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Reports of Extensive Corn Rootworm Larval Injury Common

July 13, 2001
Reports of corn rootworm larval injury are becoming increasingly common. On July 6, David Feltes, IPM educator, Quad Cities Extension Center, reported that a first-year cornfield in northern LaSalle County had plants with severe pruning and live larvae still present. Gary Bretthauer, IPM educator, Kendall County Extension Unit, reported on July 11 that several first-year cornfields in Kendall County had significant levels of corn rootworm larval injury. Reports such as these continue to indicate that the first-year corn western corn rootworm problem has become firmly established outside of east-central Illinois counties.

As we approach mid-July, much of the corn rootworm larval injury will have occurred already. Some lingering feeding will continue through July; however, I don't anticipate much injury in late July because of the early hatch that occurred this spring. On July 16, we will begin to evaluate our Urbana experimental plots for corn rootworm larval injury. John Shaw, an entomologist with the Illinois Natural History Survey, will lead this effort. Following our evaluations, we will report the root-rating results in upcoming issues of the Bulletin.

For those of you who are interested in evaluating the performance of your chosen soil insecticide, there's no easy way to accomplish this task. Relying exclusively on the intensity of lodging and yield may not offer an accurate assessment of the return on your investment of a soil insecticide purchase. Plants may lodge because of poorly rooted plants in fields suffering from compaction problems. Storms accompanied by high winds also may lead to lodged plants. In some years, yields may not reflect the level of rootworm injury. Hybrids differ considerably in their ability to compensate for rootworm larval injury. This is especially notable in wet growing seasons. The only way to accurately assess soil insecticide performance is to rate roots for larval injury. Comparing roots from treated and untreated strips within a field is essential. Unfortunately, many producers will not leave even a single check strip. Because corn rootworms do not occur at economic levels in every cornfield, some producers mistakenly assume that their soil insecticide performed exceptionally well when, in fact, very few corn rootworm larvae may have been present. We estimate that approximately 50% of cornfields support economic infestations of corn rootworm larvae.

To evaluate your soil insecticide, please consider the use of the "old" Iowa State University 1­6 root-rating scale. After you remove roots from a selected field, wash the soil from the root system and rate each individual root for scarring and pruning. It's a good idea to remove roots from several areas within a field. Obviously the more roots you examine the more reliable your estimate of overall injury is likely to be. An explanation of the rating scale follows, and schematic illustrations of root ratings 2, 3, 4, and 5 are provided in Figure 1. In addition, we encourage you to view our instructional video on this rating process at the following Web site: corn_rootworm/root_rating.html.

The Iowa State University 1­6 root-rating scale is described as follows:

1--No visible damage or only a few minor feeding scars.

2--Some plants with feeding scars but no roots eaten off to within 1-1/2 inches of the plant.

3--Several roots eaten off to within 1-1/2 inches of the plant but never the equivalent of an entire node of roots gone.

4--The equivalent of one node of roots pruned off to within 1-1/2 inches of the plant.

5--The equivalent of two nodes of roots pruned off to within 1-1/2 inches of the plant.

6--The equivalent of three or more nodes of roots pruned off to within 1-1/2 inches of the plant.

We are interested in your reports of corn rootworm larval injury throughout the state. If you find significant injury in scouted fields, don't panic. Although rescue treatments are not an option, many hybrids will compensate as long as soil moisture levels are adequate. Let us know what you find.--Mike Gray

"Goose-necked" corn due to corn rootworm larval injury.

Severely lodged corn due to corn rootworm larval injury.

Author: Mike Gray

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

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