How do sunflower pollen mixtures affect survival of queenless microcolonies of bumblebees (Bombus impatiens)?

The high pollen and nectar yield of Asteraceae flowers combined with the abundance and diversity of these plants should make them an important pollen source for both specialist and generalist bees. However, studies have found Asteraceae pollen to be a poor diet for some generalist bees, possibly owing to nutrient deficiencies or the presence of secondary metabolites or digestive barriers that prevent nutrient assimilation. Mixing pollens could allow bees to exploit these unfavourable pollens, if the different pollen sources help to complement nutrient deficiencies or alleviate the effects of toxic secondary metabolites. In our study, we examined how the proportion of sunflower ( Helianthus annuus , Asteraceae) pollen in the diet of captive-reared bumblebees ( Bombus impatiens ) affects the survival of bees in queenless microcolonies. Bees fed on sunflower pollen had significantly shorter lifespans than bees fed on broad bean ( Vicia faba , Fabaceae), rapeseed ( Brassica napus , Brassicaceae) or Cucurbitaceae pollen. However, survival on mixed pollen diets containing 50% sunflower pollen was as great as that on non-sunflower diets, which suggests that the other pollens were able to compensate for the low nutritive quality of the sunflower pollen. Due to agricultural intensification and a loss of wildflowers, farmland monocultures (e.g. sunflower crops) can be important floral resources for bumblebees. Our study suggests that providing alternative floral resources of high nutritive quality could help mitigate the potential harmful effects of a monofloral sunflower diet on bumblebees.