No. 20 Article 4/August 6, 2004

Second-Generation Corn Borer Flight Beginning

After a noticeable absence in many areas this year, the European corn borer moth appears to be present again. I noticed quite a few swarming around my porch light a few evenings ago. Reports of moth-splattered windshields have also made it to my desk this week. Light traps are seeing more activity; the Champaign light trap jumped from nearly 0 corn borers each night to almost 60 at the end of July. Increasing numbers of moths indicate the need to scout fields for egg masses. For those monitoring light traps for second-generation corn borer, feel free to submit counts to the Insect Monitoring Networkcontact me, Kelly, at (217)333-4424 or

Second-generation moths are attracted to pollinating cornfields that have fresh silks. However, they will lay egg masses in any cornfield or other hosts if pollinating cornfields are unavailable. Late-planted corn still in the whorl stages also attracts these moths, but don't forget to scout any early-planted or early-maturing varieties. Potential yield loss from second-generation corn borer is generally less than that from the first generation but depends on the time of infestation. If infestation occurs during pollen shed or when kernels are initiated, the percentage of loss per borer per plant is 4% or 3%, respectively.

Second-generation corn borers feed on pollen in leaf axils or on corn leaves. As they mature, they feed on leaf sheaths, collars, and midribs until they eventually enter the stalk. Larvae may also enter the ear or earshank. Yield losses can be attributed to physiological damage; stalk breakage; and lodging, ear dropping, and secondary invasion of stalk rots.

European corn borer in stalk.

Scouting for second-generation corn borer can be frustrating and challenging. Since the second generation is associated with an extended moth flight, the flight may last a couple of weeks. It is entirely possible to find multiple stages of corn borer on a single plant. Furthermore, scouting for egg masses is not exactly fun (there's no need to sugarcoat it). Egg masses are laid on the undersides of leaves near the midribs. They are usually concentrated on leaves in the ear zone (the ear leaf and the three ears above and below it). Moths can lay eggs anywhere on the plant, but if eggs are concentrated in that area, scouting time can be reduced by focusing on the ear zone.

European corn borer egg mass.

Management decisions can best be made with the use of the second-generation corn borer management worksheet (at right). The worksheet offers some average numbers based on research data over many years and from multiple states. However, these are just suggestions; if you have experience that suggests other numbers suitable for your area, use those. For example, we suggest an average larval survival rate of 20% (approximately four larvae per egg mass). Survival rates may decrease in extremely dry areas, so you may want to use a survival rate of 10%. Heavy storms may also reduce the survival rate of the corn borer.

Timely and frequent scouting is the key to managing second-generation European corn borer. Unfortunately, it is difficult to control all of them with only one insecticide application because of the extended egg-laying period. If the application is made just after the peak moth flight, while larvae are still feeding in the leaf-collar region, results are satisfactory. Missing later borers usually results in less yield loss, because larvae cause more injury when they attack during pollen shed rather than during kernel initiation. We estimate that insecticide treatments to control second-generation corn borer provide approximately 75% control.

Insecticides recommended for control of second-generation corn borer are included in Table 1. Products preceded by an asterisk are restricted use insecticides and may be applied only by certified applicators. Please read and follow all product labels.--Kelly Cook

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