Temperature plays an important role in effective management of the alfalfa leafcutting bee [Megachile rotundata (F.); Megachilidae], the major commercial pollinator of seed alfalfa [Medicago sativa (L.); Fabaceae] in North America. To improve our understanding of threshold and optimum rearing temperatures of M. rotundata, we examined the effect of temperature on postwintering development by using a greater number of temperature treatments than applied in previous studies (19 versus eight or fewer) and analytical tools formulated to model nonlinear relationships between temperature and insect development rates. We also tested the hypothesis that rearing temperature influences adult body lipid content at emergence, which could affect adult survival, establishment and performance as a pollinator, and reproductive success. We found that the Lactin-2 and Briere-2 models provided the best fits to data and gave reasonable estimates of lower (16–18°C) and upper (36–39°C) developmental thresholds and optimum (33–34°C) rearing temperatures for maximizing development rate. Bees successfully emerged over a broad range of temperatures (22– 35°C), but variation in development rate among individuals reared at the same temperature was lowest at 31–33°C. The optimum rearing temperature to maximize the proportion of body lipids in adults was 27–29°C. Our results are discussed in relation to previous findings and speak to the difficulties in designing practical rearing guidelines that simultaneously maximize development rate, survival, and adult condition, while synchronizing adult emergence with alfalfa bloom.