Cold tolerance of introduced emerald ash borer parasitoids

Emerald ash borer, Agrilus planipennis (Coleoptera: Buprestidae), is an invasive insect that has caused considerable ash (Fraxinus spp.) mortality in North America.  The beetle is native to eastern Asia, including China. Three Chinese Hymenopteran parasitoids have been approved for release as part of an emerald ash borer biological control program in the United States: Spathius agrili (Braconidae), Tetrastichus planipennisi (Eulophidae), and Oobius agrili (Encrytidae). The parasitoid species have successfully overwintered after some initial releases in Michigan, but the climatic limits of their distributions due to cold tolerance are unknown. The objective of this study was to measure the cold tolerance of the overwintering stage of each parasitoid species.  We briefly exposed cold-acclimated and non-cold-acclimated individuals to temperatures from 0 to -35°C to determine temperatures that initiated the freezing of body fluids (i.e., the supercooling point) and the extent of pre- and post-freeze mortality. We also measured mortality of cold-acclimated individuals over time at constant temperatures of 0, -5, -10, and -15°C. Cold acclimated S. agrili and O. agrili had the lowest supercooling points. However, cold acclimation did not cause a difference between the mean supercooling point of acclimated and non-acclimated T. planipennisi. Less than 12% mortality occurred at temperatures above the supercooling point for all species and acclimation treatments.  S. agrili and T. planipennisi individuals did not survive exposure to their supercooling point. These measures of cold hardiness, when integrated with winter climatological records, provide a foundation to predict which parasitoids might be best suited for a particular area.