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Variability of honey bees colony size

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Following a request by risk managers, EFSA published a technical report (EFSA Supporting publication 2021:EN-6518) in March 2021 to provide scientific background information on the variability of the colony size of honey bees. Ideally, this information should allow for the derivation of a threshold value that reflects acceptable effects of pesticides on honey bee colony size and will assist the risk managers in defining Specific Protection Goals for bees.

The BEEHAVE computer model simulates the development of a honeybee colony and its foraging of nectar and pollen in different landscapes. Using this model the EFSA simulated the background variability of colony size in 19 different EU environmental scenarios that covered a range of geographical, climatic, and beekeeping conditions. The dynamics of an unexposed honey bee colony were simulated over one year to elucidate different aspects of the variability distribution of colony size, and to set limits for an acceptable reduction in colony size based on the shape of the distribution curve. Subsequently, the predictive power of the model was verified by comparing the simulated variability in colony size with the variability of unexposed bee colonies observed in experimental field studies.

Results demonstrated that, although the model predictions were within the range of experimental values, there was a general underestimation of the mean variability of colony size compared to data from the field studies. Moreover, it was shown that the simulated variability in colony size increases with increasing landscape complexity (mainly due to increasing food inflow variability), so that the model, due to its simplicity, may not cover more complex scenarios in the environment. Therefore, it was concluded that the outcome of the model is likely to underestimate the variability of the honey bee colony size in the complex environment.

While this technical report provides valuable insights into the analysis of the background variability of bee colonies, no implementable threshold value was derived for an acceptable reduction of the mean colony size due to pesticide exposure. Further attempts will likely be necessary to finally establish an acceptable reduction in colony size that can be used to evaluate field studies with honey bees.

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