Northern Prairie Wildlife Research Center
Wingspan 3.4 cm. Forewing is dark gray-brown with a red-black median band, wing edge dentate; males have slightly dentate antennae with minute antennal branches (photo is of a male). This geometrid is abundant and widely distributed in western North America. Moths fly in late spring. Caterpillars feed on the foliage of conifers, particularly Pinaceae.
Similar species: Pero behrensaria is almost identical in coloration, distribution, and food plants to P. occidentalis, but with the males having filiform antennae; Pero morrisonaria with wings more mottled and yellow colors in the postmedian area of the forewing, widely distributed in wet conifer forests of the Pacific Northwest, caterpillars feed on the foliage of species of Pinaceae.