The persistence of a plant species in the face of particularly destructive invasive insects or pathogens is associated with the degree to which it attracts, resists, and tolerates attack. In the “aftermath” of emerald ash borer (EAB) in the Midwest, blue ash (Fraxinus quadrangulata) has persisted to a greater degree than other North American ash species, but the contribution of different traits to this effect are unclear. We surveyed canopy health of mature white and blue ash trees on a 5-point scale in an EAB-aftermath forest in Yellow Springs, OH in 2018. Ash trees in this preserve were first infested around 2011, and a survey in 2012 at the same site indicated that the ash trees present in this preserve were generally healthy. To address whether variation in resistance among species was related to our findings, we compared EAB larval performance on blue and white ash in cut stem bioassays. To address whether variation in adult attraction was related to our findings, we surveyed the densities of larval feeding galleries in the bark of blue and white ash trees in a nearby woodlot that exhibited similar attack dynamics.
Results/Conclusions
Canopy health ratings differed significantly between blue and white ash trees. Eighty-seven percent of surveyed blue ash trees were healthy (rating 1 or 2) whereas only 15% of white ash species were considered healthy. Conversely, only 12% of blue ash trees were unhealthy or dead (rating of 4 or 5) while 85% of white ash trees were unhealthy or dead. In cut stem bioassays, there was no difference in the hatch rate of EAB eggs or survival of larvae on white and blue ash, but larvae produced smaller feeding galleries and achieved only one third the mass on blue ash as they did on white ash. In a nearby forest exhibiting similar attack dynamics, all surveyed mature white ash were dead as a result of EAB attack and their bark exhibited an average of 60% of the phloem consumed by EAB larvae on a per area basis by the time that they died. Only one of ten blue ash trees of similar size possessed detectable larval feeding galleries at a similar position in the tree. We conclude that both reduced adult attraction and resistance to larvae contribute to the persistence of blue ash in the face of emerald ash borer.