A more diverse pollen nutrition matters for developing solitary bees but does not mitigate the negative impact of pesticides

In agricultural landscapes, bees are subjected to diminishing floral resources and exposure to pesticides. Potential interactions of nutritional stress and pesticide exposure on solitary bees are largely unknown. We investigated the development and survival of the solitary bee Osmia bicornis provisioned with different pollen nutrition and exposed to pesticides in a full-factorial design in the laboratory. We used three nutrition types characterized by a low pollen diversity and a mixture of these (higher pollen diversity). We investigated two field-realistic concentrations of the insecticides thiacloprid, sulfoxaflor and flupyradifurone, as well as of the fungicides azoxystrobin and tebuconazole. We explored whether a higher pollen diversity is beneficial for O. bicornis development and survival, how the pesticides affect various fitness measures and whether pesticide impacts are mitigated by the higher diversity pollen. We found that a more diverse pollen was beneficial for O. bicornis development time, cocoon weight, pollen efficacy and pollen consumption. Thiacloprid, sulfoxaflor and flupyradifurone elongated development time. Sulfoxaflor and flupyradifurone lowered cocoon weight and pollen efficacy, and sulfoxaflor reduced survival and pollen consumption. Our results do not support the hypothesis that a more diverse pollen mitigates negative pesticide effects, but highlight the importance of diverse floral resources for bee development and the need for further studies on the interactions of multiple stressors.