Plants and Animals

Bombus pensylvanicus American bumble bee

Key Characteristics

Queens of this species are relatively large, ranging from 2.1 to 2.5 cm. Workers are average sized at 1.4 to 1.8 cm and males are slightly larger than females at 1.6 to 2.2 cm. Body hair is short and even and ocelli are high on the face. Queens and workers are similar in appearance. Characteristics of queens and workers include a black face and top of the head; thorax with yellow along the anterior section and black posteriorly, with occasional yellow hairs mixed in; wings with a distinct black band; abdominal segments T1, T2 and T3 are always yellow with the remaining abdominal segments being black.  Males are similar in appearance to queens and workers but males tend to have more interspersed yellow hairs throughout. Additional yellow hairs can be observed on the entire abdomen and posterior sections of the thorax. Male faces are mostly yellow, with scattered black hairs (Mitchell 1962, Williams et al. 2014)

Status and Rank

US Status: No Status/Not Listed
State Status: SC - Special Concern (rare or uncertain; not legally protected)
Global Rank: G3G4 - Rank is uncertain, ranging from vulnerable to apparently secure
State Rank: S1 - Critically imperiled

Occurrences

CountyNumber of OccurrencesYear Last Observed
Allegan 3 1963
Arenac 1 1935
Barry 3 1965
Bay 1 1946
Benzie 1 1958
Berrien 4 1982
Branch 1 1924
Calhoun 1 1927
Clare 1 1972
Clinton 2 1993
Eaton 1 1918
Hillsdale 2 1979
Huron 2 1934
Ingham 3 1970
Ionia 2 1954
Isabella 1 1965
Jackson 3 1980
Kalamazoo 12 1965
Kent 2 1946
Lapeer 1 1935
Lenawee 5 1981
Livingston 4 1982
Macomb 3 1957
Manistee 1 1964
Mason 2 1992
Mecosta 1 1942
Midland 1 1935
Monroe 1 1970
Muskegon 1 1934
Newaygo 3 2020
Oakland 3 1964
Oceana 3 1970
St. Joseph 2 1959
Van Buren 1 1939
Washtenaw 7 1981
Wayne 1 1974

Information is summarized from MNFI's database of rare species and community occurrences. Data may not reflect true distribution since much of the state has not been thoroughly surveyed.

Habitat

This species favors prairies and grasslands and is rarely associated with extensive forests. Considered a foraging generalist, this bumble bee will regularly visit floral resources in dunes, marshes, forest edges, farmland and urban areas.  Known foraging resources include milk-vetches (Astragalus spp.), thistles (Cirsium spp.), dogwoods (Cornus spp.), prairie-clovers (Dalea spp.), and sunflowers (Helianthus spp.), among other species (Wood et al. 2019). Nests are above ground among tufts of long grass, but occasionally occur underground in old rodent burrows. This species can also occupy agricultural landscapes, however associated pressures such as ground management and pesticides may restrict populations in these areas. Most Bombus queens overwinter in rotten wood or underground (COSEWIC 2010).

Natural Community Types

For each species, lists of natural communities were derived from review of the nearly 6,500 element occurrences in the MNFI database, in addition to herbarium label data for some taxa. In most cases, at least one specimen record exists for each listed natural community. For certain taxa, especially poorly collected or extirpated species of prairie and savanna habitats, natural community lists were derived from inferences from collection sites and habitat preferences in immediately adjacent states (particularly Indiana and Illinois). Natural communities are not listed for those species documented only from altered or ruderal habitats in Michigan, especially for taxa that occur in a variety of habitats outside of the state.

Natural communities are not listed in order of frequency of occurrence, but are rather derived from the full set of natural communities, organized by Ecological Group. In many cases, the general habitat descriptions should provide greater clarity and direction to the surveyor. In future versions of the Rare Species Explorer, we hope to incorporate natural community fidelity ranks for each taxon.

Management Recommendations

Common and widespread in its distribution prior to the late-1990s, this species, along with other species of bumble bees, has recently experienced declines throughout its historic range (Colla et al. 2012).  In Michigan, the range of B. pensylvanicus has decreased by approximately 98% compared to historic populations (Wood et al. 2019, Rowe et al. 2019). These losses are primarily attributed to three main threats: the widespread use on cropland since the early 1990s of neonicotinoids, a pesticide group highly toxic to bees,; both nesting and foraging habitat loss due to increased urbanization and agriculture, particularly the conversion of pasture to corn and soybeans; and the spread of pathogens from bumble bees used as pollinators in commercial greenhouse operations to wild populations nearby (Colla and Packer 2008, COSEWIC 2010, Grixti et al. 2009, Cameron et al. 2011). Conservation strategies for bumble bees center around preserving healthy natural habitat areas, reducing pesticide/herbicide use, and promoting native wildflower reestablishment within urban and agricultural landscapes. Additionally, planting hedgerows and restoring native grasses along field margins, and in urban parks and residential yards provides habitat for small mammals, whose abandoned holes may in turn become bumble bee nesting and hibernating habitat (Goulson 2010).

Active Period

Breeding from first week of September to first week of October

Flight from first week of July to fourth week of September

Nesting from fourth week of May to first week of July

Survey Methods

Surveys typically involve capturing foraging bumble bees with an aerial net and identifying the individual species (Colla and Packer 2008, Grixti et al. 2009). Observational or visual surveys are also common, and generally require two to three good photos for identification.

Aerial net, visual survey

Survey Period: From fourth week of May to first week of October

Time of Day: Daytime
Cloud Cover: Clear
Air Temperature: Above 60 degrees
Wind: No Wind

References

Survey References

  • 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: 1379-91.
  • Grixti, J.C., L.T. Wong, S.A. Cameron and C. Favret. 2009. Decline of bumble bees (Bombus) in the North American Midwest. Biological Conservation 142: 75-84.

Technical References

  • Cameron, S.A., J.D. Lozier, J.P. Strange, J.B. Koch, N. Cordes, L.F. Solter, and T.L. Griswold. 2011. 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: 1379-91.
  • Colla, S.R., F. Gadallah, L. Richardson, D. Wagner, and L. Gall. 2012. Assessing declines of North American bumble bees (Bombus spp.) using museum specimens. Biodiversity and Conservation 21(14): 3585-3595.
  • COSEWIC. 2010. COSEWIC assessment and status report on the Rusty-patched Bumble Bee Bombus affinis in Canada. Commitee on the Status of Endangered Wildlife in Canada. Ottawa. 34pp.
  • Goulson, D. 2010. bumblebees: behavior, ecology and conservation. Second Edition. Oxford University Press, New York. 317pp.
  • Grixti, J.C., L.T. Wong, S.A. Cameron and C. Favret. 2009. Decline of bumble bees (Bombus) in the North American Midwest. Biological Conservation 142: 75-84.
  • Mitchell, T.B. 1962. Bees of the Eastern United States. North Carolina Agricultural Experiment Station Technical Bulletin No. 152.
  • Rowe, L.M., D.L. Cuthrell, and H.D. Enander. 2019. Assessing Bumble Bee Diversity, Distribution, and Status for the Michigan Wildlife Action Plan. Michigan Natural Features Inventory, Report Number 2019-33, Lansing, USA.
  • Williams, P.H., R.W. Thorp, L.L. Richardson, and S.R. Colla. 2014. Bumble bees of North America: an identification guide. Princeton University Press.
  • Wood, T. J., J. Gibbs, K.K. Graham, and R. Isaacs. 2019. Narrow pollen diets are associated with declining Midwestern bumble bee species. Ecology, 100(6), e02697.