Corn Earworm: Identification, Overview, and Garden Control

Corn earworm (CEW), also known as tomato fruitworm, cotton bollworm, sorghum headworm, or vetchworm, is the greatest threat to agricultural and home garden sweet corn production. Unlike beneficial garden worms, corn earworms can cause significant damage to your plants and render the food unsafe to eat. While corn and tomatoes are the preferred food of the larvae, if there is not enough corn growing nearby, the pest can also attack a wide range of other garden crops and weeds like common mallow, horsenettle, pigweed, purslane, and ragweed.

The pest occurs annually throughout North America, except in northern Canada and Alaska. In warm southern climates where it overwinters, corn earworm usually causes more damage earlier in the season than in cooler northern climates. It migrates from southern locations to the North late in the season and inflicts less damage. 

Unless you are dealing with a heavy infestation, it is usually not the corn earworm that causes the most damage for most home gardeners. The holes that the corn earworm chews into the plants make them vulnerable to other insects and disease pathogens such as fungi. 

Corn Earworm Life Cycle

The corn earworm life cycle begins with eggs laid by the female moth, hatching in three to 10 days, depending on the temperature. In warmer weather, they hatch faster. The larva goes through six instars or molts over 18 days, moving into the soil to pupate. It takes about eight to 14 days before a moth emerges. The entire life cycle takes three to four weeks; there are about four generations per year.

How to Identify Corn Earworms

Although the corn earworm is the most destructive as larvae, it is essential to know the life cycle and how it looks in all its life stages for effective pest control.

Larvae

Corn earworm larvae come in various colors, from light green or pink to maroon, brown, brownish-green, or almost black. The upper side of the body has alternating light and dark longitudinal stripes. The worms are covered with numerous black spines that look like hair. The underside of the body is lighter in color. The head is golden brown.

Mature earworm larvae grow up to 1.5 inches long. The worms look like they're covered with a protective coating (frass) that looks like sawdust to confuse predators. The larvae feed on the silks and tips first and move down to the ears afterward.

Adult Moths

The color of the corn earworm moth also varies, ranging from tan or olive green to dark reddish-brown. Male and female moths are different in color, but they share two distinctive characteristics: a dark spot in the center of the front wings that looks like a comma which is more prominent in male moths, and a dark band near the margin of the hind wings, which is light tan. The moth is about 3/4 inch long and has a wingspan of up to 1.5 inches. 

Eggs

The adult moths lay their eggs on the fresh corn silks of each ear, which is their preferred place, and the corn leaves. Although the female lays several eggs, only one egg reaches maturity. The eggs are minuscule, about half the size of a pinhead, and spherical. Initially, the eggs are white, turning reddish-brown before the larvae hatch. 

3 Ways to Get Rid of Corn Earworms 

There are generally four generations of corn earworms each year, and they often overlap so that you may find corn earworms in all development stages at any given time. Pest control measures, however, are usually focused on eradicating the eggs and young larvae from the corn ears, the favorite food of the corn earworm, as early as possible. You need to control the larvae before they move from the silk into the ears; afterward, there is no way to control them. 

Manual Removal

If there’s just an occasional ear cornworm on your plants, you might get it under control by cutting off the tips of the affected ears.

Mineral Oil

Apply mineral oil on the silks five to seven days after the silk emerges. Apply five drops (about 1/4 teaspoon) of mineral oil at the tip of each corn ear using a pipette or a dropper.

You can also mix Bacillus thuringiensis (Bt) with mineral oil in a ratio of 1:20. For example, to treat 40 corn plants, mix ½ teaspoon BT with 10 teaspoons mineral oil and apply it with a pipette the same way as described above.

Insecticides

Typical insecticides used for corn earworm control are pyrethroids such as carbaryl or permethrin. First, always make sure to positively identify corn earworm and check the product label to verify that the active ingredient indeed works on this pest. Also, corn earworm has developed a resistance to certain insecticides, so they don’t always work. 

Apply the insecticide a few days after the silks emerge and follow the mixing directions. Use a pump sprayer to insert the insecticide directly and deeply into the silks until thoroughly wetted. Applying the insecticide to any other part of the plant is unnecessary and harmful because broad-spectrum insecticides also indiscriminately kill other beneficial insects and pollinators. Repeat the treatment if needed and check the directions to see how long you can safely apply the product before harvest. 

Signs of a Corn Earworm Infestation 

Several signs can help you identify a corn earworm infestation; the earlier you detect it, the better.

On Corn

Corn earworm larvae enter the ears from the tips. A telltale sign of larval activity is the excrement they leave behind at the tip of the ear. They tend to chew off the silks first and then the rest of the ear.

There is usually only one larva per ear because corn earworm larvae feed on each other. They rarely move to another ear. Larvae also feed on young leaves, tassels, and whorls, most often in the late summer when corn earworm populations peak and the larvae might not find sufficient corn ears to feed on. 

On Tomatoes and Other Garden Crops

Tomato plants are the second favorite plant of these larvae. The larvae burrow into a tomato, feed on it briefly, and then move on to the next tomato. The larvae also feed on tomato leaves and burrow into the stems. Signs of this larvae include tunnels, damage to the stem, and the fruit ripening fast or rotting.

Other garden crops feasted on by this pest include asparagus, beans and peas, cabbage, cucumbers, eggplant, lettuce, spinach, Swiss chard, cantaloupe and watermelon, peppers, pumpkins and squashes, potatoes, and sweet potatoes. They usually burrow into firmer plant parts such as lettuce heads and bean pods, but in the absence of other food sources, they also chew on leaves. 

Secondary Damage

Detecting the indirect secondary damage from corn earworms is difficult. Signs can include holes the corn earworm has made that attract other pests, like sap beetles and fungal diseases. These secondary effects can cause more damage than one corn earworm. 

Moths

If you spot adult moths on your corn plants, they are likely corn earworms. The moths are active at night, but you might find them resting in the whorl of young corn or at the base of mature corn leaves.

What Causes Corn Earworm 

Depending on the climate, corn earworm infestations come from overwintering or a new annual pest migration. In warm climates, corn earworms overwinter as pupae in underground cells. The first adult moths emerge in March, searching for places to lay their eggs. In cooler northern climates where ear cornworms cannot survive the winter, it spreads by migrating north by riding on storm fronts. Corn earworms can arrive anytime between mid-July and September. 

How to Prevent Corn Earworms 

Corn earworms have numerous natural enemies, such as ladybugs; their numbers are usually insufficient to control an infestation. 

One way you can avoid corn earworms is by planting your corn early. In a cooler climate where corn earworms do not overwinter, corn harvested before mid-August has a better chance of avoiding corn earworms once the population density rises. The later you plant corn in the season, the higher the risk of an infestation. 

The other solution is to select corn varieties with long, tight husks because they are less prone to larvae entering the ears. Recommended corn varieties include ‘Country Gentlemen,’ ‘Staygold,’ ‘Golden Security,’ and ‘Silvergent.’

If you live in a warmer climate, practice good garden sanitation and remove all the plant residue so the pest cannot overwinter. Also, rotate your corn crops to a different spot in your garden.