Northern Prairie Wildlife Research Center
Wingspan 3.0 cm. Forewing is slightly falcate, mottled pale brown with three dark lines and a black postmedian patch; hindwing is slightly dentate. This geometrid is abundant and widely distributed in western North America. Moths fly in midsummer. Caterpillars feed on the foliage of species of Pinaceae.
Similar species: Semiothisa unipunctaria is larger, wings gray, occurs in high elevation conifer forests, caterpillars feed on the foliage of species of Pinaceae; Semiothisa sexmaculata is smaller, wings uniformly dull brown with obscure markings, abundant and widespread, caterpillars feed on the foliage of western larch (Larix occidentalis); Semiothisa ulsterata forewing is falcate, deeply notched, mostly white with a red subapical and black postmedian patch, hindwing dentate, moths are common in wet conifer forests in the Pacific West, caterpillars feed on the foliage of alder (Alnus) and birch (Betula).