An uncommon grassland and shrub-steppe species on the Sierran east slope only, often seen near vernal pools. It is only accidental in Alfalfa fields and unlikely to stand out among swarms of the two Alfalfa-feeding Colias there; on theother hand, it is likely to be the only Colias flying through sagebrush and bitterbrush at, say, Beckwourth Pass. Pale females are extremely rare. The underside of the hindwing is uniform grayuish-green, and the silver discal spot has only one (purple) rim. On our transect only at Sierra Valley, and never common.

Ubiquitous except in closed-canopy forest; one of our commonest butterflies, often reaching very high densities in alfalfa fields in midsummer to autumn and becoming conspicuous at that time; when the alfalfa is cut may emigrate en masse, even flooding into cities. This is also our most variable butterfly, seasonally and individually. Cold-season specimens are small, very heavily infuscated on the ventral hindwing, with reduced black borders above, increased basal black above on all wings, and the orange pigment concentrated into discal patches on a bright-yellow ground.

The Yellow Sulphur of the West is different in many ways from the Eastern one and may be its own species (eriphyle). Both, however, hybridize readily with C. eurytheme where they co-occur, almost always in alfalfa fields. Our Yellow Sulphur occurs only East of the Sierran crest and is only a rare stray at Donner. But it is a permanent resident in Sierra Valley and other irrigated alfalfa-growing areas (e.g., Honey Lake to the north and Carson Valley to the south, and eastward across the Great Basin). It strays only infrequently to open wildlands.