Corn Rootworm damage. J. Schuster
5 (1 = rare 5 = annual)
4 (1 = very little damage 5 = plants killed)
The adult western corn rootworm is a yellow, 1/4-inch long beetle, with black stripes and a yellow underside. 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.
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.
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. 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.