Sunflower pollen decreases bumble bee parasitism on an individual level but may increase parasite transmission across landscapes

Parasites are major drivers of bee population declines. Thus, it is critical to understand ecological factors that reduce disease transmission and increase bee health. Host diet may influence parasitism, but little research has assessed how diet affects parasite transmission. Numerous parasites infect bumble bees, including Crithidia bombi, a fecal-orally transmitted trypanosomatid gut parasite that reduces learning, reproduction, and survival. Prior research shows that a sunflower pollen diet reduces C. bombi infection in the common eastern bumble bee, Bombus impatiens. One mechanism driving this is increased excretion; however, because C. bombi is fecal-orally transmitted, increasing excretion may increase transmission. Alternatively, if sunflower pollen damages parasite cells, infection and transmission may decline. We investigated how sunflower pollen affects C. bombi transmission in B. impatiens. To test if sunflower pollen affects the transmission of excreted parasites, we infected bees using a standardized number of parasite cells from feces of either sunflower-fed or control-fed (buckwheat) bees and measured subsequent infection prevalence and intensity.

To assess differences in parasite acquisition, we compared parasite prevalence and infection intensity of bees exposed to infected sunflower-fed vs. control-fed bees. We also measured persistence times of fecal droplets from sunflower-fed and control-fed bees. We found no difference between probability and intensity of infection with C. bombi cells excreted from sunflower-fed vs. control-fed bees when cell density was controlled.

Furthermore, preliminary results show that a sunflower pollen diet increases fecal droplet persistence on flowers. These results indicate that sunflower pollen does not stop parasite transmission, and potentially increases transmission potential.