Summer Corn Insect Pests: The Silent Yield Killers
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August. The tasseled cornfields stand tall, ears filling out well. Everything appears to be going smoothly, but at that moment, some unwelcome guests can make their presence known. Yep — we’re talking about corn insect pests. August can be a revealing month, whether it’s a few chewed leaves, some plant lodging or suspicious silk damage. That’s when signs of pest activity often become more visible and farmers start getting a clearer picture of who’s been using their crop as a salad bar. From black cutworm to fall armyworm and more, these tiny troublemakers don’t take summer vacations.
Farmers face a mounting challenge every year, keeping these pests in line in ways that keep yields up and production costs under control. Maybe your customers are already experiencing some of these issues, or maybe you just want to help them stay ahead of the game. Either way, it’s time to get informed. Let’s look at some of corn’s most vigorous insect villains and the management techniques that can mitigate pest issues now and preserve crop protection tools for the future.
BLACK CUTWORM
Identification1
EGG: The white, spherical, ridged eggs are usually laid in dense, low-lying vegetation in clusters of 10–30, but may also be laid singly.
LARVA: Black cutworm larvae caterpillars are light gray to black with a dark brown head and measure two inches long at maturity. Pairs of dark bumps called tubercles run alongside the body. These wart-like bumps are specific to black cutworms and set them apart from other cutworm species that seldom cause significant injury to corn.
ADULT: Adults measure around 1 1/2 inches long, with dark brown to black wings containing a light-colored bar at each wing edge. Close to where the lightcolored bar begins, a small black dart stretches from a kidney-shaped spot. This defining feature is most apparent in young moths. It may become indistinct as the moths age or become damaged, making them more challenging to identify.
Damage
- Young larvae feed on corn leaves, creating holes but not causing significant damage.
- Large black cutworm larvae cause the damage for which they are named — typically cutting plants at or below the soil surface. One larva can destroy up to 4–5 corn plants in a lifetime.
- Large larvae bore into the stems of plants too big to cut. This boring can injure the growing point or disrupt water and nutrient f low, leading to the plant’s death.
Risk factors for infestation1
- Locations that are low-lying, prone to water accumulation, or excessively weedy can attract these pests.
- Fields managed with reduced tillage or planted with cover crops adjacent to perennial vegetation may harbor infestations.
- Late-planted fields are at risk because their vulnerable growth stages may align with plant-cutting larval growth stages.
EUROPEAN CORN BORER
Identification2
EGG: The eggs of the European corn borer are laid in small, somewhat flattened masses that overlap like fish scales. White at first, they become dotted with black when the larvae’s heads become visible.
LARVA: Newly emerged European corn borer larvae appear dirty white with black heads that turn brown later. Mature larvae reach about one inch in length. They eventually develop a pinkish-gray or pale brown color with a dark gray back stripe and four small spots atop each segment.
ADULT: Small in size, European corn borer adult moths measure only about ½ to 5/8 inches long. They appear triangular when resting. Males are darker brown, females a light tan. Both sexes have zig-zag lines across the forewings. On males, you can usually see the abdomen behind the wings.
Damage3
- Damage from boring can cause the plant to weaken and the stalk to break off below the ear at a later point.
- Yield losses may result from boring that leads to stunted corn or larvae that feed on kernels and cob.
- Corn borers tunnel into stalks and ears, making the plant susceptible to secondary insects and pathogens, which can cause stalk and ear rot later.
- Boring into ear shanks can cause ears to detach from the plant.
Risk factors for infestation2
- The largest risk for European corn borer damage comes from planting non-Bt corn hybrids.
- Early-planted, tall corn attracts first-generation adults for laying eggs.
- Sweet corn, late-planted or late-maturing, short corn attracts second-generation adults.
FALL ARMYWORM
Identification4,5
EGG: Very small (only about 1/32 inch wide), pale yellow eggs are laid in clusters of a few to several hundred. A fluffy covering makes it hard to see the individual eggs. The eggs darken over time, looking nearly black right before they hatch.
LARVA: Freshly hatched larvae are green and later vary from light tan or green to almost black. Their backs contain three thin yellowish-white lines, with a wider dark stripe on their sides next to the yellow lines. Between the eyes lies a prominent white, inverted Y-shaped mark. Unique to the fall armyworm, this mark sets it apart from corn earworm and other armyworm species.
ADULT: Moths are dark gray, patched with light and dark. Near the very end of each wing is a notable white spot.
Damage
- Feeding on leaves in whorls, larvae are capable of causing serious damage.
- At the silking stage, larvae may feed on and destroy tassels and leaves.
- Developing kernels in ears may be damaged by feeding larvae.
Risk factors for infestation
- Poor weed control practices place a field at greater risk.
- Un-silked, late-planted corn is preferred by fall armyworm.
- Warm climates and those with semi-tropical or tropical conditions attract fall armyworm.
SOUTHWESTERN CORN BORER
Identification
EGG: Flat and oval, eggs are pale yellow-green when first laid. They develop three red-orange, crosswise bars within 36 hours of laying.
LARVA: The creamy white summer phase larvae have brown or black tubercles on each segment of their bodies. Winter phase larvae have no spots but do have a reddish-brown or black head.
ADULT: Dull white in color with a wingspan of ¾ to 1 ½ inches.
Damage
- Young larvae feed on whorl leaves, making pinholes. Large larvae create bigger, elongated holes.
- Older larvae bore into stalks, which restricts the plant’s intake of water and nutrients, reducing both ear size and yield.
- Stalk lodging results from larvae girdling stalks as they prepare to overwinter.
Risk fractors for infestation
- Late-planted corn is at higher risk of southwestern corn borer infestation.
- Lack of minimum tillage can increase risk.
- Stalks left in the field during fall and winter can increase the likelihood of overwintering populations.
Wrangling these villains
These four pests attack in a variety of ways and can reduce yields significantly if not brought under control. Fortunately, there are several methods that can help mitigate these threats. Cultural methods such as tillage can help fight European corn borer and southwestern corn borer.2,6 For black cutworm control, manage early-season weeds and make sure cover crops are terminated in a timely manner.7 Black cutworm may respond to chemical treatments, but the other three either do not respond well, or timing is so critical that it makes successful application a challenge.2,4,6,7
One strategy that is effective against all of these pests is the use of Bt-traited corn. However, insects can develop resistance to Bt proteins. To combat resistance, some corn hybrids have a pyramid of traits — that is, a combination of two or more Bt proteins — to target a single pest. Pyramids are more robust than single-protein traits, thereby reducing the risk of resistance and ensuring that the pest-fighting effect of the traits lasts longer.
One example of this technology is PowerCore® Enlist® corn. It includes three proteins, Cry1A.105, Cry2Ab and Cry1F. Triple modes of action help strengthen trait resilience for improved long-term pest control. Clint Pilcher, Technical Knowledge Solutions Team Lead at Corteva Agriscience, explains further. “By diversifying your insect trait portfolio, you’re knocking off resistant insects and giving yourself more time against pests,” he says. “PowerCore traits really represent a next generation of Bt advancement.” And for farmers facing multiple pest pressures, the employment of combined Bt traits can assist in maintaining their insecticide options for a greater length of time. In addition, since multiple-trait Bt products are more robust, there’s typically a smaller refuge requirement than those for single-trait Bt hybrids. Where the original, single-trait requirements for refuge were 20%–50%, multi-trait Bt corn typically requires a refuge of only 5%–10%.
In the face of evolving pest threats, your customers have choices — in management practices, crop protection products and hybrids with multiple modes of action. With diverse modes of action, trait pyramids not only improve immediate pest control but can also help preserve the effectiveness of other crop protection tools over time. For farmers looking to stay ahead of the curve, these advanced traits can be a key part of a resilient pest management strategy.
1 “Black Cutworm,” Iowa State University, May 1, 2022, https://crops.extension.iastate.edu/encyclopedia/black-cutworm.
2 Ashley Dean and Erin Hodgson, “European Corn Borer,” Iowa State University, July 1, 2024, https://crops.extension.iastate.edu/encyclopedia/european-corn-borer.
3 “European Corn Borers (Corn),” Purdue University College of Agriculture, 2023, https://ag.purdue.edu/department/entm/extension/field-crops-ipm/corn/european-corn-borers.html.
4 “Fall Armyworms (Corn),” Purdue University College of Agriculture, 2023, https://ag.purdue.edu/department/entm/extension/field-crops-ipm/corn/fall-armyworms.html.
5 Kelley Tilmon and Andrew Michel, “Fall Armyworm in Ohio Field Crops,” Ohioline Ohio State University Extension, August, 25, 2023, https://ohioline.osu.edu/factsheet/ent-0093#.
6 G. Michael Chippendale and Clyde Sorenson, “Biology and Management of Southwestern Corn Borer,” in Radcliff’s IPM World Textbook University of Minnesota, accessed June 6, 2025, https://ipmworld.umn.edu/chippendale.
7 Laura Flandermeyer and Emily Blick, “Managing Black Cutworm in Wisconsin Corn Fields,” University of Wisconsin-Madison Crops and Soils Division of Extension, June 5, 2025, https://cropsandsoils.extension.wisc.edu/articles/managing-black-cutworm-in-wisconsin-corn-fields/.
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