What Are the Worms Inside My Corn Cobs?

Peeling back the husk of a ripe sweetcorn cob and finding a large caterpillar feeding on the kernels at the tip is a classic and very common discovery. The culprit is almost always the corn earworm (Helicoverpa zea) — also known as the cotton bollworm or tomato fruitworm — one of the most widespread and damaging corn pests in warm climates. Understanding its life cycle is the key to managing it.

The corn earworm life cycle

Adult corn earworm moths lay single eggs on the corn silks shortly after they emerge. The eggs hatch in a few days and the tiny larva crawls down the silk channel into the cob tip, where it feeds on the developing kernels. By the time you discover it at harvest, the larva may be two to three centimetres long, fat and well-fed. The damage is typically confined to the top few centimetres of the cob — the rest is perfectly edible. The larva does not spread through the cob in the way that some other pests do; it feeds at the tip and drops out when fully grown to pupate in the soil.

How to manage corn earworm

The most effective home garden technique is applying a small amount of mineral oil or vegetable oil directly to the silk channel with a dropper or pipette, about five to seven days after silks first appear. This smothers the eggs and young larvae before they enter the cob. Repeat applications every four to five days through the silk period. Use just a few drops per cob — enough to wet the silks without saturating them, which could impair pollination if done too early. Apply only once the silks have turned slightly brown at the tips, indicating pollination is largely complete.

Biological control

Trichogramma wasps — tiny parasitic wasps available commercially — parasitise corn earworm eggs and provide useful population reduction when released at the correct timing. Bacillus thuringiensis (Bt) applied as a liquid to the silk channel is also effective against young larvae before they enter the cob. Both methods are compatible with organic growing.

At harvest

A single worm in the tip of the cob does not make the rest of the cob inedible. Simply break or cut off the tip — typically the top two to four centimetres — and the rest of the cob is perfectly fine. A worm-free cob is the goal but not finding one is not a disaster.