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A medium-size (3.2 - 3.5 cm wingspan) grey, black and white moth. Forewings blue-grey, crossed by a wide, sharply defined median band, strongly curved on the basal side, less so on the distal. There is also a narrower diffuse black subterminal band and black shading near the wing base and terminal area. Hindwings white with a broad black terminal band. Body black except for the narrow white collar. Oncocnemis chalybdis cannot be mistaken for any other Alberta moth except the closely related O. piffardi Wlk., which replaces it east of the mountains. The white or partially white collar as well as the lack of a swollen base of the male clasper can be used to separate chalybdis from piffardi.

Adults are nocturnal and come to light. There is a single annual brood, with Alberta adults in late summer and fall (Aug. 10 through Sept. 30). The larvae feed on species of Spiraea, including S. douglassi (Troubridge and Crabo, 1999).

A western species, found at mid-elevations (1200-2000 m) in the mountains from western Alberta west to the east side of the Coast Ranges of BC, south to MT, WA and OR. It is replaced by the very closely related O. piffardi east of the mountains.