Mounting evidence suggests that microbes found in the pollen provisions of wild and solitary bees are important drivers of larval development. As these microbes are also known to be transmitted via the environment, most likely from flowers, the diet breadth of a bee may affect the diversity and identity of the microbes that occur in its pollen provisions. Here, we tested the hypothesis that, due to the importance of floral transmission of microbes, diet breadth affects pollen provision microbial community composition. We collected pollen provisions at four sites from the polylectic bee Osmia lignaria and the oligolectic bee Osmia ribifloris. We used high-throughput sequencing of the bacterial 16S rRNA gene to characterize the bacteria found in these provisions. We found minimal overlap in the specific bacterial variants in pollen provisions across the host species, even when the bees were constrained to foraging from the same flowers in cages at one site. Similarly, there was minimal overlap in the specific bacterial variants across sites, even within the same host species. Together, these findings highlight the importance of environmental transmission and host specific sorting influenced by diet breadth for microbes found in pollen provisions. Future studies addressing the functional consequences of this filtering, along with tests for differences between more species of oligoletic and polylectic bees will provide rich insights into the microbial ecology of solitary bees.

Red raspberry is one of the many fruit crops reliant on the European honey bee for pollination services. Recent declines among managed honey bee colonies coupled with expanding U.S. acreage have led to a marked increase in the cost of U.S. colony rentals for raspberry. The current stocking density recommendation for raspberry is 2 to 5 colonies/ha. This estimate is based on the assumption that a raspberry flower requires dozens of visits to achieve maximal drupelet set. Recent studies, however, suggest that far fewer visits are required for full drupelet set. Here, drupelet set resulting from bee visitation to a raspberry flower is assessed to determine if two prolonged visits will suffice to maximize drupelet set. For all three red raspberry cultivars examined there was no difference in drupelet set resulting from two visits when compared with openly pollinated flowers, the two visits being the first prolonged visit on each of two consecutive mornings. These findings should help refine honey bee stocking densities to better match the pollination needs of cultivated red raspberries.