Summer weather predicts overwintering survival in the European honey bee
(Apis mellifera) in Pennsylvania
The European honey bee (Apis mellifera) is both a crucial pollinator for
agricultural and natural ecosystems, and an agricultural commodity in
its own right. However, honey bees are experiencing heavy mortality in
North America and Europe due to a complex suite of factors. Weather
affects both the bees themselves and the plants that support them.
Surrounding land use, particularly proportion of agricultural and urban
areas, determines forage resource abundance and pesticide exposure risk.
Finally, management decisions, including treatment to control parasitic
Varroa destructor mites, contribute to colony success and failure. We
used three years of data from a survey of Pennsylvania beekeepers to
assess the importance of weather, topography, land use, and management
factors on overwintering mortality of managed honey bee colonies at both
apiary and colony levels. A Random Forest model for mite-treated
apiaries predicted overwintering survival with 73.3% accuracy for
colonies and 65.7% for apiaries, as determined by cross-validation.
Growing degree days was the most important predictor at both levels.
Neither topographic nor management variables were important predictors.
A weather-only model was used to predict colony survival probability
across Pennsylvania for the three years of the study, and to create a
composite map of survival probability for 1981-2019 (long-term
probability mean value of 59.5%). Although three years of data were not
enough to adequately capture the range of possible climatic conditions,
the model nonetheless performed well within its constraints. The Random
Forest approach is suited to understanding complex nonlinear drivers of
survival, and to predicting outcomes given current conditions or
projected climate changes.