Can colony size of honeybees (Apis mellifera) be used as predictor for colony losses due to Varroa destructor during winter?

For more than three decades, honeybee colonies (Apis mellifera) experience high losses during winter, and these losses are still continuing. It is crucial that beekeepers monitor their colonies closely and anticipate losses early enough to apply mitigating actions. We tested whether colony size can be used as early predictor for potential colony losses, in particular due to the parasitic mite Varroa destructor. V. destructor is one of the most important causes for these losses. Such early predictor for potential V. destructor induced losses is especially relevant as measuring V. destructor load in colonies is difficult and cumbersome. During three years, we monitored colonies with high and low V. destructor load from July until March of the next year. We found that differences in colony size were only visible after November, even though we lost almost all colonies every winter in the group with high V. destructor load. In the Northern hemisphere, November is considered to be too late for beekeepers to strengthen colonies in preparation for winter. We therefore argue that early-warning signs for potential colony losses due to V. destructor are urgently needed to allow beekeepers preventing winter losses. We discuss the role of precision apiculture to monitor the health and productivity of honeybee colonies.