Size Reduction Southward in Michigan's Mustard White Butterfly, Pieris Napi (Lepidoptera: Pieridae)

[Excerpt] The mustard white, Pieris napi (Linnaeus) is generally considered to be a circumboreal species or species-complex. Much disagreement exists regarding the classification in this group, the members of which are extremely diverse, especially in Eurasia. There is evidently "a continuous range of differentiation, from local populations, through subspecies, to species, which nomenclature cannot fully reflect" (Bowden 1972). In North America the major controversy has been whether the so-called West Virginia white, P. virginiensis Edwards, should be separated as a distinct species. Most writers today agree that it should. Of the mustard white in the Great Lakes area, the local subspecies is genelally treated as P. napi oleracea Hams (for summary ofthe taxonomicproblems, cf. Howe [1975]). This taxonoccurs extensively in Canada, and down into Minnesota, Wisconsin, and Michigan, where it becomes progressively less common southward. It is two-brooded, the members of the first brood commonly flying together with the univoltine virginiensis where their ranges overlap (Voss and Wagner 1956). Where they occur together, the two taxa are easily recognized and separated, but in the southern mountains, where napi is absent, the Appalachian form of virginiensis resembles the summer form of napi (Wagner 1978).