Effects of common agrochemicals on the health and function of honey bee colonies

Pesticides have come under scrutiny in the last two decades for their effects on European honey bees. The effects of the highest doses of insecticides encountered on crops by honey bees are often lethal and well acknowledged. There is, however, much debate surrounding the potential consequences of exposure to smaller amounts of these pesticides for bee colonies. Field studies at these levels are scarce and often report contradictory results across environments. Due to technical constraints, these studies generally lack the precision and temporal resolution necessary to elucidate what causes the effects of a pesticide to vary across environments. Much attention has been given to the neonicotinoid insecticides, because of early reports of colony failure associated with elevated amounts of these pesticides in dead worker bees. The pesticides used by beekeepers within hives against the parasitic mite Varroa destructor have, however, received much less attention. Controlled experiments on the effect of these treatments on whole bee colonies are scarce, even though they are almost ubiquitous in hives. To lift some of the technical constraints specific to honeybee studies, we developed a method to automatically and continuously measure the daily variations of the weight of hives, such that we can extract from these data the amount of nectar collected daily and indices of foraging intensity. We then developed a new analytical method to quickly measure the area of capped brood and capped honey from photos of hive frames. We applied these methods to a large field study replicated across two continents, where we treated hives with the neonicotinoid imidacloprid and the acaricide thymol, alone and in combination. We found that the effect of imidacloprid on bee colonies varied between environments and that the miticide thymol can have long-term negative effects on bee colonies. We further investigated how these pesticides affect bee colonies by studying the effects of thymol on the hygienic behaviour of honeybees, and the effect of imidacloprid on individual bee foraging performances. These results add to our understanding of bee colony growth and development and how honey bee colony performance is affected by pesticides, and will help improve honeybee health worldwide.