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Reports of European Corn Borers Begin

June 5, 2003

Reports of European corn borer moths are starting to filter in from areas around the state. In the "Hines Report" (, Ron Hines has been reporting catches of moths since the week of May 6 in southern Illinois. Mike Roegge, unit educator in crop systems, reported the first catches in Adams/Brown counties beginning May 27. Duane Frederking, field sales agronomist, Pioneer, reports European corn borer moths are noticeable everywhere in varying numbers. With all this moth activity, a little refresher on first-generation corn borers may be in order.

European corn borers overwinter as fifth-instar larvae. The larvae pupate and emerge as moths in the spring when temperatures reach 50°F. We can use the accumulation of degree-days (base 50°F) to predict different life stages or larval development (Table 1).

Degree-day accumulation begins at the first significant moth flight. Eggs hatch in about a week, and larvae move to the whorl of the plant. As moths emerge, they seek shelter in dense areas of vegetation (action sites) that provide shelter and moisture, and they mate. Females lay eggs on the undersides of leaves of corn or other hosts. Some fields are more attractive to egg-laying moths than others. Early-planted fields are at a higher risk than fields planted later in the season. Corn that is smaller than 15 to 18 inches tall is not as susceptible to corn borer injury; corn this size contains DIMBOA, a plant compound that prevents corn borer larvae from establishing. The concentration of DIMBOA decreases as plants mature. Larval survival is better on plants that are in the mid- to late-whorl stages.

As plants approach 15 to 18 inches in height, scouting for larvae and whorl feeding should take place. Larvae are small, cream-colored insects with black heads and have two distinct spots on the top of each abdominal segment. Young larvae can be found feeding in the whorl, chewing holes through the leaves, leaving what is called a "buckshot" effect. As larvae reach the third and fourth instars, they begin to tunnel in the midribs and leaf sheaths.

First-instar European corn borer larvae.

European corn borer in the midrib

When scouting, be sure to walk at least 100 feet into the field. In five random locations throughout the field, examine 20 consecutive plants (a total of 100 plants in the field). Check for fresh whorl-feeding and record the percentage of infested plants in the field. Remove two plants for every 20 plants examined and check for the presence of live corn borers by unrolling each whorl and examining the leaves. Note the stage of the larvae found. In larger fields, it is more practical to examine 25 plants in each of five random locations for every 80 acres. After the field has been scouted, use the management worksheet for first-generation European corn borer to make the appropriate control decision. If a rescue treatment is warranted, granular formulations are more effective than sprays when applied by air, and sprays are most effective when directed by ground equipment over the row rather than broadcast (Table 2).

Whorl-feeding does not always cause economic injury. It's important to remember that the mortality rate of both eggs and larvae is quite high. About 20% to 70% of larvae fail to establish due to natural enemies and environmental conditions such as high temperatures and moisture stress. Even larvae that make it into the whorl could be subject to drowning. Even if whorl-feeding is present, larval survival may be very poor. It may also indicate that the larvae have started to bore into the midrib and stalk. Larvae remain in the whorl only until they reach the third instar. Once the larvae begin to bore into the stalk, rescue treatments are no longer an option. Snapped or broken leaves along the midribs are usually an indication of larvae burrowing in the stalks. Larvae tunnel up and down the midrib and eventually leave an exit hole that weakens the leaf at that point. The life cycle and biology of the European corn borer are summarized thusly:

Moth longevity: 1 to 2 weeks

Preoviposition period: 2 to 3 days

Egg-laying period: 7 to 10 days

Each moth lays about 2 egg masses per night.

One egg mass contains an average of 23 eggs.

Eggs per female: about 4000

Egg hatch takes 3 to 7 days.

Larval survival varies from 10 to 20%.

Number of larval instars: 5

Scouting is still essential to determine the extent of feeding in the field and make decisions for treating for first-generation corn borer. Watch for future updates in the Bulletin. We look forward to hearing from you about what's happening in fields this summer.--Kelly Cook

Author: Kelly Cook

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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