Supercooling Capacity of Urophora affinis and U. quadrifasciata (Diptera: Tephritidae), Two Flies Released on Spotted Knapweed in Montana

Abstract

Freeze tests were conducted throughout the winters of 1987–1988 and 1989–1990 to determine the comparative supercooling points of Urophora affinis Frauenfeld and U. quadrifasciata (Meigen) larvae, and to assess the effects of site and midwinter temperature change on the supercooling ability of U. affinis . The supercooling capacity of U. affinis was significantly greater than that of U. quadrifasciata ; the mean supercooling point for U. affinis during the two winters was −35.1°C compared with −31.0°C for U. quadrifasciata . Regression analyses showed a significant, negative quadratic relationship between supercooling point and collection date during the winters of 1987–1988 and 1989–1990 for both species. Differences in supercooling ability among U. affinis larvae collected from six different Bitterroot Valley sites were infrequent, of small magnitude, and inconsistent over the 2 yr sampled. The supercooling ability of U. affinis maintained at +25°C was significantly less than that of U. affinis in the field but the supercooling ability of U. affinis maintained at or slightly below freezing was generally comparable with that of U. affinis in the field. Based upon this study, both fly species can be expected to survive the winter in most areas of western Montana and the Pacific Northwest, but only U. affinis should be considered for release in eastern Montana and other regions east of the Continental Divide where winter temperatures are often lower than −35°C.