Varroa destructor and deformed wing virus interaction increases incidence of winter mortality in honey bee colonies.

Winter mortality of honey bee colonies represents a major source of economic loss for the beekeeping industry. The objectives of this prospective study were to estimate the incidence risk of winter colony mortality in southwestern Quebec, Canada and to evaluate and quantify the impact of the associated risk factors. A total of 242 colonies from 31 apiaries was selected for sampling in August 2017. The presence of Varroa destructor, Vairimorpha (Nosema) spp., Melissococcus plutonius, deformed wing virus (DWV), and viruses of the acute-Kashmir-Israeli complex (AKI complex) was investigated in each colony. Management practices of the various colonies were obtained from a questionnaire. The incidence risk of colony mortality during the winter of 2017-2018 was estimated to be 26.5% [95% confidence interval (CI): 15.4 to 40.3]. In logistic regression modeling of winter mortality in colonies, an interaction was discovered between V. destructor and DWV; the detection of ≥ 1 V. destructor mites per 100 bees was associated with higher odds of mortality (3.46, 95% CI: 1.35 to 8.90) compared to colonies with < 1 mite per 100 bees, but only in DWV-positive colonies. There were more colony losses in apiaries from beekeepers owning 1 to 5 colonies than in apiaries from beekeepers owning over 100 colonies, which suggests that beekeeper experience and/or type of management are important contributors to winter colony mortality. Assuming a causal relationship, the results of this study suggest that up to 9% of all colony mortalities in the population could have been prevented by reducing the level of V. destructor to < 1 mite per 100 bees in all colonies.