A characteristic species of moist mesic forest in late winter and early spring, apparently not breeding above 5000'. It flies in dappled light and shade, often high off the ground, with rather infrequent wingbeats compared to P. rapae. In the gloom its brilliant white color often makes it look bigger than it is. This species has a very dramatic seasonal polyphenism: first-brood specimens have more or less bold, black vein-lines on the hindwing beneath, while second-brood ones (where they occur) are usually immaculate white (or pale yellowish).

Our only introduced butterfly - from the Old World - this is also one of our weediest, occurring in disturbed habitats from sea level to about 8000'. It even invades riparian woodland and montane coniferous forest habitats in summer. We are not certain how or when it reached our area. We know the species was introduced in southern Canada in the 1850s; the great Lepidopterist Samuel H. Scudder traced its spread, but was unable to resolve the history on the West Coast. It was not in San Francisco in the early 1880s, but was abundant by the time of the earthquake (1906).