European Corn Borer

Ostrinia nubilalis Huber

European Corn Borer E-series 17-W

Appearance and Life History

Female Moth Female Moth
Photo by B. Christine

The European corn borer, an introduced species, has been an important pest of corn in the Midwest since the 1920's. Besides feeding on all types of corn, European corn borer also attacks and damages hundreds of crop and weed species (e.g., peppers, apples, soybean, cotton, foxtails, pigweeds, ragweeds, smartweeds, etc.).

Mature larva in silk tunnel Mature larva in silk tunnel
Photo by J. Obermeyer

The European corn borer passes the winter as full-grown larva in corn stalks and other plant refuse such as weed stems. The mature larva is about 1 inch (25 mm) long, creamy to grayish in color, and marked by rather inconspicuous rows of small, round, brown spots running the length of its body.

Pupa inside stalk cavity Pupa inside stalk cavity
Photo by J. Obermeyer

Overwintering larvae pupate in the spring, emerging as moths in late May and early June. Female moths are pale yellow-brown with irregular darker bands running in wavy lines across their wings; male moths are distinctly darker and usually smaller. Mating takes place in early June (first generation) and in late July and early August (second generation) in dense grassy areas around corn fields. Female moths generally lay their eggs on the underside of corn leaves (often along the leaf midrib), leaf sheaths, and/or ears, depending on the generation, in masses of 15 to 30 eggs overlapping like scales of a fish. Tall, lush, early planted corn is the preferred oviposition site for the first generation moths; whereas second generation moths target actively pollinating corn, which is usually planted late. After 5 to 6 days, the eggs develop what appears to be black spots, which are actually the head capsules of young borer larvae. Once the black head is visible, hatching is imminent.

Corn Rootworm Life Cycle

Damage

Early shot hole whorl damage Early shot hole whorl damage
Photo by J. Obermeyer

First generation borers are usually present during June in the whorl of corn plants. As the larvae feed and grow, some may be found tunneled into the midrib of leaves. This damage can cause leaves to break at the point of borer entry. As the borers feed on the leaves, they typically produce a characteristic random or "shot hole" damage pattern. These holes become apparent as the leaves grow out of the whorl. By the time the first generation borers are half grown, they will have moved down the stalk and bored into it, leaving behind their sawdust-like excrement called frass at the stalk entry hole. The boring damage may weaken the plant enough to cause subsequent stalk breakage later in the season, typically occurring below the ear. Or it may cause corn to become stunted, resulting in yield reductions caused by the inability of the plant to transport water and nutrients through its damaged stalk. After boring into the stalk, the larvae usually feed inside the plant until they reach maturity and pupate. Stalk entry and tunneling by corn borer may predispose the plant to pathogens which may lead to stalk rots later in the season. They emerge as adults during July and early August beginning the second generation.

Close-up of midrib feeding Close-up of midrib feeding
Photo by J. Obermeyer

During late July and early August adults mate in grassy areas (e.g., roadsides, waterways, weed patches, etc.). Egg masses, which will produce second generation borers, are laid in corn during late July and throughout August. Newly hatched larvae typically move from the leaves to protected areas of the leaf axils and sheaths to feed on pollen and plant tissue. Second generation borers normally concentrate their attack in the ear zone, roughly the middle third of the plant. After undergoing several molts, the larvae bore into the corn plant as did the first generation borers.

Stalk entry hole and frass Stalk entry hole and frass
Photo by B. Christine

Second generation borer attack may result in stalk or tassel breakage and/or boring into the ear shanks, which may cause ears to drop off. Larvae may bore into the ears where they feed on the kernels and cob, resulting in yield losses, as well as avenues for attack from secondary insects and pathogens (e.g., ear rots). No matter where they may attack the plant, second generation borer damage can result in grain losses, harvesting problems, and poor grain quality.

Sampling Method

First Generation - Early planted corn is most likely to develop problems with first generation borers because moths are attracted to the tallest, greenest corn for egg laying. During June and early July when the corn is over 18 inches (45 cm) in extended leaf height, look for the characteristic "shot hole" leaf feeding damage in the whorl of the corn. Corn shorter than this normally has high levels of a plant aglucone, DIMBOA, which acts as a antifeedant and prevents borer establishment. If you do find signs of corn borer activity, sample to ascertain the extent of infestation.

To sample, inspect 20 consecutive plants in each of 5 areas of the field. Randomly select the first plant of each sample set. Carefully examine the whorl leaves on each plant. Count and record the number of plants showing foliar feeding damage. Total the number of plants showing such damage to determine the percentage of damaged plants. Also, determine if borers are still present and actively feeding. Pull out, carefully unroll, and examine the whorl leaves from one plant showing damage in each sample set, for a maximum of 5 plants in the field. Total the number of live borers found and determine the average number of borers per plant.

Second Generation - Because of the difficulty in readily detecting second generation borers and their damage, concentrate sampling efforts on fields that are late planted and/or actively pollinating during the period of peak egg laying. Check with your local extension personnel, as to when it is necessary to start sampling. Windshield "splatter" of corn borer moths while driving county roads after dusk will alert one to the flight, mating, and egg laying of corn borer moths in an area.

Inspect 20 consecutive plants in each of 5 areas of the field, picking the first plant of each set randomly. Carefully examine each plant for egg masses and note if live larvae are present. Since most of the eggs are laid in the middle third of the plant, check the ear leaf and the leaves at 2 nodes above and 2 nodes below the primary ear. Newly hatched larvae, which are often in the leaf axils, are difficult to see because of the accumulation of pollen and pollen anthers.

Record the number of egg masses and/or larvae found on the plants sampled. Determine the average number of egg masses per plant for the field and/or the average number of larvae per plant. Also determine the percentage of plants infested.

Management Guidelines

Corn Insect Control Recommendations: E-series 219-W (PDF)

First Generation - Use the following steps to determine whether treatment is economically justified:

  1. Preventable yield loss (bu/A) = anticipated yield (bu/A) X yield loss figure (from following table) X level of infestation (decimal) X anticipated level of control (decimal). It is probably impractical to expect 100% control. A good estimate of control might be 75%.
  2. Preventable dollar loss/A = Preventable yield loss (bu/A) X market value ($/bu).
  3. Compare preventable dollar loss/A to cost of insecticide and application to determine if treatment is warranted.

    Yield Losses Caused by European Corn Borers for Various Corn Growth Stages

Example: A field in the early whorl stage has 80% of the plants with "shot hole" feeding and an average of 2 live larvae per whorl. Anticipated yield is 150 bu/A and the crop is valued at $2.00 per bushel. The cost of the insecticide and application is $10.00 and 75% control can be expected. Would it pay to apply the insecticide?

  1. Preventable yield loss (bu/A) = 150 bu/A X .082 (8.2% loss for 2 borers/plant) X .80 (80% infestation) X .75 (75% control) = 7.38 bu/A
  2. Preventable dollar loss/A = 7.38 bu/A X $2.00/bu = $14.76/A
  3. Compare preventable dollar loss/A with cost of control/A

$14.76/A (preventable $ loss/A) - $10.00/A (cost of control) = $4.76/A return from application of control.

Second Generation - Use the following steps to determine whether treatment is economically justified:

  1. Preventable yield loss (bu/A) = anticipated yield (bu/A) X level of infestation* (decimal) X yield loss figure** (from previous table) X anticipated level of control (decimal). It is probably impractical to expect 100% control. A good estimate of control might be 65%.
    *While sampling for the second generation, consider a plant with either live larvae or egg masses as infested.
    **For each egg mass, assume a survival rate of 20% (4 borers/egg mass).
  2. Preventable dollar loss/A = Preventable yield loss (bu/A) X market value ($/bu).
  3. Compare preventable dollar loss/A to cost of insecticide and application to determine if treatment is warranted.

Example: A field that is shedding pollen has 60% of the plants infested with larvae and/or egg masses. The number of actual larvae observed averages 2 per plant. The number of egg masses averages 1/4 per plant. For 1/4 egg mass/plant, an average of 1 borer would survive per plant (assuming survival of 4 borers/egg mass). Therefore, two live borers plus one borer from egg masses equals 3 borers/plant. Anticipated yield is 150 bu/A and the crop is valued at $2.75 per bushel. The cost of the insecticide and application is $12.00 and 65% control can be expected. Would it pay to apply the insecticide?

  1. Preventable yield loss (bu/A) = 150 bu/A X .60 (60% infestation) X .081 (8.1% loss for 3 borers/plant) X .65 (65% control) = 4.74 bu/A
  2. Preventable dollar loss/A = 4.74 bu/A X $2.75/bu = $13.04/A
  3. Compare preventable dollar loss/A with cost of control/A

Transgenic or Bt Corn - Field corn producers may choose to manage corn borers through the use of hybrids with a corn borer toxin that is genetically built into the plant. These genetically modified corn hybrids contain a gene derived from a naturally occurring bacterium, Bacillus thuringiensis, which produces a protein that is toxic to corn borers. This eliminates the need for the application of a corn borer insecticide. This technology allows producers who regularly experience problems with corn borers to use this as a tool to effectively manage this insect. It reduces the need for laborious scouting for this pest, although producers should spot check areas within Bt corn fields to determine the effectiveness of this management strategy. Obviously, scouting should not be eliminated as a result of Bt corn plantings. Other above and below ground insects may be present during the season and Bt corn provides little to no protection against these.

Extensive use of this technology could result in resistance to the toxin developing in the corn borer population. To reduce the probability of this happening, corn without the Bt-like gene should also be grown in Bt corn areas. This non-Bt corn will act as a refuge for some of the corn borers, thus preserving the genetic diversity that is now in the corn borer population. If these refuge are not included in plantings of Bt corn, the technology may be short lived or seed companies will constantly have to introduce new genes, if available, or stack genes to combat this situation. With this in mind, producers should develop resistance management strategies that reduce this risk. Contact your state Cooperative Extension Service for resistance management plans. In general, it is suggested that at least 20% of the acreage in an area be grown in non-Bt corn.

If control is necessary, contact your state Cooperative Extension Service or click here for control materials and rates.

IPM Tip for European Corn Borer