Moved from Trirhabda.

...but not on pronotum! Again, that's a "strike" against sericotrachyla here (at least as far as current keys & descriptions go).

On the other hand, the pronotal spots here (and in your other shots) appear pretty much entirely piceous...which is a character of sericotrachyla. In luteocincta, the pronotal spots are supposed to have "metallic green luster", according to both Blake and Hogue...which is not the case in your photos. So that character, together with the color pattern of the venter, argues in favor of sericotrachyla.

It should be mentioned that, in the keys, the lack of pronotal pubescence together with the lack of green luster for the pronotal spots, leads to T. confusa as a candidate. But that species is described as having elytra with "long, silky pubescence"...which doesn't fit here. Also, although confusa was earlier believed to have Artemisia californica as its host plant (i.e. by Blake), Hogue and others (e.g. Swigoňová & Kjer(1)) assign rabbitbrush (Ericameria sp., formerly Chrysothamnus) as the host plant for that T. confusa...which is consistent with all the present BugGuide posts for that species.

Note that the 1982 paper of O'Brien & Ansatt mentions that T. confusa was previously recorded for the area...though they never encountered any in their study.

Moved from Frass.