I've now shot these at four sites: Riley, Peters, Irvine Park, and Santiago Oaks. The latter two are contiguous and Peters is close. Riley, where the flies have a more silver look, is a drive of about 40 minutes distant.

Fennel, aka anise, appears to be the favored adult food, though it certainly isn't exclusive. I've seen them on two unidentified plants at Santiago Oaks and a few on buckwheat at Irvine Park.

In the unlikely event I get better photos, I'll post again. These flies are very tough to shoot, typically heading deep into the flowers and out most often when it's windy.