Emerald Ash Borer again? Maybe always

The Emerald Ash Borer beetle (Agrilus planipennis) was confirmed in Lucas County in 2003 near Whitehouse.

Wood County had first sighting in 2004 near Perrysburg.

Not all the Ash (Fraxinus spp.) trees were dead by 2010. Untreated ash trees were still sub coming to the fate of the borer and eventually dying. When the various grants ran out for tree removal, and stump grinds, many Ash trees were dead fall trees and fell over during wind events.

Some of the ash trees were cut down and did not have the stumps ground or treated leading to bush type ash trees with many ground sprouts left to serve as beetle fodder. Of course, the trees 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.

With the dwindling untreated ash clumps the EAB populations crashed. However, the beetles did not completely disappear nor did the trees. EAB only kills the above-ground part of their ash hosts. This meant new growth could and did resprout from the base keeping a small reservoir of beetles around.

It is much like the relationship that the Chestnut Blight fungus (Cryphonectria parasitica) has with its primary host, the American chestnut (Castanea dentata). The fungus does not 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 reaching an impressive size before the fungus finds and kills them.

Then the cycle starts all over again.

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 stay wet.

The re-emergence of ash trees that showed no signs of EAB infestation caused some to assume the beetles had disappeared altogether. Some regenerated ash grew 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 recovery. This became clear last year with multi-stemmed ash in parts of the state beginning to show the loss of some of the stems from EAB. Also, trees that were no longer being treated for assorted reasons, including assumptions that EAB is no longer a threat, have begun to show obvious signs of infestation. Of course, trees that are no longer being treated from EAB are adding to the beetle population recovery.

An indicator of increased EAB activity is the unusually heavy woodpecker activity feeding on the larvae 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 clear on the trunks and branches of ash trees in Ohio. This 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.

The first lesson learned is that like Chestnut blight, EAB will be with us for years to come. Perhaps forever, or at least if our native ash trees continue to resprout new growth. This means that treatments to protect ash trees from EAB must continue for the near future.

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 were also considered. Also, considerations need to be made based on sentimental values. All are still justifiable considerations; however, tree owners should continue to run the math regarding removal particularly if significant defects as well as tree health issues have appeared.

The Ash tree is a native of eastern North America. It is found natively in our woodlots, stream banks, and urban areas. As a native tree should we not strive to save these trees?