Acute exposure to sublethal doses of neonicotinoid insecticides increases heat tolerance in honey bees

The European honey bee, Apis mellifera L., is the single most valuable managed pollinator in the world. Poor colony health or unusually high colony losses of managed honey bees result from myriad stressors, which are more harmful in combination. Climate change is expected to accentuate the effects of these stressors, but the physiological and behavioral responses of honey bees to high temperatures while under simultaneous pressure of one or more stressors remains largely unknown. Here we test the hypothesis that exposure to acute, sublethal doses of neonicotinoid insecticides reduce thermal tolerance in honey bees. We administered to bees oral doses of imidacloprid and acetamiprid at 1/5, 1/20, and 1/100 of LD 50 and measured their heat tolerance 4 h post-feeding, using both dynamic and static protocols. Contrary to our expectations, acute exposure to sublethal doses of both pesticides resulted in higher thermal tolerance and greater survival rates of bees. Bees that ingested the higher doses of pesticides displayed a critical thermal maximum from 2 °C to 4 °C greater than that of the control group, and a reduction in mortality from 69% to 96%. Our study suggests a resilience of honey bees to high temperatures when other stressors are present, which is consistent with studies in other insects. We discuss the implications of these results and hypothesize that this compensatory effect is likely due to induction of heat shock proteins by the insecticides, which provides temporary protection from extremely high temperatures.