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Extracted from Jonathan Koch, James Strange,Paul Williams.2012. Bumble Bees of the Western United States. A product of the U.S. Forest Service and the Pollinator Partnership
with funding from the National Fish and Wildlife Foundation Status: Uncommon
Select food plant genera: Baccharis, Cirsium, Lupinus, Lotus,
Grindelia, Phacelia Tongue Length: Medium Distribution: Coastal areas from northern Washington to
southern California Easily confused for B. vosnesenskii or B. vandykei Females (queens and workers, colors refer to ‘hair’)
• Thorax anterior to black band between wing bases
yellow, scutellum black, T1-3 black, T4 yellow, T5 and
T6 black, S3* and S4 with long yellow hairs apically,
face square. Mid leg basitarsus with the distal posterior corner
rounded. Cheek length as long as broad. Hair of the
face and top of head yellow. On the side of the thorax,
the lower anterior surface with predominantly black
hair, corbicular fringes black. Hair length medium and
uneven. *S refers to the sternites of the bumble bee, which
are found on the ventral side of the abdomen. They
are analogous to the tergites (i.e. “T”) that are found
on the dorsal side of the abdomen. Therefore S3
translates to “sternite 3”. See page 26 for an example
of this color character
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Extracted from Bumble Bees and Cuckoo Bumble Bees of California by Thorp, R. (1983).
Discussion. B. caliginosus belongs to the species group which includes B. centralis, /Javtfrons, and vandykei, and appears most closely related to B. vandykei. It differs from the California female color form of B. vandykei in the placement of the yellow hair band on the metasoma. In B. caliginosus, the yellow is on the apical half of tergite 4, while in B. vandykei it is on the apex of tergite 3. The pre-dominantly black males of B. caliginosus are dis-tinct from the predominantly yellow males of B. vandykei (cf. Figs. 115 and 124a).
This species exhibits little color variation. One male was described as a new variety (tardus) by Frison (1927b) because it lacked the yellow band on the metasoma. No further specimens of this color variant have been seen, and it was
synonymized by Stephen (1957).
An arboreal nest of this species from Berkeley was described by Pitelka (1954). Johansen (1967) noted the similarity of B. cattginosus nests in Wash¬ington to those of B. mixtm. We have taken a sur¬face nest in a garage in Eureka, Humboldt Co. It was using the cotton stuffing of a hammock stored behind some scrap lumber.
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