Bumble bees: rusty patched bumble bee (Bombus affinis)
The rusty patched bumble bee is an eastern bee whose workers 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 is an excellent pollinator of wildflowers, cranberries, and other important crops, including plum, apple, alfalfa and onion seed.
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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 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
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 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. griseocollis does 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
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. affinis with 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
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.
For an online key, photographs of specimens of the rusty-patched bumble bee and extensive identification information, visit the Discover Life website.
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.
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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). The North American bumble bees would have had no prior resistance to this pathogen. Dr. Thorp hypothesizes 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.
Where these bees were once very common, they were nearly impossible to find. B. impatiens has not shown a dramatic decline; Robbin Thorp hypothesizes that B. impatiens may serve as a carrier of an exotic strain of Nosema bombi, although it may not be as severly affected by the disease as B. affinis, B. occidentalis, B. terricola, and B. franklini. B. affinis, B. occidentalis, B. terricola, and B. franklini are closely related to each other (they all belong to the subgenus Bombus sensu stricto). B. impatiens is not as closely related, which may explain the difference in sensitivity to a pathogen. This hypothesis is still in need of validation, although the timing, speed, and severity of the population crashes strongly supports the idea that an introduced disease caused the decline of bees.
The rearing company Koppert recently applied for a permit to transport the eastern species Bombus impatiens to California for crop pollination. The Xerces Society worked with bumble bee researchers to prepare comments to the USDA/APHIS discouraging the movement of these bees into new areas.
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. Insecticide application on Forest Service managed public lands for spruce budworm has been shown to cause massive kills of bumble bees and reduce pollination of nearby commercial blueberries in New Brunswick. Broad-spectrum herbicides used to control weeds can indirectly harm bumble bees by removing the flowers that would otherwise provide the bees with pollen and nectar.
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.