Western White
Pontia occidentalis (Reakirt, 1866)

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Diagnosis: This medium-sized white has a definite checkered appearance on the upper surface, particularly in the darker female. The hindwing underside has a heavy greenish scaling along the veins. Spring and higher-altitude forms are darker. Wingspan: 33 to 48 mm.

Subspecies: The nominate subspecies is found throughout most of the Canadian range of this butterfly. Subspecies nelsoni occurs in Yukon, the Northwest Territories and western Nunavut and has smaller black spots on the upperside in the males.

Range: The Western White ranges from Northern Ontario through the Prairie Provinces to British Columbia. It is found throughout Yukon, western Northwest Territories, and Nunavut east to Bernard Harbour. In the U.S. it ranges as far south as central California.

Similar Species: The Checkered White (P. protodice). [compare images]

Early Stages: The larva is similar to that of the Checkered White and feeds on many of the same foodplants, including Tumble Mustard (Sisymbrium altissimum) and Stinkweed (Thlaspi arvense). It overwinters as a pupa.

Western White (Pontia occidentalis occidentalis). Plateau Mountain, Alta. G.J. Hilchie

Abundance: This species tends to be common to abundant, particularly during hot dry summers, but is less common east of the Prairies.

Flight Season: Two broods are found in the southern part of its Canadian range, with records from April to October in Saskatchewan. In the north, there is one brood in June and July.

Habits: The Western White occurs in a broad range of open habitats in western Canada, including prairie coulees, mountain valleys, and badlands. It is less often found around farmlands and other "domesticated" areas than the Checkered White. It also has a great tolerance for high-altitude regions and is found even on open tundra. Males often hilltop looking for females.

Remarks: This species is closely related to the Peak White (Pontia callidice Hübner) of Eurasia and has been treated as a subspecies of it by some authors. The two species are partially interfertile when hybridized in the laboratory.

© 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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