Tips for Identifying the Rusty-patched Bumble Bee (Bombus affinis)


Now that the Rusty Patched Bumble Bee is protected by the U.S. Endangered Species Act, having the ability to recognize it is invaluable – both to prevent accidental collection and promptly report new observations. While positive identification in the field may not always be possible, especially for novice observers, you can learn to distinguish this extremely rare species from other more common rust-colored bumble bees that you will encounter.



Timing: In Maine, Bombus affinis queens would likely be active from April through early June. After early to mid-June, they would remain in the nest to focus on egg-laying and you would not likely see them out foraging. In late summer and early fall, newly emerged queens would be active for a short time before hibernating for the winter.

Key Characteristics: Here’s the tricky part. Rusty Patched queens do NOT have a rusty patch! Queens of this species will be very difficult for all but experienced observers to identify in the field. If you rely strictly on color pattern, the first two abdominal segments are all yellow and the rest of the abdomen is black. Unfortunately, this is the same abdominal color pattern as several common species, including Bombus vagans, B. perplexus, B. griseocollis and B. sandersoni. A key feature to distinguish B. affinis from these species is a short malar (cheek) space – something you are not going to be able to see well, except under a microscope.

MBBA does not expect you to be able to distinguish Rusty Patched queens in the field. It will be the rare participant who has or develops the expertise to recognize one without confusing it with a similar species. As long as you are collecting outside the USFWS “photographs only” zones (see page 6 of the Participant Handbook), you don’t need to worry if you accidentally collect a queen. [REMINDER: We recommend you not collect more than one queen of each “type” (i.e., based on variations in color pattern) that you observe during a Site Visit. If you are hesitant or concerned about collecting queens, focus spring surveys on image vouchers or refrain from collecting until the workers emerge, usually starting in early June.]



Timing: Bombus affinis workers would likely emerge in late May and early June, and be present throughout the summer. Early workers would be much smaller than their sisters born later in the summer. Males would be most evident in mid to late summer through early fall.

Key Characteristics: Here’s where the rusty patch comes in! Like the queen, the first two abdominal segments are yellow and the rest of the abdomen is black. But both workers and males have a rust-colored “patch” on the upper middle part of the second abdominal segment, bordered by yellow along the sides and bottom.















There are three other Maine species with rust color on the abdomen, but with careful attention you can learn to tell them apart.

The Orange Belted or Tricolored Bumble Bee (B. ternarius) is the most common bumble bee you will see with rust color on its abdomen. This species is very abundant and found statewide, so you should easily learn to recognize it. The key distinguishing feature of this bee is that both the second and third abdominal segments are completely orange or rust colored. The orange mid-section is bounded by a complete band of yellow above and below, and just the tip of the abdomen is black. Sometimes the orange color is very pale.

The Red Belted Bumble Bee (B. rufocinctus) looks somewhat similar to the Orange Belted Bumble Bee, but it can be quite variable in color pattern. This is an uncommon species in Maine, so you are not likely to come across it often. A key feature to distinguish the Red Belted from the Rusty Patched is the rust coloring is typically confined to the third and fourth abdominal segments, which concentrates the rust on the lower half of the abdomen. Even though rust coloring can sometimes be found on other segments, there is typically much more of it on the abdomen of a Red Belted than the isolated small patch on the Rusty Patched.












The Brown Belted Bumble Bee (B. griseocollis) is known primarily from the southern half of the state, and is not a species that is frequently observed. Similar to the Rusty Patched, this species typically has a patch of rust, or more commonly BROWN, in the upper middle half of the second abdominal segment. Unlike the Rusty Patched, the brown patch is bordered by BLACK instead of yellow on the sides and bottom.











See the MBBA Participant Handbook for a list of bumble bee identification guides and on-line tools.