The Morphs of Pemphigus populitransversus: Allocation Methods, Morphometrics, and Distribution Patterns1, 2

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Abstract
A dimorphism in the gall-forming aphid, Pemphigus populitransversus Riley, is due primarily to differences in time of gall initiation. The elongate galls form during 1st bud break in spring, the globular galls are initiated 4–6 wk later. Differences in aphid and gall morphometrics and in life history stage distribution within galls are significant, but their ranges overlap sufficiently to make these criteria unreliable for distinguishing the morphs. A procedure used for identifying a large number of galls in which time of gall initiation was not known is described. A combination of statistical approaches served to allocate most of the galls to the correct morph. The 2 morphs are now shown to be sympatric over their entire range. This aphid appears to occur in North America largely east of the Continental Divide. West of the Divide it is quite rare and occasional infestations may be only local introductions. Earlier reports of its occurrence as far as California may be in error.

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