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Managing invasive insects and diseases is of great importance to everyone in the plant health care field, and to communities as well. While native tree species have co-evolved with native tree pests, reaching equilibrium over time, new pests and climate change have altered the norm. Native species are dying from pests that don’t belong here or whose damaging impact has been enhanced by the changing climate.

The Challenge of Invasive Tree Pests

By Rob Gorden


Managing invasive insects and diseases is of great importance to everyone in the plant health care field, and to communities as well. While native tree species have co-evolved with native tree pests, reaching equilibrium over time, new pests and climate change have altered the norm. Native species are dying from pests that don’t belong here or whose damaging impact has been enhanced by the changing climate.

The National Invasive Species Council (NISC) defines invasive species as, “An alien species whose introduction is likely to cause economic or environmental harm, or harm to human health,” adding that it is a species that is, “Non-native to the ecosystem under consideration.” The emphasis is placed on risk of environmental harm, native species dislocation, or potential harm to humans or other native species. We must be better prepared with systems and processes to address invasive species and with our response when they arrival.


Current state of affairs

Invasive species are primarily introduced through the action of humans. According to Invasive.org, in 2010 there were 456 invasive insect species in the United States. That number has grown by 27 new species in just the last four years.

Emerald ash borer (EAB), the most destructive tree pest ever brought to the United States, was identified in 2002, but arrived as early as 19981. New infestations may spread up to 14 miles annually. Gypsy Moths, which rampaged through New England in the 1980s, were controlled by fungi, wasps, and viruses. Other invaders include Rugose Spiraling Whitefly in the southeast; Hemlock Woolly Adelgid (HWA); Japanese Beetle; Pinewood Nematode; and Polyphagous Shot Hole Borer, which attacks 200 species of trees including Sycamore, Avocado and Sweet Gum in California.


Effects of climate change

Our changing climate has allowed native and non-native tree pests to impact broader regions, to attack previously unaffected plant species, and to obtain highly destructive populations. Examples include the Mountain Pine Beetle, which has killed more trees than wildfires; Spruce Beetle; Gold Spotted Oak Borer; Sudden Oak Death; and HWA. Increasing moisture in the east has amplified the severity of leaf diseases, needle casts and assorted blights. Droughts in the west weaken native trees, permitting explosive insect damage.


What is the role of science?

The scientific community studying these invasive pests shoulders the burden of identifying the pest and its life cycle, and then concluding how to prevent it from destroying native and urban forests. The challenges include research-funding limitations and delay in recognizing that a new pest has arrived. Pests have very specific annual life cycles, which must be studied. Experimentation with suitable treatment options, rates, and methods require years to reach scientifically verifiable conclusions, while the destruction continues unabated.


Do quarantines help?

The USDA – APHIS (Animal and Plant Health Inspection Service) is the first line of defense to prevent invasives from spreading in the United States. Their role is to eradicate these pests, not to manage them. If APHIS demonstrates success with containment and eradication, control remains with the agency. If the pest can’t be eliminated, then management of the pest defers to local government, even if federal regulation continues.

The Asian Longhorned Beetle (ALB) program has been one of APHIS’s biggest successes. APHIS has treated 1.2 million trees against ALB, and removed many infested trees, eradicating it in several state outbreaks.

The Emerald Ash Borer’s life cycle makes detection difficult, and APHIS could not stop its spread. Quarantines were imposed, but its ability to hide in firewood means we are still enhancing its spread. It has hitchhiked across two-thirds of the United States in just 17 years. Enormous resources are applied to set up, manage, and educate on the “quarantine” for each new pest.

Far less money is directed toward educating citizens and professionals on practical solutions to preserve urban forests. The consequence has been enormous delays in the implementation of practical and timely solutions. New pests can build population density, and their attack points coalesce. This delay amplifies costs to local municipalities and private citizens seeking to preserve urban forests.


Role of government

State and local governments are designated to address wide-reaching and devastating circumstances such as floods, storms and fires. However, naturally occurring invasive pest disasters are often beyond the scope of their expertise. When APHIS disconnected from the effort to eradicate EAB, the media read it as “control was not possible.” When APHIS indicated that EAB couldn’t be prevented from spreading, the interpretation was “nothing can be done,” and that EAB “would kill every Ash tree in the country.” This media message was wrong, and the effort to correct this message is now in its tenth year.

Many challenges arose at the city level when EAB management was redirected to municipalities. Erroneous beliefs as cited above, a lack of technical expertise, budgets that were severely under-funded, a struggling economy, and scant research to guide decision makers were just some of the issues municipal governments faced.

Today, some 12 years after EAB was identified, with volumes of conclusive data and strong field verification, newly infested communities are still developing EAB plans based on inaccurate, outdated or factually deficient data about costs, treatment efficacy, environmental impact, and public safety. Some city leaders, despite contrary information, have still elected to remove and/or replace healthy trees, when the science demonstrates that preserving trees is highly effective, far less costly, has a lower environmental impact, and preserves communities.


Challenges to the university extension system

The university extension system was developed to disseminate scientific research from land grant institutions on a variety of agricultural concerns to its citizen constituents. The frequency, intensity and enormity of the new pests we face are unprecedented, and extension may not receive, and is not designed to respond with, immediate solutions to these rapidly changing pest issues. Too often, extension agents, upon whom the public depends, receive information quite delayed from its release.

Because pest-specific research is often accomplished by a handful of university scientists, that research must disseminate from their institutions to a diverse group of stakeholders throughout the nation. The initial research can take years, papers are then peer reviewed, and universities might not require publication at all. It may also take years to share critical and newly validated research solutions about destructive pests with everyone who needs to know about them. Today, the release of information is not well coordinated, and new research can appear in a variety of trade journals.


Funding the response to invasive pests

Funding for trees, which are considered structural city assets, is often viewed by elected officials as a cost with little financial upside. The EAB expansion coincided with a severe downturn in the U.S. economy. With city leaders struggling to pay for basic community necessities, this tree-killing pest was not on their radar, nor in their budgets.

Tree planting feels good, and is the source of many charitable outreach efforts. Saving trees is seldom viewed in the same positive light, though one has to wonder why not? This is especially disconcerting when science demonstrates that preserving mature trees offers the greatest long-term benefit to communities, even when considering the replacement of one tree species with trees of another species2.

Most city budgets were not designed to recognize tree protection as a “capital expense,” yet tree purchasing/planting was considered an acceptable use of capital expense money. Until recently, preserving the trees you had standing was more financially challenging than removal and replacement, despite the overwhelming advantages to preservation.

Illinois has taken a leadership role in addressing the funding issue. Most municipal finance officials contacted by the tree industry hadn’t considered capitalization of tree treatment costs, and were unclear whether capitalization for treatment met the standard for generally accepted accounting principles.

Officials from the Illinois Municipal League responded that capitalization of tree treatment against invasive species is appropriate under present municipal financial guidelines and standards. A leading bonding agent who represents municipalities throughout Illinois confirmed the League’s opinion, directing that tree treatment could be treated as a capital expense.

Subsequently, several municipalities classified invasive pest treatment costs as a capital expense. Doing so provided significant flexibility in responding to the extraordinary and massively expensive municipal problem of EAB. There are now no legal or financial barriers to capitalizing tree treatment against an invasive species, and the path to bonding for treatment is unobstructed according to expert opinion in Illinois. This may be a favorable and fiscally sound direction for other states as well.


Managing the message

Adding to the political complexity of researching invasive pests is the need to partner with for-profit companies who support research with new chemistries and graduate assistantships. In an effort to stay objective, university researchers often present solutions as alternatives, instead of ranking them by their performance against a given pest. This leads to mixed signals reaching the professional community, and lack of clear direction on what decisions to make, even when most needed.

The solution requires better linkage between similar departments in all universities with land grant responsibility, horticultural or forestry programs. Some stewardship at the federal level might assure that local decisions consider the environment, livability and the impact on future generations — rather than just this year’s local budget.

We must develop a means by which productive solution-oriented research is reviewed by a national body, and guidance must be measured to consider public safety, effectiveness, environmental impacts, and both immediate and peripheral costs. These conclusions must be shared with all states, all agencies and at all levels (including the public) to assure availability of accurate and current knowledge. We must assure that small cities do not manage this burden alone with limited budgets, limited resources and knowledge. We might consider cost sharing, natural disaster relief and the development of bonding sources, as previously discussed.

Despite messages from tree industry groups, statements supporting the preservation of urban forests, and professional polls that have demonstrated citizen support for tree preservation, this message has too often fallen on deaf political ears.


Prospects for the future

During the last four years, 27 new invasive insect pests have arrived, and in the decades ahead, more will follow. It is also probable that one of the 483 invasive insects already here will pose a new risk when environmental conditions such as heat or drought weaken our native species.

The study of these pests to find solutions that can minimize their impact on our native and urban forests has been entrusted to a few universities and industry leaders. University extension creates a vital link engaging in research and in its dissemination. At this time, this research is not made available readily, universally or rapidly. The plant health care industry must also work in tandem with both federal and state agencies to quickly identify new threats, reach and share effective solutions, and then work to see them implemented, or we are destined to repeat the mistakes we have made.

Tree removal can target a single-host pest or a small group of trees within a confined or confinable geographic region. However, it is unlikely that we can cut our way to solutions that offer effective results now and preserve the promise for the future. Given the right invasive species, lack of suitable pest predators, and challenging environmental conditions, we can expect one tree species after another to come under attack. We risk destruction of our tremendously important forests — treasured by tree professionals, critical to environmental health, and taken for granted by our citizenry.

The decision to remove or protect infested or at-risk trees is a hotly debated issue, and the information used to assess whether they can or should be saved is either poorly understood or missing altogether. Tree removal strategies do not address the underlying causes of these outbreaks, and represent an unsustainable arboricultural practice. Several tree canopy assessment tools exist that consider the value of tree canopy; including heating, cooling, stormwater management, property value, and pollutant reduction, as well as the cost to treat or remove. We must use these resources in weighing our decisions and in the advice we provide to city leaders. With some new treatment products lasting two years, it has become significantly less expensive to protect the urban forest than to incur the immediate cost of removal and replacement (and the long-term changes that such removal brings to our communities).


We must continue to educate the public and legislators, neither of whom can be expected to understand the details of the challenge without our support. Of paramount importance is implementing changes to fiscal policy, which will help communities address the costs of invasive pests through contingency funds, bonding, public education and outreach. To assure a successful response to invasive tree pests and diseases, we must accelerate efforts to break down silos in the arboricultural community, and we must demand better and more widely disseminated education, and a sharing of up-to-date, peer-reviewed research. We should neither fear the science nor its conclusions, regardless of the message, as personal opinion should have little place in the preservation of our shared urban forests.


Rob Gorden is the director of urban forestry for Arborjet. His Forestry education and 32 years in both tree care and horticulture have prepared him to provide practical support to municipalities and political leaders across the country who are evaluating treatment options for invasive pests. He is a frequent speaker to city arborists, public works directors, private arborists, extension agents, plant health care applicators, and master gardeners.



1  Siegert, Nathan W., D.G. McCullough, & F.W. Telewski. 2014. “Dendrochronological reconstruction of the epicentre and early spread of emerald ash borer in North America.” Diversity and Distributions, (Diversity Distrib.) (2014) 1–12


2  VanNatta, A.R., N.M. Schuettpelz, and R.H. Hauer. 2010. “Cost Analysis of Removal and Replacement vs. Treatment of Ash Trees Susceptible to Emerald Ash Borer (Agrilus planipennis) on the UW-Stevens Point Campus.” Proceedings of the International Society of Arboriculture 86th Annual Conference. Chicago, IL, July 23 – 28, 2010. College of Natural Resources, University of Wisconsin–Stevens.

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