The postdiapause developmental rates of the European corn borer,Ostrinia nubilalis(Hübner). were investigated using collections of larvae from several selected sites in North Dakota from 1990 to 1992. Significant differences in days to pupation for postdiapausing larvae were found between different geographic locations, and between bivoltine and univoltine populations of the European corn borer in North Dakota. The differences in the time required for postdiapause development is believed to be the major contributing factor responsible for voltinism of the European corn borer population in Nort Dakota. Larvae from most of the sites had some degree of similarity to the bivoltine population, as measured by the proportion of the larvae pupating at ≤220 DD. Retention of the bivoltine trait allows a mechanism for the expression of a second generation in populations that had been considered univoltine. Retention of the bivoltine trait supports the importance of local adaptations in determining the voltinism of European corn borer populations in the northern Great Plains.