Single species may play disproportionate roles in communities via strong impacts on other interactions. Bumble bees (Bombus spp.) are common pollinators that are essential for reproduction of many native plants, and also provide valuable economic services in crop pollination. The trypanosome gut pathogen Crithidia bombi can reduce individual bee lifespan, colony reproduction, and ability to learn novel flower types. Previous work has found that nectar secondary compounds can reduce Crithidia infection, but the role of pollen, which often contains much higher levels of secondary compounds than nectar, has not been considered. We first tested the potential for different monofloral pollens to reduce Crithidia load in Bombus impatiens, the common eastern bumble bee. Upon finding a dramatic effect of sunflower (Helianthus annuus) pollen in reducing Crithidia load compared to buckwheat (Fagopyrum cymosum) and canola (Brassica campestris) pollen, we analyzed the secondary chemical and amino acid profiles of these pollens to suggest potential mechanisms. Based on these results, we tested whether triscoumaroyl spermidine and rutin could be mechanisms underlying the medicinal effect of sunflower. Finally, we sampled B. impatiens from farms varying in sunflower area and assessed Crithidiainfection, to determine whether our laboratory results could predict infection on a landscape scale.
Sunflower pollen dramatically reduced Crithidia infection relative to other treatments; two-thirds of bees fed sunflower pollen cleared their infection within one week, compared to 10% of bees consuming the other pollens. Chemical assays found no distinct differences in the amino acid profiles of sunflower compared to other pollens, but suggested several secondary compounds that differed. In manipulative experiments, we found no evidence that triscoumaroyl spermidine reduced Crithidia infection; results with rutin are pending. In field sampling we found a significant negative relationship between Crithidia infection and the amount of sunflower grown at each farm, suggesting that sunflower may play a key role influencing bee populations and pollination service via effects on pathogen loads. Future work will assess the phylogenetic, genetic and functional diversity of this medicinal effect, as well as continue to assess landscape-level consequences. Taken together, this work demonstrates the key role that plant species may play in shaping bee-pathogen dynamics.