In generalist pollination systems, what role do individual pollinator species play in plant reproductive success? Compared to generalist pollinators, oligolectic pollinators are abundant, efficient foragers, and can provide dependable pollination services to their host plant. However, they also can excel at pollen removal, due to efficient foraging and specialized morphology, and may control almost all of the pollen flow in a plant population. If specialist pollinators are not efficient at delivering that pollen to female-phase flowers, they may act as conditional parasites.

Claytonia virginica L. (Portulacaceae) is a spring ephemeral wildflower visited by a variety of insects, among them the oligolectic solitary bee Andrena erigeniae, which collects pollen exclusively from C. virginica. C. virginica’s protandrous phenology allows pollen collectors to concentrate on male-phase flowers and avoid the female-phase, while generalist visitors visit indiscriminately. Thus, the relationship between the pollen collecting A. erigeniae and its host C. virginica depends on the reward-collecting characteristics of the bee visitor in relation to those of the rest of the pollinator community. Through identification and documentation of differences in pollination effectiveness and visitation patterns among C. virginica visitors in Southeastern Pennsylvania, we determined the overall importance of the specialist pollinator to pollen transfer and reproductive success.  
Results/Conclusions

Removal efficiency differs greatly between A. erigeniae and generalist visitors. Single visits by A. erigeniae to previously unvisited male- and female-phase flowers remove relatively large amounts of pollen and deposit an average number of grains. A. erigeniae prefer the male-phase – female bees are four times more likely to visit a male-phase flower than a female one, and are four times more likely to visit male-phase flowers sequentially than transition from a male- to female-phase flower. Coupled with this sex-phase bias, high visitation rates by A. erigeniae suggest that this specialist removes most of the pollen production but delivers little. In conclusion, this specialist pollinator can play a parasitic role in plant reproduction, depending on the local abundance of all floral visitors. In order to explore the effect of this trend on bee and plant evolution, future research will examine differences between generalist and specialist cognition and behavior, local variation in pollen transfer success, and plant adaptations to intensive pollen collection.