What are European corn borers?
European corn borers (Ostrinia nubilalis) are destructive corn pests in Wisconsin. The pest was introduced to the United States in 1917 and is found in most major corn-growing areas across the country.
Larvae feed on leaves, destroy developing tassels, bore into stalks, and attack ear tips. European corn borer damage reduces grain quality, increases grain loss, and leads to harvesting problems. The widespread devastation in corn crops led to this pest being the first target of Bt corn in 1996.
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What do European corn borers look like?
Egg: European corn borer eggs are laid in small, overlapping masses. Freshly laid eggs are creamy white and approximately ¼ inch long. When eggs are close to hatching, the blackish color of the larval heads are visible inside each of the eggs.
Larvae: Larvae are dirty white with distinct black heads. Larvae mature through 5 instars, reaching 1 inch in length. Older larvae may be grayish and have small, brown spots along their body.
Pupae: The pupae are dark reddish brown with a pointed end and rounded head.
Adult: The moths are small, between ½ to 5/8 inches long. At rest, the wings form a triangular shape. Females are light tan, and males are darker brown. Both sexes have irregular wavy lines across the forewing.
What are the life cycle and biology of European corn borers?
European corn borer has 2 generations per year in most of Wisconsin (1 generation in Northern WI). The pests overwinter as 5th instar larvae in cornstalks and large-stemmed weeds surrounding fields. When temperatures reach 50°F, moths emerge and fly to aggregation sites to mate. These moths prefer dense, grassy areas that provide necessary moisture and numerous mating locations. At night, females fly to corn fields and lay egg masses on the undersides of corn leaves. In the very southern part of Wisconsin, the first generation of moths lay eggs in mid-June.
In a few days, the eggs hatch and the larvae crawl into the whorl of the corn. They feed on leaves, bore into forming tassels, and eventually enter the stalk as they develop through 5 instars. The larvae pupate and emerge as adult moths. The second generation of moths lay eggs over a long period. These eggs mature to 5th instar larvae and overwinter.
Early planted corn is most at risk of damage by the first generation of European corn borer. Late planted corn is an attractive host for the second generation even though it usually is not impacted by the first generation.
What are the symptoms of European corn borer damage?
Early feeding by European corn borer larvae presents irregular pinhole damage in leaves that grow out of the whorl. “Pinholes” found inside the whorl indicate recent feeding and the potential for live larvae. Sawdust-like frass may be visible where larvae bore into the midribs and stalks. Damage by the first generation may weaken the stalks and lead to breakage or pathogens later in the season.
Second generation European corn borer larvae cause damage to the ear zone, boring into the stalk, tassel, or ear shanks. Ears may be weakened and fall from plants. Larvae may also directly damage kernels and cobs.
How do you scout for European corn borers?
Black light traps provide useful information about the population intensity and activity in your area. WI DATCP’s Pest Survey Program has a network of black light traps across the state to monitor moth flights. Weekly reports of European corn borer counts may alert growers to emergence and abundance of the pest in their area, but use of black light traps should not be a replacement for field scouting.
It is important to scout for the first and second generations of European corn borer in your fields.
Scouting for the first generation should begin when corn reaches 18 inches of extended leaf height. Look at 10 consecutive plants in 10 areas of the field. Check for leaf feeding and the characteristic “pinholes”. Pull the whorl leaves from 2 infested plants, unroll the leaves, and check for live European corn borer larvae. Count the percentage of plants with recent feeding and the average number of live European corn borer larvae per plant.
Scouting for the second generation of European corn borer by looking for white egg masses on leaves. Scout fields weekly, searching carefully on the underside of leaves below and above the ear and near the midrib. Check at least 10 plants in 5 areas of the field.
If infestations go unnoticed, ears may drop, and stalks may break in the fall.
What is the threshold to reach for European corn borer treatment?
In fresh market sweet corn, treatment may be warranted through the early silk stage if 5% of plants have egg masses, larvae, or leaf damage. Treatment should be repeated every 3-5 days if at least one unhatched egg mass remains per 20 plants.
In processing sweet corn, treatment may be warranted during the whorl stage if 25% of the plants are infested with egg masses or live larvae or show signs of feeding. Treatment during the late tassel to silk stage may be considered if eggs or live larvae are found on 4-5% of plants. Repeat treatments every 3-5 days if conditions continue.
In late-planted processing corn, treatment may be warranted if second generation eggs or larvae are found on 4-5% of plants with emerging tassels.
Insecticide treatments for European corn borer are not always warranted. Management worksheets have been developed to help growers assess the economic benefit of treating first- and/or second-generation European corn borers.
To utilize these worksheets, it is essential to scout your fields. The first-generation management worksheet requires the number of plants infested and the number of borers per plant.
The second-generation worksheet requires the number of egg masses found per plant.
Refer to Pest Management in Wisconsin Field Crops (A3646) for the European corn borer management worksheets (p. 52).
Integrated pest management (IPM) strategies for European corn borers
Cultural Control: Cutting corn for silage or shredding it for fodder can kill overwintering European corn borer larvae if the stubble is 2 inches or less. Additionally, dry-stalk shredding may kill 80% of European corn borers.
European corn borer larvae populations may be reduced by plowing under crop stubble or shredding stalks. However, certain plowing practices increase potential soil erosion and may be unacceptable.
Biological Control: Natural enemies, such as Trichogramma wasps, may prey on European corn borer eggs and larvae. Purchasing and releasing natural enemies may not be economically feasible.
Planting transgenic Bt corn is an effective control method for European corn borer. The Bt corn expresses the soil bacterium Bacillus thuringiensis (Bt) toxin that is active against the pest.
Review “The Handy Bt Trait Table” in A3646 (p.57-58) for a list of trait packages that are marketed to control European corn borer. If you plant transgenic Bt corn, be sure to plant the required refuge and follow recommendations to delay resistance.
Chemical Control: Insecticides may be warranted to control European corn borer. Refer to Pest Management in Wisconsin Field Crops (A3646) for European corn borer insecticide recommendations (p. 68-69).
Application timing is critical. The best time to control for first generation European corn borers is between 800-1,000 accumulated, modified growing degree-days (above 50°F average). This often falls between July 1st-4th in southern Wisconsin. The best time to treat the second generation of European corn borers is right before the eggs hatch. If possible, target treatments when the black heads have developed in the eggs.
European corn borer response and management options
Monitor weather conditions and crop growth stage to determine optimal timing for insecticide applications, as needed.
Always apply insecticides according to label instructions and consider factors such as application method, rate, and pre-harvest intervals.
Consider alternative management practices in severely infested fields.
Environmental and ecological considerations for European corn borer treatment
Minimize pesticide use whenever possible to reduce potential impacts on non-target organisms, biological control organisms, and environmental health.
Implement IPM practices that promote biological diversity and ecosystem resilience.
References
Additional resources
For assistance with European corn borer management and other agricultural pest issues, contact your local agricultural extension office or entomology expert.
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