Cold tolerance of Chinese emerald ash borer parasitoids: Spathius agrili Yang (Hymenoptera: Braconidae), Tetrastichus planipennisi Yang (Hymenoptera: Eulophidae), and Oobius agrili Zhang and Huang (Hymenoptera: Encyrtidae)

Emerald ash borer, Agrilus planipennis Fairmaire (Coleoptera: Buprestidae), is an invasive insect that has caused significant ash (Fraxinus spp.) mortality in North America. Three Chinese parasitoids have been approved for release as part of a classical biological control program for A. planipennis in the United States: Spathius agrili Yang (Hymenoptera: Braconidae), Tetrastichus planipennisi Yang (Hymenoptera: Eulophidae), and Oobius agrili Zhang and Huang (Hymenoptera: Encyrtidae). This study was designed to measure the cold tolerance of the overwintering stage for each parasitoid species in the laboratory. We exposed cold-acclimated and non-cold-acclimated individuals to temperatures from 0 to −35 °C to determine temperatures that cause body fluids to freeze, mortality after brief exposure, and mortality after long-term exposure. Cold acclimation lowered the supercooling points of S. agrili (median −28.8 °C) and T. planipennisi (median −29.4 °C). Median supercooling point for Oobius agrili was −30.5 °C. Cold acclimation also increased survival of diapausing S. agrili (50% mortality at −27.3 versus −23.7 °C for non-diapausing S. agrili) during brief cold exposure. T. planipennisi and S. agrili mortality increased over long term cold exposure when held at constant temperature. Half of T. planipennisi are predicted to fail to eclose after exposure to 0, −5, −10, and −15 °C after >84, 82, 59, and 36 days, respectively, while 50% of S. agrili with diapause induced in one generation would be discolored from cold injury >84 days for all exposure temperatures. Our models characterizing parasitoid mortality due to cold exposure can be used to assess the climatic suitability of a location prior to release.