Effects of extreme low winter temperatures on the overwintering survival of the introduced larval parasitoids Spathius galinae and Tetrastichus planipennisi

Authors: Duan, Jian; BAUER, LEAH - Us Forest Service (FS); VAN DRIESCHE, ROY - University Of Massachusetts, Amherst; Schmude, Jonathan; PETRICE, TOBY - Us Forest Service (FS); CHANDLER, JENNIFER - University Of Massachusetts, Amherst; ELKINTON, JOE - University Of Massachusetts, Amherst

Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/28/2020
Publication Date: 4/4/2020
Citation: Duan, J.J., Bauer, L.S., Van Driesche, R., Schmude, J.M., Petrice, T., Chandler, J.L., Elkinton, J. 2020. Effects of extreme low winter temperatures on the overwintering survival of the introduced larval parasitoids Spathius galinae and Tetrastichus planipennisi. Journal of Economic Entomology. 1-7. https://doi.org/10.1093/jee/toaa048.
DOI: https://doi.org/10.1093/jee/toaa048

Interpretive Summary: The emerald ash borer (EAB) is a serious invasive forest pest that has spread to 35 U.S. states. Two EAB natural enemies (the parasitic wasps Spathius galinae and Tetrastichus planipennisi) were introduced from the pest's native home (northeast Asia) for EAB biocontrol in the United States between 2007 and 2015. In late January of 2019, the Midwestern and Northeastern regions of the United States experienced a wave of severe cold weather (“polar vortex”). Approximately three months after the cold wave, we examined survival rates of EAB and the two introduced biocontrol agents at locations in Michigan and Connecticut. In Michigan, which experienced colder weather in 2019, the biocontrol agents were less cold-tolerant than the overwintering EAB. This difference will likely reduce rates of parasitism of EAB larvae during the 2019 growing season in Michigan. Thus, extreme cold weather in winter can compromise the efficacy of EAB biocontrol in the United States.

Technical Abstract: Climate change has been linked to shifts in the distribution and phenology of species, although little is known about the potential effects that extreme low winter temperatures may have on insect host-parasitoid interactions. In late January 2019, northern regions of the United States experienced a severe cold wave caused by a weakened jet stream, destabilizing the Arctic polar vortex. Approximately three months later at six study sites in southern Michigan and three in southern Connecticut, we sampled the overwintering larvae of the emerald ash borer (EAB, Agrilus planipennis Fairmaire) and two larval parasitoids, Spathius galinae Belokobylskij and Tetrastichus planipennisi Yang, that are being introduced as EAB biocontrol agents in North America. At these nine study sites, EAB-infested ash trees and/or saplings were sampled, debarking, and each overwintering EAB and parasitoid larva was examined for cold-induced mortality, as indicated by a brown coloration and limp, watery consistency. In early spring in Michigan, we found 4.5 – 26% of EAB larvae, 18 – 50% of S. galinae larvae, and 8 – 35% of T. planipennisi larvae were killed by cold. In Connecticut where temperatures were more moderate than in Michigan during the 2019 cold wave, <2% of the larval hosts and parasitoids died from cold injury. Our findings revealed that cold mortality of overwintering larvae of EAB and its larval parasitoids varied by location and species, with higher mortality of parasitoid larvae compared to host larvae. The potential impacts of our findings on the management of EAB using biocontrol are discussed.