Parasites have the capacity to change host behavior and interactions with other organisms. Parasite-induced host behavioral changes are well documented in interactions with predators, but are less well studied in the context of mutualisms. Parasitoid conopid flies attack bumble bees, and may alter bee behavior. However, the effect of changes in pollinator behavior on pollination service to plants is unknown. Parasitism could potentially increase or decrease pollination service. Parasitized bees may be poor pollinators due to slow movement and low fidelity to a single flower species, or they may spend longer on an individual flower, thus transferring more pollen per visit.
I asked how parasitism by a conopid fly affected bumble bee foraging behavior using a combination of field manipulation and laboratory observations. We manipulated field colonies such that some bees were allowed to forage freely and were thus naturally exposed to parasitism while others were constrained to forage within a large cage without parasites. We then videotaped the foraging behavior of bees from these colonies on an array of red clover flowers in a laboratory enclosure. Several metrics of bee behavior were calculated from the videos and bees were dissected to determine parasitism status. To estimate effectiveness of pollen deposition, we set aside two clover inflorescences visited per bee, allowed them to mature and determined seed set and seed weight.
Results/Conclusions
I successfully manipulated conopid parasitism, resulting in a sample of 30 healthy and 18 parasitized bees. Parasitism did not effect on any metric of bee behavior we considered. For example, parasitized bees did not spend more time on individual inflorescences. However, flowers visited by conopid-parasitized bees tended to set more and heavier seeds than those visited by healthy bees, suggesting that some metric of bee behavior we did not consider was altered by parasites in a way that affected pollinator service. For instance, red clover is a composite flower made up of many florets, each of which has to be probed in order to set seed. Parasitized bees may probe more florets per flower, leading to higher seed set.
Whatever the mechanism causing this pattern, these results are unique in demonstrating that parasitism may affect pollination services for plants. Future work should scale up these effects of individual bee behavior to ask how bee parasites affect on pollination service by an individual bee’s visit to effects on seasonal or lifetime plant fitness.