Egg - The dome-shaped egg is white when first deposited but develops a reddish-brown band before hatching. Each egg is approximately half the size of a pinhead (about 0.55 mm high and 0.57 mm wide).
Larva - When fully grown, this moderately hairy larva is pale-striped, black-spotted and predominantly yellowish-green, brown or reddish-brown with a tan to orange head. When disturbed, it curls up tightly, remaining motionless for a few seconds. Early instars are yellowish-white or yellowish-green, often with orange and brown longitudinal stripes. Length ranges from 1.5 to 44 mm depending upon the instar, or developmental stage. Color plate.
Pupa - The pupa is about 31.5 mm long and 6.0 mm wide. Initially a shiny reddish-brown, it becomes dark brown before the adult emerges.
Host Plants - The corn earworm infests over 100 plants, but corn is the preferred host. In the South it occurs on at least 17 cultivated plants: alfalfa, bean, chrysanthemum, corn, cotton, geranium, gladiolus, okra, peanut, pea, sorghum, soybean, strawberry, sweet pepper, sweet potato, tobacco, and tomato. The earworm is also found occasionally on wild hosts such as toadflax.
Damage - Although the corn earworm prefers the fruiting stage of the host, it will also attack the foliage. During May and June, first-generation larvae feed in the tightly coiled blades of corn. As a result, numerous ragged holes appear when the blades unfurl. Wet, tan to brown excrement lodges in the whorl and blade axils. This condition is often referred to as "shatter worm" injury.
Since the eggs beginning the second generation are laid on corn silks, larval damage is confined largely to the developing kernels in the ear tip area. Round holes (approximately 5 mm in diameter) through the shuck are usually emergence rather than entrance holes. Third-generation damage may occur on very late corn, but larvae of this generation develop primarily on sorghum, soybeans, peanuts, and cotton, depending upon location. On sorghum, the earworm is generally found entwined in the seedhead feeding on seeds and flower stems.
Life History - In North Carolina, corn earworms overwinter as "resting" (diapausing) pupae in the soil at a depth of more than 5 cm. Adults emerge in early May, mate, and seek suitable oviposition (egg-laying) sites. A high percentage of first- generation eggs are laid on seedling corn when it is available. However, once field corn begins silking, most eggs are deposited on the silks. Later in the season, as corn silks dry up, oviposition again occurs on a variety of hosts, including sorghum. The eggs are deposited singly, each female laying from 450 to 3,000. Within 2 to 5 days, the larvae emerge and begin feeding. This insect is cannibalistic; therefore, rarely does more than one larva develop from an ear or whorl. Larvae feed for 2 to 4 weeks during which time they develop through five or six instars. They then burrow in the soil to pupate. At least four generations occur each in North Carolina.
Economic thresholds for the corn earworm vary with the crop and
with the part of the crop being attacked. In sorghum, the presence
of one medium to large worm per head justifies control. In corn and
sorghum controls are warranted when 80 percent of the whorls are
infested with one earworm or when 40 percent of the whorls have two
or more earworms. For further control information, consult the
North Carolina Agricultural Chemicals Manual.