The development of corn earworm,Heliothis zea, populations on corn was investigated in the Lower Rio Grande Valley of Texas and Tamaulipas, Mexico, during 1985 and 1986. Initial infestations by small larvae of 42.8 and 19.8% on whorl stage corn occurred during the weeks ending on day 102 (12 April) of 1985 and day 95 (5 April) of 1986, respectively. Initial trap peaks of 108 males/trap (1985) and 61 males/trap (1986) occurred from 4 d before to 3 d after the initial estimated oviposition peaks. Adults from larvae developing on whorl stage corn produced the infestation on fruiting corn during the weeks ending on days 130–137 and resulted in the largest trap peak of the year (132 males/trap in 1985 and 289 males/trap in 1986). Midpeak of trap capture occurred 9–12 d after the oviposition peak on fruiting corn. Adult emergence in fruiting corn occurred between days 150 and 182 and averaged 3.0 and 0.80/m2respectively for 1985 and 1986. The lack of a major trap peak associated with this emergence indicates that a major portion of these adults migrated from the area before mating.