Bombus affinis    Cresson, 1863

Rusty Patched Bumble Bee 

MN Status:
Federal Status:


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Minnesota range map
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North American range map
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  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 their 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, there have been consistent contemporary detections in Minnesota, primarily in the Eastern Broadleaf Forest Province, but their abundance has declined. In 2019, the Rusty Patched Bumble Bee was designated the State Bee (2019 Minnesota Statutes, Section 1.14652019) 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 (IUCN) in 2014 (Hatfield et al. 2015).


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 yellow hairs on the first and second abdominal segments, 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 Bumble Bee (Bombus sandersoni) by the combination of three main characteristics: 1. A rusty orange or red crescent shape in the middle of the upper half of the second abdominal segment, 2. Black hairs on the thorax in the shape of the profile of a thumbtack, and 3. A short face. The Rusty Patched Bumble Bee is often confused with the Brownbelted Bumble Bee (Bombus griseocollis). The Rusty Patched Bumble Bee has yellow hairs on the second abdominal segment that extend to the lateral edges, whereas the Brownbelted Bumble Bee has black hairs laterally on the second segment with more of a brown swoop in the center of the same segment. The Rusty Patched Bumble Bee is also frequently confused with the Tricolored Bumble Bee (Bombus ternarius), which has two entire abdominal segments (segments two and three) that are vibrant orange, whereas the Rusty Patched Bumble Bee only has a partial second segment that is more rust colored. One final commonly confused species is the Redbelted Bumble Bee (Bombus rufocinctus), which will have red hairs when present on abdominal segments three and four. Queen Rusty Patched Bumble Bees do not resemble workers in that they lack the rusty patch and the thumb tack outline on the thorax. They can be distinguished from the Half-black Bumble Bee queens by having short, cropped hair, a generally larger body (Rusty Patched Bumble Bee: 19-23 mm [0.75-0.91 in.]; Half-black Bumble Bee: 17-21 mm [0.67-0.83 in.]), short face, and only black hair on the face (Williams et al. 2014).


Workers and queens require consistent forage throughout the entire growing season, beginning with spring ephemerals when they first emerge around April (Mola et al. 2021), throughout the summer to provision their brood, and into the fall to build resources to overwinter. They are generalist foragers, preferring Monarda fistulosa (wild bergamot), Eutrochium maculatum (spotted Joe pye weed), Veronicastrum virginicum (Culver’s root), and Agastache spp. (hyssop) in the Midwest (Wolf et al. 2022), 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, wetlands, grasslands, roadsides, agricultural fields, and residential lawns and parks (Colla and Packer 2008). Little is known about the nesting and overwintering habitats of the Rusty Patched Bumble Bee (Boone et al. 2022).

  Biology / Life History

The Rusty Patched Bumble Bee is a highly social animal that lives in annual colonies that include a single foundress queen, female workers, males and, later in the season, new queens (gynes). 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). The foundresses will seek a suitable nesting location, usually an abandoned rodent burrow in either open grasslands or forest edges. Of the few nests reported in the literature, most were subterranean, typically 0.30-1.22 m (1-4 ft.) underground (Plath 1927; Macfarlane et al. 1994). Queens provision their developing brood with pollen and nectar, protect the nest, and tend to the first brood of workers that begin to emerge in earnest in mid-July in Minnesota. The workers then take over the responsibility of foraging for nectar and pollen for additional broods throughout the summer. One unusual characteristic of Rusty Patched Bumble Bee 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 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 1000 total bees (Boone et al. 2022), but the Rusty Patched Bumble Bee 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 bumble bee colonies, pesticide use, and habitat loss (Colla and Packer 2008; Szymanski et al. 2016). 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 Bumble Bee (Cameron et al. 2011, 2016). Other threats include habitat alterations, including those for foraging, nesting, and overwintering; insecticides, climate change, and inbreeding. It is unclear to what degree habitat management activities, such as buckthorn 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 (MNDNR) 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 MNDNR to support community science monitoring of bumble bees through the Minnesota Bumble Bee Atlas. Minnesota DNR staff will soon begin research and survey efforts aimed at better understanding the species’ 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 and guidance for reviewing actions for potential impacts to the species.


Jessica D. Petersen (MNDNR), 2023

(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.

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