Root Ratings and Evaluations of Rootworm Control--Efforts Are Under Way

July 16, 1999
During July, many people spend time in corn trying to assess the amount of rootworm injury, either in growers' fields or in research or demonstration plots. Therefore, we thought it might be worthwhile to refresh your memory about root ratings. Most entomologists in the Corn Belt still use the Iowa State University 1­6 root-rating scale that was developed in the 1960s. Although other scales have been used, the 1­6 scale is still the most universally recognized.

Click here for a video concerning root rating.

Follow these steps to assess the amount of rootworm larval injury to corn roots. After you extract roots from the field, wash off the dirt so you can see the roots and rootworm injury clearly. Examine the root systems 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.

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 l 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.

By adding all of the ratings of roots from an individual field and dividing by the number of root systems examined, you can obtain an average root rating for the field. If you are comparing the efficacy of different treatments or comparing roots from a soil insecticide­treated 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.

We began our month-long assessment of rootworm damage and effectiveness of different control tactics on July 12. We started with our trials in Urbana; we will evaluate our trials at the outlying research farms near Monmouth and DeKalb during the week of July 19. The objective of most of the trials at Urbana, Monmouth, and DeKalb is to assess the effectiveness of different formulations of registered and experimental insecticides. During the last week of July, we will join forces with a team from Purdue University to evaluate rootworm damage in the fields in our areawide management study in Iroquois County, Illinois, and Benton and Newton counties in Indiana. To wrap it all up this summer, we will evaluate roots from as many as 75 cornfields in east-central Illinois. Several extension educators, farm managers, consultants, and growers placed yellow sticky traps in their soybean fields last year to sample for western corn rootworm adults. In an effort to help us refine our thresholds for western corn rootworm adults in soybeans, cooperators will extract roots from untreated check strips in the cornfields and bring them to us for rating. The more information we have, the more confident we can be with our thresholds of two to seven beetles per trap per day.

On July 12 and 13, we observed some serious rootworm feeding damage to some untreated plots in our trials at Urbana, suggesting that rootworm densities are higher this year than they were last year. Although some variability in root ratings always occurs, average root ratings in some of our untreated check plots were close to 5 or greater. As soon as we have some data compiled, we will share some preliminary results. In the meantime, let us know what you are finding in fields in your area.--Kevin Steffey, Mike Gray

Author: Mike Gray Kevin Steffey