Preventative medicine? Examining prophylactic effects of a sunflower pollen diet in Bombus impatiens ([Hymenoptera]: [Apidae])

Widespread decline of pollinator populations is of concern for both natural and agricultural ecosystems. Pathogens have been identified as a major contributor to the decline of some bee species, making understanding host-pathogen dynamics a crucial area of research. Sunflower pollen (Helianthus annuus L.; Asterales: Asteraceae) dramatically and consistently reduces infection by a prevalent gut pathogen, Crithidia bombi Lipa & Triggiani 1988 (Kinetoplastida: Trypanosomatidae), in the common eastern bumble bee (Bombus impatiens Cresson 1863), when consumed by bees post-infection, but we do not know if sunflower can confer protection when consumed before exposure. We asked whether feeding bumble bees sunflower pollen diets prior to pathogen exposure decreases Crithidia infection compared to buckwheat pollen (Fagopyrum esculentum Moench; Polygonales: Polygonaceae). Buckwheat pollen was used as a comparison since it has a similar protein concentration as sunflower pollen, but results in higher Crithidia counts more comparable to typical wildflower pollen when consumed post-infection. Bumble bees were fed sunflower or buckwheat pollen for seven days, inoculated with Crithidia, and then fed a wildflower pollen control diet for seven more days before assessing infection. We found that consuming a sunflower pollen diet before inoculation did not reduce Crithidia cell counts compared to buckwheat pollen. Further, bumble bee survival and consumption of sucrose solution and pollen did not differ between these diets. The results show no evidence of sunflower pollen providing prophylactic resistance against Crithidia bombi infection, indicating that the timing at which sunflower pollen is consumed relative to exposure has important consequences for infection.