By: Sandy Feather ©2007
Penn State Extension
Q. I planted corn for the first time this year, and had a real problem with corn earworms. My family was totally grossed out! Can you recommend a way to control them? I prefer not to use chemical insecticides.
A. Corn earworms cause the worst of their damage by feeding on the fresh silk and boring into the tips of the ears of corn once the silk starts to dry. Although I usually see them confined to the tip, they can feed well into the middle of the ear, too. Damaged silk can interfere with pollination, which results in partially filled ears of corn. In addition to corn, corn earworms can damage tomatoes and peppers. They are also known as tomato fruitworms and cotton bollworms.
Corn earworms overwinter in the southern United States and are blown north on storms through the summer. Adults lay tiny yellow eggs singly on corn silks and on the underside of the leaves. The larvae vary in color from pink to yellowish-green to green to brown. They range in size from about one-quarter inch to two inches long. Adults are rather non-descript brownish-gray moths that you may not notice since they are active at night. We have several generations annually in our area.
The old measuring stick for corn growth:
"Knee high by the 4th of July"
Early and mid-season varieties of sweet corn are usually less bothered by corn earworms, while they are worst on varieties that silk in September. Early varieties include Seneca Horizon, Seneca Daybreak, and Sundance, while mid-season varieties include Sugar Buns, Bodacious, and King Arthur. Some references also list certain varieties of sweet corn as more resistant to this pest because they have tight husks that wrap completely around the tip, making it more difficult for corn earworms to get into. These include Country Gentleman, Silver Cross Bantam, Calumet, and Silvergent.
One of the best non-chemical controls is Bacillus thuringiensis (Bt), a bacterial stomach toxin specific to the larvae of moths and butterflies. It must be ingested by the target insect to be effective, but does not harm moths and butterflies that are not feeding on treated plants.
Bt does not have systemic activity, and must be reapplied after rain and at certain intervals, according to label directions. It is sold under trade names such as Dipel, Thuricide, and Green Step Caterpillar Control. Since it works best on small caterpillars - under one-half inch - be sure to start applying Bt when ten percent of the crop is starting to silk. It will not be effective on large corn earworms that have worked their way deeper into the ear of corn.
If the damage is confined to the tip of the ear, simply cut it off before taking it in to your family. There is nothing wrong with the undamaged portion of the ear. Make sure to dispatch the larvae you remove rather than throwing them on the compost pile so they do not mature and add to your problem.