Effect of tree diversity and ash density on emerald ash borer biological control in post-invasion forests

Emerald ash borer (EAB) (Coleoptera: Buprestidae) (Agrilus planipennis Fairmaire) is the most destructive forest insect pest to invade North America. Biological control services provided by woodpeckers (WPs) and parasitoid wasps can reduce EAB densities in infested ash.  Tree diversity has been linked with reduced pest abundance as well as increases in natural enemy abundance in many forest ecosystems. We investigated if tree species richness, ash density, and proportion ash basal area influenced ash canopy condition, EAB larval densities, WP predation of EAB larvae, and EAB parasitism in four post-invasion forests in Michigan, USA. We examined these effects in overstory (DBH > 10 cm) and recruit-sized ash (DBH = 2 -10 cm). EAB larval densities, ash canopy dieback and ash canopy transparency were not associated with tree diversity variables. WPs killed an average of 51 % of larvae in overstory ash while each parasitoid species killed less 15 %. Tree species richness supported EAB parasitism by Spathius galinae Belokobylskij & Strazanac in overstory ash but did not affect parasitism by other species nor WP predation in either strata. Ash density supported parasitism by the most common exotic species, S. galinae, but reduced parasitism by the most common native species: Phasgonophora sulcata Westwood. Proportion ash basal area supported WP predation in overstory ash but weakened WP predation in recruits. The composition of trees present in post-invasion forests may support EAB biological control by certain natural enemies, but these effects do not appear to influence EAB densities or offset ash decline.