What are corn earworms?
Corn earworms (Helicoverpa zea) are a moth species whose larvae feed on a variety of crops in the Midwestern United States. The larvae mainly affect sweet and hybrid dent seed corn. Corn earworms also feed on field corn, tomatoes, peppers, lettuce, and snap beans, and are often referred to as “tomato fruit worms” when feeding on these plants.
What do corn earworms look like?
Egg: Corn earworm eggs are very small (1/32 inch). The eggs look like flattened spheres with textural ridges. Freshly deposited eggs are light yellow. When the eggs are close to hatching, they darken to brown.
Larvae: The larvae reach around 2 inches when mature and vary extensively in color, ranging from green to pink to black. They have 3-4 dark stripes running down their backs and black spotting along their bodies. Larvae have a light head capsule with no markings. Corn earworm larvae are commonly confused with European corn borer or western bean cutworm larvae.
Pupae: The pupae are approximately 1 inch long, brown, and cylindrical. Corn earworm pupae are found in the soil.
Adult: Adult moths have a wingspan of 1.5 inches. Moths are grayish-brown and have dark, irregular markings on their forewings and a wide, dark band along the wing edge. Their hindwings are tan and have a dark edge with a light spot.
What are the life cycle and biology of corn earworms?
Winter in Wisconsin is difficult for corn earworms, and few survive. The few pupae that survive the winter emerge early in the season.
Instead of enduring Wisconsin winter, the moths prefer to overwinter as pupae in the warmer Gulf States. The pupae develop and adult moths emerge from the soil in the spring. They’re strong, night fliers and some migrate north after emergence. The first generation of moths arrives in late June and affects very early planted sweet corn. The second generation of moths, found between mid-August and early September, is more damaging to corn.
Female moths lay eggs on green and yellow stage corn silks. A single female can deposit up to 1000 eggs. Eggs hatch in 1-8 days. Larvae immediately enter the ears through the tip and start feeding. Corn earworm larvae are cannibalistic and will fight any other larvae they encounter, so it is common to find only one larva per ear.
After 2 weeks, the larvae drop to the ground to pupate. Two weeks later, adult moths emerge from the soil. The moths migrate to the warmer Gulf States at the end of the season where they overwinter in the soil as pupae.
DATCP monitors the late-season migration flights of corn earworm moths across Wisconsin using pheromone traps. In 2024, DATCP began surveying for corn earworm and reported the capture of moths during the week of July 5th. Southern counties (like Dane and Dodge counties) indicated heavy moth pressure between July 5-11th, but these counts quickly tapered off in the following weeks. In 2023, moth capture peaked from August 24th to September 21st. DATCP monitoring will continue through mid-September.
What are the symptoms of corn earworm damage?
Corn earworm larvae feed on the tips of corn ears. They typically damage the top 1-2 inches by consuming kernels and contaminating the ears with frass – insect excrement. Feeding damage can allow fungal pathogens to enter the ear of corn.
Severe infestations often damage less than 10% of the kernels at the ear tip, but who wants to find a “worm” in their corn? Although consumers may reject damaged sweet corn, ears can still be used for processing sweet corn if damaged parts are removed.
How do you scout for corn earworms?
The easiest way to scout for corn earworms is by using pheromone traps to catch male moths. Place a trap 4-6 feet above the ground on the south or west side of the field. Begin monitoring when corn is in the green silk stage. Check the trap daily starting in July for the presence of moths. Keep pheromone lures frozen until they are until needed. Remember to change the pheromone lures every 2 weeks to ensure they are effective.
DATCP has a pheromone trapping network across Wisconsin, and 2024 corn earworm moth flights can be found here.
Black light traps can also be used to monitor for corn earworms. These traps are more expensive, more difficult, and less effective than pheromone traps, consistently catching lower numbers of moths than pheromone traps placed in adjacent fields.
What is the threshold to reach for corn earworm treatment?
The number of moths caught is a good indicator of when a treatment may be necessary. If traps capture 5-10 male moths per night and the field is at 10% silking, an insecticide treatment may be warranted. Check silks for corn earworm eggs before completing an application. A second application may be needed if 5-10 moths continue to be caught 7-10 days after treatment.
If the economic threshold is met, reapplication of insecticides for corn earworms are needed because the silks continue to grow, and the ear zone needs to be protected with a chemical control to stop larval feeding. Corn should only be treated for corn earworms before the brown silk stage when the crop is vulnerable.
Integrated pest management (IPM) strategies for corn earworms
Cultural Control: If possible, plant sweet corn in the mid-season so it matures between generations of moths. Consider planting a sweet corn variety that is resistant to corn earworms, such as ‘Country Gentleman’, ‘Stay Gold’, ‘Victory Golden’, ‘Silver Cross Bantam’, and ‘Silvergent’.
Biological Control: Generalist predators, such as lady beetles, lacewing larvae, minute pirate bugs, and damsel bugs, are effective at controlling corn earworm eggs and small larvae in the Midwest. Trichogramma wasps also parasitize corn earworms.
While releases of biological controls are not currently impactful, it is important to conserve existing populations of these beneficial insects in your field. Consider reducing your use of broad-spectrum insecticides and only treat fields when necessary.
Chemical Control: When the economic threshold is met, tank mixes of an ovacide (egg insecticide) and a synthetic pyrethroid are recommended to control corn earworms in sweet corn and hybrid dent seed corn. Ensure sprays are providing adequate coverage to the ear zone where eggs are laid using water sensitive spray cards.
Application timing is critical. Larvae are protected from insecticides once they have entered the ear tip.
Refer to A3646, Pest Management in Wisconsin Crops, by following these links for corn earworm insecticide recommendations in seed corn (p. 61).
Corn earworm 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 corn earworm 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 corn earworm management and other agricultural pest issues, contact your local agricultural extension office or entomology expert.
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Managing Western Bean Cutworms in Wisconsin Corn Fields
Managing Japanese Beetles in Wisconsin Corn and Soybean Fields
