Rare Species Guide

  Basis for Listing

The Rusty Patched Bumble Bee (Bombus affinis) was once one of the most common bumble bees encountered in Minnesota. It was previously distributed across eastern North America, from Quebec to Georgia and west to North Dakota. Since the 1990s, researchers began noticing declines across the range. It is estimated that the Rusty Patched Bumble Bee's geographic distribution has been reduced by 70-87% and there is evidence of declines in abundance of 92-95% (Colla and Packer 2008; Cameron et al. 2011; Hatfield et al. 2015). Despite having essentially disappeared from a large part of its historic range, Minnesota, in particular the region from the Twin Cities Metro to the southeastern corner of the state (Eastern Broadleaf Forest Province), appears to host consistent, though low abundance, populations of Rusty Patched Bumble Bees. In 2019, the Rusty Patched Bumble Bee was designated the State Bee (Minnesota Statutes 2019 Section 1.1465) as a means of raising awareness of their need for conservation concern. It was federally listed as endangered in 2017 in the United States, endangered in Canada in 2012, and listed as critically endangered by the International Union for Conservation of Nature  (Hatfield et al. 2015).  

  Description

The Rusty Patched Bumble Bee is a medium-sized, short-tongued species of bumble bee. Workers (9-16 mm; 0.35-0.63 in.) and males (13-18 mm; 0.51-0.71 in.) have the first and second thoracic segments with yellow hairs and the remaining segments entirely black (Williams et al. 2014). They can be distinguished from similar species, such as the Half-black Bumble Bee (Bombus vagans) or Sanderson’s Bumble Bee (Bombus sandersoni), by the combination of two main characteristics: 1. A rusty orange or red crescent shape in the middle of the upper half of the second abdominal segment, and 2. Black hairs on the thorax in the shape of the profile of a thumbtack. The Rusty Patched Bumble Bee is often confused with the Brown-belted Bumble Bee (Bombus griseocollis). The rusty patched has yellow hairs on the second thoracic segment that extend to the lateral edges, whereas the brown-belted has black hairs laterally on the second segment with more of a brown swoop in the center of the same segment. The rusty patched is also frequently confused with the Tri-colored Bumble Bee (Bombus ternarius), which has two entire thoracic segments (segments two and three) that are vibrant orange, whereas the rusty patched only has a partial second segment that is more rust colored. Queens do not resemble workers in that they lack the rusty patch and the thumb tack outline. They can be distinguished from half-black queens by having short, cropped hair, a generally larger body (rusty patched:19-23 mm [0.75-0.91 in.]; half-black:17-21 mm {0.67-0.83 in.]), and only black hair on the face (Williams et al. 2014).  

  Habitat

Workers and queens require consistent forage throughout the entire growing season, beginning with spring-flower, forest-associated plants when the first emerge around April (Mola et al. 2021). They continue foraging throughout the summer to provision their brood, and into the fall to build resources to overwinter. They are generalist foragers, seeming to prefer purple flowers like wild bergamot (Monarda fistulosa) and hyssop (Agastache sp.), but foraging on a wide array of flowers, including both native and non-native flowers and crops  (Simanonok et al. 2021). Workers have been observed in a variety of habitats, including forests (fire-dependent, mesic hardwood, and floodplain forests), wetlands, grasslands, roadsides, and residential lawns and parks  (Colla and Packer 2008). Little is known about the nesting and overwintering habitats of Rusty Patched Bumble Bees due to few descriptions (Boone et al. 2022). 

  Biology / Life History

The Rusty patched Bumble Bee is a social animal that lives in annual colonies. In each colony, the foundress queen produces successive broods of female workers and eventually males and gynes in late summer. Newly produced gynes mate with males in the late summer and early fall and the remaining colony (males, workers, and foundress queens) all perish before winter. The solitary gynes then overwinter, primarily in mesic hardwoods, just a few centimeters below the soil surface. In the spring, the rusty patched queens (now considered foundresses) are some of the first bumble bees to emerge from diapause, typically in April (Plath 1922), and seek a suitable nesting location, usually an abandoned rodent burrow either in open grasslands or forest edges. Of the few nests reported in the literature, most were subterranean, typically 30-123 cm (1-4 ft.) underground (Plath 1927; Macfarlane et al. 1994). Queens will provision their developing brood with pollen and nectar, protect the nest, and tend to the first brood of female workers that begin to emerge in earnest in July in Minnesota. Queen Rusty Patched Bumble Bees have a long establishment phase, given that they emerge so early in the spring and workers are not detected until mid-summer. Workers then take over the responsibility of foraging for nectar and pollen for additional broods throughout the summer. One unusual characteristic of rusty patched nests is the construction of long cylinders of pollen, 5-6 cm (2-2.4 in.) in length (Plath 1922). In late summer, the queen produces males and new gynes and the cycle repeats itself with only the new queens over wintering, beginning around October. Reports of colony sizes vary widely from 200 to over a thousand total bees (Boone et al. 2022), but the rusty patched is thought to form some of the largest colonies of bumble bees (Macfarlane et al. 1994).

  Conservation / Management

There are many likely factors that have contributed to the decline of this species. The primary causes are thought to be pathogen spillover from commercial colonies, pesticide use, and habitat loss (Colla and Packer 2008; Szymanski et al. 2016). The long colony establishment phase creates a high need for resources in the spring. The loss of spring- flowering resources in forests thus can negatively impact rusty patched bumble bee population viability. Commercial bumble bee use in crop pollination, such as greenhouse tomato production, are thought to have contributed to transmission of pathogens in wild bumble bees including the rusty patched (Cameron et al. 2011, 2016). Other threats include habitat alterations, including those for foraging, nesting, and overwintering habitats; insecticides, climate change, and inbreeding. It is unclear to what degree habitat management activities, such as buckthorn (Rhamnus sp.) removal by machines, cattle or goat grazing, prescriptive fire, or haying may affect the short or long-term occupancy of Rusty Patched Bumble Bees.  

  Conservation Efforts in Minnesota

There is a great deal of enthusiasm and support for the State Bee in Minnesota. Several efforts are underway to monitor and conserve the Rusty Patched Bumble Bee. The Minnesota Department of Natural Resources has worked to build foraging habitat by planting diverse, native plantings and supporting healthy prairies and forests through active management and protection of natural areas. The Board of Water and Soil Resources launched a campaign called Lawns to Legumes to help homeowners and communities create pollinator gardens targeted at the Rusty Patched Bumble Bee. The University of Minnesota has partnered with the Xerces Society for Invertebrate Conservation and the Minnesota DNR to support community science monitoring of the Rusty Patched Bumble Bee through the Minnesota Bumble Bee Atlas. Minnesota DNR staff will soon begin research and survey efforts aimed at better understanding the overwintering and nesting habitats. The U.S. Fish and Wildlife Service has developed Conservation Management Guidelines for the Rusty Patched Bumble Bee to help guide conservation actions.

  Authors/Revisions

Jessica D. Petersen (MNDNR), 2023, 2026

(Note: all content ©MNDNR)

  References and Additional Information

Boone, M. L., E. Evans, J. Watson, T. A. Smith, A. Wolf, and H. Minser. 2022. Notes from rusty patched bumble bee (Bombus affinis Cresson) nest observations. Insect Conservation and Diversity 1-5.

Cameron, S. A., H. C. Lim, J. D. Lozier, M. A. Duennes, and R. Thorp. 2016. Test of the invasive pathogen hypothesis of bumble bee decline in North America. Proceedings of the National Academy of Sciences 113(16): 4386-4391.

Cameron, S. A., J. D. Lozier, J. P. Strange, J. B. Koch, N. Cordes, L. F. Solter, and T. L. Griswold. 2011a. Patterns of widespread decline in North American bumble bees. Proceedings of the National Academy of Sciences (USA) 108(2): 662-667.

Colla, S. R., and L. Packer. 2008. Evidence for decline in eastern North American bumblebees (Hymenoptera: Apidae), with special focus on Bombus affinis Cresson. Biodiversity and Conservation 17: 1379-1391.

Hatfield, R., S. Colla, S. Jepsen, L. L. Richardson, R. Thorp, and S. F. Jordan. 2015. IUCN Assessments for North American Bombus spp. for the North American IUCN Bumble Bee Specialist Group. Xerxes Society for Invertebrate Conservation 1-56.

Macfarlane, R. P., K. D. Patten, L. A. Royce, B. K. W. Wyatt, and D. Mayer. 1994. Management potential of sixteen North American bumble bee species. Melanderia 50:1-12.

Mola, J. M., L. L. Richardson, G. Spyreas, D. N. Zaya, and I. S. Pearse. 2021. Long-term surveys support declines in early season forest plants used by bumblebees. Journal of Applied Ecology 58:1431-1441.

Plath, O.E. 1922. Notes on the nesting habits of several North American bumblebees. Psyche 29: 189-202.

Plath, O. E. 1927. Notes on the hibernation of several North American bumblebees. Annals of the Entomological Society of America 20:181-192.

Simanonok, M. P., C. R. V. Otto, R. S. Cornman, D. D. Iwanowicz, J. P. Strange, and T. A. Smith. 2021. A century of pollen foraging by the endangered rusty patched bumble bee (Bombus affinis): inferences from molecular sequencing of museum specimens. Biodiversity and Conservation 30:123-137.

Szymanski, J., T. Smith, A. Horton, M. Parkin, L. Ragan, G. Masson, E. Olson, K. Gifford, and Hil. 2016. Rusty Patched Bumble Bee (Bombus affinis) Species Status Assessment. Minnesota-Wisconsin Ecological Services Field Office. Bloomington, Minnesota.

Williams, P. H., R. W. Thorp, L. L. Richardson, and S. R. Colla. 2014b. Bumble bees of North America: an identification guide. Princeton University Press. 208 pp.

Wolf, A., T., J. C. Watson, T. J. Hyde, S. G. Carpenter, and R. P. Jean. 2022. Floral Resources Used by the Endangered Rusty Patched Bumble Bee (Bombus affinis) in the Midwestern United States. Natural Areas Journal 42:301-312.