C. Richard Edwards, Corey K. Gerber, and Larry W. Bledsoe
Department of Entomology; Purdue University; W. Lafayette, W 47907-1158 USA

Growing corn in rotation with soybean has been adopted widely by corn producers in the midwestern Corn Belt of the USA. One benefit of this short rotation has been the management of western corn rootworm, Diabrotica virgifera virgifera LeConte, larval populations without the use of soil insecticides. Presently, western corn rootworms have adapted to this rotational scheme in parts of Indiana, Illinois, Michigan, and Ohio. This development has virtually eliminated crop rotation as an effective tool for managing the western corn rootworm variant As a result, a majority of producers that farm in the affected area have responded by prophylactically treating their first-year corn acreage with a planting-time insecticide application. This indiscriminate use of soil insecticides on rotated corn could have far-reaching negative implications to the environment and human health.

To determine the need for a soil insecticide in first-year corn, economic thresholds for the western corn rootworm beetle in soybean are under development. To develop these thresholds, adult rootworm populations were estimated in soybean by using the Pherocon9 AM unbaited sticky traps (Tre´ce´ Inc., Salinas, California) in one year, and rootworm larval damage was determined on corn roots in that same field the following year. Pherocon AM traps, distributed through a field in one row of 6-8 cards, were positioned so that 1/2 of the card was above the soybean canopy. After one week, the cards were removed for beetle counting and new cards were established in their place. This continued for 7 weeks. The next year, corn roots were dug from one row treated with a soil insecticide and another row without an insecticide treatment. Roots were washed to remove soil and rated using the lowa 1-6 root rating scale.

A regression analysis was performed on data collected the past three years. Although the data from 1996-1997 and 1997-1998 suggested a weak relationship between rootworm beetle populations in soybean one year and injury to corn the next, 1998-1999 data showed a strong relationship under this year's conditions.