Part XVIII: The Corn Earworm
September/October 1986
By Ron West
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It took only a few minutes work to net this catch of earworms from the author's corn patch.
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BACKYARD JUNGLE
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Here's the 18th in a series of articles that will help you tell friend from foe in your garden.
If you raise sweet corn, you more than likely know about the corn earworm ( Heliothis zea ) — the caterpillar of a tan-colored noctuid moth common throughout North and South America. This voracious field-crop and garden pest grows to about an inch and a half in length and comes in a variety of colors, with green, yellow, pink, reddish brown, and dark gray forms occurring simultaneously. The earworm is most destructive to corn, tomatoes, and cotton, but also attacks tobacco, beans, peas, peppers, squash, and other cultivated plants.
The small, white moth eggs that produce the earworm are often deposited directly on the silks of corn ears, putting the newly hatched caterpillars in position to burrow immediately through the silks and into the tops of the ears (often leaving a telltale hole). In addition to tunneling into the ears, noctuid caterpillars sometimes attack the leaf whorls of young corn plants. On tomatoes, earworms feed initially on the foliage and save the juicy fruits for dessert.
The larval stage lasts about two weeks in the case of earworms hatched in summer, and a month or more for those that emerge in spring and fall. This allows time for one or two generations to occur annually in northern regions, while four to seven generations can be completed in the South. After the larvae attain full growth, they burrow into the soil to a depth of between one and seven inches. Here they pupate — a state that can be as brief as two weeks in the summer or as lengthy as six months during winter hibernation.
Adult noctuid moths can sometimes be seen at dusk hovering about flowers in search of nectar. Each female lays about 1,000 eggs in her short lifetime, and under ideal conditions may produce as many as 3,000. Fortunately, many natural controls are at work to offset this phenomenal reproductive capacity: Birds, toads, spiders, and numerous predatory insects inflict a heavy toll on the earworm population; the minute Trichogramma wasp is an important parasite; and the earworm helps control itself through cannibalism (especially in severely infested corn ears).
The most effective active earworm control measures vary according to the species of plant being attacked. On most garden plants other than corn, earworms surface-feed and therefore are visible during the early stages of their lives, making them easy targets for plucking and crushing by hand. Additionally, the bacterial pesticide Bacillus thuringiensis (BT) can be applied periodically before the larvae start burrowing. A virus marketed (by Sandoz, Inc.) as Elcar can also be used on exposed earworms.
