No. 23 Article 3/October 8, 2004

Results of Variant Western Corn Rootworm Larval-Injury Survey

During late July and early August, an on-farm survey of western corn rootworm larval injury was conducted in 24 Illinois counties. The survey was accomplished with the generous cooperation of University of Illinois Extension Crop Systems and IPM educators. We extend our thanks! In each county, 10 first-year cornfields were selected at random and five roots were evaluated for injury on the Iowa State 1-to-6 injury scale. The established economic injury index for this scale is considered to be equal to a rating of 3.0 (some roots pruned, never equivalent to 1 node of roots). For a complete description of how to utilize this root-injury scale, please consult the following video at our University of Illinois IPM Web site. Counties included in the 2004 survey were those located primarily in northwestern and western Illinois. In addition, a few counties such as Clay and Fayette were included to track the movement of the variant western corn rootworm to the south. The results of our on-farm survey efforts are provided in Table 1.

The average root-injury ratings across the majority of counties are quite low. However, averages can be misleading. Producers are encouraged not to base their management decisions for 2005 strictly on the basis of these averages. For a more complete assessment of the variant western corn rootworm's impact in these counties, we've added two other data summary columns in Table 1: the percentage of roots with injury that equals or exceeds a root-injury rating of 3.0 and the range of root injury. The percentage of roots within a given county that equals or exceeds an injury rating of 3.0 is significant for several counties, including Bureau (30%) and Stark (72%). In addition, several counties have some isolated roots with severe pruning (roots with injury ratings of 4 or above): Clay, Henry, McDonough, Stark, and Warren. Half the counties surveyed had at least some roots with ratings equal to 3.0. We believe the results of the 2004 survey indicate that the variant western corn rootworm continues to expand its range in Illinois.

Decisions regarding the need for a soil insecticide in first-year cornfields in 2005 should be based upon scouting input and the use of thresholds. For many years, we have encouraged producers to use Pherocon AM traps in their soybean fields to monitor densities of western corn rootworm adults. We know considerable variation occurs in densities of western corn rootworm adults from field to field, even in areas where the variant is well established. We also know that many producers are not eager to use these traps. Consequently, many fields are treated that do not require soil insecticides.

In early September, Kevin Steffey and I spent part of a day in Coles County surveying cornfields for European corn borer damage. Although we found very little borer damage, we did find several fields of first-year corn with moderate levels of lodging due to root pruning. Some producers may still consider first-year cornfields to the south of Champaign County, such as Coles and Clay, out of harm's way when it comes to the variant western corn rootworm. We believe this assumption will continue to be challenged by western corn rootworms more aggressively in the future. Again, the use of Pherocon AM traps in soybean fields can help pinpoint those rotated cornfields most at risk to variant western corn rootworm larval injury.

Coles County root from first-year cornfield with corn rootworm larval pruning.

If you have any questions about the information contained in Table 1, please don't hesitate to give us a call or send us an email message.--Mike Gray, Kevin Steffey, and Kelly Cook

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