Evaluation of tree mortality and parasitoid recoveries on the contiguous western invasion front of emerald ash borer

Abstract

Abstract A recent invader to North America, emerald ash borer Agrilus planipennis Fairmaire (Buprestidae), is expanding its western range into new habitats. We examined how site factors affect the emergence and damage caused by this beetle on the western edge of its contiguous population in eastern Wisconsin, U.S.A. We characterized forest structure and quantified signs and symptoms of A. planipennis from 2010 to 2013. We evaluated the potential for establishment of three host‐specific parasitic Hymenoptera, released as biological control agents of A. planipennis. During June to September 2011, we conducted releases of 1500 Oobius agrili Zhang and Huang (Encyrtidae), 1900 Spathius agrili Yang (Braconidae) and 2700 Tetrastichus planipennisi Yang (Eulophidae) at the same location. Signs and symptoms of A. planipennis increased in abundance during the present study. Emergence of A. planipennis was greater in larger diameter trees and in subplots with more ash trees. More epicormic shoots were on the upper halves of trees in subplots with a higher percentage of ash. Fraxinus nigra exhibited a higher incidence of ruptured bark than Fraxinus pennsylvanica. In January 2013, we felled four infested ash trees and censused insect emergence from the entire trees. We recovered 441 A. planipennis, 41 T. planipennisi and no other released parasitoids. These results suggest that, of these three parasitoids, T. planipennisi has the highest likelihood of contributing to biological control in Wisconsin, although ash mortality will proceed rapidly and likely supersede the effects of the parasitoids. The best opportunity for introduced parasitoids to benefit Wisconsin may arise in post‐outbreak, recovering forests.