Ash decline and emerald ash borer biocontrol in the continental United States

Biological invasions are major drivers of the global biodiversity crisis, costing billions in ecological damage each year and instigating fundamental changes to ecosystems. Native to north-eastern Asia, the emerald ash borer (EAB) (Agrilus planipennisFairmaire) (Coleoptera: Buprestidae) is the most destructive invasive insect species in North America, threatening billions of ash trees. Classical biological control efforts aim to introduce and establish four species of parasitoid wasps from the pest’s native range. We assess whether biocontrol can mitigate ash decline across the United States. In an analysis combining continental-scale databases, we test for evidence that biocontrol can mitigate ash decline at the spatial grain of counties. We leverage the USDA/APHIS mapBioControl archive, which tracks releases and recoveries of parasitoids, the USFS Forest Inventory and Analysis (FIA), which contains demographic data for forests on permanent plots, and the USDA/APHIS county-level EAB detection year database. At county-level in 35 US states with EAB detection, we assessed trajectories of ash mortality (trees/acre), ash density (% trees/acre), and ash basal area (% basal area/acre). Using generalized additive models, we used predicted values and 95% confidence intervals to compare counties with releases of biocontrol agents (341 counties in 31 states) versus counties without releases (872 counties in 35 states). We also compared counties with parasitoid recoveries (107 counties) versus without recoveries (51 counties). Thus far, biocontrol does not significantly change ash decline trajectories at county level after EAB invasion. We suggest further studies evaluating the possible benefits of biocontrol on ash recruitment post EAB invasion.