Bumble bee banquet: Genus‐ and species‐level floral selection by MidwesternBombus

Abstract

AbstractRegionally specific flower preference data are needed to optimize conservation habitat plantings for at‐risk pollinators such as bumble bees (Bombusspp.). 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 eightBombusspecies visiting 96 flowering plant taxa. As a genus,Bombusselected flowers nonrandomly; the most strongly preferred plants includedAsclepiasspp.,Cirsiumspp., Convolvulaceae,Dipsacusspp.,Echinacea purpurea,Monarda fistulosa,Penstemon digitalis, andSilphiumspp. Only a few Fabaceae were highly selected (Baptisiaspp.,Trifolium pratense, andViciaspp.), while some were preferred only during their peak bloom (Securigera varia), and others were not preferred by bumble bees (T. hybridumandMelilotusspp.). Diets differed among habitats, and in restored meadows, bumble bees selected for native planted species such asMonarda fistulosa,Asclepias syriaca,Echinacea purpurea,Penstemon digitalis, andSilphiumspp. Diets and preferences shifted over the season, largely driven by changes in plant phenologies (e.g., in June,Penstemonwas strongly selected, in July,Asclepias, and in August,Verbena). For the three most commonBombus(B. impatiens,B. griseocollis, andB. 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 commonBombusspecies differed in their feeding niches, perhaps reducing interspecific competition. In contrast, we found high diet overlap between three rarer species—B. vagans,B. fervidus, andB. 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.