Effects of cold tolerance on potential distributions of introduced emerald ash borer parasitoids

Classical biological control of emerald ash borer, Agrilus planipennis (Coleoptera: Buprestidae), is being implemented in many areas of the United States to protect ash (Fraxinus spp.).  Three Chinese parasitoids have been approved for release: Spathius agrili Yang (Hymenoptera: Braconidae), Tetrastichus planipennisi Yang (Hymenoptera: Eulophidae), and Oobius agrili Zhang and Huang (Hymenoptera: Encrytidae). In northern states, extreme cold might be a barrier to parasitoid establishment. The objective of this study was to predict parasitoid mortality during winter in locations where the parasitoids might be released. Projections were based on our previous measures of mortality at temperatures below 0°C for cold-acclimated S. agrili and T. planipennisi. We model mortality due to cold exposure for multiple years throughout North America and compare our forecasts with published results of field releases in the northeastern United States. S. agrili had greater survivorship than T. planipennisi after brief cold exposures. Our models suggest that cold mortality was greater at more northern latitudes. However, some northern areas such as Michigan also had warmer mean minimum winter temperatures, which increased forecasted survivorship. The projected mortality of overwintering S. agrili was less than 20% in Michigan where parasitoids were recovered. These forecasted geographic distributions demonstrate where cold mortality may limit the effectiveness of biological control by a particular species. Recommendations can then be made to agencies as to what species should be released in a given location.