Effects of the fungicide captan and insecticide thiamethoxam on honey bee (Apis mellifera L.) larval development and colony survival

During crop pollination, honey bees may be exposed to numerous pesticides, including the fungicide captan and neonicotinoid insecticide thiamethoxam. Here, we tested the impact of exposure to these two pesticides individually and in combination, at a range of field-realistic concentrations as determined via our data from New York apple orchards. We used in vitro laboratory assays to monitor how individual larvae responded during development, and field trials with whole colonies to assess population growth and colony survival. Exposure to each pesticide and the combination in the laboratory led to increased larval mortality. The mortality rate of larvae treated with captan was about two times higher than that of the controls, and this effect was not dose-dependent. This increase in larval mortality from captan exposure was similar to the lowest dose of the neonicotinoid insecticide thiamethoxam. Overall, we found weak evidence for synergism (i.e., a non-additive response) between captan and thiamethoxam on larval mortality in the laboratory assays. In the field, exposure of whole colonies to the lowest doses of pesticides individually and in combination resulted in minimal impacts on population growth or colony mortality, and there was no evidence of synergism. These results suggest that colonies can potentially compensate for the detrimental effects of larval exposure to captan and thiamethoxam, at least at the low doses used in our field trial. Further work is needed to assess how compensation may occur, potentially via increased egg laying, and whether higher exposures interfere with compensation. (243 words)