Forecasting overwintering mortality of Spathius galinae in North America

[Display omitted] •On average, Spathius galinae larvae begin to freeze at −25 °C.•Most Spathius galinae larvae die after initiating freezing, but some survive.•Almost all Spathius galinae larvae died after exposure to −29 °C in the field.•Spathius galinae may not survive winters where temperatures are regularly below −28 °C. Evaluating the cold tolerance of biological control agents is often necessary to optimize their release and performance. We used field and laboratory assays to determine the cold hardiness of the parasitoid Spathius galinae Belokobylskij & Strazanac, an approved classical biological control agent of emerald ash borer (Agrilus planipennis Fairmaire) in North America. Supercooling points and lower lethal temperature of mature (cocooned) S. galinae larvae were measured in controlled cooling assays in the laboratory. Most S. galinae larvae died after reaching their supercooling point, which occurred at −25.0 °C on average. Several larvae, however, initiated freezing but later eclosed, suggesting S. galinae may be partially freeze tolerant. Supercooling points were not affected by chilling rate. In the winter of 2019 – 2020, we monitored development of mature S. galinae larvae in ash segments above and beneath the snow in three locations in Minnesota, USA. Nearly 100% of S. galinae larvae died after air temperatures reached −29 °C in Minnesota. Using models developed from our data, we forecast eclosion rates of S. galinae based on minimum winter temperatures across the range of ash (Fraxinus spp.) in North America. Our results indicate that S. galinae populations may suffer high overwintering mortality in areas where winter temperatures regularly decrease below −28 °C, but a small portion of the population may be able to survive lower temperatures.