Influence of pollen diet on parasite infection in bumble bees
Bumble bees are highly dependent on diverse floral resources, including pollen, for survival and colony reproduction. Nutrient and secondary metabolite profiles vary significantly across floral resources, making it likely that specific diets could influence bee health. Furthermore, recent studies show that some nectar secondary compounds reduce parasite loads in bumble bees. While there is evidence that concentrations of secondary compounds in pollen can be as high as in leaves, we know little about how different pollen diets could affect bee parasites. We hypothesized that pollen from different plant species could affect bumble bee pathogen loads. To test this hypothesis, we inoculated individual Bombus impatiens workers with Crithidia bombi, a trypanosome gut parasite, and then assigned bees to consume one of three different monofloral pollen diets (buckwheat, sunflower or rape; Fagopyrum cymosum, Helianthus annuus and Brassica campestrisrespectively) or a polyfloral pollen diet (equal portions by weight of each monofloral pollen) to assess how pollen diet affects parasite population growth within the host.
Parasite loads were significantly affected by pollen diets. Parasite loads in bees fed sunflower pollen were far lower than in any other treatment; loads in buckwheat-fed bees were 37 times higher than in bees fed sunflower pollen; rape and polyfloral pollen-fed bee pathogen loads were 14 and 5 times higher, respectively, relative to sunflower pollen. Our results indicate that pollen diet may play an integral and so far unappreciated role in managing Crithidia infections in Bombus impatiens. This work, combined with previous studies on nectar chemistry, suggests that both pollen and nectar composition may play important roles in mediating bee-pathogen dynamics. Further examination of amino acid, plant secondary compound and pesticide residue content in these different pollens may shed light on the mechanisms underlying their effects on bee pathogens.