No. 23 Article 4/October 7, 2005

Corn Earworm, European Corn Borer, Fall Armyworm, or Western Bean Cutworm: Which One Is Causing the Injury I'm Finding on My Corn Ears?

Injury on corn ears is a hot topic right now. Whether it's a view from the combine cab or an assessment made by foot, many producers are making their first trip into the field in quite some time. Some are finding unwelcome insect injury.

Insect injury to corn ear (photo courtesy of Duane Frederking, Pioneer).

Damaged ear tips, missing kernels, and fungal pathogens are all being reported in abundance. Several insect pests in Illinois could be the culprit. Corn earworm, European corn borer, and fall armyworm are common pests of Illinois cornfields. Their larvae all feed on the ears of corn plants. The western bean cutworm, new to Illinois, also causes injury to corn ears.

The western bean cutworm was identified in Illinois for the first time in 2004. This past summer a trapping network was set up in conjunction with the Iowa State Western Bean Cutworm Trapping program. It was our intent to monitor for the presence and distribution of western bean cutworm adults. Adult western bean cutworm moths were trapped in several counties in Illinois in 2005. The most abundant moths were found in the northwestern part of Illinois along the Mississippi River. We are examining the findings from the survey at this time, and the results will be made available in the very near future. There is a lot of confusion about whether the western bean cutworm is causing the damage to corn ears currently being observed.

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.

Corn earworm larvae.

Corn earworm injury to corn ear.

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.

European corn borer larva (photo courtesy of Marlin Rice, Iowa State University).

European corn borer injury to corn ear (photo courtesy of Marlin Rice, Iowa State University).

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.

Fall armyworm larva.

Fall armyworm injury to ear.

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.

Western bean cutworm (photo courtesy of Marlin Rice, Iowa State University).

Western bean cutworm injury to corn ear (photo courtesy of Marlin Rice, Iowa State University).

Any one or combination of the aforementioned insects could be the cause of the injury being seen in cornfields. It is important to remember, though, that corn earworm, European corn borer, and fall armyworm are all established pests in Illinois. We see injury caused by them year after year. Why so much injury in 2005? Corn plants have been stressed in many areas of the state. We have also had heavy corn earworm flights this summer, particularly the second generation. Reports of corn borer damage early in the summer are being confirmed by high numbers of second-generation injury in our annual fall survey. Fall armyworm flights have been very quiet this year, but larval injury is still a possibility. Western bean cutworm flights were abundant in northwestern Illinois. Some instances of isolated injury were reported, but reports were not widespread across the region. The possibility of injury by the western bean cutworm does exist, but based on the extent and locations of injury being reported, corn earworm, European corn borer, or fall armyworm is the likely cause.

When deciding on hybrids for 2006, particularly transgenic hybrids, growers should understand the problems faced in their areas. Know what insects you face and have appropriate expectations for how hybrids should perform against them.--Kelly Cook

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