Corn Earworm
Corn earworm/bollworm/tomato fruitworm, Heliothis zea (Boddie), Noctuidae, LEPIDOPTERA


DESCRIPTION

Adult - The corn earworm moth has a wingspan of 26 to 38 mm and is usually light yellowish-olive in color. Each forewing has a dark spot near the center. Eyes are usually light green.

Egg - The dome-shaped egg, about 0.5 mm in diameter, is pale white when first laid and develops a reddish-brown band before hatching.

Larva - The 5 to 6 larval instars vary considerably in color. Newly hatched larvae are about 1.5 mm long and yellowish-white with dark head capsules. Second instars are yellowish-green and frequently have orange and brown longitudinal stripes; their head capsules are reddish-brown to brown. Up to 44 mm long, later instars are greenish-yellow, reddish or brown with pale longitudinal stripes, raised black spots (chalazae), and brown to orange heads. All instars have 5 pairs of fleshy prolegs.

Pupa - About 31 mm long and 6 mm wide, the pupa is reddish-brown to dark brown.


BIOLOGY

Distribution - This insect is found throughout most of the Western Hemisphere.

Host Plants - The corn earworm infests over 100 plants, but corn is the preferred host. The earworm is also found occasionally on wild hosts such as toadflax and vetch.

Damage - Though foliage is attacked early in the growing season, corn earworms prefer fruiting stages of their host plants. On seedlings, corn earworms devour leaves, buds, and tender new growth. On corn, first generation larvae feed in the tightly coiled blades. 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 often is referred to as "shatterworm" injury.

As vegetable crops flower and produce fruit, larvae move to these plant parts. Entrance holes of small larvae may be barely detectable in tomatoes or completely undetectable in sweet corn ears. As infested tomatoes increase in size, small string-like scars may become apparent where the tiny larvae entered. In the corn ear, larval damage is usually confined to developing kernels in the ear tip area. Round holes (approx. 5 mm in diameter) through the shuck are usually emergence rather than entrance holes. On bean, okra, and tomato plants, large corn earworms may move from fruit to fruit feeding and leaving large gaping holes on the surface.

Life History - In North Carolina, corn earworms overwinter as "resting" (diapausing) pupae in 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 the leaves of seedling corn when it is available. However, once corn begins silking, most eggs are deposited on silks. Later in the season, as corn silks dry up, oviposition again occurs on the leaves of other hosts, including many vegetable crops. Eggs are deposited singly, each female laying from 450 to 3,000. Within 2 to 5 days, the larvae emerge and begin feeding. Because this insect is cannabalistic, 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 5 or 6 instars. They then burrow in the soil to pupate. Two to 4 weeks later, a new generation of moths emerges. At least 3 generations occur each year in North Carolina.


CONTROL

Resistant sweet corn varieties are available. These varieties are not immune to attack. Although young corn may be damaged severely by corn earworms, insecticides generally are used only from silking until harvest to protect ears. When corn is harvested in the Coastal Plain from July 10 to July 20, the infestation in ears is relatively low, even if only a fair job of chemical control is carried out.

On tomato plants, damage is also low when the fruits are harvested before July 20. The first chemical treatment should be applied when tomatoes in the first cluster are about 13 mm in diameter. Followup treatments should be applied every 7 to 10 days as necessary. In the Coastal Plain, a 5 to 7 day schedule is needed after July 20. Cultivating fields of vegetable crops after harvest kills numerous pupae in the soil and exposes many to birds and other predators.

For recommended insecticides and rates, see the current North Carolina Agricultural Chemicals Manual.

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