Impacts and mechanisms underlying sunflower pollen effects on bumble bee health

Many pollinator species are declining due to factors including parasites and pathogens, but the potential for specific plant species to structure pollinator-pathogen interactions is largely unrecognized. We previously discovered that consuming sunflower pollen (Helianthus annuus) dramatically reduced infection by the gut pathogen Crithidia bombi in the common eastern bumble bee, Bombus impatiens. However, we did not know the effects on bee infection or reproduction in field settings, or the mechanism(s) underlying effects. To assess field impacts, we placed commercial colonies at 20 farms in western Massachusetts that varied in sunflower abundance, and assessed floral resources, infection, and colony queen production over the flowering season. In the laboratory, we ascertained whether the mechanism underlying effects was chemical or mechanical by comparing extracted metabolites or exines of sunflower and buckwheat pollen (Fagopyrum esculentum), since buckwheat pollen results in high infections and has a smooth exine, while sunflower exines are spiny. We mixed metabolites or exines from sunflower or buckwheat pollen with wildflower control pollen, fed diets to infected bees for one week, and assessed infection. In the field, we found that sunflower abundance reduced C. bombi prevalence and infection intensity, and was also positively associated with greater queen production in colonies. In the laboratory, sunflower exines alone reduced infection as effectively as whole sunflower pollen, while sunflower pollen metabolites and buckwheat treatments did not. Our results indicate the potential for a single plant species to drive disease dynamics in B. impatiens, and implicate pollen spines as the mechanism reducing infection.