The emerald ash borer (Agrilus planipennis) is a wood-boring beetle that is a native of Asia, where it uses Manchurian ash (Fraxinus mandschurica) and other species as hosts. When it invaded the United States in the 1990s, however, it attacked North American ash trees, causing extensive damage to ash in the Eastern U.S. and Canada. As ash populations were being dramatically reduced, it was discovered in 2014 that emerald ash borer was also using white fringetree (Chionanthus virginicus), the first non-ash host confirmed for this beetle.
Graduate students Emily Ellison and Donnie Peterson, and their advisor Don Cipollini, Ph.D., of Wright State University, conducted a survey of the ecology of emerald ash borer interactions with white fringetrees in the midwestern U.S. This survey, a follow up to an earlier study led by Peterson, allowed these investigators to take a glimpse at the ongoing dynamics of the ecological relationships of the ash borer and its alternative host. Their findings were published earlier this month in Environmental Entomology.
Larvae of the emerald ash borer feed on vascular tissue of trees. They can girdle trees, killing them. The beetles tend to attack stressed trees that have been affected by pathogens or environmental stresses such as drought or mechanical damage. They are also more likely to attack larger trees and trees in dense concentrations.
In their first study, in 2015–2016, Peterson and Cipollini looked at 166 white fringetrees planted in ornamental landscapes in Pennsylvania, Ohio, Indiana, and Illinois. In the current study, Ellison, Peterson, and Cipollini reexamined the same trees in 2018–2019. They wanted to see if the health and infestation status of the fringetrees had changed as the wave of emerald ash borers travelled through the region. They also wanted to expand the geographic mapping of white fringetrees that had been attacked by the beetles.
In the current study, Ellison and colleagues grouped white fringetrees into four categories: newly infested, continuously infested, not re-infested, and never infested. In 2015, 26 percent of ornamental white fringetrees sampled were infested. In 2018, they found that only 13 percent of sampled trees were infested, which coincided with a drop in beetle densities in these areas. White fringetrees that were attacked tended to be larger trees, trees with sprouts growing out of their trunks (known as epicormic sprouting), and trees that had dieback in their canopies.
Emerald ash borers and other tree pests often preferentially attack larger trees for several reasons. As Cipollini explains, “One reason is that larger trees are a bigger visual and chemical target, sending a larger ‘cloud’ of volatile chemical cues that signal their presence to specialist beetles. A second reason is that bigger, older trees are more likely to be stressed from various insults incurred through their lifetime, and stressed trees become more attractive and are less resistant to beetles. A third reason is that there could be developmental changes in the chemistry of the bark, such that the bark of young trees is more resistant to the beetles than that of older trees.”
In their study, the investigators observed that, whereas some attacked trees did not survive or others declined in health, trees that weren’t re-infested were able to recover. The authors suggested this was due to variation in susceptibility and tolerance among individual trees, along with a relaxation in pressure from the beetle. Most trees were not attacked, and most previously attacked trees were not re-attacked. Ellison and collegues concluded that white fringetrees in low-density ornamental landscapes were not a significant reservoir for emerald ash borer. “Based on our findings,” the authors write, “it appears that this species will meet a better fate than most ash trees native to eastern North America.”
In terms of management possibilities, Cipollini said that horticulturists and homeowners can physically manage damaged trees. Stress is a predictor of attack, so trees subject to drought, mechanical damage, or insult by other pests are more likely to be attacked by emerald ash borers and are more likely to be compromised by the attack. If managers can help these trees be healthier, it will reduce their vulnerability to the beetles. White fringetree is also tolerant of heavy pruning, like its ornamental relatives lilac and forsythia, which can be used to select for healthy stems. Pesticides can be used to control emerald ash borers, but pesticides risk affecting non-target organisms, such as insect pollinators, populations of which are critically important ecologically.
Cipollini’s interests in future research include studying volatile communication in white fringetrees, which has never been examined. He is also interested in examining the ecological interactions of emerald ash borer with trees in the wild. The current study sampled ornamental trees. “Ornamental trees tend to be small and planted in low density situations,” Cipollini says. “The interactions of emerald ash borer with white fringetree might be very different in the wild, where there are larger populations and denser stands of trees, some of which may be growing in suboptimal conditions. These factors would tend to make the trees more vulnerable to attack and more likely to sustain populations of emerald ash borer in the absence of ash trees.” The authors documented attack on white fringetree by emerald ash borer in nine states, including five newly discovered since the initial survey, and the beetle is expanding throughout the native range of white fringetree in the southeastern U.S.
Ellison and Cipollini’s study is exciting in that it provided a glimpse into the dynamics of a host-parasite interaction within communities over a time span of several years. It will be intriguing to see how the ecological situation between these species continues to change over time, and what the population outcome will be for emerald ash borer—and for the ash trees that it originally decimated following its North American invasion.