A phototaxis assay to measure sublethal effects of pesticides on bees

In agricultural areas, bees are pervasively exposed to pesticides at sublethal concentrations. Methods to establish whether these concentrations may cause negative effects are needed to assess the potential hazards of field-realistic levels of exposure. Bee risk assessment relies on a single species, the highly social Apis mellifera and, due to fundamental life history differences, available sublethal tests cannot be applied to solitary bees. We provide a simple phototaxis assay to detect sublethal pesticide effects on bees. The assay is highly effective (86–97% response) and provides an unambiguous binary outcome (bees either walk straight to the light source or walk erratically across the arena). We validate the assay by conducting two experiments. First, we build dose–response curves and estimate ED50 and BMD values of an insecticide (acetamiprid) on Osmia bicornis and Osmia tricornis. Second, we assess the effects of sublethal insecticide-fungicide mixtures (acetamiprid-tebuconazole) on Osmia cornuta and A. mellifera. These experiments show that our assay can detect effects of field-realistic levels of acetamiprid exposure as low as 1–30 ng/bee. The phototaxis assay can be used to obtain relevant ecotoxicological endpoints at low sublethal concentrations in both solitary and honey bees, thus contributing to fill an important gap in bee risk assessment.