Corn earworm is a pest that feeds on corn kernels inside the husk. The larva is a brown-headed caterpillar with alternating dark and light stripes running lengthwise on the body. They can be green, brown, or black, and are 1.5 inches when fully grown. They are cannibalistic, so usually only one is found per ear of corn. The pupa is cylindrical, brown, about 1-inch-long, and is found in the soil. Adult moths are tannish brown with a 1 1/2-inch wingspan. The front wings are marked with a distinct dark spot in the center and darker bands near the outer margins.
In Utah, there are up to three generations of corn earworm each year. The first generation of moths either come from overwintering pupae (southern and central Utah), or migrate each season from the south. Females lay eggs only on fresh corn silks. Once silks are brown and dried, they are not suitable egg-laying sites. The larva crawls into the ear, feeds for 10 to 14 days, and then exits and drops to the ground. They burrow 2 to 5 inches deep into the soil and pupate, and moths emerge to form the 2nd generation. Pupae formed in late summer may overwinter in warmer climates, otherwise they are killed by cold winter temperatures.
- Tunnels in kernels
- Direct damage to the ear tip
- Mold growth within the ear
- Opening in the husk at the ear tip
Some general practices include the following:
- Plant sweet corn early so that it will silk before major moth activity occurs and may escape injury.
- Corn that silks between first and second generation flights (July 20 to August 5 on average in northern Utah) may also avoid damage.
- In locations where corn earworm pupae survive the winter, fall tillage is an effective practice.
- Trichogramma is a wasp that parasitizes corn earworm eggs. It occurs naturally in Utah, or can be purchased from vendors and released in the corn field. The timing of releases and maintenance of adequate wasp populations are critical to success.
In corn, good control is dependent on preventing larvae from entering the ears. About half of the eggs are laid within 2 days of silk emergence, and the remainder of the eggs are laid by 9 days later. If an insecticide spray is used, start applications within 2 days of the beginning of silking, and reapply to keep an active residue on new silk. Silk grows about 1/2 inch per day, and once silks turn brown, they are no longer attractive as egg-laying sites.
Some growers have had some success with applying drops of mineral oil plus Bt (Bacillus thuringiensis) onto silk. Other options to apply to the silk include neem oil or spinosad.