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. The spring brood is very small, with narrow black borders and intense infuscation on the ventral hindwing. White females cannot always be told from C. eurytheme. Hybrids have very even, intermediate coloration (as contrasted with cold-weather forms of eurytheme, in which the orange contracts to distinct patches). Hybrids occasionally exceed 10% of the population.
Three broods at Sierraville, March to October (occasionally November). Cold-weather animals hug the south or SW-facing sides of hills; the males fly very low and directly. In summer, males patrol just over the top of the alfalfa and often mate with teneral females. Both sexes visit Alfalfa flowers freely, but also Composites of various kinds (including Rabbitbrush) and others.
Bright yellow individuals are occasionally taken in Orange Sulphur populations west of the crest, including the Central Valley. These appear to be mutant individuals of local origin.