Cold hardiness in Spathius galinae (Hymenoptera: Braconidae), a larval parasitoid introduced for biocontrol of emerald ash borer in North America

Abstract

The ability of a biocontrol agent to acclimate to and survive the climate of intended introduction locations is a critical attribute for successful biological control of an invasive pest. We evaluated the cold tolerance of Spathius galinae Belokobylskij & Strazanac, a braconid parasitoid native to the Russian Far East introduced to the United States for biocontrol of the ash (Fraxinus spp.) pest, emerald ash borer (EAB), Agrilus planipennis Fairmaire, by measuring the supercooling point (SCP) of mature S. galinae larvae exposed to winter temperatures at four different field locations that span a gradient of plant hardiness zones. We observed a significant effect of overwintering location on SCPs of S. galinae larvae collected from field populations, with lower SCPs observed at locations with lower average minimum ambient temperatures. We also tested the SCP of three stages (early-instar, late-instar, and mature cocooned larvae) of lab-reared parasitoids and found that SCP did not significantly differ between stages of lab-reared S. galinae. Our findings provide strong evidence that S. galinae can reduce SCP in response to below-freezing temperatures. The increase in cold hardiness of S. galinae in response to below-freezing temperatures should be considered in delineation of the optimal geographic range for biocontrol releases against EAB in North America.