As adults, these small insects resemble the moth fly, except they are pure white in color and tent their wings over the body when at rest.

Kaufman Field Guide to Insects of North America reports about a hundred species north of Mexico. Insects of the Los Angeles Basin notes five species of prime local importance. My guess is that those shown here are the Greenhouse Whitefly (Trialeurodes vaporariorum) which feeds on the leaves of a large variety of plants and appears to be the most widespread in this area. Another possibility is the Crown Whitefly (Aleuroplatus coronata), which is the most common species on oak, and these were found landing on live oak.

I’ll check back in hopes of finding immatures, as these, unlike the adults, are easily differentiated. The Greenhouse Whitefly has translucent green pupa, while nymphs of the Grown Whitefly form distinctive crown patterns.

Additional information, as well as photos of Greenhouse Whitefly can be seen at:
http://mamba.bio.uci.edu/~pjbryant/biodiv/hemipt/Aleyrodes.htm