Are floral visitation records enough to characterize bumble bee diet breadth?

There is a substantial body of evidence demonstrating that many North American bumble bee species (Bombus spp.) are declining in their range and relative abundance. Some of the basic obstacles to understanding and protecting bumble bees in North America include a lack of data on bumble bee occurrence, habitat preferences, or resource use. Using pollen collected from six bumble bee species across 51 sites, we examined whether floral-visitation records largely reflected patterns of pollen use. We also examined the degree of overlap in the plant use (pollen + visitation records) of individual species to identify plants that may add practical value to conservation or restoration plantings. We found that floral-visitation networks often contained more species than were detected via pollen analysis, though this difference depended to some extent on underlying floral richness. We also identify several plant species whose pollen was collected by all bumble bee species we observed. Our results suggest that although bumble bees rely on a variety of plant species for their dietary needs, the plants they collect pollen from are a small subset of what’s available and co-occurring species may overlap strongly in their use of pollen.