By John Kmitta
Photo courtesy of Dave Roberts, Ph.D.We have covered the Emerald Ash Borer (EAB) in depth in recent years, documenting the unrelenting spread of this devastating pest. But according to Dr. Dave Roberts, plant pathologist and academic extension specialist at Michigan State University, it is amazing how many communities are still adopting a “cut and chip” approach, because they did not know that there were any treatment options available.
In Roberts’ leaflet titled, “The Plant Doctor’s Landscape Tips,” he points out that options for treating ash trees can involve spraying, soil/trunk treatment, or trunk injection. Through research and field observations, virtually all of these methods have successfully protected ash trees from EAB destruction…and all have failed. Failure of treatments is usually associated with misapplication of chemicals and/or treatment after the EAB has infested specific trees, he stated.
“Managing EAB is about good science and good chemistries,” said Joe Doccola, director of research and development at Arborjet, Woburn, Mass. “There are exciting things coming down the road. We need to continue to work with researchers and develop sound chemistries.”
Dr. Dan Herms, professor of entomology at Ohio State University, has been conducting research of EAB in three areas: resistant species of ash, ecological effects of the infestation, and control with new and existing insecticides.
In terms of pest-resistant species of ash, Herms points out that Manchurian ash are the most resistant, and the trees are being researched to determine the mechanisms of resistance, the natural chemistry that leads to resistance, as well as the genetic markers.
According to Herms, Jennifer Koch at the USDA Forest Service in Delaware, Ohio, is working on hybridizing North American ash with Asian species of ash. She is examining back-crossing hybridization that would be 95 percent North American ash crossed with 5 percent Asian ash utilizing the resistance mechanisms and genetic markers of Asian species of ash.
“We have identified resistant germ plasm, and identification of the genetic markers should not be too difficult,” said Herms. “So the biggest challenge might be the hybridization.”
According to Herms, resistance breeding is a long-term solution and will require ongoing research. Although it would introduce new varieties of ash that would be resistant to the pest, it would obviously not be an answer for existing ash trees that are impacted by the pest.
“In terms of systemic insecticides, we are examining soil applications and trunk injection methods to better understand why they work and how best to use these insecticides,” said Herms. “Emamectin benzoate has shown two years of control per application. Imidacloprid is also effective, but should be applied every year.”
According to Herms, soil treatments with imidacloprid are less effective as the tree gets larger. Control becomes less dependable as the diameter increases, because the insecticide is essentially diluted as the size of the tree increases. Trees greater than 15-inch dbh require two applications of soil injections, either in the fall and spring or twice in spring (four to six weeks before emergence and then right at adult emergence). Herms also pointed out that researchers have found that adult EAB emergence corresponds with the timing black locust trees bloom, and therefore the blooming of the black locust provides a good time reference for the application of insecticides.
According to Herms, for soil treatments the base of the trunk is the best location (rather than injecting on a grid out to the drip line). The base of the trunk has a high density of roots, and therefore the tree uptakes a higher volume of the insecticide.
According to Shawn Bernick, director of research and tech support at Rainbow Treecare Scientific Advancements, Minnetonka, Minn., the approach to EAB is constantly being refined and modified, and a big part of that is examining the effectiveness of various application methods.
“We are learning on the fly how to manage EAB,” said Bernick. “Arborists have been managing bronze birch borer and two-lined chestnut borer successfully for years, so we think it’s a matter of getting them comfortable with managing EAB. What we can do is slow the spread of EAB, and preserve and protect ash trees with chemical controls.”
According to Doccola, arborists need to realize that trees are at risk, and they can do things in a proactive manner.
Herms doesn’t recommend treatment unless EAB has been identified as a threat in your county, because imidacloprid treatments don’t last more than one season anyway.
According to Herms, studies of the natural forest at ground zero in Michigan show that ash mortality there was 30 percent per year during the last four years and is now at 100 percent. At 114 monitoring sites in natural forests, ash mortality is more than 99 percent. “The impact of this pest is what can be expected if it spreads unabated which it probably will,” Herms added.
According to Herms, native borers are quite common, so arborists should become proficient at the identification of borers, he said. Misdiagnosis of this pest for several years is what led to it getting out of hand. Arborists are likely to be the first to diagnose this pest, so proper identification is important.
“My feeling is that this pest is always going to be here and always killing ash trees,” said Roberts. “But the more information we can offer about the pest, the better. That way, people can make their own, informed, decisions.”
For a PDF of “The Plant Doctor’s Landscape Tips,” visit www.arborage.wpengine.com/Media/MediaManager/Roberts.pdf
Photos courtesy of Dr. Dave Roberts