Mustard White (Pieris oleracea) (Harris, 1829)
Diagnosis: This species is chalky white on the upper surface, with a dusting of black scales at the wing bases, along the costa, and at the forewing apex. There are two seasonal forms. In the spring form the underside is pale yellow on the hindwing and near the forewing apex, and the veins in these areas are lined with contrasting dark green shading that shows through the wings onto the upperside. In the summer form there is less dark shading on the upperside and the underside is white, with little if any dark shading on the veins. Wingspan: 32 to 50 mm.
Subspecies: The nominate subspecies occurs in most of the Canadian range of this butterfly. Subspecies frigida occurs in Labrador and adjacent Quebec (Bradore Bay); it has brighter yellow shading on the underside and dark shading on the veins on the upperside in females.
Range: The Mustard White occurs across Canada from Newfoundland west to the Rocky Mountain foothills in Alberta, northern and central British Columbia, western Northwest Territories as far north as treeline, and the coast of Nunavut at Coppermine and Arviat.
Similar Species: The West Virginia White (P. virginiensis) occurs with the spring form of the Mustard White in southern Quebec and Ontario; the dark scaling on the veins of the hindwing beneath is a diffuse grey brown, not a sharply defined dark green as in oleracea. Where the range of oleracea meets that of the Margined White (P. marginalis) in the Rocky Mountain foothills and in northern British Columbia, marginalis can be recognized by the paler, more diffuse shading on the veins of the underside and by the darkly marked females. The Arctic White (P. angelika) occurs with oleracea in western Northwest Territories (e.g., Norman Wells, Aklavik); its smaller size, dark scaling on the veins along the outer margin of the upperside in males and the very dark shading in females can be used to distinguish angelika from oleracea. [compare images]
Early Stages: The larva is green with a white or pale yellow lateral line and tiny black spots.Foodplants are a variety of plants in the mustard family (Brassicaceae), especially rock cress (Arabis spp.) and toothwort (Dentaria spp.).
Abundance: Although it has declined in numbers in many areas owing to loss of habitat, the Mustard White is still common in most of its range.
Flight Season: Adults are on the wing from late April to mid-September, with two overlapping generations in most areas. In southern Ontario there are three and occasionally four generations, while in the north there is a single generation in June and July.
Habits: This is primarily a butterfly of woodlands and open areas near woodlands.
Remarks: Until recently the "Mustard White" was considered to be a single species, with numerous North American populations treated as subspecies of the Eurasian Pieris napi (Linnaeus). Attempts to divide this "species" into several constituent species based on wing markings (e.g., Eitschberger,1983), have proved unsatisfactory. Recently, however, Geiger and Shapiro (1992), using enzyme electrophoresis to measure genetic similarity among populations of "napi" have demonstrated that there are three species in the napi complex in Canada and at least one more in Alaska, none of which are the same as Old World Pieris napi The two additional species in Canada are the Margined White (P. marginalis) and the Arctic White (P. angelika).
© 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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