Ecologist Jutta Burger spotted these yellow-faced bumblebees in the park on January 13. One was gathering pollen on a Bermuda buttercup (Oxalis pes-caprae, aka sourgrass) and the other was doing the same on the flower of an iceplant (Carpobrotus edulis).
The yellow-faced bumblebee (Bombus vosnesenskii) has a complex history, according to Wikipedia.
Historically, Bombus occidentalis, the so-called “western bumble bee” was the most common species, with a distribution all the way from California to British Columbia and Alaska, but diseases introduced by commercial rearing operations in the eastern United States brought B. occidentallis to the brink of extinction, and B. vosnesenskii has filled the gap.B. vosnesenskii‘s success in the vacuum left by B. occidentalis has not been the perfect story of nature finding balance after disturbance, however; in the San Francisco area, the frequency of B. vosnesenskii appears to inversely correlate with the species richness of bees in the area, indicating that B. vosnesenskii outcompetes other bee species for space and resources. This may be due to the species’ early emergence during the season, allowing it to overtake and monopolize available nest spaces.
In other words, this bee’s appearance here in mid-January is not altogether something to cheer. Yes, she is doing a good job pollinating plants and gathering food for the baby bumblebees she will soon produce. But by doing so, she may be making life more difficult for other bees that emerge later. Early bee gets the pollen.
This bumblebee nests in holes in the ground, “typically some sort of rodent burrow,” according to the same source. Add another species that benefits from the work of the common ground squirrel: not only Burrowing Owls but Yellow-Faced Bumblebees.
Also worth noting is that this species “is an extremely important pollinator for commercial agriculture” and is “the primary pollinator for greenhouse tomatoes.”