Multiple stressors interact to impair the performance of bumblebee (Bombus terrestris) colonies

The data corresponds to a semi-field experiment on bumblebees (<i>Bombus terrestris</i>) to study the combined effects of chronic exposure to field-relevant doses of three abiotic stressors, the neonicotinoid thiamethoxam, the pyrethroid cypermethrin, and the EBI-fungicide tebuconazole, as well as the biotic stressor, <i>N. ceranae</i>. We used a stepwise removal approach to determine the relative effect of each stressor on bumblebees at individual and colony levels. First, we evaluated the prevalence of <i>N. ceranae</i> in inoculated bumblebee colonies to assess if pesticide exposure alters immune response mechanisms and can lead to increased susceptibility to <i>N. ceranae</i> infection. Next, to further explore this effect we measured gene expression in four AMPs (abaecin, apidaecin, defensin and hymenoptaecin), a signal molecule of the Toll (pelle), and Imd immune pathways (relish), and an immune recognition/effector gene (hemomucin) in order to evaluate the immune response of bumblebees to <i>N. ceranae</i>, and investigate interactions between pesticides in the immune response. We also studied the expression of two genes related to P450-mediated detoxification in bumblebees to test if exposure to the EBI-fungicide tebuconazole alters the P450 enzymatic detoxification mechanisms in bumblebees, and can exacerbate the effect of insecticides. Finally, the fitness and reproductive success of the colonies was monitored throughout the study in order to determine which combinations of stressors are more harmful to bumblebees.