What Is Tunnelling Into My Corn Stalks?

Finding small round holes in corn stalks with sawdust-like frass (droppings) around them, or discovering broken tassels and shot holes in leaves, points almost certainly to the European corn borer (Ostrinia nubilalis) — one of the most economically significant corn pests globally and an increasingly common problem in home gardens. The larvae tunnel into the stalk, weakening it and disrupting the movement of water and nutrients to the cob above.

How corn borer attacks

Adult corn borer moths lay egg masses on the underside of corn leaves, usually on the widest leaf near the middle of the plant. The hatching larvae initially feed on young leaf tissue in the whorl, causing a characteristic row of round or oval shot holes when the leaves unfurl — the "shot hole" pattern is the first visible sign of corn borer. The larvae then bore into the mid-stalk or shank of the developing ear and tunnel inside, where they are protected from most sprays. Stalk entry wounds are visible as round holes with pushed-out frass. The tunnelling weakens the stalk and can cause the plant to break at that point in windy weather.

Timing of management

The most effective control window is before the larvae enter the stalk. In the whorl stage, Bacillus thuringiensis (Bt) applied as a liquid drench into the whorl kills the young caterpillars before they bore in. Spinosad-based products are also effective at this stage. Once inside the stalk, chemical treatments cannot reach them. Inspect plants from mid-June and apply Bt or spinosad at first sign of shot-hole damage in the whorl.

After harvest: break the cycle

Corn borer larvae overwinter in corn stalks and debris. Destroying all crop residue — shredding, deep-burying or removing stalks after harvest — dramatically reduces the population that will overwinter and attack next year's crop. Never compost infested stalks unless the compost reaches very high temperatures consistently. Rotating corn to a new site each year also helps.