By Donald Lewis
Iowa State University Extension
One of the pure pleasures of summertime in Iowa is eating sweet corn fresh from the garden or farmers market. To the dismay of gardeners, growers and sweet corn aficionados, however, there is the matter of an occasional pest to consider. The most important and best known insect pest of sweet corn in Iowa is the pudgy, hairless “worm” found at the tip of an infested ear, the corn earworm.
Corn earworms come in a variety of colors, ranging from light green, to tan, brown, pink or nearly black. The caterpillar’s body is marked with light and dark stripes running lengthwise and the skin texture is coarse due to microscopic spines that cover the surface. Earworms are only in the ear for three to four weeks but during that short time they grow to nearly 1 .5 inches in length. Infestations may be present throughout the summer but are generally worse in late summer.
Unlike hardy residents of the state the corn earworm does not survive Iowa winters. Instead moths that lived and grew in southern states on either corn or cotton last year are blown here during May and June each year to reinfest the state.
These recent-arrival-moths fly after sunset and reproduce by depositing their eggs on the fresh, green silks of the sweet corn ear. These eggs hatch in two to six days and within an hour the tiny, young larvae crawl into the silk channel and move to the tip of the developing ear. The larvae feed on the silk and developing kernels and foul the ear with excrement. About three weeks after silking the sweet corn is ready to harvest and eat, and there, waiting for you at the end of the ear is the much-grown earworm caterpillar.
The amount of corn earworms in the sweet corn crop varies from place to place, from year to year and with the time of the year. Mostly the damage is determined by the number of moths in the vicinity which depends on the weather and other factors. Some varieties of sweet corn are more or less susceptible to earworm attack, and genetically modified varieties are available that produce their own defense against caterpillar attack.
Growers and gardeners who want “clean” sweet corn must work to prevent the earworms from getting into the silks. If the caterpillars are already crawling toward the ear tip it is too late to stop them. A typical preventive management strategy is to spray insecticide on the corn ears throughout the entire period when green silks are present.
Insecticides for the home gardener include azadirachtin (Neem), Bacillus thuringiensis, carbaryl (Sevin), or permethrin. Spray at the first sign of silk emergence (one or two days after tassels appear) and again two days later after silks have elongated. For complete protection, especially in later plantings, spray a third time three days after the second spray. After the silks turn brown there is no benefit to spraying.
Admittedly, this is an extensive amount of insecticide but it is currently the most practical method for assuring worm-free sweet corn. The alternative is to not treat at all. Instead, cut off the damaged tip of infested ears and enjoy the remainder of the ear.