University of Illinois

No. 24/November 7, 1997

Preliminary Economic Threshold for Western Corn Rootworms in Soybeans

On-farm research conducted during 1996 and 1997 has led us close to the release of a preliminary economic threshold for western corn rootworms in soybeans. In 1996 and 1997, Matt O'Neal, an entomology graduate student, worked closely with a group of producers in east-central Illinois. In each producer's field, Matt, along with cooperating farmers, placed Pherocon AM traps (yellow sticky traps) throughout 16 soybean fields. Traps were replaced once per week over a 3- to 4-week sampling period, from late July to mid-August. In 1997, roots were evaluated for rootworm larval injury in first-year cornfields throughout the length of each of four treated and untreated strips. Preliminary analyses indicated very good correlations between western corn rootworm beetle counts (1996) on sticky traps and root injury this past season (1997). At the 1998 Agricultural Pesticides Conference, we will offer producers a look at a preliminary economic threshold. In 1998, we hope to refine the threshold by rating roots in producers' fields and determining how well the beetle captures (Pherocon AM traps) of 1997 predict the level of larval injury. Bottom line: Producers who are interested in monitoring their soybean fields for western corn rootworm adults in 1998 will have more guidelines regarding whether or not a soil insecticide might be needed in 1999.
Mike Gray and Kevin Steffey, Extension Entomology, (217)333-6652