Survival of reciprocal populations of European corn borer,Ostrinia nubilalis(Hübner), equaled or was intermediate to natural populations and an introduced low-diapause population (exotic), which survived less well. When peak natural second-generation hatch (31 July–3 August) north of 39°00′ N lat. was approximated, most of seven natural populations approached 100% diapause. However, only 0.25–22.95% of the exotic population diapaused. Diapause by progenies of reciprocal crosses was greatly influenced by this exotic parent. At the most northern locations, Ankeny, Iowa, and St. Paul, Minn., influence of the parental male was particularly evident. When peak natural third-generation hatch (13–16 August) south of 39°00′ N lat. was approximated, five reciprocal populations diapaused similarly to their corresponding natural populations (≈85%). Late-season hatch allowed larval diapause of the exotic population to increase to 46%. Under these late-season conditions, influence of the parental male was evident only at the most southern locations, Portageville, Mo., and Tifton, Ga. This apparent contradition of parental male influence between midsummer and late-summer hatch is because heterogeneity for diapause remains in a low-diapause population. Introductions of a low-diapausing population north of 39°00′ N lat. during late July apparently encourage resultant progeny from reciprocal crosses to avoid larval diapause and to perish in late autumn.