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Root Ratings from 2003 Corn Rootworm Control Trials in Illinois

September 5, 2003

Through our Insect Management and Insecticide Evaluation program, coordinated by Ron Estes in the Department of Crop Sciences, we obtain quite a bit of efficacy data for products intended to control some of the major insect pests of alfalfa, corn, and soybeans in Illinois. Among the first data we provide, before our full report is completed, are the root-rating data from our corn rootworm control trials. We conduct three rootworm control trials at outlying farms operated by the Department of Crop Sciences: Northern Illinois Agronomy Research Center (Shabbona, near DeKalb); Northwestern Illinois Agricultural Research and Demonstration Center (near Monmouth); and Crop Sciences Research and Education Center (Urbana). Each year we establish trap crops of late-planted, mixed-maturity corn hybrids, sweet corn, and pumpkins to attract corn rootworm females that will lay eggs in the plot areas. The following year we establish our product efficacy trials where the trap crops were planted the previous year. Consequently, we usually have substantial rootworm larval damage in our trial areas, ensuring that the rootworm control products are "put to the test."

Table 4 shows the average root ratings from our three rootworm control trials in 2003. The root ratings are from the 1-to-6 scale, introduced by Hills and Peters at Iowa State University in 1971 (Hills, T. M., and D. C. Peters. 1971. A method of evaluating post-planting insecticide treatments for control of western corn rootworm larvae. Journal of Economic Entomology 64:764-765). The whole number root ratings are as follows:

  1. no damage, or only a few minor feeding scars;
  2. feeding scars evident, 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 destroyed;
  4. one node (or the equivalent) of roots completely destroyed;
  5. two nodes (or the equivalent) of roots completely destroyed;
  6. three or more nodes (or the equivalent) of roots completely destroyed.

Most entomologists use a root rating of 3.0 to distinguish between acceptable root damage (<3.0) and less-than-acceptable root damage (>3.0). However, the relationship between root ratings and yield may be altered depending on environmental conditions and corn hybrid. For example, when corn plants are stressed by a lack of moisture, root ratings less than 3.0 may result in some yield loss. On the other hand, when corn plants have plenty of moisture and the corn hybrid has the ability to compensate for rootworm in-jury, root ratings greater than 3.0 may not result in yield loss. With this explanation of root ratings as background, you can place the performance of rootworm control products in our trials in 2003 in context.

The rootworm larval damage in all of our untreated checks at all three locations in 2003 resulted in root ratings greater than 4.0. Rootworm damage at our Monmouth and Urbana locations was particularly severe in the untreated checks. Please note that we used two hybrids as untreated check plots this year--a DeKalb hybrid and a Northrup King (NK) hybrid. We intend to take yields from these 4-row plots, so we needed to include a nontransgenic DeKalb isoline to fairly compare yield with the MON 863 event in the DeKalb YieldGard Rootworm hybrid. At this point, I cannot easily explain the significantly different average root ratings in the two untreated checks at Monmouth and Urbana.

For the most part, rootworm control products performed very well in our three trials in 2003. All products held the average root rating to less than 3.0 at the Urbana site (planted on May 13), a fairly impressive feat given the severe damage in the untreated check. I suspect that the rainfall pattern at the Urbana site was beneficial for all products tested. We will examine and share rainfall data in our full report to be published on the Web later this year.

The rootworm larval damage at the Monmouth site (planted on May 16) was quite severe, and under the conditions at that site, most products performed quite well. However, the average root ratings for plots treated with Capture 2EC, Cruiser FS, Empower (a granular formulation of bifenthrin, the same active ingredient as in Capture), and Poncho 1250 were greater than 3.0. The average root rating for plots treated with Capture 2EC (3.2) was not significantly different from several products that held the average root rating to less than 3.0.

The rootworm larval damage at the DeKalb site was less than at the other two sites, probably because rainfall prevented us from planting on a reasonable
planting date. The trial at DeKalb was planted on May 28, right about the time when rootworm larvae were hatching. Consequently, all products but the seed treatments Cruiser FS and Poncho 1250 held the average root ratings to less than 3.0. In fact, the average root ratings in plots treated with Cruiser and Poncho were not significantly different from the average root ratings in the untreated check plots. It's possible that the active ingredients of Cruiser and Poncho did not have enough time to come off the seed and get into the soil to provide optimal control of rootworm larvae.

Although we have received a fair number of reports of unacceptable performance of soil insecticides in producers' fields this year, the granular insecticides applied in our trials (with the exception of Empower) continued to provide the most consistent control of rootworm larvae. Control of corn root-worm larvae with Capture 2EC was acceptable at all three locations. And as we have experienced in the past, the performance of the insecticidal seed treatments was challenged by heavy rootworm larval pressure, with the notable exception of their performance at the Urbana site.

The MON 863 transgenic event for rootworm control provided excellent control of corn rootworms, although it's worth noting that some root injury in the MON 863 plots at the Monmouth site led to a root rating of 3.0 on individual roots. Because MON 863 is a low- to moderate-dose event, some rootworm larval injury on Yield-Gard Rootworm hybrids is to be expected.

These data clearly indicate that the interaction among products, environment, and rootworm larvae can create situations in which some products do not provide consistent control. When we prepare our report, we will include percentage consistency of all products in the trials.

One more note: Our planting times were not to our liking this year. In fact, one could argue, and justifiably so, that our mid- to late-May planting dates are not reflective of producers' planting dates. We agree, and we intend to plant our plots earlier in 2004. It's possible that insecticides applied in early April will not hold up well against severe rootworm pressure when larvae begin feeding in late May and early June. Consequently, we need to get our trials in the ground earlier than we have done in the past. In addition, we will include a time-of-planting study among our trials to determine how control of rootworm larvae compares among soil insecticides, seed treatments, and transgenic hybrids. We want to make certain that results from our trials are useful for the people with whom we share the information.--Kevin Steffey

Author: Kevin Steffey

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

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