To Treat or Not to Treat Bees? Handy VarLoad: A Predictive Model for Varroa destructor Load

Abstract

The parasitic Varroa destructor is considered a major pathogenic threat to honey bees and to beekeeping. Without regular treatment against this mite, honey bee colonies can collapse within a 2-3-year period in temperate climates. Beyond this dramatic scenario, Varroa induces reductions in colony performance, which can have significant economic impacts for beekeepers. Unfortunately, until now, it has not been possible to predict the summer Varroa population size from its initial load in early spring. Here, we present models that use the Varroa load observed in the spring to predict the Varroa load one or three months later by using easily and quickly measurable data: phoretic Varroa load and capped brood cell numbers. Built on 1030 commercial colonies located in three regions in the south of France and sampled over a three-year period, these predictive models are tools designed to help professional beekeepers' decision making regarding treatments against Varroa. Using these models, beekeepers will either be able to evaluate the risks and benefits of treating against Varroa or to anticipate the reduction in colony performance due to the mite during the beekeeping season.

Keywords: Apis mellifera; Varroa destructor; beekeeping; decision-making tool; predictive model; treatment.

Figure 1

In this figure, 5 scenarios are presented with increasing risk (from left to right) taken by the beekeeper to not treat when the model predicts it was necessary or to treat when it was unnecessary. The risk is inversely proportional to the measure of quantile Q. For each level of risk, four cases are represented: (1) Hives with vp_t_x (i.e., Vp at t = 0) < = 0.7 and vp_t (i.e., Vp three months later) >3; (2) Hives with vp_t_x (i.e., Vp at t = 0) < = 0.7 and vp_t (i.e., Vp three months later) < = 3; (3) Hives with vp_t_x (i.e., Vp at t = 0)> 0.7 and vp_t (i.e., Vp three months later) < = 3; (4) Hives with vp_t_x (i.e., Vp at t = 0)> 0.7 and vp_t (i.e., Vp three months later) < = 3.