephila (formerly Nephila) clavata) is — like so many introduced organisms — from East Asia (Japan, China, Korea, and Taiwan) (Hoebeke, Huffmaster and Freeman 2015; full citation at the end of the blog).

The spider was originally found in 2013 at several locations in three counties of northeast Georgia. All were near warehouses and other facilities associated with Interstate-85, a major transport corridor (Hoebeke, Huffmaster and Freeman 2015).

The Joro spider is one of about 60 species of non-indigenous spiders (Araneae) that have been detected in North America. The majority originated in Europe and Asia (species list posted here; see Araneae).

The Joro spider is one of the golden orb-web spiders, a group with conspicuously large and colorful females that weave exceptionally large, impressive webs. One species of the genus, N. clavipes (L.), occurs in the Western Hemisphere. It is found throughout Florida, the West Indies, as far north as North Carolina, across the Gulf States, through Central America, and into South America as far south as Argentina. It is also known as the “banana spider” or “golden silk spider.” (Hoebeke, Huffmaster and Freeman 2015)

Hoebeke, Huffmaster and Freeman (2015) describe both the spider’s discovery in Georgia (by Huffmaster) and how to distinguish it from other large spiders in the southeastern U.S. South Carolina has posted a fact sheet here.

In Asia and northeast Georgia, the spider apparently overwinters as eggs. Spiderlings emerge from the egg cocoons in the spring. Males reach maturity by late August. Females become sexually mature in September and early October. Oviposition occurs from mid-October to November resulting in the production of only a single egg sac. Large, mature females were first observed beginning in late September and persisted until mid-November when temperatures began to cool significantly. Most spiders were found in large webs attached to the exterior of homes near porch lights, on wooden decks, or among shrubs and flowering bushes near homes (Hoebeke, Huffmaster and Freeman 2015). By 2021 the webs were so numerous as to be consider major nuisances.

Probable Introduction Pathways

Hoebeke, Huffmaster and Freeman (2015) think the spiders are frequently transported (as adults or egg masses) in cargo containers, on plant nursery stock, and on crates and pallets. If accidental transport were to occur in late August to early October from East Asia, then the spiders’ reproduction would be at its height and there would be a greater likelihood that egg masses might be deposited on structures or plant material being exported.

This thought is supported by an email sent to Hoebeke in 2016 that a Joro spider had been seen on the outside of a freight container in Tacoma, Washington. There has been no report of additional sightings in Washington State (Hoebeke pers. comm.)

Spread within the United States

By 2021, the Joro spider had been detected in at least 30 counties in north and central Georgia, adjacent South Carolina; Hamilton and Bradley counties in Tennessee; and Rutherford and Jackson counties in North Carolina (Hoebeke pers. comm.). See the map here.

Spread in the United States is probably associated with major transport routes. The original detections were 64 km northeast of Atlanta near a thriving business location on the I-85 business corridor,

It is also possible that spiderlings balloon, that is, ride air currents to move some distance. This distance can be miles, depends on the spider’s mass and posture, air currents, and on the drag of the silk parachute (Hoebeke, Huffmaster and Freeman 2015). The 2014 Madison County detection in northeast Georgia was not near transport corridors but in a rural mixed farm landscape, downwind from the other sites. Males also use ballooning to find females for mating (Gavriles 2020).

How might the Joro spider affect the local ecosystem?

Many questions exist about the Joro spiders’ impact. Will they outcompete other orb weaving spiders – either native or nonnative? Will they reduce other insect populations through predation? Scientists do not yet see indication of displacement of native spiders or depletion of prey species (Gavriles 2020; Hoebeke pers. comm.)

Potential Range – update

In March 2022, two University of Georgia scientists (Andy Davis and Benjamin Frick) published a study that evaluated the Joro spider’s cold tolerance by studying the spider’s physiology and survival during a brief (2 minute) freeze. They found that the Joro spider’s more rapid metabolic and heart rates means it could probably survive throughout most of the Eastern Seaboard. The scientists reiterate earlier information that the Joro spider does not appear to have much of an effect on local food webs or ecosystems.

SOURCES

Cannon, J. Palm-sized, invasive spiders are spinning golden webs across Georgia in ‘extreme numbers’ https://www.usatoday.com/story/news/nation/2021/09/29/scientists-say-invasive-joro-spiders-here-stay-georgia/5917913001/ accessed 21-11/5

Gavrilles, B. Like it or not, Joro spiders are here to stay. October 26, 2020 https://news.uga.edu/joro-spiders-are-here-to-stay/

Hoebeke, E. Richard. University of Georgia Department of Entomology

Hoebeke, E.R., W. Huffmaster, and B.J. Freeman. 2015 Nephila clavata L. Koch, the Joro Spider of East Asia, newly recorded from North America (Araneae: Nephilidae) PeerJ https://peerj.com/articles/763/#

Posted by Faith Campbell

Asian giant hornet in Washington State: Update

Asian giant hornet; photo by Yasunori Koide; Wikimedia commons

They’re still here … and perhaps more widespread than thought last year. What officials have learned is that colonies are often high up in trees, not in the ground, as expected based on behavior in Asia. This makes detection and control especially difficult.

In June a citizen found a dead Asian giant hornet (AGH) male in Snohomish County. This county in the Seattle metropolitan area is separated from Whatcom County (site of last year’s detections) by a third county, Skagit County. The Washington State Department of Agriculture (WSDA) responded by setting up traps in Snohomish and King counties, and urging citizens to be alert and report any hornet sightings.

Equally worrying, the dead wasp was determined by appearance and genetics to be unrelated to the colonies detected in 2019 and 2020 in Washington and British Columbia. Trapping in the areas found no additional specimens (S. Spichiger pers. comm.)

In July, WSDA designated the hornet genus Vespa as a quarantine pest; this action confirms WSDA authority to control access to nest sites.

Nests Found and Destroyed

Starting in late summer, citizens began reporting sightings and officials succeeded in tracking hornets to their nests. However, it was not easy! Eradicating Asian giant hornets demands lots of resources and commitment. While all these nests were in Whatcom County – site of last year’s detections — it is clear that several colonies had been established. It seems to me highly unlikely that they have all been detected.

Detection of the first nest in 2021 came in August, following several visual detections of the hornets attacking nests of paper wasps. WSDA staff captured and tagged three hornets over a couple of days. They succeeded in tracking the third hornet when it reappeared a week later. The nest was destroyed (after removal of all hornets) on August 25^th. This nest held nine layers of comb with 292 eggs, 422 larvae, and 563 prepuae. Nearly 200 adult hornets were killed. One queen was found. [Hornet Herald 21.07 Sept. 8 2021] The nest was at the base of a dead alder tree in rural Whatcom County, east of Blaine, just 400 metres south of the Canadian border.

The second and third nests were detected on September 8 and 10, 2021. In these cases, tagging and tracking the hornets was easier than in August. Nest eradication was not easy, however. Both nests were high inside dead alder trees, making access difficult. Both nests held multiple combs with hundreds of larvae, eggs, and pupae. Fortunately, only one queen was found in each. [Hornet Herald 21.08; October 5, 2021]

No detections have occurred since these.

WSDA also collected data on foraging behaviors of wasps in the third nest. Data include information on periods during the day when the wasps are active, and what materials they bring back to the nest – which includes wood pulp for nest comb construction and insect thoraces for feeding the pupae. [Hornet Herald 21.08; October 5, 2021]

It is encouraging that only one queen was found in each nest; in 2020, the single nest officials destroyed held 200 queens!

Trapping in British Columbia

Although British Columbia officials increased the number of traps in 2021, and urged citizens to also set out traps, no confirmed AGH finds were made in British Columbia until early November, when one was caught in a trap set for Japanese beetles. This hornet was on the border with Washington, so officials are trying to determine whether it came from one of the nests already discovered there.

There were a couple of unconfirmed sightings. On October 22 a single, aged specimen was found in a Japanese beetle trap about 1.2 km north of the first hornet nest extracted this year in Washington. The beetle trap had been serviced one month earlier. Canadian government entomologists are analyzing the DNA of this specimen to see if it was related to the Washington State nests.

At least one citizen said he had seen an Asian giant hornet in July, but officials said they could not investigate until they had either a picture or a specimen.

Asian giant hornet with radio tag developed by USDA APHIS scientists

Intriguing wrinkle

Mattila et al. (2021) describe an “impressive array of strategies” Asian honey bees use to protect nests from attacks by hornets in the genus Vespa, including a previously unknown use of auditory and perhaps chemical signals to warn nest mates. The authors suggest that this diverse alarm repertoire is similar to alarms issued by socially complex vertebrates such as primates and birds.

SOURCES OF INFORMATION

USDA Agriculture Research Service: https://scientificdiscoveries.ars.usda.gov/highlights/asian-giant-hornet/

Washington State Department of Agriculture https://agr.wa.gov/hornets

Mattila, H.R., H.G. Kernen, G.W. Otis, L.T.P. Nguyen, H.E. Pham, O.M. Knight, N.T. Phan. 2021.

Giant hornet (Vespa soror) attacks trigger frenetic antipredator signalling in honeybee (Apis cerana) colonies. R. Soc. Open Sci. 8: 211215. https://doi.org/10.1098/rsos.211215

Posted by Faith Campbell

For a detailed discussion of the policies and practices that have allowed tree-killing pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at http://treeimprovement.utk.edu/FadingForests.htm

The Lacey Act – Can It Protect US from Invasive Species?

Sean Connery as Hotspur, Shakespeare Henry IV Part I (BBC, “Age of Kings”)

[Starlings – one of the agricultural pests that prompted adoption of the Lacey Act – were introduced to the U.S. because they were mentioned by Shakespeare: Hotspur says “Nay, I’ll have a starling shall be taught to speak nothing but ‘Mortimer,’ and give it him, to keep his anger still in motion.”]

Americans are increasingly aware of the damage caused by invasive species. The law that ostensibly protects our environment from most potentially invasive animals is the Lacey Act – more specifically, the “injurious wildlife” sections of the law, now known as 18 U.S.C. 42 or title 18.

When it was adopted 120 years ago, the Lacey Act was not intended to protect the environment from the full range of possible animal bioinvaders. While Congress amended it several times in the first 60 years of its existence, the law still has many gaps that impede its usefulness for that purpose.

Rep. John F. Lacey via Wikimedia Commons

When first adopted in 1900, the injurious wildlife provisions of the Lacey Act prohibited importation only of wild mammals and birds that posed a threat to agriculture and horticulture. The statute was quite broad in that it prohibited importation of any wild bird or mammal without a permit; there was no requirement that a species be designated as “injurious” to be regulated. The Act was then administered by the U.S. Department of Agriculture. [For a detailed discussion of the Lacey Act’s changing provisions, see Jewell 2020; full reference at the end of this blog.]

In 1960 the Act was amended to expand the list of taxa eligible for designation as “injurious” to include fishes, mollusks, crustaceans, reptiles, and amphibians. Congress also expanded the justifications for listing a species as injurious. It added harm to people, to forestry, or to wildlife or US wildlife resources to the law’s original concerns for agriculture and horticulture. This second change brought the purposes of the Lacey Act closer to the mandate of the U.S. Fish and Wildlife Service (USFWS) – which had assumed responsibility for implementing the Act in 1939.

Unfortunately, Congress simultaneously took other action that greatly weakened USFWS’ ability to use the Act to protect the environment from introduced animals. First, it dropped the requirement that the Secretary approve, with a permit, any importation of a wild bird or mammal.

Second, the 1960 amendment clouded the originally clear prohibition of movement of listed species across state lines. The new language prohibits “any shipment between the continental United States, the District of Columbia, Hawaii, the Commonwealth of Puerto Rico, or any possession of the United States …”

For the next 57 years, the USFWS and Congress sometimes interpreted that language as continuing to prohibit transport between the states within the continental United States. However, this situation could not last. In 2017, acting in a case that had challenged the 2012 listing of several nonnative constrictor snakes as “injurious,” the D.C. Circuit court found that the plain language of §18 U.S.C. 42(a)(1) does not prohibit the transportation of injurious wildlife between states within the continental United States. So now, transportation of injurious wildlife among the continental states is not prohibited by the statute in most circumstances.

Burmese python; photo by R. Cammauf, Everglades National Park via Flickr

The Law’s Strengths

Some aspects of the law have been strengths. Since the term “injurious” has never been defined, the USFWS has been able to use its discretion to list species that are not necessarily invasive themselves but that might cause harm in some other way. For example, the salmon family and 20 genera of salamanders have been listed because they are vectors of harmful wildlife pathogens.

In addition, USFWS has listed entire genera or families of organisms – as long as each species within the taxon has been shown to possess the “injurious” trait(s). This flexibility has probably helped listings aimed at precluding importers from switching from the species that initially raised concerns to related species.

The Law’s Inherent Weaknesses

1) Legal shortfalls

Due to the confusion created by the 1960 amendment, the USFWS now lacks authority to prohibit interstate transport of species listed as “injurious”. This gap undermines the law’s efficacy in controlling spread of listed species once they are established within the U.S.

Also, the law does not prohibit other human actions that pertain to the presence and spread of species listed as “injurious,” e.g., sale, possession, or intra-state transport. Addressing these other aspects of invasive species policy was left to other players, such as states or resource managers.

2) Funding shortfall

Neither the Executive Branch nor Congress has ever provided specific funding for implementation of the Lacey Act. Only one USFWS staffer has the job of listing species under the Act. This situation might change now, since the American Rescue Plan Act adopted in spring 2021 does provide funding over the next five years for listing species that can vector pathogens harmful to people.

Staff’s Evaluation of Its Implementation of the Lacey Act

Since USFWS took over implementation of the Lacey Act in 1939, 36 taxonomic groups have been added to the “injurious wildlife” list. Seven of these listings comprise multiple species – either as genera or families.

Two mammals have been listed since the late 1960s – brushtail possum in 2002 and raccoon dog in 1983. Recent listings have strongly focused on aquatic organisms. This is because the staff is housed in the Fish and Aquatic Conservation program and their expertise is in these species.

silver carp; photo by University of Illinois

Listing activity appeared to be building in the second decade of the 21^st Century, with multi-species listings of fish, snakes, and salamanders between 2012 and 2016. However, there has been only one listing action since 2016 – and that was by an act of Congress (listing of the quagga mussel).

In two peer reviewed papers, the USFWS’ Jewell and Fuller provide a history of the Lacey Act’s injurious wildlife title and analyze the effects of listing of 307 species (those listed since 1952). They conclude that 98% of the species listings were “effective” because the listed species either had not been introduced subsequent to listing [288 species; 94% of the total number of listed species] or had not spread to additional states [12 species, 4% of the total]. Another way to calculate the latter figure is to say that 63% of all established species have remained within the state(s) where they were established at the time of listing. Only three species have been spread to additional states by human actions. In these cases, Jewell and Fuller considered the Lacey Act measures to be “ineffective”. For further details on the Jewell and Fuller evaluations of listing efficacy, see their article – full citation given at the end of this blog.

Jewell and Fuller do not evaluate the impacts of animal species introduced to the U.S. after 1960 that have never been listed under the Lacey Act, or speculate about whether listing those species might have minimized the risk of their introduction.

Jewell and Fuller consider listing of species not yet established in the U.S. to be most effective for two reasons. First, listing minimizes the probability that the species will be imported intentionally or unintentionally. Second, listing provides states with risk analyses and other information on which to rely in adopting their own restrictions, including possible prohibitions on sale or possession.

Jewell and Fuller also argue that even in the absence of legal authority to regulate interstate transport of listed species among the continental states, it is still worthwhile to list species that are already established in the U.S. They give six reasons. I summarize those reasons (placing them in my order, not Jewell and Fuller’s):

1) Listing can protect the islands of Hawai`i, Puerto Rico, and the Caribbean and Pacific territories. All are extremely vulnerable to invasive species.

2) If a species shares the traits of injuriousness with other species, particularly those in the same genus or family, then including the already-invasive species demonstrates why the related species should also be listed.

3) Many imported animals carry parasites and pathogens harmful to native species, and stopping the continued importation can reduce those threats that cause disease.

4) Prohibiting further importation of the invasive species can prevent individuals from being introduced to new areas where the species would not otherwise have arrived and can reduce propagule pressure that could introduce hardier individuals.

5) Listing can provide states and other jurisdictions with the technical information they need to pursue additional restrictions not federally authorized under 18 U.S.C. 42, such as transport into a state, possession, and sale.

6) Listing reduces propagule pressure and might enhance the efficacy of any eradication or control measures.

How to Improve the Lacey Act

1) Amend the Lacey Act to restore authority to regulate interstate movement of listed species – including among the continental states and emergency listing authority. Also establish a more streamlined listing process.

2) Strengthen implementation of the law by providing a specific, adequate appropriation to hire additional staff. Utilize the enhanced resources to assess species proactively using risk assessment tools.

It is not yet clear whether the Biden Administration will initiate a more active listing process, especially beyond the zoonotic disease vectors that are the subject of the American Rescue Plan Act.

Note: The “injurious wildlife” section of the Lacey Act (18 U.S.C. 42, or title 18) is separate from another part of the Lacey Act (16 U.S.C. 3371-3378) that is has always been more widely known. This provision regulates wildlife trafficking across State lines. It was later broadened to include plants and trafficking of wildlife and plants from foreign countries.

SOURCES

Jewell S.D. (2020) A century of injurious wildlife listing under the Lacey Act: a history. Management of Biological Invasions. Volume 11, Issue 3: 356–371, https://doi.org/10. 3391/mbi.2020.11.3.01 https://www.reabic.net/journals/mbi/2020/3/MBI_2020_Jewell.pdf

Jewell S.D., P.L. Fuller (2021) The unsung success of injurious wildlife listing under the Lacey Act. Management of Biological Invasions. Volume 12, Issue 3:527-545 https://www.reabic.net/journals/mbi/2021/3/MBI_2021_Jewell_Fuller.pdf

Alternative view – that Lacey Act implementation has failed to protect the U.S. – presented by the following authors:

Fowler, A.J., D.M. Lodge and J. Hsia. 2007. Failure of the Lacey Act to protect US ecosystems against animal invasions. Frontiers in Ecology and the Environment.

Springborn, M. C.M. Romagosa and R.P. Keller. 2011. The value of nonindigenous species risk assessment in international trade. Ecological Economics

Jenkins, P.T. 2012. Invasive animals and wildlife pathogens in the United States: the economic case for more risk assessments and regulation. Biological Invasions

Posted by Faith Campbell

Two More Key Studies: Forest Pests and Carbon

hemlock woolly adelgid – a pest that has spread north as result of warmer winters; photo from bugwood.org

I recently posted a blog reviewing impacts of insects and pathogens on efforts to sequester atmospheric carbon in forests. I want to add two other studies. The first, by Weed, Ayers, and Hicke (2013; full citation at end of this blog), delved more deeply into three mechanisms by which climate and atmospheric changes associated with increasing greenhouse gases influence biotic disturbances: (1) effects on the physiology of insects and pathogens that cause changes in their abundance and distribution; (2) effects on tree defenses and tolerance; and (3) effects on interactions between disturbance agents and their own enemies, competitors, and mutualists. They also looked at interaction of tree-killing pests with other sources of forest disturbances – e.g., wildfires, drought, bioinvasions by organisms other than insects and pathogens, and human conversion of forested land to other uses. Tree-killing pests can promote destabilizing positive feedbacks with these other sources of forest disturbances. Weed, Ayers, and Hicke (2013) express the concern that recurrent forest disturbances caused by insects and pathogens might counteract carbon mitigation strategies. [This concern is similar to findings by Quirion et al. (2021) cited in the previous blog and by USDA Forest Service scientists studying disturbance agents in western forests (Barrett et al. 2021).

A second study by Clark and D’Amato (2021) looks intensively at forest growth and change in four types of secondary forests in New England to discover climate change dynamics and their resulting relative ability to sequester atmospheric carbon.

A 2013 Study by Weed, Ayers, and Hicke

Weed, Ayers, and Hicke (2013) begin from the premise that epidemics of forest insects and diseases (native and introduced) are the dominant sources of disturbance to North American forests. They note that, on a global scale, bioinvasions might be at least as important as climate change as threats to the sustainability of forest ecosystems. As agreed by most authorities, they find that the underlying cause of bioinvasions is propagule pressure from global transport, not climate change. However, climate change is strongly connected to management of continuing invasions.

Weed, Ayers, and Hicke (2013) review 79 studies published 1950 – 2012 which addressed a total of 27 insects and 22 diseases. Despite their opening focus on introduced pests, and the fact that six of the insects and nine of the diseases are nonindigenous, most of the research they were able to review has been on native organisms, principally on two species: the mountain pine beetle and southern pine beetle. Less is known about pathogens’ interaction with changes to climate than about insects’. A further complicating factor is the need to study both the insect and the pathogen when considering diseases vectored by insects (e.g., beech bark disease, oak wilt, Dutch elm disease, black stain root diseases, laurel wilt, thousand cankers disease, and pitch canker). [Profiles of most of these diseases are posted here; click on “invasive species”.] It is no surprise, then, that Weed, Ayers, and Hicke (2013) identify several areas where there is insufficient research. They state that despite scientists’ broad knowledge of climate effects on insect and pathogen demography, we still lack capacity to predict pest outbreaks under climate change.

Changing climatic conditions can exacerbate pest-caused disturbances by reducing winter mortality of insects and by increasing the development rate of insects and pathogens during the growing season. The changing conditions can also alter leaf maturation (which affects insect feeding) or synchrony of the life cycles of bark beetles. Contrary to the authors’ expectations, drought does not appear to cause a universal reduction in trees’ creation of defensive chemicals.

Due to pests’ host preferences, these disturbance agents typically alter the composition of tree species within stands – which can change forest types. For example, Weed, Ayers, and Hicke (2013) mention how mountain pine beetles shifted western forests from five needle pines toward subalpine firs. They do not mention balsam woolly adelgid or other fir pests.

The authors expect warming and increases in atmospheric CO₂ to promote faster forest maturation in many US regions. Drought, however, will probably slow maturation rates in arid areas such as the southwest and intermountain regions. Climate change increases the likelihood that forest stands will be exposed to different and less suitable climates than those under which the current stands matured, making more stands susceptible to pests. (The USFS report on western forests said the same — Barrett et al. 2021). These changes tend to reduce the extent of mature forests and can adversely affect ecosystem services. They note the need for increased capacity to predict future patterns of biotic disturbances and integrate this knowledge with forest ecosystem science and the socioeconomics of human land use.

Weed, Ayers, and Hicke (2013) raise an interesting point regarding the impact of disturbance factors on trees’ ages and sizes. They mention specifically reduction in numbers of large-diameter beech trees due to beech bark disease and elms due to Dutch elm disease. Several large-growing trees, e.g., American chestnut and white pines, have been virtually eliminated from much of their historical ranges. They express the fear that emerald ash borer, sudden oak death, butternut canker, and laurel wilt are in the early stages of having a similar effect on their hosts. [Profiles of most of these pests are posted here; click on “invasive species”.] Weed, Ayers, and Hicke (2013) note the importance to wildlife of this shift – the loss of mature forest habitat changes availability of food supplies, nest cavities, etc. The authors do not relate these specific pest-mediated changes to the climate change-caused alterations. However, they do note that pest impacts exacerbate a situation already arising from loss of mature forests due to human land use patterns.

Weed, Ayers, and Hicke (2013) mention changes in elemental cycling and hydrologic processes resulting from pest-caused mortality; they refer to several studies by Lovett, especially Lovett et al. (2006). These changes can have long-lasting effects on productivity, biodiversity, and elemental cycling. Among them are effects on water transpiration and increased soil moisture and runoff. I had blogged earlier about these impacts as they pertain to black ash swamps. At high elevations, snow accumulates more deeply on the ground while snowmelt is more rapid because loss of canopy will decrease interception of snow by the canopy (leading to reduced sublimation and redistribution of snow) and increase solar radiation to the forest floor.

Weed, Ayers, and Hicke (2013) anticipate that pest outbreaks under climate change will commonly produce persistent changes in the feedbacks that connect biotic communities and elemental cycling.

Weed, Ayers, and Hicke (2013) summarize their findings as follows:

1) Epidemics of forest pests (native and introduced) exceed other sources of disturbance to North American forests.

2) Insect populations are highly responsive to climate change due to their physiological sensitivity to temperature, high mobility, short generation times, and explosive reproductive potential. Pathogens and declines are also strongly influenced by climate change due to their sensitivity to temperature and moisture. These effects have proven to be more dramatic than expected in the case of pine bark beetles. There is no discussion of whether other insect-host relationships might differ substantially.

3) Changes in biotic disturbance regimes have broad consequences for forest ecosystems and the services they provide to society.

4) Climatic effects on forest pest outbreaks might beget further changes in climate by influencing the exchange of carbon, water, and energy between forests and the atmosphere.

5) In some areas, climate-induced changes might result in increased or decreased disturbance risk.

eastern white pine; photo by F.T. Campbell

A 2021 Study by Clark and D’Amato

Clark and D’Amato (2021) focused on a research site in New England which provided 69 years of data on succession dynamics. The site has four types of secondary forests. Clark and D’Amato (2021) found that mixed hardwood (oak)-pine systems dominated by large diameter eastern white pine (Pinus strobus) exhibited the greatest increase in biomass over the 69-year period and thus performed best as carbon sinks. These forests also had the greatest structural complexity.

However, these “mixedwood” systems are largely an artifact of past clearing for agriculture and are naturally trending toward greater domination by hardwoods. In fact, new trees growing in all four forest types were predominantly shade-tolerant beech (Fagus grandifolia) and hemlock (Tsuga canadensis). Clark and D’Amato (2021) note that these species are both less compatible with predicted future climatic conditions and are under attack by non-native pests — beech bark disease and hemlock woolly adelgid, respectively. The article makes no mention of possible complications from two other pests of beech, beech leaf disease and beech leaf weevil. [All three pests have profiles here.]

They conclude that if the goal is to maximize carbon sequestration in forests – while maintaining structural complexity – managers must adopt silvicultural strategies intended to maintain the pine component. This strategy is not without risk. Mature white pine constitutes 68% of the biomass in the mixedwood stands. Clark and D’Amato (2021) note that a strategy relying so heavily on one species exposes that strategy to a high risk of catastrophic losses due to stochastic disturbance-related mortality, emerging forest health issues, and/or selective timber harvests targeting the largest trees. Of course, eastern white pine has already survived one pest, white pine blister rust.

SOURCES

Barrett, T.M. and G.C. Robertson, Editors. 2021. Disturbance and Sustainability in Forests of the Western United States. USDA Forest Service Pacific Northwest Research Station. General Technical Report PNW-GTR-992. March 2021

Clark, P.W. and A.W. D’Amato. 2021. Long-term development of transition hardwood and Pinus strobus – Quercus mixedwood forests with implications for future adaptation and mitigation potential. Forest Ecology and Management 501 (2021) 119654

Lovett, G.M., C.D. Canham, M.A. Arthur, K.C. Weathers, and R.D. Fitzhugh. 2006. Forest Ecosystem Responses to Exotic Pests and Pathogens in Eastern North America. BioScience Vol. 56 No. 5 May 2006)

Lovett, G.M., M. Weiss, A.M. Liebhold, T.P. Holmes, B. Leung, K.F. Lambert, D.A. Orwig, F.T. Campbell, J. Rosenthal, D.G. Mclimate changeullough, R. Wildova, M.P. Ayres, C.D. Canham, D.R. Foster, S.L. Ladeau, and T. Weldy. 2016. Nonnative forest insects and pathogens in the United States: Impacts and policy options. Ecological Applications, 26(5), 2016, pp. 1437-1455

Poland, T.M., Patel-Weynand, T., Finch, D., Miniat, C. F., and Lopez, V. (Eds) (2019), Invasive Species in Forests and Grasslands of the United States: A Comprehensive Science Synthesis for the United States Forest Sector. Springer Verlag.

Weed, A.S., M.P. Ayers, J.A. Hicke. 2013. Consequences of climate change for biotic disturbances in North American forests. Ecological Monographs, 83(4), 2013, pp. 441–470

Posted by Faith Campbell

New Study on Forest Carbon and Pests: The Picture is Ugly

lodgepole pines killed by mountain pine beetle in British Columbia; photo courtesy of Wikipedia

Natural systems, especially forests, could provide as much as 37% of the near-term mitigation necessary to meet Paris global climate goals. In the US, conservation, restoration, and improved land management could provide carbon sequestration equivalent to an estimated 21% of current net annual emissions.

However, the current U.S. forest carbon sink, which includes soils and standing and downed wood as well as live trees, might be in jeopardy due to increasing levels of disturbance, conversion, and/or declining sequestration rates in old growth stands.

Insects and plant diseases are one such disturbance agent. Acting alone or in combination with other forest stressors, they can damage or kill large numbers of trees in short periods of time, thereby reducing carbon sequestration and increasing emissions of stored carbon through decomposition of wood in dead or injured trees.

Historically, native and introduced insects and diseases have impacted an estimated 15% of the total U.S. forest cover annually. This impact is likely to increase. One study (Fei et al., 2019) found that an estimated 41% of the live forest biomass in the contiguous U.S. could be impacted by the 15 most damaging introduced pests already established in the U.S. Continuing introductions of new pests and exacerbated effects of native pests associated with climate change portend worsening losses of live trees. These rising impact of pests, combined with more frequent and severe fires and other forest disturbances, are likely to negate efforts to improve forests’ carbon sequestration capacity.

Sources of information about introduced pests’ impacts is available from, inter alia Campbell and Schlarbaum Fading Forests II and III, Lovett et al 2016, Poland et al. 2021, many blogs on this site, and pests’ profiles posed here under “invasive species” tab. Chapter 4 of Poland et al. (2021) provides a summary of what is known about interactions between invasive species and climate change – both climate impacts on bioinvaders and bioinvaders’ effect on carbon sequestration.

The United States and other major polluting countries have certain advantages. Their strong economies have the scientific and financial resources needed to implement effective invasive species prevention and forest management strategies. At the same time, many of them receive the most new forest pests – because they are major importers. These introduced pests pose the most serious and urgent near-term ecological threat to their forests and all the ecosystem services forests provide.

So, reducing insect and disease impacts to forests can simultaneously serve several goals—carbon sequestration, biodiversity conservation, and protecting the myriad economic and societal benefits of forests. See the recent IUCN report on threatened tree species.

A Major New Study

A new study by Quirion et al. (2021) takes another step in quantifying the threat to U.S. forests’ ability to sequester carbon by analyzing data from National Forest Inventory plots. Unfortunately, the re-measurement data for the period 2001 – 2019 are not available in the NFI for the Rocky Mountain states, which represents a critical data gap in the NFI program. This gap might not have had a significant impact on the national findings, however, because while the insect damage level (measured by an earlier inventory round) was quite severe in the Rocky Mountain States, the relatively slow growth of trees in that region means carbon sequestration rates are low.

Forest stand productivity – and carbon sequestration — will typically decline immediately after pest outbreaks, then recover or even increase beyond pre-outbreak levels depending on the productivity and maximum achieved biomass of replacement plant species and related soil characteristics. However, when prevalence of the disturbance increases, by, for example, more frequent pest outbreaks, carbon stocks in standing trees and sequestration rates can be reduced for extended periods.

Findings

Nationally, insects and diseases have decreased carbon sequestration by live trees on forest land by 12.83 teragrams carbon per year. This equals ~ 9% of the contiguous states’ total annual forest carbon sequestration and equivalent to the CO₂ emissions from over 10 million passenger vehicles driven for one year.
This estimate includes the impacts of both native and introduced insects and diseases, because the NFI database does not distinguish between them.
Insect-caused mortality had a larger impact than disease-caused mortality (see below). Forest plots recently impacted by insect disturbance sequestered on average 69% less carbon in live trees than plots with no recent disturbance. Plots recently impacted by disease disturbance sequestered on average 28% less carbon in live trees than plots with no recent disturbance.
Ecoprovinces in which the greatest annual reductions in live tree carbon sequestration due to pests were the Southern Rocky Mountain Steppe, Cascade Mixed Forest, Midwest Broadleaf Forest, and Laurentian Mixed Forest. (Ecoprovinces are outlined – but not named – in Quirion et al. 2021; more complete information is provided in the supplementary material.)

If this study had been carried out in the 1920’s, when chestnut blight and white pine blister rust were spreading across vast areas and killing large trees, the impact of diseases would have been much higher. Today, the most widespread impacts of diseases are on either small trees (e.g., redbay succumbing to laurel wilt) or slow-growing, high-elevation trees (e.g., whitebark and limber pine to white pine blister rust). As long as no equivalents of those earlier diseases are introduced, insects will probably continue to have the larger impacts.

western white pine killed by blister rust; photo from National Archives

Quirion et al. 2021 note that their estimates should be considered conservative. The USFS’s inventory records only major disturbances. That is, when mortality or damage is equal to or exceeds 25% of trees or 50% of an individual tree species’ count on an area of at least 0.4 ha. This criterion largely excludes less severe pest disturbances, including those from which trees recover but which might have temporary negative effects on carbon sequestration.

The study’s authors note that their work has important limitations. The dearth of data from the Rocky Mountain states is one. Other factors not considered include transfers of carbon from live biomass to dead organic matter, soils, and salvaged or preemptively harvested wood products. As trees die from pests or diseases, their carbon becomes dead wood and decays slowly, producing a lag in the carbon emissions to the atmosphere. A small fraction of the carbon in dead wood might be incorporated into soil organic matter, further delaying the emissions. A full accounting of the carbon consequences of pests and diseases would require assessment of these lags, probably through a modeling study.

affects of mountan pine beetle on lodgepole pine in Rocky Mountain National Park, Colorado photo from Wikimedia

Actions to Maintain Carbon Sequestration

Quirion et al. (2021) outline several actions that would help protect the ability of America’s forests to sequester carbon. These suggestions address both native and introduced pests, since both contribute to the threatened reduction in capacity.

Concerning native pests, the authors call for improved forest management, but warn that measures must be tailored to species and environmental context.

Concerning introduced insects and pathogens, Quirion et al. (2021) call for strengthening international trade policies and phytosanitary standards, as well as their enforcement. The focus should be on the principal pathways: wood packaging (click on “wood packaging” category for on this blog site) and imported plants (click on “plants as vectors” category for on this blog site). Specific steps to reduce the rate of introduction of wood-boring insects include enforcement to increase compliance with the international treatment standard (ISPM#15), requiring trade partners – especially those which have repeatedly shipped infested packaging – to switch to packaging made from alternative materials. Introductions via the plant trade could be reduced by requiring foreign shippers to employ integrated management and critical control point systems (per criteria set by the U.S.) and using emergency powers (e.g., NAPPRA) to further restrict imports of the plants associated with the highest pest risk, especially plant species that are congeneric with native woody plants in North America. See Lovett et al 2016; Fading Forests II & III

As backup, since even the most stringent prevention and enforcement will not eliminate all risk, the authors urge increased funding for and research into improved inspection, early detection of new outbreaks, and strategic rapid response to newly detected incursions.

To reduce impacts of pests established on the continent – both recently and years ago – they recommend increasing and stabilizing dedicated funding for classical biocontrol, research into technologies such as sterile-insect release and gene drive, and host resistance breeding.

Thinning is useful in reducing damage by native bark beetles to conifers. However, it has not been successful in controlling introduced pests for which trees do not have an evolved resistance. Indeed, preemptive harvesting of susceptible species can harm forest ecosystems directly through impacts of the harvesting operation and indirectly as individual trees that may exhibit resistance are removed, reducing the species’ ability to develop resistance over time.

Further research is needed to clarify several more issues, including whether introduced pests’ impacts are additive to, or interact with, those of native species and/or other forest stressors.

SOURCE

Quirion BR, Domke GM, Walters BF, Lovett GM, Fargione JE, Greenwood L, Serbesoff-King K, Randall JM & Fei S (2021) P&P Disturbances Correlate With Reduced Carbon Sequestration in Forests of the Contiguous US. Front. For. Glob. Change 4:716582. [Volume 4 | Article 716582] doi: 10.3389/ffgc.2021.716582

SOURCES of additional information

Campbell, F.T. and S.E. Schlarbaum. Fading Forest reports at http://treeimprovement.utk.edu/FadingForests.htm

Lovett, G.M., M. Weiss, A.M. Liebhold, T.P. Holmes, B. Leung, K.F. Lambert, D.A. Orwig, F.T. Campbell, J. Rosenthal, D.G. McCullough, R. Wildova, M.P. Ayres, C.D. Canham, D.R. Foster, S.L. Ladeau, and T. Weldy. 2016. Nonnative forest insects and pathogens in the United States: Impacts and policy options. Ecological Applications, 26(5), 2016, pp. 1437-1455

Posted by Faith Campbell