The scurfy dusting of leaves, stems, and inflorescence, and the flower form here all point to a member of the Goosefoot family, Chenopodiaceae...probably in the genus Chenopodium. Compare with images of C. berlandieri here, which appears to be one of the more common native members of the genus in your area (see map here). Non-native weedy species in this genus are also common. Here's a link to Chenopodium occuring in the southwest.

I don't know why the beetles were crawling around on this plant...it's quite distinct from known hosts plants for any species of Trirhabda. Interesting.

If, by chance, you regularly visit the locale where these photos were taken, keep an eye out for Brickellia (host plants for T. schwarzi) or Eriodictyon angustifolium (the only host of T. eriodictyonis in your area).

But even without supporting host plant info, I'm thinking you have T. schwarzi here...both from the keys, descriptions, and illustrations in Hogue(1) and Wilcox, and from comparing with the images on BugGuide. (For instance, like T. schwarzi, your beetles appear a bit wider & shorter than the comapartively longer, thinner T. eriodictyonis posts.)