1) Subspecies sayi always has some black maculation on the pronotum...varying from two distinct small and somewhat oblique spots, to larger spots which can become confluent, forming an inverted-"v" shape. The other two subspecies have pronotum entirely red, with no black on the pronotum. 2) Subspecies blandi differs from discoideus in its: smaller size; more narrow form; more sparse punctation (especially the pronotum); and more rounded apex (the apex of discoideus is described as "feebly truncate").
Associated with composites in the genera Gutierrezia & Haplopappus; larvae bore in the root crown while adults are found on the flowers and foliage. --T.C. MacRae
Internet References
Curated images of 1) ssp. discoideus; 2) ssp. blandi; and 3) ssp. sayi...from "The Photographic Catalog of the Cerambycidae of the New World"(2).