Corn earworms,Heliothis zea(Boddie), are a major pest in the major sweet corn seed-growing area of the United States. In an effort to predict corn earworm infestations, populations per 100 ears were recorded over a period of 8 yr, 1971–78. Growing degree-days (GDD) were calculated (thresholds of 13 and 35°C) for 1970–78. Chronological dates for photoperiod necessary for diapause induction were also determined. Correlations of peak corn earworm populations with GDD during the previous year were calculated. Highest degree of correlation occurred between GDD on 23 September and the peak population the following year. A theory is presented to explain the relationship of accumulated GDD on 23 September and the peak corn earworm population the following year.