Journal archives for April 2020

April 09, 2020

Yellow-faced Bumble Bees & Other Pyrobombus Species in the SF Bay Area

@alexis_amphibian (Alexis Babyan) provided this very helpful information about the difficulties in distinguishing bumble bees in the Pyrobombus subgenus from each other.

Bombus vosnesenskii, Yellow-faced Bumblebee, is by far the most common bumblebee in the Pyrobombus subgenus likely to be seen in this region (California Coastal Range). It has a dark body with yellow hairs on the face and what I (Alexis Babyan) call the "shoulders"- the front part of the thorax- and has a band of yellow hairs towards the rear of the abdomen.
BugGuide page:

The nearly identical Bombus caliginosus also appears in this range (CA Coastal Range), and can rarely be told apart by photographs. Bombus caliginosus, Fog-Belt Bumblebee, is very tricky: it looks basically the same as Bombus vosnesenskii - a dark body with yellow hairs on the face and "shoulders"- and has a band of yellow hairs towards the rear of the abdomen. BugGuide has a whole section of photos of bumblebees that are either B. vosnesenskii or B. caliginosus - even the experts can't say for sure.

From a recent research article, "A contemporary survey of bumble bee diversity across the state of California" (1):
"Specifically, in B. vosnesenskii, the malar space is not longer than wide; T4 (T = tergum) is completely yellow; S3–4 (S = sternum) have only black hairs; and there are many large pits on the lower central area of the clypeus. In B. caliginosus, the malar space is longer than wide; the leading edge of T4 has many black hairs medially; S3–4 have yellow hairs; and there are only small or only a few large pits on the lower central area of the clypeus. "

From a Bay Nature article (2):
"The difference between Bombus vosnesenskii and Bombus caliginosus, according to the insect reference BugGuide: 'Females with malar space relatively short and S4 usually black as compared with caliginosus, and males with different antennal proportions. These characters are rarely visible in photos so those taken at coastal sites where both could occur are rarely identifiable.' ”

Other possible distinguishing characteristics:
A second partial yellow stripe towards the end of the abdomen (at T5?) is characteristic of male B. vosnesenskii, although male B. caliginosus sometimes also have this pattern. B. vosnesenskii is said to have a neater, tidier-looking coat of hairs overall compared to B. caliginosus, which runs a little shaggier.

The two species can only be definitively distinguished under the microscope. B. caliginosus has some additional yellow hairs on its underside that B. vosnesenskii doesn't have, and there is apparently something different about the proportion of the malar region (cheek area) and the proportion of the antennae of the males. These are subtle enough differences that they won't be visible in regular size photos of live, active, individuals out in the fields. Of all the thousands of photos of bees in the Backyard Pollinators project, there are currently only 7 photos that claim to be Bombus caliginosus, and currently only one of those photos has attained "research grade." That photo is of a dead individual, not on a flower, but I left it in the project because confirmed photos of B. caliginosus are so scarce:

Bombus melanopygus, Black-tailed Bumblebee, is the second-most common likely to be seen in this region, but it is easy to distinguish from B. vosnesenskii because in addition to having yellow hairs on its face and a yellow stripe towards the rear of its abdomen, it has an additional very wide yellow stripe around its midsection (the rear part of its thorax and the front part of its abdomen.) Bombus melanopygus in some regions also have orange/red hairs on their abdomens, but the ones we get in our region only have black and yellow hairs. (The different color forms of B. melanopygus were previously believed to be different species, but they are currently understood to be a single species.)
BugGuide page:

Bombus vandykei is also tricky. It also looks basically the same as B. vosnesenskii and B. caliginosus- at least, the females do- the males are much more yellow overall. A female B. vandykei looks the same as a B. vosnesenskii to my eye, but apparently the yellow stripe towards the rear of the abdomen is positioned at a different body segment.
B. vandykei is rarely seen in the SF Bay area, but there are occasional observations of it in Sonoma County, Santa Clara County and Solano County.
BugGuide page:

Bombus flavifrons, Yellow-fronted Bumblebee ( is also in the same subgenus and might be confused with the less common Bombus sitkensis, Sitka Bumblebee (

Bombus bifarius, Black-notched Bumblebee, is another less-common member of this subgenus, and could be mistaken for Bombus melanopygus.
BugGuide page:

John Kehoe (@jkehoe) has made several educational videos about native bees:
Bee Video: Mason, Carder and Leafcutter Bees

(1) "A contemporary survey of bumble bee diversity across the state of California," Kaleigh Fisher, Kristal M. Watrous, Neal M. Williams, Leif L. Richardson, Sarah Hollis Woodar (Ecology and Evolution, March 18, 2022)

Posted on April 09, 2020 03:31 AM by truthseqr truthseqr | 4 comments | Leave a comment