[View the full-size image (1920 x 1821) at this link.]

This series show a bee that was repeatedly visiting flowers of the Metcalf Canyon Jewelflower (Streptanthus glandulosus ssp. albidus...formerly S. albidus ssp. albidus)...a very rare, state & federally listed endangered species in the Mustard family (Brassicaceae). The very fast-flying bee was so regular in its visits to the raceme in the image here that, by focusing on its preferred flowers and waiting for it to return, I was able to get a number of shots.

My best guess is that this is a female of Anthophora urbana...compare with images of that species here.

A California Native Plant Society conference talk on S. g. ssp. albidus indicated bumblebees are its primary pollinators. At minute 7:36 in this video of the talk it's stated that based on 65 hours of video camera surveillance of flowering S. g. ssp. albidus, it was estimated that over 99% of the visiting pollinators were Bombus vosnesenskii. However, during a visit of a few hours to this locale on the afternoon of 5/10/15 I noted and photographed exactly four different individual bee visitors, of which only a single one (i.e. just 25% of my sample) appeared to be a Bombus (vosnesenskii or californicus?). The other 3 bees appear below:

     

I'm guessing the discrepancy

All the bee visitors were very rapid and wary flyers, and I wasn't able to get as diagnostic images as I'd like (e.g. no clear wing venation shot in this series). Nevertheless, I'm hoping John Ascher or others may be able to discern enough in the photos I did get to make a tentative ID. It would be helpful to know more about the pollinator guild for this species.