Several insect species regularly attack corn, from tiny aphids on the leaves to fat caterpillars burrowing into the ears. Whether you’re growing sweet corn in a backyard garden or managing a large field, the pests you’ll encounter fall into a few major categories: ear-feeding caterpillars, root-destroying larvae, sap-sucking insects, and grain-infesting weevils. Knowing which bug you’re dealing with tells you exactly how to respond.
Corn Earworms
The corn earworm is the single most recognizable corn pest in North America. The adult is a tan or olive-colored moth that lays eggs directly on fresh corn silks. When the tiny larvae hatch, they immediately begin feeding down the silk channel into the ear. You’ll rarely find more than one larva per ear because corn earworms are cannibalistic, so the first one to establish itself kills any competitors.
Feeding damage is usually limited to the tip of the ear, where kernels fail to develop fully. You might peel back a husk and find a plump, striped caterpillar (ranging from green to brown to nearly black) surrounded by chewed kernels and dark frass. Beyond the cosmetic damage, earworm injuries create entry points for fungal pathogens that can raise levels of aflatoxins and fumonisins, toxins that pose genuine health risks to humans and animals.
For sweet corn growers, monitoring matters. Pheromone traps that catch the adult moths help you decide whether treatment is needed. The general threshold is one to three moths per trap per night while green silks are still present. Once 90% of silks have turned brown, earworm pressure drops and spraying is no longer worthwhile.
The Mineral Oil Trick for Home Gardens
If you’re growing a small patch of sweet corn, there’s a low-tech method that has been recommended by the USDA for decades. A few drops of plain mineral oil (the kind sold as a laxative at pharmacies) applied to the silk channel will kill earworm larvae already present and block new ones from entering. One pint treats roughly 600 ears.
Timing is critical. Wait at least three days after silks first emerge from the ear tip, when the silk tips are starting to wilt and turn brown. Applying too early interferes with pollination and you’ll get a poorly filled ear. Waiting longer than seven days after silking gives larvae time to reach the kernels where the oil can’t touch them. Use an ordinary medicine dropper, fill it about half full for small ears and three-quarters full for large ones, and insert it about a quarter inch into the silk mass at the husk tip. The oil spreads through the silk channel on its own. Mark treated ears with a strip of cloth or a crayon so you don’t lose track, since not all ears silk at the same time.
Corn Rootworms
While earworms attack from above, corn rootworms do their damage underground. The western corn rootworm is one of the most economically destructive corn pests in the United States, responsible for up to $1 billion in combined yield loss and control costs in bad years. Adults are small yellowish-green beetles with black stripes that you’ll see on leaves, silks, and tassels in midsummer. They lay eggs in the soil near corn roots in late summer, and those eggs hatch the following spring.
The larvae, which are small white grubs, feed on the root system. Early damage shows up as discolored root tips and brown lesions. In heavy infestations, primary and secondary roots get completely pruned away, and the plant can no longer absorb water effectively. Aboveground, you’ll see what’s called “goosenecking,” where stalks bend at odd angles near the base as the plant tries to right itself without a stable root system. Severe lodging (plants tipping over) makes mechanical harvest nearly impossible and can reduce yields by 10% or more. Damaged roots are also more vulnerable to fungal diseases that compound the problem.
Crop rotation is the classic defense. Rootworm eggs are laid in cornfields, so rotating to soybeans or another non-corn crop breaks the cycle. Some rootworm populations have adapted to rotation patterns, which is why monitoring adult beetle numbers in late summer helps predict next year’s risk. If you’re finding an average of one or more adult beetles per plant, the next season’s larvae could cause significant damage.
Corn Borers
European corn borers are caterpillars that tunnel into corn stalks, shanks, and ear tips. Unlike earworms that enter through the silk channel, borers chew directly through the plant tissue. You’ll notice small holes in the stalk with sawdust-like frass pushed out of the entry point. Inside, the tunneling weakens the stalk structurally, increasing the risk of breakage in wind or during harvest. Borers can also tunnel into the ear itself through the shank, causing kernels to rot. The larvae are pale with small dark spots running along their bodies, and they overwinter inside corn stalks left in the field, which is why chopping or tilling crop residue after harvest reduces next year’s population.
Aphids and Leaf-Feeding Beetles
Corn leaf aphids are small blue-green insects that cluster in the whorl (the rolled-up center leaves) and around the tassel. They suck plant sap and excrete a sticky residue called honeydew, which can coat the tassel and interfere with pollen shed. In most years, natural predators keep aphid numbers in check, but heavy infestations during tasseling can reduce pollination and hurt yield.
Southern corn leaf beetles are tiny (about 3/16 of an inch), grayish-brown beetles that often appear dusty because they’re covered in soil particles. They chew narrow grooves into leaf surfaces. While individual beetles cause minor damage, large populations early in the season can stunt young plants.
Bugs in Stored Corn
Corn pests don’t stop at the field. Maize weevils are small, dark beetles with a distinctive long snout that specialize in attacking whole dried grains. They look nearly identical to rice weevils and granary weevils (telling them apart requires a microscope), but since control is the same for all three, precise identification doesn’t matter much. Adults chew into individual kernels and lay eggs inside. The larvae develop entirely within the grain, hollowing it out from the inside, so an infestation can be well established before you notice it.
Maize weevils don’t typically develop in flour or finely ground products, but they will infest shaped grain products like macaroni, cereal, and even dog treats. Outside the kitchen, decorative corn, stored birdseed, and bean bags are common breeding sites that people overlook.
Control comes down to exclusion and sanitation. Store susceptible grains in airtight, insect-proof containers. If you discover an infestation, start by checking whole-grain items like corn, rice, and wheat, then expand your search to processed products and non-food grain items. Discard anything infested, clean shelves thoroughly, and rotate stock so nothing sits long enough to become a breeding ground.
Natural Predators That Help
Not every bug on your corn is a problem. Minute pirate bugs, tiny black-and-white insects barely visible to the naked eye, are among the most effective natural enemies of corn earworms. They prey on earworm eggs and can destroy 50% or more of them before they ever hatch. Both the immature nymphs and adults also eat aphids, thrips, and spider mites, consuming 30 or more spider mites per day each.
Trichogramma wasps are another powerful ally. These parasitic wasps, so small you’d barely notice them, lay their own eggs inside the eggs of caterpillar pests like corn earworms and borers, killing them before they develop. Trichogramma species are the most widely used biological control agents in the world, commercially produced and released in agricultural programs across Europe and Asia. They’re available to home gardeners in the U.S. as well, sold as cards containing parasitized eggs that you hang among your corn plants during silking season. Avoiding broad-spectrum insecticide sprays whenever possible helps both of these beneficial species survive and do their work.