Forecasting overwintering mortality of Spathius galinae in North America

Authors: WITTMAN, JACOB - University Of Minnesota; AUKEMA, BRIAN - University Of Minnesota; Duan, Jian; VENETTE, ROBERT - Us Forest Service (FS)

Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/18/2021
Publication Date: 6/20/2021
Citation: Wittman, J.T., Aukema, B.H., Duan, J.J., Venette, R.C. 2021. Forecasting overwintering mortality of Spathius galinae in North America. Biological Control. 160. https://doi.org/10.1016/j.biocontrol.2021.104694.
DOI: https://doi.org/10.1016/j.biocontrol.2021.104694

Interpretive Summary: The emerald ash borer (EAB) is a serious invasive forest pest that has currently spread to 35 U.S. states. A biocontrol agent (the parasitic wasp Spathius galinae) collected from the pest's native home (Russian Far East) has been approved for releases against EAB in the U.S. and Canada since 2015. We evaluated the ability of this introduced biocontrol agent to survive below-freezing winter temperatures in the upper midwestern state Minnesota. We showed that the wasp may suffer high overwintering mortality in some areas of Minnesota, where winter temperatures regularly decrease below -28°C. Our findings will allow us to determine the optimal geographic range for field releases of the wasp against EAB in North America.

Technical Abstract: Evaluating the cold tolerance of biological control agents is often necessary for optimizing their release and performance. We determined the cold hardiness of the parasitoid Spathius galinae Belokobylskij & Strazanac, which is a component of the biological control program targeting emerald ash borer (Agrilus planipennis Fairmaire) in Minnesota, USA, using field and laboratory assays. We deployed ash trunk segments containing mature (cocooned) S. galinae larvae at three field sites in Minnesota from December 2019 - March 2020. At each location, half of the trunk segments were buried underneath the snow and half were placed above the snow by tying them to tree trunks. Supercooling points and lower lethal temperature of mature S. galinae larvae were measured in controlled cooling assays in the laboratory. We also chilled the mature larvae at different rates to determine if chilling rate affected estimated supercooling points or lower lethal temperatures. Spathius galinae larvae died after reaching their supercooling point, which we estimated in the lab to be -24.5°C, indicating that the insects were not freeze tolerant. Supercooling points were not affected by chilling rate. We found that S. galinae larvae had near 100% mortality at air temperatures of -29°C in Minnesota. High temperature variation also contributed to mortality of larvae. Using models developed from our data we forecast eclosion rates of S. galinae across the USA based on minimum winter temperatures. Our results indicate that S. galinae populations may suffer high overwintering mortality in areas where winter temperatures regularly decrease below -28°C.