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Colias occidentalis Fabricius, 1807
Sulphurs; Western Sulphur
Family: Pieridae (Whites, Marbles, and Sulphurs)
Species account authors: Crispin Guppy and Jon Shepard.
Extracted from Butterflies of British Columbia.
Introduction to the Lepidoptera of BC
Introduction to the Butterflies of BC
The Families of Lepidoptera of BC

Photograph

© Norbert Kondla     (Photo ID #6161)

Map

Click on map to view a larger version of this map.
Distribution of Colias occidentalis in British Columbia.
(Click on the map to view a larger version.)
Source: Butterflies of British Columbia by Crispin Guppy and Jon Shepard © Royal BC Museum

Species Information


Adult

Western Sulphurs are always yellow with black wing borders, and show little variation in BC. They usually lack all submarginal spots, although one or two may be faintly visible on the ventral hindwing. The ventral hindwing discal cell spot has only a single red ring around it, usually has a pink centre, and seldom has a satellite spot. Unlike those of Alexandra's and Christina's Sulphurs, the dorsal hindwings of male Western Sulphurs are an even, clear yellow except for the black wing border and a characteristic extensive dusting of black scales near the dorsal wing bases. The females are pale yellow, with a poorly defined and reduced black wing border.

Immature Stages

Eggs are shiny and whitish at first, becoming rosy red with an ivory white tip as they mature. Mature larvae are dark velvety green, with many black dots surrounded by green and with white hairs thickly distributed on the sides of the body. The spiracular line is thin but conspicuously white; the white ring around the black dots is replaced with green. There is a faint pink suffusion along the spiracular line. The pupal head has a projecting beak, and there is a decided hump on the top of the thorax. The pupae are smooth, emerald green at first but becoming darker and assuming a yellowish colour towards maturity. The beak is dark green above and yellow below. The three abdominal segments beyond the tip of the wing cases each have two small black dots. The spiracular line is distinctly yellowish (Hardy 1960).

Subspecies

Only the nominate subspecies occurs in BC. Western Sulphurs were originally described by Scudder (1862) from "Gulf of Georgia (A. Agassiz), Fort Simpson, British America (W.H. Edwards)." There was a Fort Simpson on the coast near Prince Rupert, BC, at the time Scudder described C. occidentalis. It is clear, however, that the Fort Simpson referred to is the one on the Mackenzie River, NT, because Edwards (1868-72), who supplied the Fort Simpson specimens, states that the type series included specimens from "Mackenzies River." The female types may have included the one illustrated by Edwards (1868-72) as the female for C. occidentalis, which appears to be a Colias philodice vitabunda.

Genus Description


Colias is the name of a promontory on the coast of Attica where there was a tem pie of Aphrodite. There is no obvious relationship to the butterfly, but the name may be a pun (Emmet 1991). An alternative explanation is suggested under Pontia. The common name "sulphurs" is derived from the yellow "sulphur" colour of most species.

Sulphurs in BC are generally medium-sized butterflies that are yellow, orange, white, or (one species) yellow green with black markings. The wings of males always have a solid black border, with the exception of the Arctic Sulphur. The black borders of females contain extensive pale areas, or may be greatly reduced or absent. There are several multivoltine species that show considerable seasonal variation in wing colour.

There are about 70 species of Colias in the world. The centre of distribution in North America is BC, with more species (13) than any other province or state. Colias species may all be inter-fertile, with natural hybrids known for most species combinations where they occur together in the wild. The species have behavioural, ecological, and physiological differences that maintain separation of the species in the wild (Hovanitz 1963).

Eggs are laid singly on the leaves of the foodplants, and are pale yellow green to cream, later turning orange. Young larvae are slender, yellowish or green, and smooth-skinned with a thin coat of fine hairs. Mature larvae are yellow green or green with fine black dots all over, and stripes of various colours running along the back and sides. Sulphurs hibernate as second to fourth instar larvae (except Canadian Sulphurs, which hibernate as fifth instar larvae), and then complete development in the spring. There are five larval instars in all Colias (Ae 1958a). Pupae are fastened head up with a girdle around the middle.

Members of the genus utilize a wide range of foodplants, although each species specializes to a greater or lesser extent. Larvae of sulphurs feed on plants in three groups: legumes (Fabaceae), Vaccinium (Ericaceae), and Salix (Salicaceae). Sulphurs occur in a wide range of habitats, including arid sagebrush areas, alfalfa fields, meadows, alpine tundra, and forest bogs.

Sulphurs always rest with their wings folded over their backs, and bask in the sun by leaning to the side to allow the sun to warm the underside of their wings. It has been demonstrated for several species (C. meadii, C. nastes, C. philodice, and C. eurytheme) that the darker the pigmentation on the underside of the wings, the more heat can be absorbed from the sun while basking, permitting greater flight activity in cold environments (Kingsolver 1985).

There is relatively little variation in wing pattern between many species, making identification difficult. The key characters mentioned in the species discussions are shown in the figure.

Biology


Western Sulphurs are univoltine, and fly from June, at low elevations on Vancouver Island and the mainland, to September in subalpine habitats on Vancouver Island. Eggs hatch a week or so after oviposition, and the egg chorions are partially or entirely eaten by the larvae. Third instar larvae commence aestivation in the fold of a shrivelled leaf about two weeks after the eggs hatch, and remain quiescent until early April the next year. Pupation occurs about a month after feeding recommences in the spring. Pupation occurs with the head up and a girdle around the thorax. The adult emerges about 17 days after pupation. When disturbed, fifth instar larvae raised their thoracic segments in a sphinxlike position (Hardy 1960).

The larval foodplants are unknown in BC, but Lathyrus nevadensis var. nuttallii is a likely one on Vancouver Island (Hardy 1960). Oviposition has been recorded on sweet white clover, lupines, and Vicia sativa in Yakima Co., WA (Newcomer 1964a).

Habitat


Western Sulphurs inhabit low-elevation dry grassy slopes and forest edges on the lee side of the Cascade and Coast ranges on the mainland of BC. On Vancouver Island they occur in sea-level forest openings and edges, and upward into subalpine and alpine meadows.

Distribution

Distribution

Western Sulphurs are restricted to Vancouver Island and the Cascade Mountains of BC, western WA, western OR, and northwestern CA.

Status Information

Origin StatusProvincial StatusBC List
(Red Blue List)		COSEWIC
NativeS4YellowNot Listed



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