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Preliminary Root-Rating Results for DeKalb and Monmouth Corn Rootworm Trials

August 4, 2000

On July 24 and 25, a crew led by John Shaw, Illinois Natural History Survey, dug and washed roots from experimental corn rootworm trials in Monmouth and DeKalb, respectively. We rated the roots for larval injury and present some of the preliminary results in Tables 1 and 2. Some of the highlighted products (Table 1), such as Warrior T, are not labeled for corn rootworm larval control. Please consult the appropriate product label for rate and application instructions before making any treatment with these insecticides.

The Monmouth and DeKalb corn rootworm insecticide-efficacy trials were planted on May 16 and 17, respectively. From May 16 to July 24, a total of 13.46 inches of rain fell at Monmouth (May 16-31, 2.94 inches; June, 7.07 inches; July 124, 3.45 inches). The June rainfall total for Monmouth was more than 3 inches greater than the monthly average. Total precipitation at DeKalb from May 17 to July 25 was 10.29 inches (May 17-31, 1.55 inches; June, 5.76 inches; July 1-25, 2.98 inches). Rainfall for June at the DeKalb site was 1.58 inches greater than average. Overall, June was wet at both of these locations. Recall that the 6-week period from early June to mid-July is "prime time" for corn rootworm larvae. Most of the standard soil insecticide products we tested at these locations held up reasonably well to moderate rootworm pressure. Controls (no soil insecticide used) at each location were above a root rating of 4.0 (one node of roots or the equivalent destroyed). When corn plants have root injury equal to a rating of 4.0 or greater, they are more likely to lodge. This is especially true when soil moisture is plentiful and plants are top heavy with "fat" ears. I suspect that lodging will increase in the upcoming weeks across many fields in central and northern Illinois counties. Although many of the products performed satisfactorily in our trials, consider that we did not plant corn until mid-May. Many of the so-called "rootworm complaint" calls that we've received are from producers who planted in very early April. In essence, soil insecticides were applied nearly 2 months before larval feeding began. As we look back on this summer and examine how products held up to rootworm pressure in producers' fields, it becomes obvious that insecticide performance varied considerably from field to field. Many interacting factors are involved, including planting date, rainfall patterns and amounts, product characteristics, accuracy of application (calibration accuracy), and method of application (band vs. in-furrow). The results in Tables 1 and 2 also clearly show that seed treatments did not hold up very well at either location. Let us know how your products are performing in your fields.--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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