Moved from Eutreta.

I have no doubt this is Eutreta angusta, as you suggested.

Indeed, it keyed out to E. angusta in Foote, Blanc, & Norrbom(1)(1993)...and the species distribution maps from that reference, as well as Essig Museum data, indicate the only species with records along the California coast are E. angusta and E. hespera. But E. hespera has the whitish-hyaline spots "coalesced" in the central-basal part of the wings, and the subapical spotless dark band is wider than the white apical cresent (vs. narrower, as in angusta and your specimen).

As mentioned in your remarks, there are 3 distinct collections from the Oso Flaco locale at the Essig Museum (here, here, and here). And Ambrosia chammisonis is mentioned as the host plant associate in another Essig record from relatively nearby in coastal Santa Barbara County.

In fact, references indicate Ambrosia (aka "ragweed") is the primary native host genus of E. angusta. In particular, the host plant list on pg. 93 of Foote & Blanc(2)(1963) indicates records on Ambrosia for only 3 CA tephritids, with E. "pacifica" (= angusta) as the only member of Eutreta appearing there.

Among diagnostic characters are: the dark spots visible on the head in your 2nd image; tawny 3rd antennal segment; and various subtle details of the wing pattern (best illustrated in Fig 9 on pg. 8 of Curran(1932), but also on pg. 183 of Foote, Blanc, & Norrbom(1)(1993); Fig. 37 on pg 111 of Foote & Blanc(2)(1963); and Fig. 157 on pg. 179 of Stoltzfus(1974)).

Eutreta are gall-makers...it would be neat to photo-document the gall, but apparently it's a root gall on Ambrosia, so may be hard to find (it's recorded as making stem or crown galls on other host genera). And for all tephritids, if you're lucky, you may encounter interesting courtship displays.

Moved from ID Request.