Diversity of Flower-visiting Bees and their Pollen Loads on a Wildflower Seed Farm in Montana

During a two-year survey on a wildflower seed farm in southcentral Montana, we collected similar to 50 species of bees from 18 genera in sweep samples on cultivated wildflowers and weeds. The two cultivated plant species most intensively sampled attracted different assemblages of bee visitors. Slender white prairie clover (Dalea candida) attracted 27 species, 94% of visitors being Apis mellifera (73%), Lasioglossum spp., Colletes phaceliae, and Bombus spp. Prairie coneflower (Ratibida columnifera) attracted 20 species, the majority being Halictus rubicundus and three Melissodes species; only 3% of visitors to this plant were A. mellifera, despite the fact that the coneflower field was closer to an apiary than were the prairie clover fields. Other apparently non-random plant-bee associations included A. mellifera on Onobrychis viciaefolia, Bombus spp. on Astragalus cicer, and Halictus ligatus and a Melissodes sp. on Symphyotrichum chilensis. Analysis of pollen loads suggests high flower constancy for A. mellifera, Bombus spp., and many of the native solitary bee species foraging on cultivated plants. The low numbers of honey bees on certain plants suggest that native, non-managed bees of such genera as Bombus, Melissodes, Halictus, and Lasioglossum may be critical for plant species for which honey bees show relatively low preference (especially when highly-preferred species such as D. candida are abundant).