There are over 100 species of underwing moths (Genus Catocala) in America north of Mexico. Many look similar and are difficult for the amateur to tell apart. Based on range, physical traits and larval hosts, I believe this specimen is Catocala faustina.
Faustina underwings are found in the Great Basin, the Pacific States, British Columbia, Montana and Colorado. Preferring dry habitats, they do not occur east of the Rockies.
Adult faustina underwings have forewings which are tan-grey with darker areas. Their red-pink hindwings have a black marginal and medial band with a white to yellow fringe. The medial band does not reach the inner margin of the wing. The body is also grey-tan. The adults fly at night in late summer and fall. Underwings are rarely seen during the daylight for they rest well camouflaged on tree trunks with their hindwings hidden.
In the fall underwings deposit their eggs in tree bark where the eggs overwinter. The larvae hatch in the spring. Willows, poplars and cottonwoods are the host plants for faustina underwing larvae. Larvae also rest by day on tree trunks, limbs or under debris on the ground and are not often observed. Faustina underwings pupate just under the surface of the ground. There is only one brood a year.
Leonard found this bedraggled faustina underwing in very late August on our Lookout CA ranch (Modoc County). It was out during the day and was almost dead, probably having recently laid its eggs.
I learned a new word while reading about faustina underwing moths: “Eclosion” which is the act of emerging from a pupal case or hatching from and egg.