Emerald Ash Borer Two Decades Later

Fun fact: What happened on July 9, 2002? If you guessed a birthday for Rich Hauer, Tom Hanks or the late Nikola Tesla, congratulations. You win and score a point. If not, no worries. Perhaps you said it was the anniversary of Johnny Cash’s signing with Columbia Records in 1958, or the last Grateful Dead concert in 1995? If so, score another point for each correct response. However, if you guessed this was the date that emerald ash borer (EAB) was first positively identified in the Michigan region of the United States, you are the winner.

While July 9, 2002, serves as the official date for emerald ash borer’s discovery in North America, it was likely introduced onto the continent in the mid-1990s (Herms and McCullough, 2014). Since its discovery, the composition of many urban forests across the U.S. and Canada has changed – often dramatically. Similarly, our understanding of this noxious pest and its management has evolved over the past two decades (Sadof et al., 2023). While some lessons learned are tied to the biology of EAB and ash-tree species, lessons learned can be applied to other infestations or outbreaks that are sure to arise in the future.

Emerald ash borer damage
Photo 1: An end result of emerald ash borer when left to the insects’ timeframe. Photo courtesy of Richard Hauer.

A sustainable EAB management approach

You may have experienced how EAB moves through a region. If not, know that within 10 to 15 years of initial discovery, most native ash trees will be dead if left unprotected. (Photo 1) Before this, a noticeable decline in a population is observed about five years after EAB is first discovered in a new locale. The epidemiology of the insect includes a building phase (approximately years one to five), an exponential increase in tree mortality (approximately years five to seven) and then a declining rate of loss (approximately years seven to 12), because fewer ash trees remain (Herms and McCullough, 2014).

When a noxious and fast-moving threat like EAB emerges, you can let it control how you manage trees or you can take steps to incorporate strategic control into your management efforts (Hauer et al., 2020; Hauer and Peterson, 2017). With the latter approach, the best time to plan is before EAB has been discovered in your community, or at least once the presence of the insect has first been confirmed.

What management approach you choose is up to you and the community you work with. This can range from a do-nothing (beyond removal) approach to an active approach that may include some combination of preventative treatment, staged removal and strategic replanting. To guide decision-making, a sustainability model that integrates economic, societal and ecological principles is a useful starting point.

Economic considerations

Many communities past and present face a financial decision when EAB comes to town. When the city of Milwaukee was confronted with the impending EAB epidemic in the late 1990s, they quickly realized they would face a heavy financial burden with whatever option they chose. As pathologist Mark Stennes would often say when discussing the management of pests, “Whether you like it or not, it will cost you money.” The city decided to take an active approach and treat their 32,000-plus ash street trees at that time.

The annual cost of treatment was $1 million – a fraction of the near $30 million they estimated they would need to spend to remove ash and replant. Given these two potential management costs, and then factoring in the loss of ecosystem services tied to the mature ash populations they stood to lose, the city realized that treatment, along with staggered removal and replacement of low-value ash trees, was a much better choice both financially and for the wellbeing of Milwaukee’s residents (Vannatta et al., 2012).

Societal considerations

“Should we apply pesticides or not?” is an important question to ask with any pest-management approach. Each manager of a tree population has to answer this and should do so after engaging their clients – whether they are individuals or groups of individuals, such as a homeowners’ association. In the case of a municipal forestry program, the clients are the residents of the community.
Acceptance of chemical controls will vary by client, and over the past several decades, some communities have decided that a non-pesticide approach is the one they are most comfortable pursuing. There is no one way that is better than the other. However, a pesticide approach is really the only option currently available to protect ash trees.

Social science says that given a choice, people tend to favor nonchemical approaches such as sanitation first (Arnberger et al., 2017). This works well for some pest problems, such as Dutch elm disease (DED). However, we currently have no effective sanitation approach with EAB. A chemical treatment option to preserve ash is rated higher than doing nothing (Schlueter and Schneider, 2016), as people like trees and prefer living trees to dead ones.

Ecological effects

The loss of ash-tree canopy, whether by preemptive removal of ash trees or through EAB-induced tree mortality, has important ecological ramifications. Existing decision-making models such as i-Tree can be used to estimate potential losses in ecological benefits such as carbon sequestration, particulate filtration and stormwater abatement. When the city of Milwaukee was weighing its options for managing its urban forest infested by EAB, staff used historic aerial imagery capturing the loss of American elm and found it took 40 years to recover and regrow the loss of elm-tree canopy (Hauer et al., 2020).

When the change in tree cover was run through the i-Tree Eco model, the city found that the loss of stormwater-mitigation services alone doubled the costs associated with DED sanitation. As such, the hydrology benefit associated with retaining mature ash trees was more than sufficient to cover the costs associated with chemical treatment. This was before considering the monetary benefit associated with air-pollution filtration, energy conservation, carbon sequestration and aesthetics.

Long-term tree injection
Photos 3a, b and c: Long-term tree injection resulted in a low level of internal wood discoloration in green ash trees injected five or six times. Most trees appeared as the one on the left, rather than the one on the right. And for all trees, on average less than 10% of the wood area was discolored as a result of injection with emamectin benzoate. Photos courtesy of Abigail Tumino.

Tree injection and EAB management

The injection of ash trees is an EAB management approach that allows communities to maintain mature ash trees even in the face of an infestation. It took less than a decade of testing treatment approaches to find viable methods that work (Herms and McCullough, 2014). As an example, several studies found that emamectin benzoate showed a two-year efficacy, which is on the current label rates. More recent findings from Indiana and Wisconsin have shown a three-year treatment cycle had the same efficacy as a two-year cycle (Sadoff et al., 2023). A bit of caution, though; in the third year of treatment, it is advised to re-treat ash trees as soon as possible at the start of the growing season when starting the next round of injection.

It is well understood that untreated ash trees that are susceptible to EAB have a near 100% mortality rate. An occasional ash tree does survive, but this is uncommon. We also know that ash trees can be treated multiple times with few outward indications that injury has occurred. However, until recently we really didn’t know what level of internal tissue damage occurred as a result of multiple tree injections.

bark cracking and separation
Photo 2: Following manufacturers’ recommendations is excellent advice to heed. Without doing so, bark cracking and separation may occur with tree injection. Photo courtesy of Richard Hauer.

Early tree-dissection studies found little internal damage as measured by wood discoloration and external damage, such as bark cracking after one or two injection treatments (Herms and McCullough, 2014). A study of a longer treatment cycle found external damage was higher on trees treated with a closed system, with a plug, rather than an open system, when the injection site had nothing left in the drill hole after treatment (Hauer et al., 2022). An interesting finding, however, was that failure to follow the manufacturer’s recommendations explained approximately 75% of the external wounds that were observed. (Photo 2)

More recently, a study conducted at The Morton Arboretum in Lisle, Illinois, found little internal discoloration and damage for 40 trees that were injected five to six times over the past 10 to 12 years. (Photos 3a, b and c) Since little internal damage was found, a catch phrase developed, as in, “See ya in 10 years,” when another follow-up tree dissection and measurement of damage should occur. In the meantime, we found little cause for concern with current treatment approaches, especially if you abide by what a manufacturer states with its treatment protocol.


Let the insect control you, or control the insect! The lessons learned with EAB can be applied to other pests. Understand the biology of the pest, experiment with management approaches and apply sustainability principles to guide your actions. Over the past 20 years, much has been learned about EAB management. Yes, new and exciting discoveries are likely yet to occur. However, with what is currently known, adopting an active management approach is the key to best management of the pest.
If you are new to this and EAB has just been found in your area, don’t panic, as you do have a year or so to make the decision that best fits your situation. If EAB is not in your area yet, develop your plan in anticipation of the pest and, once it arrives within about 15 miles, implement your preferred measures. If EAB has come and gone, then you have lived the story and know well which approaches worked and others you wish had been applied.

Arnberger, A., Schneider, I. E., Ebenberger, M., Eder, R., Venette, R. C., Snyder, S. A., … & Cottrell, S. (2017). Emerald ash borer impacts on visual preferences for urban forest recreation settings. Urban Forestry & Urban Greening, 27, 235–245.
Hauer, R. J., Ball, J. J., & North, E. (2022). Observation of External Wounding on Green Ash (Fraxinus pennsylvanica ‘Marshall’) Trees Associated with Tree Injection Systems. Forests, 13(11), 1802.
Hauer, R. J., Hanou, I. S., & Sivyer, D. (2020). Planning for active management of future invasive pests affecting urban forests: The ecological and economic effects of varying Dutch elm disease management practices for street trees in Milwaukee, WI, USA. Urban Ecosystems, 23(5), 1005-1022.
Hauer, R. J., & Peterson, W. D. (2017). Effects of emerald ash borer on municipal forestry budgets. Landscape and Urban Planning, 157, 98-105.
Herms, D. A., & McCullough, D. G. (2014). Emerald Ash Borer Invasion of North America: History, Biology, Ecology, Impacts, and Management. Annual Review of Entomology, 59(1): 13–30.
Sadof, C. S., McCullough, D. G., & Ginzel, M. D. (2023). Urban ash management and emerald ash borer (Coleoptera: Buprestidae): facts, myths, and an operational synthesis, Journal of Integrated Pest Management, Volume 14(1): 1–13, https://doi.org/10.1093/jipm/pmad012
Schlueter, A. C, & Schneider, I. E. (2016). Visitor Acceptance of and Confidence in Emerald Ash Borer Management Approaches. Forest Science, 62(3): 316–322, https://doi.org/10.5849/forsci.14-229
Vannatta, A. R., Hauer, R. H., & Schuettpelz, N. M. (2012). Economic analysis of emerald ash borer (Coleoptera: Buprestidae) management options. Journal of Economic Entomology, 105(1), 196-206.
Richard Hauer, Ph.D., is director of urban forestry, CN Utility Consulting, a 15-year TCIA member company based in Des Moines, Iowa.
Jake Miesbauer is arboriculture scientist at the Morton Arboretum in Lisle, Illinois.
Andrew Koeser is associate professor of environmental horticulture at the Gulf Coast Research and Education Center, University of Florida, Wimauma.
Blake Thilmony is a research scientist with Rainbow Ecoscience, a 25-year TCIA member company based in Minnetonka, Minnesota.
Abigail Tumino is a research assistant at the Morton Arboretum.
This article is a follow-up to Richard Hauer’s presentation on the same topic during TCI EXPO ’22 in Charlotte, North Carolina. To listen to an audio recording created for that presentation, go to TCI Magazine online at tcimag.tcia.org and, under the Resources tab, click Audio. Or, under the Current Issue tab, click View Digimag, then go to this page and click here.

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