Varroa destructor is the main culprit for the death and reduced populations of overwintered honey bee (Apis mellifera) colonies in Ontario, Canada

The relative effect of parasite levels, bee population size, and food reserves on winter mortality and post winter populations of honey bee colonies was estimated. More than 400 colonies were monitored throughout three seasons in Ontario, Canada. Most of the colonies were infested with varroa mites during the fall (75.7%), but only 27.9% and 6.1% tested positive to nosema disease and tracheal mites, respectively. Winter colony mortality was 27.2%, and when examined as a fraction of all morbidity factors, fall varroa mite infestations were the leading cause of colony mortality (associated to > 85% of colony deaths), followed by fall bee populations and food reserves. Varroa-infested colonies, with weak populations and low food reserves in the fall, significantly decreased spring colony populations, whereas varroa infestations and Nosema infections in the spring, significantly decreased bee populations by early summer. Overall, results suggest that varroa mites could be the main culprit for the death and reduced populations of overwintered honey bee colonies in northern climates.