Western Sulphur (Colias occidentalis) (Scudder, 1862)
Diagnosis: The upperside of this species is bright yellow in males with a solid black border. The female is paler yellow (very rarely yellowish white), with the black border pale but prominent. The underside of the hindwing is dark yellow with a dusting of grey; the discal spot is white with a prominent pink rim, and there is usually a trace of submarginal dark dots. Wingspan: 43 to 50 mm.
Subspecies: Only the nominate subspecies occidentalis occurs in Canada.
Range: This is a species of the Pacific Northwest with a limited range in Canada. It occurs on eastern Vancouver Island and in arid areas on both sides of the Cascades Mountains in the Fraser and Similkameen River Valleys.
Similar Species: Queen Alexandra's Sulphur (Colias alexandra) is frequently confused with the Western Sulphur where their ranges meet in southwestern British Columbia. In this area, males of alexandra have the outer third of the hindwing a darker yellow than the forewing (this is the ultraviolet reflective area discussed below under Remarks), and the underside of the hindwing is a greyish orgreenish yellow. Females have virtually no black on the upper side of the forewing. The Clouded Sulphur (C. philodice) has prominent dark submarginal spots on the underside of both fore- and hindwings, and the silver spot in the centre of the hindwing underside has a double pink rim. The Pink-edged Sulphur (C. interior) is smaller, the underside of the hindwing is bright yellow with little grey dusting, and the spot in the middle of the hindwing above is orange rather than yellow. [compare images]
Abundance: Very localized and spotty in distribution, but can be common where it occurs.
Flight Season: Adults are on the wing from late May until early July.
Habits: The Western Sulphur inhabits dry, open coniferous forests, particularly in and near Douglas-fir forests. It is rarely found in cultivated areas (Tilden and Smith, 1986).
Remarks: The Western Sulphur, and the next two species, belong to a complex of species that has been poorly understood in the past. The occurrence of intermediate-looking forms between each of these species led to the belief that there was extensive hybridization wherever their ranges met and they were treated as subspecies of a single species. Most of these species, as in many other Colias,have patches of specialized scales on the wings of males that are visible to us only under ultraviolet light, but are visible to the butterflies themselves and are important in courtship and mate selection. Through the use of ultraviolet photography Clifford Ferris has studied this complex for many years andhas published a series of papers on the group. He was able to show that the Western Sulphur differed from the other species in being non-ultraviolet reflective, and later used the patterns of ultraviolet reflectance to arrange the remainder of the group into four species, three of which occur in Canada. We now know that many of the suspected hybrid zones actually reflect variation within species rather than hybridization between species, although the species do occasionally hybridize. A recent review ofthe group by Ferris (1993) forms the basis for our treatment of these species.
© 2002. This material is reproduced with permission from The Butterflies of Canada by Ross A. Layberry, Peter W. Hall, and J. Donald Lafontaine. University of Toronto Press; 1998. Specimen photos courtesy of John T. Fowler.
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