No. 23 Article 4/October 6, 2006

Sorting Out the Caterpillars in 2006

At the end of the 2005 growing season, many people reported injury to corn caused by several different species of caterpillars. This year, if anything we received even more reports of caterpillar injury to corn, especially to ears, than we did in 2005. Consequently, it seems appropriate to reprint part of an article written by Kelly Cook for issue no. 23 of the Bulletin last year (October 7, 2005). Although Kelly has departed the Department of Crop Sciences for a new position with the Illinois Natural History Survey, the information she wrote last year is just as viable today. The title of the article last year was ">Corn Earworm, European Corn Borer, Fall Armyworm, or Western Bean Cutworm: Which One Is Causing the Injury I'm Finding on My Corn Ears?" . A substantial excerpt follows.

"So how does one determine the cause of ear damage this late in the season? The answer is simple: You really can't. At this time in the season, it is rare to find any larvae still feeding on corn ears. Without larvae, you can't be positive if injury was caused by earworms, corn borers, fall armyworms, or bean cutworms, as they cause very similar injury. Let's look at each insect individually.

"Corn earworm. Two generations of corn earworm infest Illinois cornfields each year. Because earworms generally do not overwinter in Illinois, summer populations arise primarily from immigration of moths from southern states in late spring and early summer. Infestations of earworm larvae can cause injury to corn plants, including slight defoliation of leaves, damage to the tassel, and consumption of silks and kernels. The second corn earworm generation usually occurs during pollination. Larvae enter the ear primarily through the silk channel, unlike European corn borer and fall armyworm, which enter through the husks or cob. As silks dry, corn earworm begin feeding on kernels. Larvae feed at the tip and along the sides of the ear near the tip, continuing to feed until they mature. At that time the larvae drop to the ground to pupate. When leaving the ear, corn earworm may drop from the ear tip or create exit holes by chewing through the husk. These exit holes can be mistaken for entrance holes caused by other larvae.

"European corn borer. Two to three generations of European corn borer occur in Illinois each year. Injury to corn ears is caused by the second and third generations. Loss of grain to larvae's direct feeding on kernels is usually not important in field corn, but in sweet corn and seed corn, losses can be significant. Larvae feed on pollen and silks before entering the ear. Entry to the ear is also gained by tunneling through the shank and cob. Ear feeding by corn borer larvae is not focused on any one area. Injury can be found at both ends and along all sides of the ear. Larvae feed until mature; they overwinter as fifth-larval instars in stalks and plant debris.

"Fall armyworm. Like the corn earworm, fall armyworm moths migrate north into Illinois each year. Fall armyworms are a concern for cornfields from mid- to late summer. They cause serious leaf-feeding damage and feed directly on corn ears. Late-planted or later-maturing hybrids are more susceptible to fall armyworm injury. Most common is pretasseled corn. Larvae consume large amounts of leaf tissue, but as corn plants develop, larvae move to the ear. Unlike the corn earworm, the fall armyworm feeds by burrowing through the husk on the side of the ear. Larvae also enter at the base of the ear, feeding along the sides and even tunneling into the cob. They usually emerge at the base of the ear, leaving round holes in the husks.

"Western bean cutworm. A mid- to late-summer pest of corn, western bean cutworm moths begin to emerge in early July. Though some leaf feeding occurs, larvae feed primarily on silks, tassels, and developing kernels. Larvae of the western bean cutworm are not cannibalistic, and several larvae may infest one ear. Entry to ears is gained through silk channels or by chewing through husks, injuring the tip, base, and sides of the ear. Larvae feed on kernels until about mid-September, when they exit through husks."

We urge corn producers to be aware of the amount of injury caused by these different caterpillars in their areas this year. Although the western bean cutworm will catch a lot of the blame because it is a pest new to us, it's very likely that much of the injury to corn ears this year was caused by corn earworms and fall armyworms, not to mention the larger numbers of European corn borers being found this fall. As producers plan their corn insect management program for 2007, they will need to keep these pests in mind. Although Bt corn will not be the solution for everyone, the benefits and limitations of Bt corn should be considered.--Kevin Steffey and Kelly Cook

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