Overwintering mortality of Urophora affinis and U-quadrifasciata (Diptera : Tephritidae) on spotted knapweed: Effects of larval competition versus exposure to subzero temperatures

The overwintering survival of Urophora affinis Frauenfeld and U. quadrifasc iata (Meigen) was examined in a 5-yr (1989-1993), multiple-site field study in western Montana. These two tephritid ny species are introduced biologic al control agents of spotted knapweed, Centaurea maculosa Lamarck, and diff use knapweed, Centaurea diffusa Lamarck. Significant differences in overwin tering mortality of U. affinis and U. quadrifasciata larvae (whether occurr ing conspecifically or heterospecifically within the same capitulum) were f ound among sites. Overwintering mortality of U. affinis and U. quadrifascia ta (whether occurring conspecifically or heterospecifically within the same capitulum) showed no relationship with site elevation. Overwintering morta lity of U. affinis and U. quadrifasciata showed no relationship with the de nsities of conspecific or heterospecific galls found within the capitula. E xtended periods of extremely cold temperature were found to explain most of the variation in overwintering mortality of both fly species. We conclude that periods of extremely cold temperature is the most important mortality factor affecting overwintering survival in U. affinis and U. quadrifasciata .