The lead image for this Alert of an Emerald Ash Borer (EAB) (Agrilus planipennis, family Buprestidae) adult was taken on June 1, 2023, in Butler County, OH. According to our Ohio State Phenology Calendar, 50% of adult emergence occurs when the accumulated Growing Degree Days reach 1000. As of today, the accumulated GDD for Cincinnati is 998.
The picture of the EAB adult is not entirely in focus owing to how difficult it is to take a good picture of the beetles. They have excellent vision and rapidly respond to threats by buzzing off.
A little photo-rust should be understandable since the photographer has not taken a picture of an EAB adult in southwest Ohio since 2012. It was amazing back then how rapidly untreated ash trees (Fraxinus spp.) went to ashes owing to EAB.
The beetle had only been confirmed to be in Hamilton County in 2007. Of course, all the trees weren’t dead by 2012, but untreated ash trees were circling the drain and the annual ever-tightening downward spiral eventually led to few trees left to serve as beetle fodder.
EAB populations crashed. However, the beetles didn’t completely disappear nor did the trees. EAB only kills the above-ground portion of their ash hosts. This meant new growth could and did resprout from the base.
It’s much like the relationship that the Chestnut Blight fungus (Cryphonectria parasitica) has with its primary host, American chestnut (Castanea dentata). The fungus doesn’t kill the tree’s root system. The disease ran rampant from the early 1900s until no mature American chestnuts remained. However, we still find chestnut sprouts in the woods with some attaining an impressive caliper before the fungus finds and kills them. Then the cycle starts all over again.
Ash regeneration became evident in southwest Ohio as early as 2008 with regrowth from the basal portions of dead trees and stumps forming multi-stemmed “bush ash.” Of course, they were attacked as soon as the new stems reached a sufficient girth to support EAB larvae. This provided a persistent supply of beetles although the overall number of beetles continued to dwindle owing to the loss of larger-diameter trees that supported larger numbers of beetles.
Another food source for EAB arrived over time in the form of seedlings. Our native ash trees were always a significant player in forest succession with the ash trees often being the earliest colonizers of open fields particularly where soils remain wet. The early succession of ash seedlings has been observed and photographed in southwest Ohio for the past several years.
The re-emergence of ash trees that showed no signs of EAB infestation caused some to assume the beetles had disappeared altogether. Indeed, some regenerated ash grew very large and showed no evidence of dieback from EAB. Of course, that was because the beetle populations were extremely low, but they never went to zero.
However, over the past few years, EAB has experienced a reversal of fortune. This became evident last year with multi-stemmed ash in the southwest part of the state beginning to show the loss of some of the stems from EAB. The dieback has accelerated as shown in the image below.
Also, trees that were no longer being treated for various reasons, including assumptions that EAB is no longer a threat, have begun to show obvious signs of infestation. This is illustrated by the image below. Of course, trees that are no longer being protected from EAB are adding to the beetle population recovery.
One of the most dramatic EAB larval reveals was associated with a lightning strike that occurred last fall. The strike exposed EAB larval galleries and presumably fried the larvae. However, we don’t recommend this EAB treatment method.
Early on, as EAB populations increased, unusually heavy woodpecker activity was a good indicator of the presence of larvae that were serving as a built-in woodpecker buffet. Woodpeckers made small holes and their hyperactivity caused the bark to flake off giving the tree a blonde, or lighter, appearance. This is occurring again with the jagged holes becoming evident on the trunks and branches of ash trees in southwest Ohio.
Finally, EAB adults produce leaf-notching feeding damage on ash leaflets. As shown in the images below, suspected EAB leaf-feeding damage has become apparent over the past few years indicating the beetles are returning.
The population rebound was predicted long ago by many including Cliff Sadof, Professor of Entomology and Extension Fellow at Purdue Entomology. He illustrated it nicely with his “Invasion Wave Model” published in 2017 to guide management decisions.
Lessons Learned
The first lesson learned is that like Chestnut blight, EAB will be with us for years to come. Perhaps forever, or at least as long as our native ash trees continue to resprout new growth.
This means that treatments to protect ash trees from EAB must continue for the foreseeable future. While the time between treatments may be extended based on the product’s active ingredient and localized EAB populations, the current situation in southwest Ohio teaches us that eventually, we must return to the original treatment protocols.
Making treatment decisions means we also need to revisit whether to treat or replace. Healthy, mature ash trees were commonly targeted for treatment based on their significant environmental services which would take years to match with a replacement tree. The cost of removing large trees as opposed to the cost of treatments was also considered. Both remain justifiable considerations; however, tree owners should continue to run the math regarding removal particularly if significant defects as well as tree health issues have emerged.
Protective treatments are also justifiable based on sentimental value as well as other requests of the tree’s owner. Ash trees planted by a lost relative, or as a memorial tree, remain a highly valued part of landscapes. Older tree owners may want their ash trees to remain protected so the trees continue to live throughout the owner’s lifetime. It’s an understandable perspective.
However, relatively small trees were sometimes included in EAB treatment plans for no other reason than the insecticides could provide protection. This is illustrated by the image below with the small ash tree receiving annual treatments until the owners moved. Of course, treatment decisions should always be revisited periodically to determine if it makes sense (cents?) to continue treatments over the tree's lifetime, or if replacement is the best option.
Lingering Ash
For reasons not yet clearly understood, some native ash trees were able to survive the EAB onslaught. These have been called “lingering ash” and are defined as healthy ash trees greater than 4” DBH and growing in a location known to have been infested with EAB for several years and where over 95% of the ash trees have been killed by EAB.
Lingering ash trees were first noted in 2010 and a research project was initiated by scientists with the USDA Forest Service, Northern Research Station, with much of the work centered at the Forestry Sciences Laboratory in Delaware, OH. The project has also included scientists working in several universities in states where EAB has ravaged native ash trees.
An article published in 2020 in Science titled “Rising from the Ashes – An Effort to Breed North American Ash Trees That can Resist a Deadly Beetle and Could Accelerate the Return of a Forest Icon” summarizes the research efforts to that point. You can access the article by clicking on this hotlink: https://www.science.org/content/article/can-ambitious-breeding-effort-save-north-america-s-ash-trees