Floral Resources Used by the Endangered Rusty Patched Bumble Bee (Bombus affinis) in the Midwestern United States

Abstract

The once-common rusty patched bumble bee (RPBB, Bombus affinis) has disappeared from most of its original range in eastern North America. As a result of this dramatic population decline, RPBB was listed as federally endangered in 2017. Unlike many endangered species, remnant populations of the RPBB often occur in urban/suburban parks and natural areas. This paper summarizes photographic records of RPBB floral use collected largely by volunteer community scientists in the midwestern United States, with a focus on Wisconsin, one of the species' remaining strongholds. RPBB were documented in 37 of Wisconsin's 72 counties. We identified flowers to genus in 772 of 803 digital images (some bees were not on flowers). Although 87 plant genera were identified, 76% of the flowers represented just 13 genera. Over half of the flower records (54.6%) were from Monarda, Eutrochium, Veronicastrum, Agastache, or Solidago. Incidental surveys from other states show a similar pattern, although additional genera have been shown to be used by RPBB. Our results support existing recommendations for plantings and habitat management favoring specific plant species, generally associated with native grasslands and savannas. Because the active period of RPBB covers a large part of the growing season, however, we also provide evidence for the importance of spring ephemeral woodland wildflowers (e.g., Dicentra, Anemone, Hydrophyllum) and late-flowering species of wetlands and wet meadows (e.g., Eutrochium, Eupatorium). A landscape that includes woodland, native grassland, and wet meadows provides floral resources that support all RPBB life history stages, including early-season queens, late-season gynes, and males.