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Update on Corn Rootworm Injury

July 20, 2001
On July 16, John Shaw, Illinois Natural History Survey, and our summer research crew began to evaluate roots for corn rootworm larval injury at our trial located just south of Urbana. Roots from our experiments are still being rated for injury. We hope to share the preliminary root injury ratings for the various treatments in an upcoming issue of the Bulletin. The severity of injury in our Urbana experiments has been impressive, with many of the roots from our control plots missing two to three nodes of roots. This is the level of corn rootworm pressure that we seek in our trials to effectively evaluate registered and experimental compounds. In addition to our own experiments, many of our readers have responded to our request for information regarding root injury on their respective farms. Some of the reports suggest that producers are not pleased with the level of root protection offered by their chosen soil insecticide. As we've indicated in some previous issues of the Bulletin, even though severe root pruning is detected, many hybrids have remarkable abilities to regenerate root tissue. This is especially true when soil moisture is adequate.

Chris Pierce, a graduate student in the Department of Entomology, reported on July 18 that he was observing a large surge in the number of adult western corn rootworms in several producers' fields in Iroquois County. Chris also indicated that the densities of female western corn rootworm adults had increased significantly from the previous week. This suggests that egg laying will begin in earnest very soon. Chris' dissertation research during the past three growing seasons in Iroquois and Champaign counties indicates that the development of corn and soybeans greatly influences the level of egg laying in these crops. Although soybeans appear to be the preferred egg-laying site for western corn rootworms in east-central Illinois, corn remains susceptible to oviposition as well, especially late-planted corn. For much of east-central Illinois, rotated and continuous corn remain at risk to corn rootworm larval injury. It's easy to understand why soil insecticide usage has risen so rapidly in these counties. Silvia Rondon, a Ph.D. student in the Department of Crop Sciences, has confirmed that oviposition by western corn rootworms in east-central Illinois also occurs in other crops, such as alfalfa and oat stubble. We continue to work on this challenging pest management problem and invite our readers to attend the upcoming Agronomy Field Day (August 23) on the South Farms where we will provide some additional information on this topic.

To more effectively target where soil insecticides should be used during the spring of 2002, we urge growers to begin final preparations for monitoring their soybean fields with Pherocon AM traps. For complete information on monitoring procedures and economic thresholds please visit the following Web site: infosheets/1-wcornr/wcornr.html.

We suggest that producers deploy 12 Pherocon AM traps throughout their soybean field no later than the last week of July. After 1 week, remove the 12 traps and place 12 new traps in the same field. This process should be repeated for the first 3 weeks of August. Traps should be placed just above the soybean canopy. We've used metal stakes in our plots; however, others have used wooden stakes or even plastic pipes for this purpose. At the conclusion of the monitoring period in late August, calculate the average number of adults caught per trap per day. If five adults per trap per day are captured, this suggests that average root injury the following season in rotated corn may equal 3.0 (some pruning) if the field is left untreated (no soil insecticide used). If 10 adults per trap per day are caught, this indicates that severe pruning (root rating of 4.0, one node of roots destroyed) may occur the following year if the rotated cornfield is left untreated. Because not all cornfields support economic infestations of corn rootworms, an investment of your time in using these traps could save you the expense of a soil insecticide next season.

In the upcoming weeks, we'll let you know how the root-rating results from our experimental trials turn out.--Mike Gray

Author: Mike Gray

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

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