Cold hardiness of emerald ash borer parasitoids Spathius agrili and Tetrastichus planipennisi.

Spathius agrili (Hymenoptera: Braconidae) and Tetrastichus planipennisi (Hymenoptera: Eulophidae) are parasitoids native to China and are approved for release as part of a biological control program for emerald ash borer, Agrilus planipennis (Coleoptera: Buprestidae), in the United States. Populations of both species successfully overwintered after initial release in Michigan. However, overwintering field data is sparse, so the geographic limits of the parasitoids’ distribution due to cold are unknown. We exposed late-instar, pupae, and adult parasitoids to cold temperatures in a lab to determine two measures of cold hardiness: the supercooling-point (SCP), lower lethal temperature, and (LLTemp). To measure SCP, parasitoids were cooled on thermocouples until an exotherm was detected due to freezing of body fluids. To measure LLTemp, we cooled parasitoids to target temperatures based on mortality at the SCP, returned them to room temperature, and assessed mortality. Initial observations indicate that for both species late instars have lower SCP than adults. Further differences in cold hardiness among species, life stage, and rearing temperature treatments will be discussed. Determination of the cold tolerance for each species allows for targeted parasitoid release of the appropriate species in areas where mortality due to cold exposure can be minimized.