Corn Borer

The corn borers that we find in our gardens are European Corn Borers (Ostrinia nubilalis). They are not native to North America. They were brought here, unintentionally, prior to 1917 in broom corn (Sorghum bicolor), which was imported to make brooms.

Corn borer larvae overwinter in the plant debris in your garden. They hatch into pale brown moths with zigzag markings in June. The female moths lay eggs from the end of June to the middle of July in clusters of 15 to 20 on the undersides of leaves of their favorite vegetables. The eggs hatch within a week, and the larvae feed for an additional three to four weeks. The young larvae feed on leaves, tassels, and under the husks while older larvae burrow into the stalks of your corn or the stems of your other crops. A major pest of corn, the European corn borer (Ostrinia nubilalis) will also feed on over 300 different garden plants including peppers, snap beans, potatoes, tomatoes, apples and gladiolus. Damage to corn is caused by the young larvae which chew leaves and tassels. Later they tunnel all parts of the stalks and ears, resulting in reduced plant vigor, broken stalks, poor ear development and dropped ears. Other crops are damaged primarily by the tunneling of the stalks, pods or stems by the larvae.

Corn borer larvae overwinter in the plant debris in your garden. They hatch into pale brown moths with zigzag markings in June. The female moths lay eggs from the end of June to the middle of July in clusters of 15 to 20 on the undersides of leaves of their favorite vegetables. The eggs hatch within a week, and the larvae feed for an additional three to four weeks. The young larvae feed on leaves, tassels, and under the husks while older larvae burrow into the stalks of your corn or the stems of your other crops.

Once burrowed into the stalks and stems, the larvae pupate and hatch into a second generation of moths which begin the cycle again. In warm climates, thanks to the longer growing season, it is possible to have up to three generations of moths in one growing season.

Fully grown corn borer larvae (3/4 – 1 inch long) are extremely destructive flesh-colored caterpillars with a reddish or dark brown head and several distinct spots on the top of each abdominal ring or segment. The adult borer is a night-flying yellowish-brown colored moth (1 inch wingspan) with dark wavy bands across its wings.

Life Cycle

Fully grown larvae pass the winter concealed in corn stubble or other plant parts on which they have been feeding. Pupation takes place in late spring with the adult moths appearing in May and June. When mature, the females begin laying clumps of white eggs on the undersides of the lower leaves of host plants. (Adult females may lay up to 500 eggs over their short lifetime.) Under ideal conditions, these first generation eggs hatch within 3-7 days. Tiny caterpillars begin feeding on host plants and complete their development in 3-4 weeks. Pupation occurs deep inside the corn stalks and second generation moths emerge and begin laying eggs in early summer. Produces 1-3 generations per year depending upon the climate.

How to Control

  1. Shred and plow under cornstalks in or near fields where borers overwinter. This should be done in fall or early spring before the adults emerge.
  2. Use pheromone traps to determine main flight period for moths, then release trichogramma wasps to destroy eggs.
  3. Beneficial insects, such as ladybugs and lacewing larvae, will consume a large number of borer eggs.
  4. Treat silk frequently with  Garden Dust (Bt-kurstaki) or Monterey Garden Insect Spray (spinosad) to kill young larvae. Repeat applications every 4-5 days until tassels turn brown.
  5. Use organic insecticides only as a last resort.

Note: Ladybugs will consume almost 60 borer eggs a day. Stink bugs, damsel bugs, spiders and hover fly larvae feed on young caterpillars.