Rapid Spread of an Introduced Parasitoid for Biological Control of Emerald Ash Borer (Coleoptera: Buprestidae) in Maryland

Emerald ash borer (Agrilus planipennisFairmaire (Coleoptera: Buprestidae)), an invasive phloem-feeding beetle native to Asia, has devastated North American ash forests since its detection in Michigan, United States in 2002. As the emerald ash borer has continued to spread, the potential for successful long-term management hinges upon the release, establishment, and spread of introduced larval and egg parasitoids for biological control. Here, we focus on the establishment and evidence for spatial spread of introduced larval parasitoid, Spathius agriliYang and Spathius galinaeBelokobylskij & Strazanac (Hymenoptera: Braconidae) in the state of Maryland. To assess each species, we analyzed historical release and recovery data and resampled previous release sites and nonrelease sites for establishment. We found little evidence of establishment or spread for S. agrili, despite a comparatively large number of release locations, events, and individuals. By contrast, despite much lower propagule pressure and shorter history of releases, we detected multiple established populations of S. galinaeat release sites and at sites up to 90 km from the nearest release point approximately 3 yr after its most current release. Our findings show that S. galinaehas established and spread rapidly following field releases whereas its congener, S. agrilihas not. Although it may still be too early to evaluate the level of population control and ash protection afforded by S. galinae, these findings indicate the need for continued investment in S. galinaefor emerald ash borer classical biological control efforts.