Foraging behavior of bee pollinators structured by parasites and plant chemistry
Pollination mutualisms are mediated by partner traits such as bee foraging behavior and floral nectar chemistry. Expression of these traits may be influenced by competitive or trophic interactions with third parties; for example, parasites can reduce bees’ ability to learn foraging tasks, which could affect their effectiveness as pollinators. Similarly, secondary metabolites found in floral nectar can reduce disease load of parasitized bees, but may also have deterrent or toxic effects on consumers. Despite the importance of pollination mutualisms, we currently lack understanding of how parasite infection could affect bees’ foraging behavior, and how bees might balance the benefits of secondary metabolite consumption, such as reduced parasite load, with physiological and other costs. To ask whether bees seek out plant chemicals that reduce their parasite load, in multiple sites and years we quantified parasite and parasitoid load of two bumble bee pollinators of Chelone glabra, a plant whose nectar and pollen contains iridoid glycoside secondary compounds known to reduce bee parasites. In a laboratory experiment using microcolonies, we then crossed bee diet (with or without iridoid glycosides) and parasite infection (parasitized or not) and measured subsequent benefits and costs to bees of feeding on iridoid glycosides.
At the start of the flowering season, bees foraging at C. glabra were more likely to be afflicted with parasites but not parasitoids than those foraging at other plant species, suggesting that parasitized bees preferentially foraged for nectar and pollen that could lower disease load. Parasite load of C. glabra foragers declined over the plant’s flowering season, yet that of bees visiting other plants remained constant over time. In the laboratory experiment, bees consuming iridoid glycosides had lower parasite loads and higher survival than controls, but they consumed less nectar and pollen, suggesting bees experience both benefits and costs when they consume these compounds. These results demonstrate that pollinator parasites can influence host foraging in the field and that the costs and benefits of nectar secondary metabolite consumption for bee pollinators are dependent on the multispecies trophic context in which these interactions occur. Our study suggests that both parasites and plant chemistry can influence bee foraging behavior in ways that could shape the outcomes of pollination mutualisms.