Corn rootworm feeds on the silks, roots of sweet corn.

Adult western corn rootworm is a yellow, 1/4-inch-long beetle, with black stripes and a yellow underside. Its wings do not cover the posterior part of the abdomen. The adult northern corn rootworm is a greenish, 1/4-inch-long beetle. The larva of each species is slender, white, and up to 1/2 inch long.

Damage Caused by Corn Rootworm

Adults feed on the green silks of sweet corn, preventing pollination and ear fill. They also damage cucumber and squash blossoms. Usually none of this adult damage is significant in commercial crops. The larvae feed on the roots of corn, reducing yield and causing stalks to fall over. This lodging can reduce yield if it occurs before pollination is complete. Later larvae (mid-June through mid-July) tend to cause the most damage.

Life Cycle of the Corn Rootworm

Eggs overwinter in the soil and hatch in early summer. The larvae feed only on the roots of corn; if eggs hatch where no corn roots are present, they die. Larvae pass through three instars before pupating in the soil and emerging as adults. Females of both species lay eggs in late summer and fall in corn. Western and northern corn rootworms develop through only one generation per year.

Management of Corn Rootworms

Gardeners: Rotate where you grow sweet corn. Apply pesticides to sweet corn when silks are green every 2-3 days while beetles are present and clipping silks. Stop treating when silks turn brown.

Commercial: In much of the state, crop rotation (with corn following any crop except corn) prevents root damage from corn rootworms.

• In areas where the adults lay eggs in the soil of other crops, damage is likely even in rotated fields.
• Counts from yellow sticky traps during the latter part of the previous summer can be used to determine the need to treat.
• The reduction in adult population is important for reducing the number of eggs laid which will hatch into larvae and start feeding on corn roots the following year.
• Apply a soil insecticide if corn is planted in the same field as last year and no earworm or corn borer control programs were followed.
• Where foliar insecticides for earworms or corn borers are used fairly often in sweet corn, they also control rootworm beetles and prevent egg laying.

Contact your county Extension office for current pesticide controls.