Link to Western Wallflower host plant observation: https://www.inaturalist.org/observations/209199989
Link to confirmed observation/better photos of Small Milkweed Bugs mating: https://www.inaturalist.org/observations/181862100
Western Small Milkweed Bug (Lygaeus kalmii ssp. kalmii) A.k.a. Common Milkweed Bug. Adults are 8-12mm long. They are found on Milkweed and other plants, sometimes in large numbers.
Field Guide to California Insects, by Kip Will, J. Gross, D. Rubinoff , J. Powell, 2nd ed., 2020, pp. 129-130, Fig. 129
BugGuide: https://bugguide.net/index.php?q=search&keys=Lygaeus%20kalmii%20kalmii
Kaufman Field Guide to Insects of North America, Eaton and Kaufman, 2006, p. 118-119.
on groundsel (Baccharis halimifolia)