Esterase polymorphism in the corn earworm, Heliothis zea (Boddie): A survey of temporal and spatial allelic variation in natural populations

Allelic variation at the Est-II locus was investigated in 22 natural populations of Heliothis zea from maize, separated spatially and temporally. Of the four alleles observed, Est-II^b was the predominant one in every population. The relative frequency of Est-II^b ranged from 0.57 to 0.85, with a mean of 0.72±0.06, and proved to be remarkably stable in space and time. Temporal changes in Est-II allele frequencies were observed at two of the sampling localities; however, principal component analysis and multiple correlation of genetic and environmental components have provided persuasive evidence that abiotic environmental or geographic variation is not related to differences in Est-II allele frequencies. Possible factors responsible for the maintenance of this polymorphism are discussed and include possible effects of the chemical composition of host plants and pesticides.