Tree water stress and insect geographic origin influence patterns of herbivory by borers in green (Fraxinus pennsylvanica) and Manchurian (F. mandshurica) ash

Abstract

Emerald ash borer (EAB), Agrilus planipennis (Coleoptera: Buprestidae), is a devastating buprestid beetle introduced to North America from Asia. Asian ash trees exhibit resistance to EAB, likely due to a shared co-evolutionary history. Resistance to one pest, however, does not necessarily confer resistance to others. Should Asian ash be highly susceptible to North American herbivores, the utility of such species for hybridization, breeding programs, and establishment in managed landscapes could be compromised. Common urban problems, such as drought stress, can increase borer attack and survival and may further complicate the search for resistant plant material. The objectives of this study were to examine the relative susceptibility of green (Fraxinus pennsylvanica) and Manchurian (F. mandshurica) ash to EAB and indigenous borers and whether that susceptibility changed with drought stress. In a common garden experiment, EAB occurred more frequently and reached higher abundances in green than Manchurian ash. The frequency and abundance of bark beetles (Curculionidae), North American native clearwing borers (Sesiidae), and longhorn beetles (Cerambycidae) were similar in the two ash species. Generation time of EAB was uniformly one year and did not depend on ash species or water stress. Although borers increased as expected in stressed trees, the relative susceptibility of green and Manchurian ash to borers did not change. The findings suggest Manchurian ash may be resistant to several classes of borers, regardless of insect geographic origin, although these conclusions should be viewed with some caution until the results can be verified in larger trees.