Corn Rootworms and Bt Corn:
Assessing the Impact, Managing the Pest

Dr. Kevin L. Steffey
Professor and Extension Specialist in Entomology,
Department of Crop Sciences,
University of Illinois

Dr. Michael E. Gray
Professor and Extension Specialist in Entomology,
Department of Crop Sciences,
University of Illinois

Interim Assistant Dean,
University of Illinois Extension,
Agriculture and Natural Resources Programs

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Corn rootworms are the most economically important insect pests of corn in North America and have also become established in Europe. Corn rootworms have been referred to as “the billion dollar bugs” due to the annual costs of control + the value of yield losses. With the practice of growing continuous corn beginning in the 1950s, the western corn rootworm spread from western Nebraska and Kansas to the east coast by the mid-1980s. Corn rootworm larvae cause damage by feeding on corn roots, which reduces the efficiency of water and nutrient uptake and predisposes the corn to lodging. The injury caused by corn rootworm larvae usually has been assessed by using one or another root-injury rating scales, both of which were developed at Iowa State University. The current most widely used rating scale is the 0-to-3 node-injury rating scale. However, the root rating scales were developed during a time when soil insecticides were the primary tactics for rootworm control. With the advent of transgenic Bt corn for rootworm control, there is evidence to suggest that these root-rating scales are no longer relevant for comparing rootworm injury to Bt corn with rootworm injury to non-Bt corn. As the use of Bt corn for rootworm control increases, many more acres will be planted with Bt corn, placing significant selection pressure on corn rootworm populations. As the acres of Bt corn and “triple stack” hybrids increase to meet the demands for corn, insect resistance management and integrated pest management practices will be critical to ensure the continued use of transgenic technology.

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