Recently, I was at an event with several young agriculture professionals when I realized some may have never seen Emerald Ash Borer (EAB) before. While there is an awareness of Ash dying in forests and landscapes, many newer professionals are coming into the industry post Ash-Apocalypse and may not be entirely familiar with the ins and outs of this menace. While it may be “old news” to some, it is important to keep informed about our invasive pests because, even though the initial infestation wave has long passed, it is still present in our forests and landscapes infesting new ash seedlings and resprouting ash. There are still lingering ash at risk, and any trees that were previously protected by insecticides could be at risk if treatment is discontinued.
EAB refresher
Emerald Ash Borer (Agrilus planipennis Fairmaire) is an introduced invasive species that arrived in Detroit, Michigan area in the late 1990’s and was first detected in 2002. Since then, this little beetle (1/2 inch long and 1/8 inch wide) has been responsible for decimating hundreds of millions of native ash trees in North America. The larva of this beetle bore into the cambium, xylem and phloem layers of trees to feed. As it chews it creates circuitous larval galleries filled with frass (sawdust and bug waste). These galleries destroy the tree’s vasculature and eventually girdle the tree, severing the flow of nutrients up and down the trunk. This leads to canopy dieback that starts at the top of the tree followed by suckers sprouting from the trunk base. As feeding progresses bark will start to pop off the tree trunk where you will see winding galleries and tunnels visible underneath. Another sign of EAB infestation is the classic “D”-shaped exit holes where adult beetles have emerged from the tree. All this eventually leads to the death of the tree. Other clues and signs include woodpecker damage on Ash trees as they hunt for the juicy larva under the bark!
The adult beetle is a metallic iridescent green and is about ½ inch long. Adults emerge in early summer from infested trees leaving behind a “D”-shaped exit hole. They mate and lay eggs on all native ash species. Eggs hatch and larva bore into the bark and feed all summer through fall. They overwinter within trees as larva. In spring they pupate and emerge again in summer.
EAB has spread across much of the U.S. and it is still expanding and finding new populations of Ash to feed upon. Here in Ohio, we were very close to the epicenter. But as you can see by the most recent USDA APHIS map below, it is still spreading and is relatively new in some states, including Oregon where the Oregon ash (Fraxinus latifolia) is at particular risk. Blue ash (Fraxinus quadrangulata) is considered more tolerant to EAB feeding (Cippolini & Morton, 2023), but is still susceptible. White ash (Fraxinus americana) is thought to have some slight resistance found in some lingering specimens but still sees 80-90% mortality (Steiner et. al. 2019), and green (Fraxinus pennsylvanica) and black ash (Fraxinus nigra) are the most susceptible. Genetic variability does exist in these populations and some "lingering ash" have been identified and are being propagated and studied. Among these, studies have documented reduced larval feeding, reduced adult preference for leaves, and significant differences in larval weight gain on selections of lingering ash suggesting there are some variables at play that allow trees to, if not fully survive, at least survive longer than more susceptible counterparts (Koch et al 2015). You may find some still living lingering ash in Ohio. And some landowners may be treating their Ash trees on their property to protect them long term. In addition to Ash trees, EAB can develop in white fringetree (Chionanthus virginicus) and cultivated olives (Olea europaea) which may surprise you. White fringetree is more likely to survive EAB than ash. EAB grows slowly on fringetree. It is not as preferred as ash, but can suffice. Research suggests that ornamentally planted white fringetree in low densities will not be a significant reservoir for EAB. Research is still ongoing for interactions of EAB with wild populations of fringetree (Roche 2020). So far it appears young olive trees are resistant but older stressed olive trees may support larval development (Cippollini 2025) which may pose a problem now that EAB has reached the west coast. All of these hosts are members of the olive family, Oleaceae.
Treatment
Treatment is available to protect Ash trees IF they are not too far gone. Once the tree is about 30% defoliated, the effectiveness of these treatments goes down. These trees may be too far gone to rescue. Systemic pesticide treatments are available, such as those containing the active ingredients imidocloprid, dinotefluran, or emamectin benzoate that protect the vasculature from the feeding of the larva. However, they must be used early in the infestation and repeated yearly to every 2 or 3 years, depending on the product and tree size, to maintain adequate protection. Depending on size, product may be applied as a soil drench to the root zone to be taken up by the roots. Another application option is direct injection, but this must be performed by an arborist. This is necessary for larger diameter trees. These methods rely on the trunk's ability to transport liquids up and down the trunk to allow the product to penetrate the whole tree and canopy. If the tree's vasculature is compromised by excessive feeding of EAB or other girdling insect, that transport cannot happen and treatment will be ineffective. Follow all label instructions when using insecticides for any purpose as the label is the law. Pay attention to the tree diameter limitations on product labels. After a certain size, Ash trees are best protected by a professional who can inject the tree with appropriate products that may last longer than over the counter options.
There are also non-native parasitoid wasps that have been researched extensively and released in the U.S. to attempt to moderate EAB populations. These four wasps (Oobius agrili, Tetrasticus planipennisi, Spathius agrili, and Spathius galinae) were brought from EAB’s native range and exclusively target EAB. One is an egg parasitoid (O. agrili) and the other three are larval parasitoids. While they will never eliminate EAB, they can be a component of reducing severity and abundance of EAB in the wild in hopes of preserving some Ash in the environment.
In addition to the research on biological control organisms, research being conducted by USDA-Forest Service, universities and other institutions are continuing to study, select, and breed ash that may be resistant to this pest.
References
Cipollini, D. (2025). The Potential Risk Posed by Emerald Ash Borer to Cultivated and Wild Olive Trees. Forests, 16(2), 357. https://doi.org/10.3390/f16020357
Cipollini, D. & Morton, E. (2023) The persistence of blue ash in the aftermath of emerald ash borer may be due to adult oviposition preferences and reduced larval performance. Agricultural and Forest Entomology, 25(4), 584–589. Available from: https://doi.org/10.1111/afe.12582
Koch JL, Carey DW, Mason ME, Poland TM, Knight KS (2015) Intraspecific variation in Fraxinus pennsylvanica responses to emerald ash borer (Agrilus planipennis). New For 46:995–1011
Steiner, Kim C.; Graboski, Lake E.; Knight, Kathleen S.; Koch, Jennifer L.; Mason, Mary E. 2019. Genetic, spatial, and temporal aspects of decline and mortality in a Fraxinus provenance test following invasion by the emerald ash borer. Biological Invasions. 21(11): 3439-3450. https://doi.org/10.1007/s10530-019-02059-w.
