Genetic Differentiation and Its Bearing on Migration in North American Populations of the Corn Leaf Aphid, Rhopalosiphum maidis (Fitch) (Homoptera: Aphididae)

A survey of genetic variation at 21 electrophoretic loci in 15 natural populations of the corn leaf aphid Rhopalosiphum maidis reveals 8 loci useful for population comparisons. Within-population polymorphism levels range from 0–19% with Hillsboro, Tex., the most variable population sampled. Fixed genetic difference between northern (Illinois), Bristow (Oklahoma), and southern (all other) populations suggest regional populations are clonally derived. In addition, the lack of heterozygosity in Illinois indicates that relatively few females may found northern populations. These genetic differences offer a new tool to study annual migration in R. maidis with preliminary evidence suggesting Illinois populations were not founded by aphids from the southern regions sampled. The origins and maintenance of this variation pose questions of evolutionary interest.