Flowering resources modulate sensitivity to a common fungicide in Bombus terrestris

Bees are exposed to various stressors, including pesticides and lack of flowering resources. Despite potential interactions between these stressors, the impacts of pesticides on bees are generally assumed to be consistent across bee-attractive crops, and regulatory risk assessments of pesticides neglect interactions with flowering resources. We assessed the interactive impacts of the globally used azoxystrobin-based fungicide Amistar and three types of flowering resources (purple tansy, buckwheat, and a floral mix) on Bombus terrestris colonies in a full-factorial semi-field experiment with 39 large flight cages. Fungicide exposure through purple tansy monocultures reduced population (colony) growth, production of males, and adult worker body mass, while Amistar had no impact on colonies in buckwheat or floral mix cages. Furthermore, buckwheat monocultures hampered survival and fecundity irrespective of fungicide exposure. This shows that flowering resources modulate pesticide impacts and that B. terrestris requires access to complementary flowers to gain both fitness and fungicide tolerance.