The Rusty-patched bumble bee Bombus affinis

The Rusty-patched Bumble Bee
Bombus affinis Cresson 1863

B.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 steeply declined 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.

IDENTIFICATION

In order to properly identify bumble bees, you need to first determine whether the bee you are examining is male or female. There are three different types of bumble bees you will encounter: workers, queens, and males. Both queens and workers are female. In most cases, queens and workers have similar coloration and physical features, except that queens are much larger. Males can differ in coloration from females of the same species, as well as other physical characteristics. Once you have determined the sex of your bee, you will want to identify whether it is a Rusty-patched Bumble Bee or another species of bee.

Male	Female	Is your bumble bee male or female?
		Males have thirteen antennal segments versus twelve antennal segments in females. Males of some species will have larger eyes, or longer hair than females of the same species.
		Males differ from females in having seven abdominal segments, whereas females have only six abdominal segments.
		Males do not have pollen baskets. A pollen basket is a broad concave shiny segment rimmed with long hairs that is found on the back legs of a female bumble bee. Pollen baskets are used to carry pollen back to the nest.

Is it the Rusty-patched Bumble Bee?
The Rusty-patched Bumble Bee (Bombus affinis) 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.

Rusty-patched Bumble Bee
Bombus affinis worker

Half-black Bumble Bee
Bombus vagans worker

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.

Rusty-patched Bumble Bee
Bombus affinis worker

Brown-belted Bumble Bee
Bombus griseocollis worker

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.
		Rusty-patched Bumble Bee Bombus affinis worker	Common Eastern Bumble Bee Bombus impatiens worker

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.
		Rusty-patched Bumble Bee Bombus affinis worker	Two-spotted Bumble Bee B. bimaculatus worker

For an online key, photographs of specimens of the Rusty-patched Bumble Bee and extensive identification information, visit the Discover Life website.

DISTRIBUTION

Historic range of the Rusty-patched Bumble Bee	Current range of the Rusty-patched Bumble Bee

In the past, the Rusty-patched Bumble Bee was common and widespread across the northeast; map from Milliron 1971, A monograph of the western hemisphere bumblebees.	Where it was once common, the Rusty-patched Bumble Bee has not been found, or has been found only in very small numbers; map adapted from Milliron 1971, A monograph of the western hemisphere bumblebees.

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, and portions of North 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 in Illinois and a single site in Wisconsin.

Please contact us if you have any records of the Rusty-patched Bumble Bee from the last 10 years. If you do research on bumble bees, have incidental bumble bees in your collection, or have student insect collections from the past few years, it would help us to know if you have or have not seen these bees. It is as important for us to document where these bees were formerly common, but not recently collected, as it is to document where they were collected.

Download a WANTED poster (pdf, ~2 MB) for the Rusty-patched Bumble Bee

Read the status review for B. affinis, B. terricola and B. occidentalis on our Red List of Pollinator Insects. Note, this status review is currently being updated with more recent information.

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, 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.