Oviposition of the corn earworm,Heliothis zea(Boddie), on corn in Idaho was observed to decrease from July 1–15 and then to increase again. A simple model based on heat units above 55°F predicted when the mid-July oviposition would begin. A thermodynamic model (WATBUG) was used to simulate seasonal development of the population, produce life tables, and evaluate the effect of control treatments.