Development of Corn Rootworms Is Progressing

June 25, 1999

· Some corn rootworm larvae in corn planted after soybeans

· Corn rootworm pupae observed near West Lafayette, Indiana

· Emergence of adult corn rootworms is imminent

In last week's issue of the Bulletin (No. 13, June 18, 1999), Mike Gray discussed how to sample for corn rootworm larvae and summarized the questions people ask about assessing performance soil insecticides for control of corn rootworms. This is just a brief reminder to encourage you to make plans for assessing corn rootworm damage in July and to prepare you for the emergence of corn rootworm adults.

On June 16, Jeff Bunting, American Cyanamid Company, found some corn rootworm larvae in a cornfield in northern Ford County. The field had been planted to soybeans in 1998. Obviously, the problem with western corn rootworms in corn planted after soybeans has not gone away. (No, we really didn't think it would.) We are very interested in documenting where the problem exists to determine if this new biotype of western corn rootworm has extended its range north, west, or south in Illinois. The spread of this new biotype to the east occurred relatively rapidly; we are fortunate that the affected area in our state is still reasonably confined. However, we want to hear from you if you encounter the problem beyond the known affected counties: Champaign, DeWitt, Douglas, Edgar, Ford, Grundy, Iroquois, Kankakee, LaSalle, Livingston, Macon, McLean, Piatt, Vermilion, and Will. We suspect or have heard rumors that the problem has been verified in Christian, Coles, Logan, Moultrie, Sangamon, and Woodford counties. Please help us keep accurate records by reporting the problem. The only positive verification is the presence of rootworm larvae in the field or corn rootworm larval damage to the roots. Try to ignore all of the claims of rootworm problems that actually are problems associated with other causes (such as wind-lodged corn).

We will assess many fields of corn planted after soybeans for rootworm larval damage this summer. In fact, we have an ambitious on-farm research effort under way, an effort that involves many extension personnel, farm managers, company representatives, consultants, and growers. We are attempting to refine our current economic injury index and determine the extent of the problem in Illinois. As the data accumulate, we will inform you of our findings.

The entomologists at Purdue University have found at least one corn rootworm pupa in one of their plots near West Lafayette, Indiana. This means that emergence of corn rootworm adults is imminent. In fact, I suspect that astute observers could find them in some fields in southern or south central Illinois right now. By the time you receive this issue of the Bulletin, corn rootworm adults may be emerging in central counties. Let us know when you find the first adult. And as the summer progresses, let us know about the densities of these pests you find.--Kevin Steffey

Author: Kevin Steffey