Throughout the U.S. corn belt, the No. 1 group of insect pests is the corn rootworm beetles. 

There are actually three species of corn rootworms in Kentucky, the northern, western and southern corn rootworms. While they each damage corn in a similar way, by chewing on the roots of the developing plant, they have some distinct differences in their biology and management.

The adult of each of the three species is a small green beetle about ¼-inch in size. The western corn rootworm is the most common species attacking corn in Kentucky and can be recognized by the three distinct black stripes on its pale green body. 

The southern corn rootworm (also known as the spotted cucumber beetle) is more common than the western, but is only an occasional pest of corn. The southern corn rootworm is recognized by the 11 prominent black spots on the green wing covers. 

The least common of the species in Kentucky is the northern corn rootworm which is lacking in any black markings on the wing covers.

Biology

What is common with each of the corn rootworms in Kentucky is that their eggs hatch in late spring and the larvae are destructive to corn roots. Generally the majority of damage occurs during the month of June.

What can differ among the species is when the eggs are laid. With the western and northern species the eggs are laid in the soil at the base of corn plants during the previous summer. These species have a very narrow host range, feeding on corn and a few other grass species.

The eggs over-winter to hatch the following May. The young larvae can only move a very short distance in the soil to find roots and begin their development. For this reason, in Kentucky the western and northern corn rootworms are only problematic in continuous corn.

The southern corn rootworm over-winters as an adult and will lay its eggs in the spring. The southern corn rootworm has a very wide host range and will lay its eggs around the bases on many plant species. It generally does not concentrate on just corn.

Damage

The damage done by the larvae to corn roots can be extensive. Corn plants develop roots on progressive rings, the corn rootworm larvae can destroy three or more complete rings of roots during June if populations are severe.