Sunflower pollen diets reduce infection in closely related bumble bee species

Parasite growth and infection success are often influenced by host diet. Consuming sunflower pollen (Helianthus annuus) dramatically reduces infection by the trypanosomatid gut parasite Crithidia in the model bumble bee species Bombus impatiens, as well as two of its relatives in the subgenus Pyrobombus. However, only five Bombus species out of more than 250 species globally have been assessed, making results difficult to generalize. Expanding our understanding of diet-mediated infection outcomes beyond model species is important given that many bumble bee species are in decline. We examined how consuming sunflower pollen affects Crithidia infection in two previously untested species of bumble bees in the subgenus Pyrobombus, Bombus perplexus and Bombus ternarius, compared to wild-caught B. impatiens. We hypothesized that consuming sunflower pollen would reduce Crithidia infection in these species, similar to B. impatiens, since dietary sunflower pollen reduced Crithidia infection in other members of the subgenus Pyrobombus. Infected bees were fed either monofloral sunflower pollen, or monofloral buckwheat pollen (Fagopyrum esculentum) as a control for seven days, after which we counted the number of Crithidia cells in the gut. We found that sunflower pollen significantly reduced Crithidia infection in B. impatiens, with comparable significant effects in B. perplexus and B. ternarius. These results demonstrate that, at least for its close relatives, B. impatiens is a good model species for understanding the effects of diet on host interactions with a common gut parasite.