This is one of the most common bumble bees on the west side of the PNW. It is fairly large, and mostly black dorsally, with a yellow band near the end of the abdomen (on tergal segment 4, or T4, to be technical), a yellow face, and yellow on the pronotum (the dorsal plate that covers the thorax in most insects) from the wing base to the head. It is superficially similar to a pair of other yellow faced bumble bees, but can be differentiated from Bombus caliginosus by the yellow fringed hairs on the 3rd and 4th segments of the sternum (S3-4) of that species (which are all black on B. vosnesenskii), and from B. vandykei by the yellow abdominal band of that species being on T3 rather than T4.
Yellow-faced Bumble Bees are the earliest emergers of our bumble bees, becoming active in February. But that early flight period, coupled with habitat loss, may be one of the causes of lower diversity amongst Bombus spp, since B. vosnesenskii apparently out competes other Bombus for prime nesting sites and limited nectar resources.
Like many bumble bees the Yellow-faced is a eusocial ground nester. Their colonies may contain as many as 300 bees once all of the eggs have hatched.
Queens emerge in the early spring and attempt to find an appropriate nesting site.(One of the primary problems of habitat loss is the concurrent paving over of the abandoned rodent burrows which are their preferred nesting site). They then lay a clutch of eggs which become the first group of workers. After that clutch has matured they relieve the queen of nectar and pollen gathering duties, and she concentrates on reproduction. After several clutches of workers (infertile females) and drones (reproductive males) have reached maturity a new generation of queens is reared. During this time, with many of the workers reaching the end of their lifespan, the queen once more takes up foraging duties. The new generation of queens sallies forth to mate, and having done so will hibernate through the winter to emerge in the spring and start the cycle again. Since bumble bees do not make and store honey, the colony dies out when the food runs out after the first hard frosts.
Yellow-faced Bumble Bees are an important agricultural pollinator, and bumble bees in general are the most important pollinators for tomatoes. Though they will sting they are not aggressive, and the vast majority of stings are merely because someone trapped one against their skin, or was disturbing a nest.
Size- Queen-21-25 mm; workers 11-20 mm; drones 13-18mm
Habitat- Open grassy areas, parks, shrub and chaparral
Range- Mostly west of the Cascades in our region
Eats- Wide variety of nectar and pollen sources
Flight Season- February to October