We studied the oxygen consumption of two megachilid bees (Hymenoptera: Megachilidae), Megachile rotundata (F.) and Osmia lignaria Say, at selected, biologically relevant intervals throughout their respective life cycles. The U-shaped oxygen consumption curve and the static weights of wintering (nonfeeding) prepupae that we observed during the life cycle of M. rotundata support previous arguments for a winter diapause similar to that observed in other Hymenoptera. For O. lignaria, which overwinters as an adult, we found stepwise increases in oxygen consumption and continuous weight loss throughout the wintering period. However, our observations on adult O. lignaria wintering requirements are consistent with the previously published results for overwintering M. rotundata prepupae and reveal sharply increasing survival rates when wintered for a minimum of 3 mo. We interpret the greatly reduced survival in both M. rotundata and O. lignaria, as an indication that a critical biological process, diapause, is disrupted among individuals wintered for <3 mo. In the continued development of these two species as commercial scale pollinators on an ever-increasing list of target crops, any similarities or contrasts observed between the “summer bee,” M. rotundata, and the “spring bee,” O. lignaria, although of interest from a biological perspective, will probably have important implications in the continued development of sustainable population management protocols.