By Brandon M. Gallagher Watson
Emerald ash borer draws frequent comparisons to Dutch elm disease. In many respects, they are similar. Both are fatal conditions, both are caused by an exotic agent, and both affect a prominent species in the urban forest. When Dutch elm disease was first identified in the United States, it was 1928. Ten years after its discovery, the science and the management plans were still very rudimentary. It wouldn’t be until the 1980s, more than 50 years after the first elm was found dead — when a predictable management protocol was finally developed.
We are just past the 10-year mark since the discovery of emerald ash borer (EAB), so where are we at? What has changed in the past 10 years in terms of our understanding the insect, the management tools arborists have available to them, and the public perception of this problem?
When Dutch elm disease reached North America, the fungus had been killing elms in Europe for more than 20 years. There was some knowledge at this point that the trees were dying from a fungus — not a reaction to wartime chemicals, as originally thought. There was also some knowledge that bark beetles were vectoring the disease, but, beyond that, there was still plenty unknown. For nearly 40 years, management efforts would focus on trying to stop the beetles. This plan did little to slow the nationwide spread of the disease, and spraying DDT from helicopters likely contributed to the misuse of insecticides in the mid-20th Century. In fairness to those attempting to manage Dutch elm disease at that time, what else could they do? Vascular wilt diseases were poorly understood, and fungicides at the time were often metal-based and formulated for foliar spraying only. Even if injectable fungicides were available, there was no application technology to predictably inject the treatments into the trees. It would be decades and millions of dead trees before technology caught up with the problem.
Fast forward to 2002. Ash trees in southeast Michigan were dying; but, unlike elms in 1920s Europe, it was pretty apparent what was killing these ash trees. One peek under the bark, and researchers could see the culprit was an insect. A little comparative anatomy, and they realized this insect — although not from around here — was related to pests we were familiar with managing. The pest was identified as Agrilus plannipenis, a flatheaded beetle known as emerald ash borer. Cousins of this insect, the bronze birch borer (Agrilus anxius) and two-lined chestnut borer (A. bilineatus) were well known to arborists as pests of birch and oak trees. Unlike these native pests, which tend to be secondary pests that attack only stress-weakened trees, EAB was taking out any ash tree in sight, regardless of health. By the time it was discovered, EAB had been in Michigan for several years and thousands of trees were standing dead or dying.
Responding fairly quickly, USDA-APHIS implemented quarantines on the movement of ash trees and raw ash wood. At this time, they also began partnering with universities such as Michigan State and Ohio State to develop a management protocol and evaluate possible treatment options. Research would also focus on the biology of the insect, understanding the damage potential of this pest, and going over to Asia to understand EAB in its native ecosystem. Within just a few years of discovery, EAB was one of the better-understood invasive pests, certainly better understood than Dutch elm disease was shortly after its discovery. According to Dr. Dan Herms, one of the first scientists contacted to research emerald ash borer, an accelerated scientific understanding of EAB was possible due to greater background knowledge of similar pests, a greater number of entomologists available, a greater understanding of management tools, as well as a significantly higher scientific capacity than we had in the 1920s.
After the quarantines were established, they were almost immediately violated, and infested nursery stock was shipped from Michigan to Maryland and Virginia. Although the beetles are capable of flying on their own to a new location, humans have certainly done their part to help this insect spread. Firewood, nursery stock, and timber harvesting have all contributed to EAB moving from town to town at 65 mph. It would take Dutch elm disease 50 years to travel about 800 miles from the first find. Emerald ash borer moved that far — in every direction — in under 7 years.
Management tools have changed significantly during the past decade through the efforts of industry and university scientists. At the time of discovery, the management tools of related flathead borers consisted primarily of soil-applied imidacloprid and trunk sprays of pyrethroids; but emerald ash borer would prove more challenging than the current tools could handle. Initial recommendations from state authorities were that treatments were ineffective and costly, leaving homeowners and urban foresters with a sense of helplessness. New management tools, such as the introduction of emamectin benzoate (TREE-age), would be a ray of hope that trees could be protected from EAB, and, in some cases, even saved if already infested. Field experience was showing soil applications of imidacloprid were effective on smaller trees, but failing on larger trees. New research would lead the EPA to approve new application rates to protect larger trees with imidacloprid (Xytect). Another tool, dinotefuran (Safari, Transtect), would become available as a faster-acting soil-applied treatment, and also open up systemic bark-spray applications as an option for arborists.
With a multitude of active ingredients, trade names, application methods, and marketing claims, both homeowners and arborists alike were often left wondering what were the best options available to them. To help alleviate some of the confusion, the universities of Ohio State, Michigan State, Illinois, Wisconsin-Madison, and Purdue produced a document establishing the treatment options best supported by the research data. This helped further the understanding of what treatments were available and what to consider when settling on one. Around the same time, research began in Canada on the use of azadirachtin (TeeAzin), the first organic, OMRI-listed treatment option for EAB. There are now well-established protocols for managing EAB on trees of all sizes and in all growing situations.
It has been said that emerald ash borer is one of the easiest pests for arborists to manage. So, with a full suite of options for treating and protecting trees from EAB, why are ash trees still dying by the tens of millions? One reason, said Dr. Dan Herms, is although we have a toolbox of treatment options, every one of them aims to protect an individual tree, not the population of ash trees. We do not have tools available that are sprayed from helicopters to protect an entire neighborhood; each tree must be protected one at a time. We also have a limited set of tools to help reduce the population of the beetles themselves. Natural predators of EAB, such the parasitoid wasps that are beginning to be released, may perhaps have an effect on stabilizing the population of emerald ash borers, but long-term benefits of these efforts have yet to be realized.
Public perception of EAB, along with the actions and reactions of municipalities, has contributed to the decline of ash populations as well. When EAB first arrived, the initial agency and municipal leader response was “nothing could be done, cut all the ash trees down.” Homeowners, and arborists alike, were getting mixed messages on what could and should be done. In an attempt to combat this misinformation, a group of scientists, industry leaders and arborists joined in 2011 to form the Coalition for Urban Ash Tree Conservation. This consensus group’s goal was to inform government leaders that cost-effective, environmentally sound EAB treatment protocols are available to preserve ash trees through peak EAB outbreaks.
A new understanding about how EAB-affected ash populations at the city-wide scale began to emerge around this time as well. First thought to be akin to a wildfire, where one spark would erupt in an uncontrollable inferno, it was becoming apparent that EAB followed a different pattern. Following the initial discovery, EAB would continue to kill ash trees at a steady rate for a few years. The tree death would then begin accelerating at an exponential rate, where 80 percent of the remaining ash population would be exterminated in a matter of a few years. Dubbed the “exponential ash tree death curve,” this model has greatly altered how municipalities manage EAB.
The “exponential ash tree death curve” model has greatly altered how municipalities manage EAB. Photo courtesy of Rainbow Treecare Scientific AdvancementsNo longer are the options “cut them all down” or “save them all.” The death curve model has shown municipal leaders that an effective plan incorporates removal of low-value ash trees and preservation of high-value ash trees for continued community benefits. New EAB cost calculators from Purdue University and University of Wisconsin-Stevens Point take into account these new, integrated approaches to municipal EAB management. Trees can be preserved for far less cost to the city than removal and replanting, while maintaining the benefits of a mature forest canopy.
So, how has emerald ash borer understanding and management changed in the past ten years? Although we are clearly decades ahead of Dutch elm disease research in terms of understanding the biology, the effects on municipal budgets, and a full toolbox of options to combat the problem, where has that gotten us? New infestations still pop up all the time, trees are still dying every year in unimaginable numbers, the public perception is still somewhere between misinformation and complacency, and, all the while, municipal leaders are reluctant to act. So, are we better off now than 10 years ago? In this arborist’s opinion, certainly. While we, no doubt, have a long way to go for the public and the leaders to comprehend the issue like tree care professionals do, we are making a difference. More mature ash trees are still thriving in EAB-infested areas than would be without the actions of arborists. And although not everyone listens, municipal leaders are beginning to understand the community benefits these mature trees provide.
Emerald ash borer — while garnishing comparisons to other prominent urban forestry epidemics — is truly an unprecedented event. Stories in 2004 predicted the extinction of every North American ash species, as did 1960s articles on American elms. Only time will tell if either prediction was prophetic, or sensationalistic. In the meantime, all we can do is continue to educate the public on the options and save one tree at a time.
Brandon M. Gallagher Watson is director of communications at Rainbow Treecare Scientific Advancements, and is an ISA Certified Arborist (#MN-4086A).
Sometime in the mid-to-late 1990s
Emerald ash borer arrives in Michigan on shipping crates of automotive parts from China.
EAB first detected in Canton, Mich.
USDA-APHIS, USDA-FS begin to fund research with Universities.
Federal quarantine established.
EAB found in Maryland and Virginia, both from imported nursery stock.
Found in Ohio.
Found in Indiana.
Found in Michigan’s Upper Peninsula.
Emamectin benzoate (TREE-AGE) becomes available with 24C Label. Research shows 2 years of control and promise for saving infested trees.
Rate trials begin for treatments on larger trees.
Found in Illinois.
Chicago is first major city to implement a citywide ash program that incorporates treatments and removals.
Found in Pittsburgh
Five universities produce the first consensus document on the treatment options for EAB.
Research trials on treatment options, rates, and application timing begin in Hazel Crest, Ill.
Found in Missouri.
Canadian Forest Service begins research on azadirachtin, as an OMRI-listed, organic treatment for EAB.
Found in Minnesota.
Found in Kentucky.
Found in New York.
EPA approves new Xytect (imidacloprid) rates to prevent EAB on larger-diameter ash trees.
Found in Iowa.
Bark-spray-applied dinotefuran (Safari, Transtect) becomes available, offering an alternative application to injection or soil treatments.
Two-thirds of Pennsylvania believed to be infested.
Ohio abandons in-state quarantine.
Research shows ash populations die at an exponential rate and certain cities have experiences losing the majority of their ash trees in a few years, changing the way many cities manage their ash trees.
Stingless wasps, a natural parasitoid of EAB, released in the Twin Cities.
A consensus group of scientists, industry leaders, and arborists form the Coalition for Urban Ash Tree Conservation, to inform government leaders that cost-effective, environmentally sound EAB treatment protocols are available to preserve ash trees through peak EAB outbreaks.
Found in Montreal.
Found in Massachusetts.
Found in Connecticut.
Estimates for the number of dead trees reach the hundred million mark across the 18 infested states and Canada.
Azadirachtin (TreeAzin) available in the US for first time.
Found in ???