Colias occidentalis
Western Sulphur

Family Description:
This is really a complex of species or subspecies, including astraea, christina, krauthii, pheudochristina, wasatchia, and possibly others.
Caterpillar: The caterpillar is yellowish green, marked lengthwise on each side with a white line dashed with orange, and dottedwith black.
Adult: The butterfly is medium-sized, with a wingspan of 1 to 2 inches. Its appearance varies, both between and within the sexes: Males: They can be one of two types, or morphs. The first type is bright yellow on the upperside, with a black border on the edges of the wings, a small spot on the forewing and an orange spot on the hindwing; underneath is yellow, darker on the hindwing which also has a ringed spot, and fringed with pink. The second type is orange on the upperside, with yellow on the wings at the edges and near the body; underneath is greenish gray to orange. Females: They can be one of two morphs, as well. The first type is light yellow to white on the upperside, with the dark wing border either interrupted or missing; underneath is yellow, darker on the hindwing which also has a ringed spot, and fringed in pink. The second type is orange on the upperside with black wing borders and dark wing veins; underneath is greenish gray to orange.

This species ranges from Alaska, south through the western portion of Canada and the northwestern quarter of the U.S., and east to North Dakota. The yellow morphs occur in the northern portion of Idaho, while the orange morphs occur mainly in the central and western portions of the state.


It frequents open areas, including meadows, prairies, sagebrush steppe, and forest openings.


Caterpillar: Caterpillars feed on members of the pea family (Fabaceae), such as lupines (Lupinus spp.), vetch (Vicia angustifolia), and white sweet clover (Melilotus alba).
Adult: Butterflies drink flower nectar.


There is one generation of caterpillars each summer. Older caterpillars overwinter in a physiological state called diapause, and emerge in the spring to feed and pupate. Adults generally fly from May to August, with most flying during June and July.

Males actively patrol in search of receptive females. Females lay eggs singly on the leaves of host plants.

Idaho Status: Unprotected nongame species.
Global Rank:

G3; vulnerable to population decline and extinction.

Opler, P. A., H. Pavulaan, and R. E. Stanford. 1995. Butterflies of North America. Jamestown, North Dakota, USA: Northern Prairie Wildlife Research Center Home Page. (Version 05Nov98).

Opler, P. A. and A. B.Wright. 1999. A Field Guide to the Western Butterflies.   Second Edition.  Peterson Field Guide Series. Houghton Mifflin Company, New York, New York, USA, 540 pp.

Pyle, R. M. 1981. National Audubon Society Field Guide to North American Butterflies. Alfred A. Knopf, Inc., New York, New York, USA, 924 pp.

Scott, J. A. 1986. The Butterflies of North America. Stanford University Press, Stanford, California, USA, 583 pp.

Stanford, R. E. and P. A. Opler. 1993. Atlas of Western U.S.A. Butterflies (Including Adjacent Parts of Canada and Mexico). Published by authors, Denver, Colorado, USA, 275 pp.