Diet composition and quality can affect an organism’s ability to survive, grow and reproduce, and an organism's dietary needs may change depending on its health status. Bees are critical pollinators that strongly influence plant reproduction and agricultural production, and their survival is in part determined by diet quality and health. Furthermore, diet could influence bee performance and health through nutritional quality or the presence of secondary compounds with medicinal effects. Previous research found that sunflower (Helianthus annuus) pollen dramatically reduced infection by the gut parasite Crithidia bombi in the common eastern bumble bee, Bombus impatiens, relative to other monofloral pollens including buckwheat (Fagopyrum cymosum). However, this previous work did not evaluate impacts of diet and infection on bee performance. In this study, we assessed the impacts of two pollen diets, sunflower and buckwheat, and infection status (infected or not with C. bombi), on bee survival and colony performance using microcolonies of queenless workers. Microcolonies in each treatment combination were terminated 35 days post egg laying, all eggs, larvae and pupae were counted, and all surviving worker bees were dissected to quantify infection.
Sunflower pollen significantly reduced C. bombi parasite load; bees that consumed sunflower pollen had 2.3 times less infection on a log scale than bees that consumed buckwheat pollen. Sunflower pollen also improved microcolony performance relative to buckwheat pollen; microcolonies fed sunflower pollen laid eggs significantly more quickly than microcolonies fed buckwheat pollen, and were more than twice as likely to have laid eggs at any given time. Per bee usage of sunflower pollen was significantly higher than per bee usage of buckwheat pollen, and bees maintained usage of sunflower pollen over the course of the experiment, whereas usage of buckwheat pollen decreased over time. There was a significant interaction between infection and pollen in the number of eggs produced that suggests a medicinal effect of sunflower pollen on C. bombi infection; infected buckwheat microcolonies produced significantly less eggs than any other treatment. Microcolonies fed sunflower also produced more larvae and were more likely to have produced pupae by the end of the experiment. These results suggest that sunflower pollen is a crucial diet component for bee health and colony performance. Future studies will examine potential chemical, morphological or nutritional mechanisms by which sunflower pollen reduces C. bombi infection. This study highlights an important tritrophic interaction between plants, pollinators and parasites.