Nectar and pollen are rich in phytochemicals, some of which have been shown to reduce disease in pollinators. These effects have the potential to influence the ecological and evolutionary relationships between plants, their pollinators, and parasites that cause pollinator disease. However, prior studies have shown inconsistent and sometimes contradictory effects of plant-derived chemicals on infection in live bees, where differences in bee genotype, abiotic conditions, and parasite strain could influence results. We used a standardized, high-throughput, cell culture-based procedure to (1) characterize the growth-inhibiting properties of phytochemicals on a bumble bee parasite, and (2) describe how phytochemical resistance varies among parasite strains. We tested nine different floral phytochemicals on four strains of the intestinal trypanosome parasite of bumble bees, Crithidia bombi.
We found that C. bombi showed resistance beyond naturally occurring levels to the phenolics gallic acid, caffeic acid, and chlorogenic acid; a survey of published literature showed that C. bombi resistance to these compounds exceeded that of bloodstream trypanosomes by several orders of magnitude. However, thymol and eugenol inhibited growth at concentrations naturally occurring in pollen and nectar. C. bombi strains varied in resistance to phytochemicals, suggesting that phytochemical resistance and parasite population dynamics could be influenced by natural selection for tolerance to phytochemicals of locally common flowers. Overall, our results demonstrate that the direct effects of phytochemicals on C. bombi, either within bees or during transmission of the parasite via flowers, have the potential to influence infection dynamics and host-parasite evolution.