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One of the largest underwing moths found in Alberta (7.0 -7.8 cm wingspan). The forewings are pale grey, mottled with darker grey and whitish patches. There are poorly defined but noticeable black streaks at the wing base, in the fold and in the sub-apical area. The area just inside the reniform spot is white, and the space immediately below the reniform forms a large, rather prominent squared white spot. A third pale area is located midway down the subterminal band. The hindwings are salmon or orange-pink, crossed by a rather narrow and cleanly defined black median band. There is a wide black terminal band bordered by a narrow and somewhat scalloped orange-white fringe. Adults are alike, and both sexes have simple antennae. The large size, patchy looking grey forwings and salmon-colored hindwings will help to identify parta.

Adults are nocturnal and come to light, but they are best collected using sugar baits. The adults emerge in late summer and early fall, and the eggs overwinter. The larvae, which are solitary defoliators, hatch in May and can be found until early August. There is a single brood each year.

Nova Scotia south to Maryland and Kentucky, west to southern Saskatchewan and Alberta, western Montana and Utah. In western Canada, it appears to be restricted to the wooded parts of the prairies. In Alberta, it has been collected in the arid grasslands region, north to Dinosaur Provincial Park.