Interactions between woodpecker attack and parasitism by introduced parasitoids of the emerald ash borer

Abstract

Agrilus planipennis, the emerald ash borer (EAB), is an invasive forest pest causing significant economic and ecological impacts. EAB is attacked by some native natural enemies, the most significant being woodpeckers, which can remove between 30 and 95% of the EAB larvae in a tree. However, despite high woodpecker predation rates, EAB populations continue to grow. Management of EAB focuses on classical biological control, with the introduction of four parasitic wasps from Asia. Because both woodpeckers and larval parasitoids attack mature EAB larvae, their impacts overlap and potentially interact. This study examines the relationship between native woodpecker predation and parasitism by the introduced parasitoids S. agrili and T. planipennisi. We established a cohort of parasitized larvae on select ash trees and then used screening to exclude woodpeckers from some sections of the tree. We show that while woodpeckers had no effect on observed parasitism rates of individual trees, the presence of parasitized larvae changed woodpecker foraging behavior and resulted in significantly lower overall foraging by woodpeckers. We hypothesize these changes are due to these larval parasitoids being a lower quality food reward. Parasitism of EAB larvae could contribute to a decrease in patch quality for woodpeckers such that they would quit foraging sooner than expected if it were a high-quality, un-parasitized patch. This study fills a gap in our understanding of the complex relationship between woodpecker mortality and parasitism mortality of EAB and our results demonstrate that this relationship may have broad implications for the EAB biological control program.