Plants may play critical but largely unrecognized roles mediating bee-pathogen dynamics. Growing evidence suggests that pollen has an important role in reducing disease transmission and infection intensities. Pollen is the sole source of lipids and protein for bees and varies wildly in nutritional content, morphology, and secondary chemistry. Pollen also affects the expression of host immunity genes and upregulation of detoxification genes. Here, we conducted a series of laboratory experiments and a large-scale field experiment to test how pollen diet and supplementation affects bee disease and health. We first tested the potential for different pollen diets to reduce Crithidia bombi, a trypanosome gut-parasite, in experimentally infected bumble bees (Bombus impatiens). We then explored the generality of these results across different strains of the parasite, different sources of pollen, and different time periods over which we fed bees different pollen diets. Finally, we investigated how whole-colony supplementation of particular pollen diets affected bee disease and colony health in the field.
We found that sunflower pollen had dramatic beneficial effects in reducing parasite infection in bumble bees. Crithidia infection levels were 20- to 50-fold lower in bees fed sunflower pollen than canola or buckwheat pollen. Moreover, two-thirds of sunflower-fed bumble bees had no detectable infection after one week of consuming the treatment pollen. This effect was robust across experiments comparing variation in parasite strains and sunflower cultivars. We found no effect of pollen diet on bee survival, suggesting minimal mortality costs of consuming sunflower pollen. Moreover, these beneficial effects of sunflower pollen were evident even when compared to feeding bees a wildflower pollen mix. Initial supplementation of field bumble bee colonies with sunflower vs. wildflower pollen found that supplementation with sunflower pollen reduced microbial growth in the colonies. Additional analyses will explore the role of field supplementation of sunflower pollen on disease intensity, acquisition of other diseases and viruses, and colony health. Here we document that a single flowering species can play a pivotal role in reducing disease in economically important pollinators. As a domesticated crop and native wild species, sunflower could be easily included in agroecosystems and native habitat to potentially help manage bee health.