No. 25 Article 5/December 3, 2004

Consistency Ratings for Corn Rootworm Control Products

On September 2, Ron Estes, University of Illinois manager of the Insect Management and Product Evaluation Program, reported the root-rating results of our corn rootworm insecticide trials in issue no. 22 of the Bulletin). Root ratings serve as one tool to evaluate product performance.

Another approach that can be used is to calculate a consistency rating for each product. We often receive questions from farmers about consistency of a given insecticide over many growing seasons and locations. Table 1 provides consistency percentages for products tested in our experiments at DeKalb, Monmouth, and Urbana during the 2004 growing season. For each product, 20 roots (4 replicates, 5 roots per replicate) were rated for larval injury on the Iowa State 1-to-6 scale. Root ratings of 1 to 2 signal that very minor larval injury or root scarring occurred. A rating of 3.0 is the commonly accepted economic injury index and indicates that some pruning (light to moderate) has taken place but never the equivalent of one node. Ratings of 4, 5, or 6 indicate that 1, 2, or 3 nodes of roots, respectively, have been destroyed. Typically, when root injury is 4 or greater, plants become more predisposed to lodging, goosenecking, and ultimately may suffer significant yield losses.

Consistency ratings in Table 1 reveal the percentages of plants that were rated from 1 to 3 on the Iowa State 1-to 6-scale. The greater the percentage, the more consistently a given product performed at a satisfactory level. We believe that to assess the overall performance of a rootworm control product, it makes sense to look at the actual root-injury ratings as well as the consistency percentages. We also know that yields are the bottom line. However, yield data are often difficult to interpret because of complicated interactions involving severity of root injury, root regeneration differences among hybrids, soil moisture levels, percentage lodging, and summer temperatures (particularly during anthesis). In determining which root protection products are likely to perform the best under intense rootworm larval pressure, we encourage you to look at both root ratings and consistency percentages. In addition, we encourage our readers to look at data sets on corn rootworm product efficacy maintained at other land-grant institutions.

table 1

The consistency percentages in 2004 reveal a theme that we have repeated for several years. It remains clear that the insecticidal seed treatments (Cruiser and Poncho 1250) did not provide the level of consistency that producers seek in a corn rootworm control product. The pyrethroid products Capture 2EC (liquid bifenthrin) and Empower (granular bifenthrin) also did not provide consistent levels of root protection. The YieldGard Rootworm (Golden Harvest H-8588RW) treatment provided 100% consistency in the DeKalb and Monmouth experiments. In Urbana, the YieldGard RW treatment dropped in consistency to 60%. The level of rootworm pressure at all three locations was similar, so the reduction in consistency at the Urbana site was somewhat of a surprise. The planting date at Urbana was the earliest, albeit only by 1 week. We will continue to examine the reduced performance of the YieldGard RW treatment at the Urbana site.

If you have any questions concerning these experiments, don't hesitate to give us a call or send us an e-mail message.--Mike Gray

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