The pest occurs annually throughout North America with the exception of 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. Its migration from southern locations to the north might be so late in the season that it does not get the chance to inflict much damage. Unless you are dealing with a heavy infestation, for most home gardeners, it is usually not the corn earworm itself that causes the most damage. It is the holes that the corn earworm chews into the plants that makes them vulnerable to other insects and disease pathogens such as fungi.
What Do Corn Earworms Look Like?
Although the corn earworm is the most destructive as larvae, it is important to know the life cycle and how corn earworm looks in all its life stages for effective control of the pest. The eggs laid by the female moth hatch in three to 10 days depending on the temperature. In warmer weather, they hatch faster. The larva emerging from the eggs go through six instars, which takes about 18 days. For the last stage of the life cycle, the mature larvae moves into the soil to turn into pupa, a process that takes about eight to 14 days before a new generation of moths emerges. The entire life cycle takes three to four weeks; there are about four generations per year. Corn earworm larvae come in a variety of 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 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 are light tan in color. The moth is about ¾ inch long and has a wingspan of up to 1.5 inches. The adult moths lay their eggs on the fresh corn silks of each ear, which is their preferred place, but also on the corn leaves. Although the female lays several eggs, only one egg reaches maturity. The eggs are minuscule, about half the size of a pin head, and spherical in shape. Initially the eggs are white, then they turn 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 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; afterwards, 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. Using a pipette or a dropper, apply five drops (about ¼ teaspoon) of mineral oil at the tip of each corn ear. 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
Common insecticides used for corn earworm control are pyrethroids such as carbaryl or permethrin. First, always make sure to positively identify corn earworm, and also 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 they are thoroughly wetted. Applying the insecticide to any other part of the plant is unnecessary and harmful because broad-spectrum insecticides indiscriminately kill other beneficial insects and pollinators as well. 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
There are several signs that can help you identify a corn earworm infestation and the earlier you detect it, the better.
On Corn
The preferred food of corn earworm larvae are corn ears. After the larvae hatch from their eggs, they chew off the silks first. After they are done with those, they move down to the tip of the ears. If the silks have not yet emerged, the larvae may also move directly into the ears. Corn earworm always enter the ears from the tips. A telltale sign of their feeding on the kernels are the excrements they leave behind at the tip of the ear. To protect them from predators, the worms are covered with a layer of frass that looks like sawdust. Depending on their development, they may only feed on the tip of the ear, or they remain in the ear long enough to move halfway down the ear before they start their next larval stage. There is only one larva per ear because corn earworm larvae feed on each other. One larva usually only feeds on one corn ear and rarely moves to another ear. When the larva is mature, it bores a small ear in the side of the ear to leave the ear, so it can drop to the soil, where they pupate. The pupa do not feed, so you won’t be able to detect any damage at this stage. In addition to the ears, larvae also feed on young leaves, tassels, and whorls but that generally does not affect the quality of the corn harvest. This feeding happens more 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
While corn is the preferred host plant for the moths to lay their eggs, tomato plants are the second favorite. Of all the plant parts, the larvae prefer to burrow into a tomato, feed on it briefly, and then move on to the next tomato—unlike corn, where they usually remain in the same ear. The larvae also feed on tomato leaves and burrow into the stems. If there is no corn nearby, the larvae also feed on other garden crops. This includes 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 earworm is more difficult. The holes that the corn earworm has made in the corn ears can attract other pests such as sap beetles, as well as make the plants prone to fungal diseases. This can cause more damage than the corn earworm itself.
Moths
If you spot adult moths on your corn plants, there will most likely be corn earworms. The moths are active during the 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 migration of the pest. In warm climates, corn earworms overwinter as pupae in underground cells. The first adult moths emerge in March and start searching for places to lay their eggs. In cooler northern climates where ear cornworm cannot survive the winter, it spreads by migrating north by riding on storm fronts. Corn earworm can arrive anytime between mid-July to September.
How to Prevent Corn Earworms
Corn earworms have numerous natural enemies such as ladybugs; however, their numbers are usually not sufficient to keep a infestation under control. One way you can avoid corn earworms is by planting your corn early. In cooler climate where corn earworms do not overwinter, corn harvested before mid-August have 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—remember that the larvae start feeding on the silks and only move down to the ears afterwards. 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.
