As we near the end of July, the "Leps" of summer will soon be descending on cornfields throughout the state. Corn earworm, southwestern corn borer, fall armyworm, and second-generation European corn borer are four of the prominent moth pests of corn. Corn earworm flights are already heavy in some areas. Dan Fournie, Collinsville, has seen an increase in moth numbers and reported a one-night catch of 189 corn earworm moths on July 10. Ron Hines, senior research specialist, Dixon Springs Agricultural Center, also reports this week that flights of corn ear-worm, fall armyworm, and European corn borer are coming on strong, while a second flight of southwestern corn borer moths is beginning.
As flights increase, be aware that moths are searching for locations to lay eggs. Corn earworm moths will be most attracted to cornfields that are currently silking. Moths prefer to lay eggs on newly emerged silks but will also lay eggs on the upper and lower leaf surfaces, tassels, and stalks of corn plants. In Illinois, the economic importance of the corn earworm is generally limited to sweet corn and seed corn. Early instars feed on leaves, leaving small pinholes on the leaf surface. Older larvae, ranging in color from light green or pink to dark brown or nearly black, feed on leaves, causing defoliation. More important, they feed on silks and developing kernels. Earworm larvae that are able to penetrate the ear not only destroy kernels but also predispose the ear to secondary pests and microorganisms that may produce mycotoxins. It's also of importance to note that the corn earworm is cannibalistic, and rarely will more than one larva survive per ear. Damage of kernels due to earworm feeding can result in loss of yield. Injury such as this in sweet corn results in an unsalable product, especially in the fresh market. Sweet corn processors often accept damage to some extent; corn earworm feeding is commonly limited to the ear tips, which can be cut off before processing.
Figure 1. Corn earworm adult.
Figure 2. Corn earworm larva.
The southwestern corn borer does not occur in much of the Corn Belt. In Illinois, it has been found as far north as Effingham, with economic populations occurring as far north as State Highway 50. In most years, economic populations are limited to areas south of Interstate 64. Injury caused by second generation southwestern corn borer is caused by larval feeding on the ear, ear shoots, and leaf sheaths. Larvae will also tunnel in the stalk. In the late fall, larvae chew out the inside of the stalk in a thin ring just above ground level. They tunnel into the root, which can cause the stalk to lodge. Treatment should be considered when 25% of the plants have egg masses or larvae on the leaves. Other factors such as yield, value of corn per bushel, and the cost of insecticide also should be considered in any management decisions.
Figure 3. Southwestern corn borer larva in stalk.
Fall armyworm larvae are similar in appearance to true armyworm larvae. Color varies from light brown to green, with three yellow stripes down its back. The fall armyworm can be distinguished from the true armyworm by a distinct white or yellow inverted "Y" on its head. Fall armyworm larvae cause damage similar to that of the corn earworm. Tassel, ear, and leaves of the upper part of the plant may be partially or completely destroyed. They also feed on kernels as the ear develops. Infestations of fall armyworm are generally not economical. There is some uncertainty to how much yield loss is truly caused by fall armyworm feeding, but the current threshold recommends treatment when 75% or more of the plants are damaged and worms are still present.
Figure 4. Fall armyworm larva.
Figure 5. Fall armyworm head. Notice the inverted "Y."
Finally, even though we saw little activity from the first generation of European corn borers, the second generation is on its way. Pay attention for egg laying to occur in late-planted or late-maturing fields. Second-generation larvae will feed on leaf sheaths, collars, and midribs until they enter the stalks. Yield loss can occur from lodging and stalk breaking, ear dropping, and secondary invasion of stalk rots. An in-depth article about second-generation European corn borers will be in next week's Bulletin. Stay tuned for updates on these summer pests of corn.--Kelly Cook
Figure 6. European corn borer in stalk.