Technical Abstract: Osmia cornuta adults require exposure to wintering temperatures followed by incubation in the spring to emerge. We studied winter survival, weight loss, emergence time, and post-emergence longevity in O. cornuta exposed to a variety of pre-wintering and wintering treatments. Few bees emerged after exposure to wintering durations <30 days. Maximum survival and longevity were obtained with wintering durations of 90-150 days. Time to emergence following incubation declined with increasing wintering duration and increasing wintering temperature. Pre-wintering treatment had a dramatic effect on bee fitness. Bees pre-wintered for 15 days lost less weight, had higher survival and lived longer than bees pre-wintered for 45 or 75 days. In general, larger bees were more likely to survive the winter and lived longer than smaller bees. We discuss the relevance of these results to survival in wild O. cornuta populations and the management of populations used in orchard pollination.