Bombus occidentalis, the Western Bumble Bee, has several color forms or patterns. Color forms refer the coloration of the bumble bee with respect to the location of the hairs on its body. Solid areas of colors often present as bands; sometimes they can also take the form of patches or spots. If two hair colors intermingle in the same location on the body, the hair appearance is often referred to a “mixed.” Bumble bee hair colors include black, yellow or white (sometimes both), orange or brown.
A specific color form can be associated with the ecosystem where it occurs. For example, a bumble bee may have one color form near a coastal area, and different color form inland. Sometimes color forms mix or overlap in a region. Two color forms may both occur in an area that straddles two habitats; for example, in a transitional zone where coastal habitat and inland foothills merge. Even within a specific color form, there may be some variability from one individual to the next. Aside from their color forms, queens, worker females and males differ from each other in appearance, anatomy, size and length, according to their biological and task-related roles.
Identification guides are helpful for sorting through the variables found in the field. Two guides I regularly refer to are: Western Bumble Bees of the United States1 and the BugGuide website. BugGuide is a reference tool that covers all insects, spiders and kin. The BugGuide end note for this article directs the reader to the Bombus occidentalis species page.2 I am a generalist, an enthusiastic amateur in the study of native bees, with no formal training in entomology. I find these guides to be essential, refer to them often and am always looking for more additional resources.
A guide should not be taken literally. The actual bumble bee one sees in the field does not necessarily correspond exactly to the diagrams or photographs provided in a guide. Depending on when the guide was last updated, there may be new identification and taxonomic information out there on a species that has not yet been distributed and incorporated into the references or guide(s) of choice. When I am confused because the color form of a bumble bee does not seem to match my go-to reference guides, I try to consider other factors such as floral resources, the shape and size of the bumble bee’s head and face, body size, tongue length (if I can see it). I consider what other species are known occur in the location around the same timeframe. Perhaps I am looking at a different species altogether. Maybe I am still learning how to look at bumble bees and at every other life form I encounter. If that be so, then I am happy to remain a student, to continue my studies as long as I can still breathe.
I use iNaturalist to log my observations. Last summer as a volunteer participant in the California Bumble Bee Atlas project, I also logged bumble bee observations in Bumble Bee Watch. Both of these platforms represent a collaboration between generalists and specialists. The latter group are scientists who possess the knowledge to make expert identifications if 1) the photos submitted are of high enough in quality; and 2) identification does not require examining the organism under a microscope. (Note: Some native bees are particularly difficult to identify to species without very high quality macro photography or microscopic examination.)
Scientific studies that report new information about a species often sit behind a formidable subscription paywall, but some of them are free to read. It can be informative to use a search engine to check what is out there. The actual bumble bee one encounters may not look exactly like any of the diagrams or detailed macro photo examples she has available for reference.
The top two photographs included here are of a male B. occidentalis that was feeding on Common Woolly Sunflower. I took the photos along the shore of Odell Lake near Crescent Lake, Oregon in the Deschutes National Forest, on July 18, 2021.
The bottom two photos I took on August 12, 2024 at the Yampa River Botanic Park in Steamboat Springs, Colorado. This bumble bee is also a male. If your travels take you to Steamboat Springs, I recommend a visit to the botanic park. I returned to visit it four times during the 10 or days I was there. While is small compared to some parks, it is maintained with great care by group of local volunteers. Lots of bumble bees, native bees, butterflies and birds in a concentrated area–and of course flowers, shrubs and trees. Nearby Emerald Mountain is great for searching for bees along single track trails and fire roads.
The reader will notice some differences between the color patterns of the two bees. The bee in the bottom set of photos has thicker bands of light yellow or ivory hair interspersed with black bands. The light haired bands begin at the scutum.3 The wing band is black and the posterior end of the thorax, the scutellum, has an ivory band of hair. On the abdomen, from the first terga, or segment and down to as far can be seen in the photo, possibly the sixth terga, the lighter hair bands are predominant. Male bumble bees have seven tergum, but I think that the seventh terga is not visible in the photos. The bee also has a distinctive black band located mid-abdomen, with what appears to be an oval spot of lighter hairs nestled within that wide band.
The bee on the Woolly sunflower has a black head and a dark yellow-haired scutum. The scutum is the band located at the front of the thorax just behind the head but in front of the wing band. The rest of the bee is mostly black but the first terga seems to have some light hairs and then there are at least two more bands of light hair culminating in a thicker light yellow or ivory band at the sixth terga. I do not think the seventh terga is visible here, but it is probably the same color as the sixth terga based on my favorite reference guides. The specialist who first identified my Steamboat observations of B. occidentalis wrote a a couple comments that suggest that the color form of this bumble bee is indeed specific to the local habitat. Makarii Loskutov, a Russian entomologist who specializes in bees and primarily bumble bees wrote: “‘Coloradensis’…Interesting…”4 and, on a separate observation he confirmed, he wrote: “Local colour form.”5
If your travels take you to Steamboat Springs, I recommend a visit to the botanic park. I returned to visit it four times during the 10 or days I was there. While is small compared to some parks, it is maintained with great care by group of local volunteers. Lots of bumble bees, native bees, butterflies and birds in a concentrated area–and of course flowers, shrubs and trees. Nearby Emerald Mountain is great for searching for bees along single track trails and fire roads.
- Koch, Jonathan, et al. Western Bumble Bees of the United States, PDF. Xerces Society for Invertebrate Conservation, U. S. Forest Service and the Pollinator Partnership, 2012, xerces.org/publications/identification-monitoring-guides/bumble-bees-of-western-united-states. ↩︎
- Ascher, John S., and Nathaniel Green. Species Bombus Occidentalis – Western Bumble Bee, Iowa State University, Department of Plant Pathology, Entomology and Microbiology, 9 Sept. 2021, bugguide.net/node/view/356800. Website accessed February 1, 2025. ↩︎
- Rolfs, D., Robinson, L., Jennings, D., (n.d.). Major Body Parts of a Bumble Bee. Washington’s Bumble Bees. Website accessed on February 5, 2025. ↩︎
- iNaturalist observation: https://www.inaturalist.org/observations/240875763 ↩︎
- iNaturalist observation: https://www.inaturalist.org/observations/240876473 ↩︎
