Evaluating Corn Roots for Rootworm Larval Damage

July 7, 2000
In issue no. 13 of the Bulletin (June 23, 2000), I wrote an article for individuals who were interested in evaluating corn roots for rootworm larval damage. However, at that time, I indicated that it was a bit too early to offer an explanation for the Iowa State University 1-6 root rating scale. Well, now it's time to talk about rating roots for rootworm damage. During July, entomologists at land-grant universities and with agricultural companies spend a great deal of time evaluating corn roots extracted from research and demonstration plots. In fact, we will begin about 4 weeks of this activity beginning the week of July 10. We anticipate that rootworm larvae will have completed their feeding, for the most part, during mid-July. We want to examine the roots soon to determine the level of injury before corn hybrids have much time to "regrow" roots in response to rootworm larval damage. Excessive compensatory root growth makes assignment of a root rating more difficult.

Healthy corn roots (left) and corn roots severely damaged by corn rootworm larvae (right).

The Iowa State University 1-6 root rating scale dates back more than 30 years, although the paper explaining the root rating scale was published in 1971. The process is rather simple. After you extract roots from the field, wash off the dirt so 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, we have a brief video of the root rating process on the IPM web site. 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.

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 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.

Lodged corn as a result of severe rootworm larval damage.

Goosenecked corn stalks as a result of rootworm larval damage.

Ria Barrido with Growmark, Inc., in Bloomington visited some cornfields in western Illinois during the week of June 26 and noted some severe rootworm larval damage. Some of the fields had been treated with a soil insecticide at planting time. As July progresses, we may receive more reports of rootworm larval damage. In fact, if rains continue to loosen the soil and if wind becomes a factor, you may encounter fields of corn plants that lodge as a result of severe rootworm larval damage. However, always remember that other factors can cause corn plants to lodge; only by examining the roots can you verify the presence or absence of rootworm damage. Good luck with your evaluations this month, and let us know what you find.--Kevin Steffey and Mike Gray

Author: Mike Gray Kevin Steffey

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

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