Protection of North American ash against emerald ash borer with biological control: ecological premises and progress toward success

The emerald ash borer (EAB), Agrilus planipennis (Coleoptera: Buprestidae), a phloem-boring beetle native to Asia, was first discovered in 2002 in the USA and Canada, causing widespread mortality of ash trees (Fraxinus spp.). A classical biological control program against EAB was implemented with the first regulatory approvals for environmental releases of three hymenopteran parasitoids from China in 2007: Tetrastichus planipennisi (Eulophidae), Spathius agrili (Braconidae), and Oobius agrili (Encyrtidae), and a fourth parasitoid, Spathius galinae (Braconidae) from the Russian Far East in 2015. We analyzed literature from the Scopus Database to examine the ecological premises of protection of North American ash trees with biocontrol, with a particular focus on the population dynamics of EAB and its biocontrol agents and implications for protecting and conserving native ash in the aftermath of EAB invasion. To date, the introduced parasitoids have been released in over 360 counties in 31 EAB-infested states, Washington D.C., and three Canadian provinces. Three of the parasitoids, T. planipennisi, S. galinae, and O. agrili, have successfully established self-sustaining populations in many release areas in the northeastern and Midwestern USA. In several regions where releases were made early, these agents have now spread to nearby forests and resulted in significant suppression of the target pest to low densities. Survival of regenerating ash due to suppression of EAB by parasitoids has also been observed in some sites with early parasitoid releases. The suppression of EAB is likely to expand geographically and thus contribute to North American ash recovery.