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Corn Rootworm Larval Injury Evaluated in First-Year Cornfields

August 11, 2000

In late July, Susan Ratcliffe, Extension entomologist, coordinated a large root-retrieval project with producers, numerous agribusiness personnel, and Extension Unit leaders and educators. We extend our thanks to all those who participated in this large project. Digging roots from cornfields in late July and early August is not much fun. Ultimately, we hope to refine our economic threshold even more when these data are further evaluated.

To begin this task, cooperators dug corn roots from untreated check strips (no soil insecticide used), and we arranged for their journey to Urbana, where they were washed and rated on the Iowa State 1-to-6 injury scale (see Table 1). Root injury from these on-farm plots will be compared with adult capture data collected in soybean fields during 1999 with Pherocon AM traps (yellow sticky traps) and the threshold adjusted as needed.

A cursory examination of these preliminary data reveals considerable variation in root injury from field to field. Root ratings in Livingston County (eight fields evaluated) ranged from 1.3 (light scarring) to 4.9 (two nodes of roots destroyed). Similarly, a wide range of injury occurred in McLean County (1.7 to 4.3). By using Pherocon AM traps in soybean fields this season, producers should be able to more accurately pinpoint those first-year cornfields most at risk to corn rootworm injury in 2001. Data such as these, year after year, strongly document that economic larval injury does not occur in every rotated cornfield of east-central Illinois. Remember, even continuous cornfields support economic infestations of corn rootworm larvae only about half the time in any given year. These data show that only 13 of 36 fields exceeded the economic injury index of 3.0 in untreated check strips. The bottom line is that 64% of these first-year cornfields did not exceed the economic injury index. Before an investment is made in a soil insecticide for rootworm control in rotated corn, we encourage you to evaluate the potential need of these pest-management tools.--Mike Gray and Kevin Steffey

Author: Kevin Steffey Mike Gray

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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