Megachile rotundata (F.), an adventive, gregarious, cavity-nesting, leaf cutting bee, is used throughout North America for the pollination of alfalfa, Medicago sativa L., seed crops. We examined the influence of various postcocooning (prewintering) temperature regimes on development, survival, emergence time, and longevity in both nondiapausing and diapausing forms of this species. Diapausing male and female M. rotundata required 27–30 d and 29–32 d, respectively, to develop from prepupae to emergence after incubation at constant 29°C, regardless of postcocooning treatments to which they were exposed. Likewise, longevity after emergence was not related to postcocooning temperatures and ranged from 5 to 6 d and 4 to 6 d for males and females, respectively. Elevated postcocooning temperatures were associated with a higher prevalence of nondiapausing individuals, those completing development from eggs through emergence as adults in the same summer season. Relevance of our results to the commercial production of alfalfa leafcutting bees is discussed.