The rusty patched bumble bee is a species of bumble bee native to eastern North America. Its workers and males have a small rust-colored patch on the middle of their second abdominal segment. This bee was once commonly distributed throughout the east and upper Midwest of the United States, but has declined from an estimated 87% of its historic range in recent years. The rusty-patched bumble bee was once an excellent pollinator of wildflowers, cranberries, and other important crops, including plum, apple, alfalfa and onion seed.
Responding to a petition filed by the Xerces Society in 2013 to list the rusty patched bumble bee as an endangered species under the US Endangered Species Act (ESA), the U.S. Fish and Wildlife Service (USFWS) finalized the ruling and gave the rusty patched bumble bee endangered status under the ESA in January of 2017.
In order to properly identify any bumble bee, you need to first determine whether the bee you are examining is a female worker, a queen or a male bee. Then, you can begin to determine whether your bee is a Rusty-patched Bumble Bee or some other species of bumble bee. The rusty-patched bumble bee workers have a distinctive rusty patch on the front half of their second abdominal segment. The first abdominal segment and the rear half of their second abdominal segment are both yellow. All other abdominal segments are black. The hair on the heads of B. affinis workers is mostly black throughout. On the thorax, black hairs extend from a central patch in the middle of the thorax out towards the wings and centrally in a narrow V towards the rear. The coloration of Bombus affinis queens and males differ from the workers in their lack of a strong rusty patch on the second abdominal segment. Queens also differ in having the thorax mostly yellow except for a small central bare patch. Similar bumble bees that occur in the same region are B. vagans, B. griseocollis, B. impatiens, and B. bimaculatus.
Distinguishing B. affinis from B. vagans
Bombus affinis (left) and Bombus vagans (right)
B. vagans have a longer face than B. affinis. B. vagans workers and queens have yellow hair on the first two abdominal segments and black on the rest of the abdominal segments. There is no rusty patch on their abdomen. B. vagans have a patch of yellow hair the top of their heads in contrast with B. affinis patch of black hairs.
In addition to the lack of the rusty patch, male B. vagans can be distinguished from B. affinis by yellow hairs along the margins of their abdominal segments and some yellow hairs mixed in among the black hair of the more apical abdominal segments.
Distinguishing B. affinis from B. griseocollis
Bombus affinis (left) and Bombus griseocollis (right)
Bombus griseocollis can be distinguished from B. affinis by several key features. B. affinis have a stripe of black hairs that extends between the wings on the thorax. B. griseocollis have a central bare black spot with only a few black hairs at the edges of this spot. The hair on the thorax of B. griseocollis workers is predominately yellow. B. affinis have yellow hairs extending to the lateral margins of the second abdominal segment. B. griseocollis have black hairs along the sides of the second abdominal segment. B. griseocollisdoes have a rusty brownish patch in the middle of its second abdominal segment but this patch is flanked by black hairs along the rear and the sides of the segment.
Male B. griseocollis are easily distinguished from B. affinis by their large eyes and a prominent patch of dense yellow hairs on the front of their faces.
Distinguishing B. affinis from B. impatiens
Bombus affinis (left) and Bombus impatiens (right)
B. impatiens queens, workers, and males have yellow on only the first abdominal segment, with the rest of the segments black, whereas B. affinis have yellow on the first and second abdominal segments. Also, B. impatiens have a bare patch in the middle of the thorax surrounded by yellow hair, as opposed to B. affiniswith their patch of black hair extending between the wing bases. Male B. impatiens have a prominent patch of yellow hair on the front of their face, as opposed to B. affinis with mostly black hair on the front of the face.
Distinguishing B. affinis from B. bimaculatus
Bombus affinis (left) and Bombus bimaculatus (right)
B. bimaculatus have longer faces than B. affinis. B. bimaculatus queens, males, and workers have black along the sides of their second abdominal segment, whereas B. affinis have yellow hairs that extend to the sides. B. bimaculatus have yellow hairs in a central notched pattern on the second abdominal segment. Workers of B. bimaculatus also have a bare patch in the middle of the thorax surrounded by predominately yellow hair, as opposed to B. affinis with their patch of predominately black hair extending between the wing bases. Male B. bimaculatus have a prominent patch of yellow hair on the front of their face, as opposed to B. affinis with mostly black hair on the front of the face.
All bumble bees belong to the genus Bombus within the family Apidae. The family Apidae includes the well-known honey bees and bumble bees, as well as carpenter bees, cuckoo bees, digger bees, stingless bees, and orchid bees. B. affinis belongs to a sub-genus of Bombus, Bombus sensu stricto.
Bumble bees are important pollinators of wild flowering plants and crops. As generalist foragers, they do not depend on any one flower type. However, some plants do rely on bumble bees to achieve pollination. Loss of bumble bees can have far ranging ecological impacts due to their role as pollinators. In Britain and the Netherlands, where multiple bumble bee and other bee species have gone extinct, there is evidence of decline in the abundances of insect pollinated plants.
Bumble bees are also excellent pollinators of many crops. Bumble bees are able to fly in cooler temperatures and lower light levels than many other bees, and they perform a behavior called “buzz pollination”, in which the bee grabs the pollen producing structure of the flower in her jaws and vibrates her wing musculature causing vibrations that dislodge pollen that would have otherwise remained trapped in the flower’s anthers. Some plants, including tomatoes, peppers, and cranberries, require buzz pollination.
Historically, the rusty-patched bumble bee was distributed along the east coast of the United States from southern Maine south through Georgia with an extension west along the northern states through Minnesota. A few individuals have been found as far west as North Dakota. The former range of the Rusty-patched bumble bee includes these states: Minnesota, Wisconsin, Indiana, lower Michigan, Ohio, Pennsylvania, New York, Vermont, New Hampshire, Maine, Maryland, Virginia, North Carolina, South Carolina, Georgia, Connecticut, Massachusetts, Delaware, New Jersey, West Virginia, and portions of North Dakota, South Dakota, Iowa, Illinois, Kentucky, and Tennessee. A number of surveys have been done, but the Rusty-Patch Bumble Bee has not been found in most of its range since 2003 with the exception of a few isolated areas.
Although this species was formerly commonly found through most of its range, surveys between 2003 and present have found very few B. affinis. Recently, B. affinis has been found in small numbers in isolated areas primarily in the northern part of its range. In 2017 this species was listed as endangered under the U.S. Endangered Species Act. In 2014 the state of Vermont listed B. affinis as an endangerd species and in 2015 this species was listed as Critically Endangered on the IUCN Red List. In Canada B. affinis was protected under the federal Species at Risk Act (SARA) in 2010.
There are a number of threats facing bumble bees, any of which may be leading to the decline of the rusty-patched bumble bee. The major threats to bumble bees include: spread of pests and diseases by the commercial bumble bee industry, other pests and diseases, habitat destruction or alteration, pesticides, invasive species, natural pest or predator population cycles, and climate change.
Commercial bumble bee rearing may be the greatest threat to Bombus affinis. In North America, two bumble bee species have been commercially reared for pollination of greenhouse tomatoes and other crops: B. occidentalis and B. impatiens. Between 1992 and 1994, queens of B. occidentalis and B. impatiens were shipped to European rearing facilities, where colonies were produced then shipped back to the U.S. for commercial pollination. Bumble bee expert Robbin Thorp has hypothesized that these bumble bee colonies acquired a disease (probably a virulent strain of the microsporidian Nosema bombi) from a European bee that was in the same rearing facility, the Buff-tailed Bumble Bee (Bombus terrestris). Dr. Thorp hypothesized that the disease then spread to wild populations of B. occidentalis and B. franklini in the West (from exposure to infected populations of commercially reared B. occidentalis), and B. affinis and B. terricola in the East (from exposure to commercially reared B. impatiens). In the late 1990’s, biologists began to notice that B. affinis, B. occidentalis, B. terricola, and B. franklini were severely declining.
Many parts of this hypothesis were supported by recent scientific evidence which documented that commercial bumble bees were responsible for the spread and amplification of Nosema bombi, and that this likely has had a profound effect on wild bumble bees, including the rusty patched bumble bee. They did not find evidence that the pathogen had been introduced from Europe. Bumble bees are reared commercially for use as pollinators of agricultural crops and it has been clearly documented that these commercial bumble bees carry high pathogen loads, and regularly interact with wild bumble bees near greenhouses and in open field settings.
Besides the threat posed by the commercial bumble bee industry, there are many other threats to wild bumble bee populations. Bumble bees are threatened by many kinds of habitat alterations which may destroy, alter, fragment, degrade or reduce their food supply (flowers that produce the nectar and pollen they require), nest sites (e.g. abandoned rodent burrows and bird nests), and hibernation sites for over-wintering queens. Major threats that alter landscapes and habitat required by bumble bees include agricultural and urban development. Livestock grazing also may pose a threat to bumble bees, as animals remove flowering food sources, alter the vegetation community, and likely disturb nest sites. As bumble bee habitats become increasingly fragmented, the size of each population diminishes and inbreeding becomes more prevalent. Inbred populations of bumble bees show decreased genetic diversity and increased risk of decline.
Insecticide applications on farms poses a direct threat to foraging bumble bees. Specifically, the use of highly toxic insecticides known as neonicotinoids continues to grow throughout the range of the rusty patched bumble bee. A number of scientific articles clearly document the lethal and sublethal effects that these chemicals are having on bees and other pollinators, and their use has intensified extensively within the rusty patched bumble bee’s range during the same time period that declines have been observed. Moreover, the massive increase in the past two decades in the use of the herbicide glyphosate on genetically modified corn and soybean fields has been effective at eliminating milkweed from the agricultural landscape. It is likely that other wildflowers have also been eliminated from farm edges – and it is reasonable to assume that a major loss of floral resources from the Upper Midwest could have had an effect on the rusty patched bumble bee. While no direct link has been made from the use of these pesticides to the declines observed in the rusty patched bumble bee there is little doubt that stressors like pesticides at the very least put increased pressures on an already imperiled bumble bee, especially when one considers the scope at which these chemicals are being adopted and used.
Bumble bees are threatened by invasive plants and insects. The invasion and dominance of native grasslands by exotic plants may threaten bumble bees by directly competing with the native nectar and pollen plants that they rely upon. In the absence of fire, native conifers encroach upon many meadows, which removes habitat available to bumble bees. The small hive beetle (Aethina tumida) is an invasive parasite of the honeybee, yet it also infests bumble bee colonies. Its actual impact on bumble bee colonies could be severe, although it has not been well studied.Global climate change also poses a real threat to bumble bees; anecdotal evidence has suggested that some of the bumble bee species adapted to cool temperatures are in decline, whereas warmer adapted species are expanding their ranges. Baseline data and long term monitoring are needed to better understand the true impact of climate change on bumble bees.
Much of the content for this page was developed from a status review, co-authored by professor emeritus Robbin Thorp (U.C. Davis Department of Entomology), Elaine Evans, Sarina Jepsen and Scott Hoffman Black (Xerces). Bee illustrations were provided by Elaine Evans.
Funding for our efforts to conserve bumble bees in decline has been generously provided by the CS Fund and Xerces Society members.