Facing multiple threats: Diet-pesticide interactions in bumblebees.

Global change presents a threat to pollinators, with declining populations trends jeopardising the delivery of pollination services. In agroecosystems, the reduction of floral resources and exposure to pesticides have been identified as stressors of bee pollinators. The present study investigated the interaction between nutritional and agrochemical stresses on sublethal endpoints in the bumblebee Bombus terrestris. We used two pollen diets differing in quality and quantity and exposed micro-colonies to various concentrations of two insecticides (sulfoxaflor and cyantraniliprole), a fungicide (Amistar®; active ingredient azoxystrobin), or an herbicide (glyphosate) in a crossed experiment comprising 29 experimental conditions. Regardless of the pollen regime, there were negative impacts of the insecticides on both brood development and proteome; however, there was no significant impact on brood development following exposure to Amistar® or glyphosate. The proteome was affected by all pesticides when the colonies were nutritionally stressed, indicating that pesticides induced a molecular differentiation between groups fed on different diets. Nutritionally stressed colonies were less developed compared to control colonies, but this effect was not synergistically increased by exposure to sulfoxaflor, Amistar®, or glyphosate. The development of colonies exposed to cyantraniliprole was affected similarly, regardless of the nutritional condition and therefore did not benefit from a quality pollen diet. Overall, our results highlight that the effects of agrochemicals on bumblebees can vary depending on diet and can be detected via proteome analyses. We therefore urge for conservation action supporting pollinators with suitable and diverse floral resources alongside improved risk assessment regarding pesticides.