Seasonal honey bee (Apis mellifera) physiology differentially impacts toxicity to pesticides used to control honey bee pests

Honey bee (Apis mellifera L.) toxicology research is crucial to understanding the risks of different chemicals and for finding new compounds to decrease pests and pathogens in honey bee colonies. However, the results of toxicological studies are not always consistent. In this study, we tested compounds from 7 chemical classes to compare topical LD50 values for honey bees that were reared in the winter versus honey bees that were reared in the spring. The compounds included acetamiprid, amitraz, coumaphos, fipronil, fluvalinate, indoxacarb, and hydramethylnon, all of which are periodically used by beekeepers to control honey bee pests. Fluvalinate was 4× more toxic in the spring than in the winter. Conversely, acetamiprid, amitraz, coumaphos, fipronil, and indoxacarb were all more toxic in the winter than the spring by 37×, 7×, >87×, 6×, and 4×, respectively. Coumaphos had the highest discrepancy between the seasons, while hydramethylnon was not significantly different between winter and spring. Overall, our results suggest that toxicology research on honey bees should be accounting for seasonal variation in the honey bees they are testing. The honey bee's susceptibility to each compound may change depending on what time of year they are collected and is unique to each chemical class being tested. This work provides evidence that honey bee susceptibility to pesticides needs to be evaluated in the context of season in future research to properly evaluate their impact on honey bee health.

insecticide; neonicotinoid; organophosphate; pollinator health; pyrethroid.