Corn Earworm
 Heliothis zea (Boddie), Noctuidae, LEPIDOPTERA

DESCRIPTION

Adult - The moth has a wingspan of 25.5 to 38.5 mm. The forewings of the male are usually a light yellowish-olive; those of the female are yellowish-brown to pinkish-brown. Each forewing has a dark spot near the center. The hind wings are white with a broad, dark brown, outer marginal band and, usually, a narrow, brown, intermarginal band.

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.

BIOLOGY

Distribution - This insect is found throughout most of the Western Hemisphere. Annually it is among the most destructive insect pests in the southern United States and has been found as far north as Saskatchewan. It has recently been introduced into the Hawaiian Islands.

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.

CONTROL

Plant high quality, certified corn seed of varieties noted for their "husk-tightness." Such varieties help prevent the entry of earworms by the silks. An early planting, which allows the corn to silk before earworm populations peak, also reduces the extent of earworm damage. The expense of chemically controlling this pest in field corn and sorghum is prohibitive except under very rare circumstances.

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 current North Carolina Agricultural Chemicals Manual.