This is the only underwing (genus Catocala) in North America with white and black hindwings. Others are usually black with brightly-colored bands of orange, yellow, pink, or red, with a number sporting all-black hindwings. Underwings rest by day, camouflaged on the bark of tree trunks. If they are disturbed into taking flight, the contrasting patterns of their hindwings will flash out, hopefully startling predators and allowing escape. This species ranges widely, living in forests coast to coast. In Montana, White Underwings fly in August and September. Larvae feed on leaves of aspens, cottonwoods, and willows.  

Size: Forewing length 34 – 40 mm

Photos by: Adam Potts on 8/19/21 near Bozeman, MT