Monitoring and Habitat Assessment of Declining Bumble Bees in Roadsides in the Twin Cities Metro Area of Minnesota
Elaine Evans, Michelle Boone, Dan Cariveau
Report no. MnDOT 2019-25
Several bumble bee species have declined dramatically, including the endangered rusty-patched bumble bee, Bombus affinis. Roadsides offer a unique opportunity to increase habitat for these declining species. The objectives of this study are to: (1) characterize the bumble bee community and floral availability within roadsides in the Minneapolis and Saint Paul, Minnesota, metro area, (2) estimate detection probabilities and occupancy for bumble bees using occupancy modeling, (3) determine the effort needed to detect rusty-patched bumble bees, and (4) examine the relationship of the bumble bee community to the surrounding landscape. We use rapid and broad-scale sampling at randomly selected locations. Despite overall low floral abundance, many bumble bee species, including rare and declining species, use roadsides. Occupancy models predict rusty-patched bumble bees occupy 4% of sites, with a 30% chance of detection if it is at the site. We recommend performing nine surveys in a single season to be 95% sure that B. affinis is detected if it is there. Bumble bee abundances and species numbers increase with more wooded area and floral cover. Crops are negatively associated with bee abundance, species numbers, and the presence of rare bumble bees. Our management recommendations for roadsides to support rare and declining bumble bees are: (1) incorporate additional bumble bee forage, (2) when weed control requires elimination of flowering plants, replace with bumble bee forage, (3) use our estimates for occupancy and abundance as a baseline to assess conservation efforts for bumble bees within roadsides in the metropolitan area of Minneapolis and Saint Paul.