Technical Abstract: 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 (non-feeding) prepupae that we observed for 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 increasing oxygen consumption temperature sensitivity 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 for both species when wintered for a minimum of three mo. In spite of the observed trends in temperature sensitivity of wintering O. lignaria, 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 less than three 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, while being of interest from a biological perspective, will likely have important implications in the continued development of sustainable population management protocols.