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Rootworm Insecticide Performance Is Poor in Some Areas of Illinois

July 12, 2002
During the past couple of weeks, we have received more than a typical number of reports of poor performance of soil insecticides for rootworm control. Although this is unfortunate, it's not very surprising. The densities of western corn rootworm adults in north-central, northeastern, and eastern Illinois in 2001 were extremely high. Obviously the weather during the winter did nothing to suppress rootworm populations. And then to add insult to injury, corn was planted late or grew slowly during the cool, wet conditions that prevailed in May and early June. Although late-planted corn often escapes severe rootworm injury in the Midwest, the cool soil temperatures also delayed hatch of rootworm larvae and slowed larval development. The end result of all of this is that a whole lot of larvae survived and began feeding on corn plants when the root systems were not well established. And finally, the recent hot, dry weather has compromised efficacy of insecticides applied at planting time.

Reports of very heavy infestations of corn rootworm larvae are numerous, especially in northeastern and north-central Illinois. John Lilienthal, with Pioneer Hi-Bred International, found as many as 46 larvae per root system in a field near Kankakee. Others have noted similar numbers.

Unfortunately, when farmers experience poor control of rootworms with soil insecticides, they often discover it too late to do much about it. Some people have applied "rescue" treatments in fields where the insecticide applied at planting did not work, and others are considering it. I want to reemphasize that this is not a good idea, especially now. In cornfields where an insecticide can be directed to the bases of the corn plants and cultivated into the soil in June, rescue treatments offer some protection. In tall cornfields with mature rootworm larvae in June, rescue treatments usually are a waste of money. This is especially true when the soil is dry. Although folks may get a little satisfaction from the prospects of revenge, I strongly discourage spending any more money for rootworm larval control at this time of year. I know this is tough news to hear and dispense, but research data suggest that attempting to control rootworm larvae in dry soils in July is fruitless.

As people continue looking for rootworm larval damage throughout July, it might be useful to be able to categorize the damage. Entomologists have used root-rating scales for years to assess rootworm larval damage to individual plants. The "old" 1-6 root-rating scale was developed at Iowa State University, which dates back more than 30 years (published in a scientific journal in 1971). The process is simple. After you extract roots from the field, wash off the dirt so that you can see the roots and rootworm injury clearly. Examine the roots for the overall amount of injury and assign a rating to each root. An explanation of the rating scale follows, and schematic illustrations of root ratings 2, 3, 4, and 5 are shown in Figure 1. In addition, you can access a brief video of the root-rating process from the Web: videos/corn_rootworm/root_rating.html. We hope the video, which was prepared and edited in 1999, adds some instructional insight.

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

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

2--Some roots 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.

More recently, entomologists at Iowa State University have developed a "new" node-injury root-rating scale to assess the level of rootworm larval damage to individual plants. The node-injury scale describes the degree of root pruning more precisely than the 1-6 root-rating scale. The node-injury scale includes both the number of nodes of roots completely destroyed (0 to 3, the number to the left of the decimal) and the percentage of a node eaten (0.01 to 0.99, the number to the right of the decimal).

Regardless of the root-rating scale you use, you should assess the level of damage within a given field by examining several root systems. Add all of the ratings of roots from an individual field and divide by the number of roots examined to obtain an average root rating or node injury for the field. If you are comparing the efficacy of different treatments or comparing roots from a soil insecticide-treated area of the field with roots from an untreated check strip, follow the same procedure to obtain averages for the different treatments. Such comparative root ratings may provide insight for future reference.

In general, a root rating of 3.0 or higher on the 1-6 root-rating scale and a node-injury rating of 0.5 or higher on the 0-3 node-injury scale suggests that economic damage may have occurred. However, as most of you know, these economic indexes may differ among hybrids and during stressful environmental conditions. A few years ago, Mike Gray and I conducted a detailed, multiyear study of the interaction of different hybrids, rootworms, and environmental conditions in DeKalb and Urbana. We learned that different hybrids respond differently to rootworm larval damage and that the response was affected by environmental conditions. For example, during one year when growing conditions were stressful, economic damage occurred with some hybrids when the average root rating was only 2.5 (on the 1-6 root-rating scale). When growing conditions were more favorable, economic damage did not occur with the same hybrids until the root ratings were 4.0 or higher.

As you assess rootworm larval damage and performance of insecticides, try to consider as many factors as possible to determine whether yield will be affected. When we have information from our rootworm insecticide efficacy trials, we will share the results.--Kevin Steffey

Author: Kevin Steffey

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

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