Two-form bumble bee (Bombus bifarius)
Two-form bumble bee (Bombus bifarius)
The two-form bumble bee (Bombus bifarius) is primarily found at higher elevations. I have only encountered it in the Cascade Mountain Range. Its most distinctive feature is the black, inverted triangle on the thorax. The T2 and T3 segments of the abdomen may display a black or red-black mixture.
For more details, refer to page 78 of Bumble Bees of the Western United States.
These photos, taken along the Pacific Crest Trail in August, show pollen-carrying individuals, most likely workers.