Prepupae of the rose galling Diplolepis spinosa from areas with relatively cold winters in southern Canada, and Diplolepis variabilis from a milder locale in western Canada, were used to test the hypothesis that mild winter temperatures are detrimental to the survival and potential fecundity of insects. Prepupae of D. spinosa held within or removed from their galls were exposed to simulated overwintering temperatures (−22, 0, 5, or 10°C) for approximately four months before measuring their survival, body size, and potential fecundity. Similar studies were conducted using prepupae of D. variabilis that were removed from their gall and subjected to 0°C or 10°C treatments. Diplolepis spinosa, with or without their galls, averaged 66% more mortality at 10°C than at 0°C. Female D. spinosa that survived the 10°C treatment had 32% fewer eggs than those held at 0°C. In contrast, there was no difference in survival or numbers of eggs between D. variabilis held at 0°C and 10°C. Body size of adult females and size of eggs did not differ among temperature treatments for either species. We conclude that mild overwintering temperatures may be detrimental for insects by raising their metabolism, and consequently reducing energetic reserves needed for development to the adult stage and subsequent production of eggs the following spring. J. Exp. Zool. 298A:23–31, 2003. © 2003 Wiley-Liss, Inc.