Food Limitation Affects Parasite Load and Survival of Bombus impatiens (Hymenoptera: Apidae) Infected With Crithidia (Trypanosomatida: Trypanosomatidae)

Bumble bees (genus Bombus) are globally important insect pollinators, and several species have experienced marked declines in recent years. Both nutritional limitation and pathogens may have contributed to these declines. While each of these factors may be individually important, there may also be synergisms where nutritional stress could decrease pathogen resistance. Understanding interactions between bumble bees, their parasites, and food availability may provide new insight into the causes of declines. In this study, we examined the combined impacts of pollen and nectar limitation on Crithidia, a common gut parasite in Bombus impatiens Cresson. Individual worker bees were inoculated with Crithidia and then assigned in a factorial design to two levels of pollen availability (pollen or no pollen) and two nectar sugar concentrations (high [30%] or low [15%] sucrose). We found that lack of pollen and low nectar sugar both reduced Crithidia cell counts, with the most dramatic effect from lack of pollen. Both pollen availability and nectar sugar concentration were also important for bee survival. The proportion of bees that died after seven days of infection was similar to 25% lower in bees with access to pollen and high nectar sugar concentration than any other treatment. Thus, nectar and pollen availability are both important for bee survival, but may come at a cost of higher parasite loads. Our results illustrate the importance of understanding environmental context, such as resource availability, when examining a host-parasite interaction.