Consuming sunflower pollen reduced pathogen infection but did not alter measures of immunity in bumble bees

<p>Certain diets can benefit bee health by reducing pathogens, but the mechanism(s) driving these medicinal effects are largely unexplored. Recent research found that sunflower (<em>Helianthus annuus</em>) pollen reduces the gut pathogen <em>Crithidia bombi</em> in the common eastern bumble bee (<em>Bombus impatiens</em>). Here, we tested the effects of sunflower pollen and infection on two bee immune metrics to determine if sunflower pollen diet drives changes in host immunity that can explain this medicinal effect. Bees were infected with <em>C. bombi </em>or not and given either sunflower or wildflower pollen. Subsequently, bees received a benign immune challenge or were left naïve to test the induced and constitutive immune responses, respectively. We measured hemolymph phenoloxidase activity, involved in the melanization cascade, and antibacterial activity. Sunflower pollen reduced <em>C. bombi</em> infection, but we found no significant pollen diet effect on either immune measure. Phenoloxidase activity was also not affected by <em>C. bombi</em> infection status; however, uninfected bees were more likely to have measurable constitutive antibacterial activity, while infected bees had higher induced antibacterial activity. Overall, we found that sunflower pollen does not significantly affect the immune responses we measured, suggesting that the mechanisms underlying its medicinal effect do not involve these bee immune parameters.</p>