A characteristic species of the East slope and Great Basin, sporadically breeding at Donner Pass but not persistent and not descending the West slope. Its green underside mottling is distinctive and unique in our fauna. The early-spring brood is small and very intensely-colored, with the underside mottling dark blackish-green. It flies slightly later than the Western White in most places.

Very similar to the Checkered White, which it largely replaces at higher elevations. Our Western White is probably conspecific with the Old World Pontia callidice, and alpine or early-season specimens of ours are indistinguishable from the Himalayan taxon P. c. kalora.

In the 1970s this species was often abundant at low elevation along the transect, overwintering only locally and sporadically-mostly on dredge tailings along the American River. It has since become much rarer and in most years is seen only in September and October. At Sierra Valley it overwinters unpredictably but colonizes each year from the desert in May or June, and usually becomes common by late summer. It has been recorded at all sites but is not a permanent resident at any of them at this time!

Always single-brooded early in the season, this is a species of rocky, unforested sites including serpentine barrens and the alpine zone. It also occurs on lava flows in Sierra Valley, straying to adjacent roadsides and alfalfa fields. The interrupted vein-lines on the underside of the hindwing are diagnostic, as is the dumbbell-shaped black spot at the end of the forewing cell. Many females are distinctly pale yellow. Higher-altitude specimens average slightly smaller, with crisper and blacker markings than foothill ones. A very infrequent stray to the Sacramento Valley.