Corn earworm activity typically peaks in late-August to early-September. As the remnants of tropical storms and hurricanes make their way North, they often bring large numbers of “hitch hikers” with them. These hitch hikers are dropped back down to earth as the trade winds hit the cooler air of the Great Lakes.
Due to the extremely small size of the earworm eggs, it is virtually impossible to scout for this pest. Sweet corn fields are susceptible to infestation as long as there are green silks present in the field.
Control measures must be taken during the narrow window of opportunity between when the egg hatches and when the larvae enter the protection of the husk. To further complicate matters, this pest has developed resistance to the pyrethroid family of insecticides. Depending on the year, the level of resistance can vary from 40-60% of the corn earworm population.
Research in the mid-western US suggests that the timing of the spray application is essential for good control. Apply a protective insecticide at first silk, followed by a second application 3-4 days later. Coragen Insecticide is an effective alternative to the pyrethroid insecticides.
 Hutchison et al. 2007. Evidence for Decreasing Helicoverpa zea Susceptibility to Pyrethroid Insecticides in the Mid-Western United States. Plant Management Network, 19th July, 2007.