Population dynamics studies of Heliothis zea (Boddie) developing on corn were conducted in the Lower Rio Grande Valley which includes both south Texas and northern Tamaulipas, Mexico. These studies revealed that an estimated 2 to 6 billion adult H. zea were produced from the area corn crop. Adult emergence from corn was synchronized with a buildup of H. zea on the area cotton crop; however, meteorological events occurring during the emergence period indicate that long range dispersal from the area may play an important role in depleting this population.