Bumble bee banquet: Genus- and species-level floral selection by Midwestern Bombus

Regionally specific flower preference data are needed to optimize conservation habitat plantings for at-risk pollinators such as bumble bees (Bombus spp.). Current tools for selecting flowers for plantings rely on raw bee flower visits, which can be biased toward abundant flowers. To assist in planning habitat enhancements for bumble bees, we quantified genus- and species-level floral preferences using a selection index that accounts for floral availability. Through 477 h of observation in Ohio, USA during the summers of 2017 and 2018, we recorded 22,999 observations of eight Bombus species visiting 96 flowering plant taxa. As a genus, Bombus selected flowers nonrandomly; the most strongly preferred plants included Asclepias spp., Cirsium spp., Convolvulaceae, Dipsacus spp., Echinacea purpurea, Monarda fistulosa, Penstemon digitalis, and Silphium spp. Only a few Fabaceae were highly selected (Baptisia spp., Trifolium pratense, and Vicia spp.), while some were preferred only during their peak bloom (Securigera varia), and others were not preferred by bumble bees (T. hybridum and Melilotus spp.). Diets differed among habitats, and in restored meadows, bumble bees selected for native planted species such as Monarda fistulosa, Asclepias syriaca, Echinacea purpurea, Penstemon digitalis, and Silphium spp. Diets and preferences shifted over the season, largely driven by changes in plant phenologies (e.g., in June, Penstemon was strongly selected, in July, Asclepias, and in August, Verbena). For the three most common Bombus (B. impatiens, B. griseocollis, and B. bimaculatus), rarefaction analysis indicates that we were able to detect almost all plants in their summer diets. However, for five less common species, even our extensive sampling was insufficient to fully characterize their diets. The common Bombus species differed in their feeding niches, perhaps reducing interspecific competition. In contrast, we found high diet overlap between three rarer species-B. vagans, B. fervidus, and B. pensylvanicus, suggesting that these at-risk species might benefit from different floral communities than would the common species. Five of eight species (including one that is currently under review for federal listing) most strongly preferred one or another non-native plant, presenting managers with a conservation conundrum concerning how to balance the needs of bees with the preservation of native plants.

Automated summary

Regionally specific flower preference data are needed to optimize conservation habitat plantings for at-risk pollinators such as bumble bees. Current tools for selecting flowers for plantings rely on raw bee flower visits, which can be biased toward abundant flowers. To assist in planning habitat enhancements for bumble bees, we quantified genus- and species-level floral preferences using a selection index that accounts for floral availability. Through extensive observation, we found that bumble bees nonrandomly selected certain flowers and their diets and preferences varied across habitats and seasons.