Illinois Natural History Survey - University of Illinois

Corn Rootworm Injury: Reducing Prophylactic Soil Insecticide Treatments

 The environmental and economic benefits of reducing the dependence of farmers on soil-applied corn rootworm insecticides are numerous. Although soil insecticides don't leach as readily as many of the commonly used herbicides, these products have the potential to contaminate surface and ground water resources, especially in the spring if planting is followed by heavy precipitation.

Corn roots injured by rootworms.

Illinois corn producers grow an estimated 2.8 million acres of continuous corn (corn grown in the same location year after year with no crop rotation). Approximately 2.5 million acres (88 percent) are treated with a soil insecticide each spring during planting. The great majority of the acreage is treated in a prophylactic fashion -- pesticides are applied without first examining acreage to determine if rootworms are present in sufficient numbers to warrant application. The estimated expense to Illinois farmers for soil insecticide applied during planting for corn rootworms is $26.5 million. Based on on-farm investigations in 1990 and 1991, it was discovered that as many as half of the growers would not have required an insecticide treatment based on the level of root injury observed on their farms.

If the majority of producers in Illinois were aware of and could select tolerant varieties of corn instead of using a soil insecticide, the economic ($26.5 million saved), environmental (reduction of the threat to nontarget organisms), and personal health and safety benefits would be substantial. Virtually no contemporary information is available regarding the ability of corn hybrids to compensate for rootworm injury. Kevin Steffey, an entomologist with the Illinois Natural History Survey, demonstrated that a hybrid used in a University of Illinois trial was able to respond to rootworm injury by regrowing new root tissue. When yields were measured, no meaningful differences were observed among treatments despite the wide range of root injury found. In 1993, a hybrid evaluation experiment was conducted near DeKalb, Illinois. Results from this study have thus far shown that hybrids don't always respond in precisely the manner anticipated. For instance, one hybrid that had the largest root volume had the lowest yield. Conversely, the three top yielding hybrids had the smallest root volumes. Our preliminary data suggest that some hybrids expend resources growing new roots in response to rootworm injury rather than increasing grain yield. The interactions among rootworms, corn plants, and environmental conditions are complex and additional research is required in order to attempt to clarify these relationships.

A great deal of interest is currently being focused on the granular formulation of soil insecticides. This concern is being triggered in part because of the Environmental Protection Agency's concern over avian safety in treated cornfields. In addition, with the current emphasis of the Clinton administration on reducing pesticide use, soil insecticides come under immediate scrutiny. Because of the renewed emphasis on seeking pesticide alternatives, identifying potential roles for host plant resistance is becoming more imperative. Obtaining knowledge about how corn plants respond to corn rootworm injury may aid in the development of resistant corn hybrids.

Michael E. Gray and Kevin L. Steffey, Center for Economic Entomology

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Last Modified 3/19/96

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