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
Distribution: Pacific coastline west to the Colorado Rocky
Mountains, associated primarily with high elevations in the
Intermountain West; dark form (B. flavifrons dimidiatus) found in
western portions of the range; red form (B. flavifrons flavifrons)
in eastern portions of the range
Can be confused with B. centralis and B. sitkensis
Females (queens and workers, colors refer to ‘hair’)
Thorax with black and yellow hair intermixed
especially anteriorly, T1-2 yellow at least
apicolaterally, T3-4 red or black, T5 black
sometimes with yellow apically.
Mid leg basitarsus with the distal posterior
corner rounded. Cheek length longer than broad.
Hair of the face and top of head with black and
yellow intermixed. On the side of the thorax, the
lower anterior surface with long yellow hairs,
corbicular fringes black with red admixture
along posterior margin. Hair length long and
Extracted from Bumble Bees and Cuckoo Bumble Bees of California by Thorp, R. (1983).
Discussion. B. flavifrons belongs to the group of species which includes B. caliginosus, centralis, and varufykeiy and is most closely related to B. centralis. In California, it differs from centralis in having predominantly black instead of red hair on metasomal tergites 3 and 4, and in the admixture of black to the yellow hair on the anterior scutum. Males of B. flavifrons in California may have yel¬low hairs on much of tergites 3 and 4, thus resem¬bling faded males of centralis or yellow males of vandykei. They can be separated from both by the presence of some black hairs basally on tergites 3 and 4.
The California color form ("cHmidiatus" popula¬tions) exhibits some color variation to include females with some reddish hair apically on tergites
3 and 4, thus tending toward the nominate color form. The males exhibit more variation, especially in the amount of yellow hair on tergites 3 and 4.
Nests of the nominate color form in Alberta, Canada, are discussed by Hobbs (1967b).
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