Corn Insects

Western Corn Rootworms, Diabrotica virgifera, and Northern Corn Rootworm, Diabrotica barberi.

wcr

Western corn rootworm adult.

ncr

Northern corn rootworm adult.

Nature of Rootworm Damage

Northern corn rootworm adults are about ¼-inch long with pale green to yellow coloration. Western corn rootworm adults are about the same size as northerns or slightly larger. Overall coloration when viewed from above is yellow with a black stripe around the margin of each wing cover. Westerns frequently have a dark stripe extending part way up the center of the wing covers. Southern corn rootworm adults are about 3/8 inch long and have 12 black spots on a chartreuse background. Mature rootworm larvae are white and slender, about ½ inch long, with brown heads and a dark plate on the top side of the terminal segment. Southern corn rootworms, Diabrotica undecimpunctata howardi, are of little concern, in part because their eggs apparently do not overwinter in Kansas.

Significant production problems in continuous cornfields are most likely caused by western corn rootworms in Kansas. Damage is caused by larvae, which tunnel within corn roots and prune them as they feed. Severe damage causes the plants to “lodge” or lean over, which greatly reduces the ability of the leaves to harvest sunlight and reduces the plant’s ability to tolerate moisture stress, effectively limiting the amount of soil nutrients brought into the plant. Research has concluded that rootworm-infested corn that lodges will yield less than rootworm-infested corn that doesn’t lodge.

Occasionally rootworms can cause problems when the corn follows very weedy soybeans or soybeans infested with volunteer corn that persisted through July. In some upper Midwestern states, northern corn rootworms can survive a non-corn crop by remaining in the egg stage for two winters before hatching, but this phenomenon has not been confirmed within Kansas. In another form of 'rotation resistance', eggs may be laid in soybean fields in late summer where this crop is usually followed by corn. Western corn rootworms, by far the state’s most significant species, are generally not known to lay many eggs in non-corn fields in Kansas, so crop rotation still works well here.

Management

Rootworm insecticides are rarely needed in first year corn fields. Rootworms are less of a problem in sandy soils and in southeast Kansas, south of U.S. Highway 54. Previous year counts of rootworm beetles can be used to establish the potential for economic damage and the need for insecticide protection the next growing season. Insecticides are best applied before rootworm damage has become severe (mid-May through mid-June), sometimes before or close to egg hatch. Adequate precipitation or soil moisture must be available to move the products into the rootworm feeding zone. Control may be improved if the insecticide is cultivated into the soil. Some materials are registered for chemigation.

Silk Clipping Protection

Adult beetles may prevent pollination by early silk clipping, but clipping after pollination does not affect yield. Foliar spray treatments are probably justified if there are eight to 10 beetles per plant and 10 percent of the silks are beginning to show.

Please refer to the most recent version of the Corn Insect Management Guide for control recommendations.

More information on corn rootworms in Kansas can be found here:

KSRE publication MF845 (1995): Corn Rootworm Management in Kansas Field Corn.

KSRE publication MF2642: Western Corn Rootworm Kansas Areawide Management Program Summary (2004)

Page last updated 11/04/2013 by J.P. Michaud.