Corn earworm is the caterpillar that ruins sweetcorn from the inside. The moth lays its eggs on the fresh silks, the tiny caterpillar crawls down the silk channel and into the tip of the cob, and there it feeds, safe from sprays, birds and most predators, until you husk the cob and find the tip eaten away and full of droppings. It also attacks tomatoes, where it is known as the tomato fruitworm, boring into the fruit and leaving it to rot. It is a frustrating pest, because by the time you see the damage, the caterpillar is already inside the fruit and out of reach.
This guide explains how to identify the corn earworm, what damage it causes, and organic-first control in order — with an emphasis on timing, since the control that works is the one applied before the caterpillar gets inside. If the damage on your corn does not match, the pest identifier helper can help you compare, and our caterpillars on plants guide covers caterpillar control in general.
How to identify corn earworm
The adult is a moth, about 35–40 mm across the wings, pale brown or olive with a dark spot on each forewing. It flies at night and is rarely seen in the day. The eggs are tiny, white or pale yellow, laid singly on the fresh silks of corn, or on the leaves and fruit of tomatoes. The caterpillar is the damaging stage: it varies in colour from pale green to brown to pink, with pale stripes along the body and dark spots, and it grows to about 40 mm long. The colour varies so much that it is often confused with other caterpillars — the reliable identification is the host and the damage.
The damage is the giveaway. On sweetcorn, the tip of the cob is eaten away, with dark droppings in the silk channel and a caterpillar, or several, inside the tip. On tomatoes, the caterpillar bores a neat hole into the fruit, usually near the stem end, and feeds inside, leaving the fruit to rot. On beans and other crops, it feeds on the pods and the young growth.
Lifecycle in general terms
In warmer regions the corn earworm can produce several generations a year, and it migrates north in summer in many areas, arriving with warm weather. The moth lays eggs on the fresh silks of corn, or on tomato leaves and fruit. The eggs hatch within a few days, and the tiny caterpillars crawl down the silk channel into the cob, or bore into the tomato fruit, where they feed for two to three weeks. They then drop to the soil to pupate, and the adults emerge a few weeks later. The final generation pupates in the soil and overwinters there.
Because the caterpillar feeds inside the cob or the fruit, it is protected from almost everything once it is in. The control window is short: the few days between the eggs hatching and the caterpillar entering the cob. That is why timing, and the silk treatments described below, matter so much.
Damage signs
On sweetcorn, the first sign is often a caterpillar or droppings in the silk channel, and at harvest the tip of the cob is eaten away. The damage is usually confined to the tip — the rest of the cob is often still edible once the damaged tip is cut off — but in a heavy attack several caterpillars may be in one cob, and the feeding lets in fungi that rot the kernels. On tomatoes, the caterpillar bores a hole into the fruit, the fruit rots from the inside, and it drops early. On beans, the pods are eaten and the seeds inside are damaged.
The damage is often worst in late summer, when the moth population is highest and the corn is silking. Early corn often escapes with little damage, while late corn can be heavily attacked. If you grow sweet corn or tomatoes, check the silks and the fruit weekly from the time they appear.
Organic-first control, in order
1. Inspect first. Check the fresh silks daily once they appear, looking for tiny white eggs and small caterpillars. On tomatoes, check the fruit for entry holes and the leaves for eggs. Note which plants are affected and how bad it is — a few caterpillars on a large planting need little action, while a heavy attack on young fruit does.
2. Hand-pick and remove. On sweetcorn, the most effective step is to treat the silks: once the silks have wilted and turned brown, a few drops of mineral oil or vegetable oil applied to the silk channel can smother the eggs and young caterpillars before they enter the cob. This is the traditional home-garden method, and it works best when applied to every ear, a few days after the silks appear. Pick off any caterpillars you find on tomatoes by hand and drop them in a bucket of soapy water. Remove and destroy damaged fruit rather than leaving it to rot on the plant.
3. Barriers. Fine insect mesh or row cover over the corn, put in place before the silks appear, keeps the moths from laying eggs — but corn is wind-pollinated, so the cover must allow pollen to move, or the corn must be hand-pollinated. This is practical for a small planting but difficult for a large one. For tomatoes, row cover over young plants helps, but it must be removed once the plants are flowering.
4. Traps. Pheromone traps catch male moths and tell you when they are flying, which helps you time your inspections and your silk treatments. They will not control a caterpillar population on their own, but they are a useful early warning, especially for sweetcorn.
5. Beneficial insects and birds. Parasitic wasps attack the eggs and the young caterpillars, and birds eat the caterpillars they can reach. Encourage wasps by planting small-flowered herbs and flowers nearby, and avoid broad-spectrum sprays, which kill the wasps. Do not rely on predators alone for this pest — the caterpillar is inside the cob or the fruit, out of reach, so timing and silk treatment matter more.
6. Least-toxic options last. Bt — Bacillus thuringiensis — sprayed onto the silks and the tomato foliage kills young caterpillars when they eat it, and it is harmless to people, pets, birds and most other insects. It works best on young caterpillars, so apply it early and thoroughly, and reapply after rain. Spray in the evening, and check your local extension service for the right product and timing. Avoid broad-spectrum insecticides: they kill the parasitic wasps and other beneficial insects, and they cannot reach the caterpillar inside the cob anyway.
Prevention
Prevention is about timing and hygiene. Plant sweetcorn early, where possible, so it silks before the moth population peaks in late summer. Plant a succession of small plantings rather than one large one, so not all the corn is silking at the same time. Choose varieties with tight, long husks, which make it harder for the caterpillar to reach the cob — some varieties are bred for this, and your local extension service or seed supplier can advise. Clear away crop debris at the end of the season, and dig the soil over, since the pupae overwinter in the soil. Rotate corn and tomatoes to fresh ground each year. Check the planting calendar for zone 5 in April for sowing times that may let you grow an early crop.
Frequently asked questions
Can I eat sweetcorn with earworm damage? Yes, once the damaged tip is cut off. The rest of the cob is usually fine. Cut away the eaten tip and any rotting kernels, and use the corn promptly rather than storing it.
Will the earworm attack my tomatoes too? Yes — on tomatoes it is known as the tomato fruitworm, and it bores into the fruit. The same moth lays eggs on both crops, so an attack on the corn can be followed by an attack on the tomatoes, and vice versa.
What is the oil treatment for corn silks? Once the silks have wilted and turned brown, a few drops of mineral oil or vegetable oil applied to the silk channel can smother the eggs and young caterpillars. Apply to every ear, a few days after the silks appear. It is a traditional method and it works best as part of a plan that includes early planting and inspection.
Does Bt work on corn earworm? It works on young caterpillars, if it is applied to the silks and the foliage they are eating, before they get inside the cob or the fruit. Timing matters — apply early, thoroughly and in the evening, and reapply after rain. Check your local extension service for advice.
Why is late corn always worse? The moth population builds up through the summer, so corn that silks in late summer faces far more moths than corn that silks in early summer. Planting early, or planting a succession, spreads the risk.
Corn earworm is a pest where timing beats strength. Plant early, check the silks daily, treat them once they wilt, and pick the caterpillars off the tomatoes — and the damage will shrink to a tip you can cut off. For caterpillar control in general, see our caterpillars on plants guide, and for general help use the pest identifier helper.