Pathogens are one of the factors driving pollinator declines. Diet can play an important role in mediating pollinator health and resistance to pathogens. Sunflower pollen ( Helianthus annuus ) dramatically reduced a gut pathogen ( Crithidia bombi ) of Bombus impatiens previously, but the breadth of this effect was unknown. We tested whether pollen from nine H. annuus cultivars, four wild H. annuus populations, H. petiolarus, H. argophyllus and two Solidago spp., reduced Crithidia in B. impatiens compared to mixed wildflower pollen and buckwheat pollen ( Fagopyrum esculentum ) as controls. We also compared hand- and honeybee-collected pollen (which contains nectar) to assess whether diet effects on pathogens were due to pollen or nectar. All Helianthus and Solidago pollen reduced Crithidia by 20–40-fold compared to buckwheat pollen, and all but three taxa reduced Crithidia compared to wildflower pollen. We found no consistent differences between hand- and bee-collected pollen, suggesting that pollen alone can reduce Crithidia infection. Our results indicate an important role of pollen diet for bee health and potentially broad options within the Asteraceae for pollinator plantings to manage bee disease.

1. Several bee species are declining due to multiple factors, including pathogens. Ingestion of sunflower (Helianthus annuus) pollen can dramatically reduce the bumble bee gut pathogen Crithidia bombi, but little is known about how timing and exposure to sunflower pollen consumption affects pathogen load.2. Two experiments were carried out to investigate how exposure to sunflower pollen relative to pathogen exposure affects Crithidia bombi in Bombus impatiens. Foraging trials with pollen-producing and male-sterile (pollen absent) sunflower lines were performed to investigate whether sunflower pollen affected pathogen transmission in a single foraging bout, and 7-day laboratory trials were done to investigate whether timing and duration of exposure to sunflower pollen after infection affected C. bombi.3. In foraging trials, pollen presence on inflorescences inoculated with C. bombi did not affect transmission (pathogen cell counts of foraging workers) 1 week later, suggesting that a brief experience with sunflower pollen concurrent with pathogen exposure is insufficient to reduce infection. In laboratory trials, consuming sunflower pollen for the first 3.5 days or all 7 days after infection reduced cell counts compared with a negative control pollen, but consuming sunflower pollen starting 3.5 days after infection did not. Consuming sunflower pollen for 7 days was significantly and substantially more effective than any other treatment. Thus, both duration and timing of exposure to sunflower pollen may affect pathogen load.4. These results are important for understanding ecological disease dynamics in natural settings with free-flying bumble bees, and may inform decisions about using medicinal diets to manage bumble bee health commercially.

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