A phototaxis assay to measure sublethal effects of pesticides on bees

Pesticides are considered a main driver of world-wide bee declines. In agricultural areas, bees are exposed to combinations of pesticides at low concentrations. The extent to which these low levels may cause sublethal effects remains unknown. Laboratory methods to detect sublethal effects are needed as a first step in assessing the potential hazards of pesticides at low concentrations. Current bee risk assessment schemes rely on a single species, the highly social Apis mellifera , and provide insufficient coverage of sublethal effects. Due to fundamental life history differences, available 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 (only 6.63% of the bees failed to respond) and provides an unambiguous binary response (bees either walk straight to the light source or walk erratically across the arena). We validate the assay by conducting two experiments. In the first one, we estimate dose-response curves and calculate ED50 and benchmark dose (BMD) values of an insecticide on Osmia bicornis and O. tricornis . In the second one, we assess the effects of three insecticide doses, alone and in combination with a fungicide, in O. 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.