The alfalfa leafcutter bee, Megachile rotundata (Fab.) (Hymenoptera: Megachilidae), was introduced into the lowbush blueberry, Vaccinium angustifolium and V. myrtilloides (Ericaceae), agro-ecosystems of Atlantic Canada and the state of Maine in the early 1990's. As a managed pollinator of this crop, this typical “summer-active” bee is released approximately one month earlier than for alfalfa pollination and therefore is exposed to a different set of climatological conditions, primarily night time temperatures which can reach as low as −8°C during flowering. The objectives of this study were to investigate aspects of cold-hardiness in post-diapausing M. rotundata, and to determine the potential effects of subzero temperatures on adult females through studies of supercooling capacity and survival at low temperatures. Post-diapausing M. rotundata maintain a supercooling capacity below −23°C until pupation, after which all developmental stages freeze between −15°C and −17°C. Fed and unfed adult females show significant differences in their supercooling capacity; −8°C and −16°C, respectively. Bees which have fed showed very little susceptibility to temperatures above −5°C, but mortality significantly increased at −10°C, and was 100% at −15°C. As such, M. rotundata populations are likely susceptible to unusually cool night time temperatures when used for lowbush blueberry pollination. The implications of earlier release times on bee biology and population recovery for pollination are discussed.