FWS Focus



The rusty patched bumble bee is one of about 21 species of bumble bees in eastern United States, and was listed as federally endangered in 2017.

Historically, the rusty patched bumble bee was broadly distributed across the eastern United States and upper midwest, as well as in southern Quebec and Ontario, in Canada. This historical range continued south to the northeast corner of Georgia and reached west to the eastern edges of North and South Dakota, as noted in the species status assessment in 2016.

Prior to its listing as endangered in 2017, the species experienced a widespread and steep decline. The exact cause of the decline is unknown, but evidence suggests a synergistic interaction between an introduced pathogen and exposure to pesticides, specifically insecticides and fungicides, which was also noted in the species assessment. The species status assessment notes that the remaining populations are exposed to several interacting stressors, including pathogens, pesticides, habitat loss and degradation, non-native and managed bees, the effects of climate change climate change
Climate change includes both global warming driven by human-induced emissions of greenhouse gases and the resulting large-scale shifts in weather patterns. Though there have been previous periods of climatic change, since the mid-20th century humans have had an unprecedented impact on Earth's climate system and caused change on a global scale.

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and small population biology. These stressors likely operate independently and synergistically. For example, dietary stress due to insufficient floral resources may reduce an individual’s resiliency to pathogens and pesticides, exposure to insecticides can reduce resistance to disease and exposure to fungicides can increase insecticide toxicity.

The rusty patched bumble bee is a social species with an annual cycle that starts in early spring when colonies are initiated by solitary queens that emerge from overwintering sites. This cycle progresses with the production of workers throughout the summer, and ends with the production of males and new queens in late summer and early fall. Survival and successful recruitment require food from floral resources from early spring through fall, undisturbed nesting habitat in proximity to foraging resources and overwintering habitat for the next year’s queens.


The U.S. Fish and Wildlife Service wrote a species status assessment for the rusty patched bumble bee in 2016, listed the species in 2017 and finalized the recovery plan for the rusty patched bumble bee in 2021. Along with our partners, we are actively engaging in conservation of this species. This work includes, but is not limited to: surveys and monitoring, conservation planning, research, habitat management and enhancement, as well as outreach.

Additional resources

We also developed mapping tools for federal agencies, researchers and others who are interested in rusty patched bumble bee conservation:

Along with our partners, we've developed a wide selection of other resources to guide the conservation community as will all work to protect this species. These resources include survey guidance, survey reporting spreadsheets, voluntary guidance for conservation management, habitat assessment guidance, plant lists, identification and photography guidance, guidance for project proponents, as well as many other resources.

Scientific Name

Bombus affinis
Common Name
Rusty-patched bumble bee
Rusty patched bumble bee
FWS Category

Location in Taxonomic Tree

Identification Numbers



Characteristic category



Rusty patched bumble bees have been observed in a variety of habitats, including prairies, woodlands, marshes, agricultural landscapes and residential parks and gardens, as documented by S.R. Colla and L. Packer in 2008 and later by S.R. Colla and S. Dumesh in 2010. The rusty patched bumble bee requires areas that support sufficient food, including nectar and pollen from diverse and abundant flowers, as well as undisturbed nesting sites that are in proximity to those floral resources. These bees also require overwintering sites for hibernating queens, as documented by D. Goulson and others in 2015 and Potts and others in 2010.

Rusty patched bumble bee habitat can be divided conceptually into nesting and wintering, as well as foraging habitat types, based on the relative timing of pollen and nectar availability. The locations of pollen and nectar sources for the rusty patched bumble bee may vary throughout the growing season.


We assume that the rusty patched bumble bee nests in upland grasslands and shrublands that contain forage during the summer and fall and as far as 30 meters into the edges of forest and woodland. In 2019, J. Lanternman and others summarized 451 observations of nest-searching behavior by queens of nine bumble bee species. Although the rusty patched bumble bee was not among the nine species observed, their observations may shed some light on how the species searches for nest sites. J. Lanternman and others observed queens searching for nesting sites in open grassland habitats, but nest-seeking queens favored woody transitional habitats over open habitats.  

Rusty patched bumble bee nests are typically 1 to 4 feet underground in abandoned rodent nests or other mammal burrows and occasionally at the soil surface or aboveground, as documented by O.E. Plath in 1922 and later by R.P. Macfarlane in 1994. Among the 43 rusty patched bumble bee nest records cited by Macfarlane in 1994, 95% were underground. Queens may locate abandoned rodent burrows by using olfactory or chemical cues, as documented by Lanternman and others in 2019.


Little is known about the overwintering habitats of rusty patched bumble bee queens, but based on observations of other species we assume that rusty patched bumble bee queens overwinter in upland forest and woodlands. Other species of Bombus typically form a chamber in loose, soft soil, a few centimeters deep in bare earth, moss, under tree litter or in bare-patches within short grass and may avoid areas with dense vegetation, as documented by A.V. Alford in 1969 and later by A.R. Liczner and S. Colla in 2019. Overwintering habitat preferences may be species-specific and dependent on factors such as slope orientation and timing of emergence. Most queens in England were found in well-drained soil, shaded from direct sunlight in banks or under trees and free from living ground vegetation, as documented by A.V. Alford in 1969. A recent review of published literature shows that overwintering queens have been found mostly in shaded areas, usually near trees and in banks without dense vegetation, as noted by A.R. Liczner and S. Colla in 2019. The only known documented overwintering rusty patched bumble bee queen, discovered in a maple oak-woodland, which was about 0.5 kilometers into the woodlands, was in Wisconsin in 2016. It was found under a few centimeters of leaf litter and loose soil, as documented by B. Herrick from the University of Wisconsin-Madison.


Land on which the natural dominant plant forms are grasses and forbs.


A dense growth of trees and underbrush covering a large tract.


Of or relating to cities and the people who live in them.


Environments influenced by humans in a less substantial way than cities. This can include agriculture, silvaculture, aquaculture, etc.


Areas such as marshes or swamps that are covered often intermittently with shallow water or have soil saturated with moisture.

Characteristic category

Life Cycle

Life Cycle

Bumble bees live in colonies – cooperative groups that include the offspring of one female and one male. Healthy rusty patched bumble bee colonies are large and may include more than one thousand workers, or non-reproductive females. The workers protect the colony, forage for nectar and pollen and care for the young. Healthy colonies with many workers can produce dozens to hundreds of new queens, as documented by R.P. Macfarlane in 1974 and later with others in 1994.

Initially, colonies include only foundress queens, or original, but grow to include workers, males and new queens. In spring, queens emerge from their overwintering chambers to initiate colonies, having stored sperm from mating the previous autumn to fertilize eggs. Access to blooming flowers and a suitable nesting site – typically a rodent burrow – enables the queen to rear the first workers on her own. Lanternman and others noted in 2019 that a “continuous supply of floral resources is required to support the nest-founding stage…because each queen must forage for food as well as tend the nest, potentially limiting her mobility." Colony survival and productivity relies on continual access to blooming plant species throughout the spring, summer and early fall and protection from outside threats. Workers facilitate the production of the males and queens, which disperse from the nest to mate with reproductive progeny from other colonies that comprise the population, as documented by O.E. Plath in 1922, as was later confirmed by R.P. Macfarlane and others in 1994, as well as S.R. Colla and S. Dumesh in 2010. Before winter, the foundress queen, workers and males all die. Only the new queens, referred to as gynes, overwinter to initiate new colonies in the spring.

Life Span

Rusty patched bumble bees have an annual life cycle. Workers can be seen in the field several weeks after nest establishment throughout the summer and into early fall, late June through September. Males are in typically in flight during late summer and fall, August through September. The timing for observing queens depends on your geographical location. For example, in southern Wisconsin, queens, which are distinguished by their larger size and other characteristics described in the physical characteristics section, are in flight in spring, roughly April through May, and then again in late summer and fall.


Colonies produce reproductive males and queens in the late summer. The reproductive individuals leave the nest and mate. The workers and males die and only the queens enter diapause and overwinter. Little is known about rusty patched bumble bee mating behavior. The female emerges from diapause in the spring and then searches for a suitable nesting site. After establishing the nest, the queen lays eggs which hatch after approximately four days. The larvae feed on pollen and nectar and go through four instars before pupating, further developing and eventually hatching as full-size adults. Development time can take approximately five weeks, but can vary with temperature and food, as documented by D.V. Alford in 1975. Rusty patched bumble bee larvae live in cells and are fed individually by adults, which is called pollen-storer behavior, as documented by the Committee on the Status of Endangered Wildlife in Canada in 2010.

Characteristic category

Similar Species

Similar Species

Rusty patched bumble bees can be confused with:

  • Brown-belted bumble bee (Bombus griseocollis)
  • Half-black bumble bee (B. vagans)
  • Lemon cuckoo bumble bee (B. citrinus)
  • Confusing bumble bee (B. perplexus)

Rusty patched bumble bees are commonly confused with brown-belted bumble bees, because the second abdominal segment of the brown belted bumble bee has brown hairs towards the front and black hairs on the sides and back.

Queen rusty patched bumble bees, which do not have the distinctive rusty patch, can be confused with B. vagans, because the first two abdominal segments are yellow in both species.

There are 21 species of bumble bees in the eastern United States and 48 species across the country. If you think you have seen a rusty patched bumble bee, please try to take photographs - photos from the top, side and head are helpful - and upload to BumbleBeeWatch.org, BeeSpotter.org, iNaturalist.org or another community science site. Most of those sites provide or verify identifications, if it's possible, from the submitted photographs. Standard insect anatomy terms are used to describe bees for identification

Learn more about bumble bee anatomy for identification purposes.

Characteristic category



Bumble bees are generalist foragers, meaning they gather pollen and nectar from a wide variety of flowering plants, as noted by the Xerces Society in 2013. The rusty patched bumble bee is a short-tongued species, as described by J.T. Medler in 1962, so they are not able to easily access the nectar in flowers with deep corollas, meaning all of the petals of a flower. The species is one of the first to emerge early in the spring and the last to go into hibernation. That said, rusty patched bumble bee requires a constant and diverse supply of flowers that bloom throughout the colony’s long-life cycle to meet its nutritional needs, as documented by MacFarlane and others in 1994. The nectar from flowers provides carbohydrates and the pollen provides rusty patched bumble bees with protein. The number of queens that a colony can produce is directly related to the amount of pollen that is available, as documented by I. Burns in 2004. Rusty patched bumble bee may also be dependent on woodland spring ephemeral flowers, because they emerge early from their winter habitat in the spring, and is often associated near woodland habitats, as documented by S.R. Colla and S. Dumesh in 2010. The availability of floral resources is dependent on the proper soil and precipitation conditions to sustain them. Extended periods of drought, for instance, may lessen the availability and diversity of flowering plants in a given area because plant phenology is primarily driven by temperature, precipitation and the timing of snowmelt in the spring, as documented by D.W. Inouye and F.E. Wielgolaski in 2003, as well as G.H. Pyke and others in 2016.

Characteristic category



Seasonal activity

Based on a review of rusty patched bumble bee observation records, in most years, the rusty patched bumble bee may only be active above ground between about March 15 through October 10 and April 10 through October 10, south and north of 42º latitude, respectively. Although air temperatures may be conducive to activity later in the fall, cessation of flight “appears to be timed with the passing of native fall flowers," as noted by D.F. Schweitzer and others in 2012.

Daily activity

The rusty patched bumble bee is active under a broad range of conditions, but remains inactive when conditions are too cold or rainy. A study that included four bumble bee species found minimum calculated air temperature for activities that ranged from 3.6 to 12.6°C. There is no similar data for the rusty patched bumble bee, but it’s reasonable to assume that the species could be active between dawn and dusk at temperatures as low as about 4°C (39°F) within the seasons described above. Bumble bees do not typically fly when conditions are foggy, rainy or drizzling. Sunny days with low wind speeds, of less than 8 miles per hour, may be optimal, but they will fly during sub-optimal conditions.

Overwintering and nesting periods for rusty patched bumble bee

  • Latitude <42degrees N: Overwintering period is October 10 through March 15. Nesting Period is March 16 through October 9
  • Latitude >42degrees N: Overwintering period is October 10 through April 10. Nesting Period is April 9 through October 9
Characteristic category

Physical Characteristics

Size & Shape

Rusty patched bumble bees range in size within and between castes.


  • Queen length: 19 to 23 mm
  • Female worker length: 9 to 16 mm
  • Male length: 14 to 17 mm
Color & Pattern

All rusty patched bumble bees have a mostly yellow upper thorax, with a black spot or band between the wings that may extend toward the back in a v-shape, resembling a thumb tack. The bottom of the thorax is black. In workers and males, the first abdominal segment is yellow and the second has a patch of rusty hairs on the front portion of the segment, with yellow hairs on the back and sides. Rusty patched bumble bee queens are entirely yellow on the first two abdominal segments and the rest of the abdominal segments are black.



Rusty patched bumble bees historically occurred in the eastern and midwestern United States and southern Canada. The species used to occur broadly across the eastern United States, upper midwest and southern Quebec and Ontario. Since about 2007, a decline has been documented across its range in the United States. Similar declines have occurred in Canada, where it was listed as endangered on Schedule 1 of the Species at Risk Act in 2012.

Additional resources

We also developed mapping tools for federal agencies, researchers and others who are interested in rusty patched bumble bee conservation:

Launch Interactive Map


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