On July 13, a crew led by John Shaw, Illinois Natural History Survey, dug and washed roots from an experimental corn rootworm trial located just south of Champaign-Urbana. We rated the roots for larval injury and present some of the preliminary results in Table 1. Some of the highlighted products (Table 1), such as Warrior T, are not labeled for corn rootworm larval control. Please consult the appropriate product label for rate and application instructions before making any treatment with these insecticides.
The Urbana experimental corn rootworm insecticide efficacy trial was planted on April 28, late by today's standards. In general, most products performed very well and kept larval injury below a root rating of 3.0 (some light pruning of corn roots) on the Iowa State 1-to-6 injury scale. The level of rootworm "pressure" in the experiment was quite good, with over two nodes of roots destroyed in the untreated control. With the exception of the seed-treatment products, Furadan 4F, Fortress 5G (band application), NuFos 15G, and Warrior T (two reduced rates), the great majority of insecticide treatments kept root injury below the commonly accepted economic injury index of 3.0 on the Iowa State injury scale (several roots eaten off to within 1.5 inches of the plant but never the equivalent of an entire node of roots destroyed). Although the banded application of Fortress 5G failed to provide adequate root protection, the in-furrow treatment of this product offered a high level of root protection. The two lower application rates of Warrior T also failed to keep root injury below a root rating of 3.0; however, this product is not currently labeled for larval control. The higher rate of Warrior T (0.09 lb a.i./acre) did keep root injury below a rating of 3.0. NuFos 15G, which has the same active ingredient (chlorpyrifos) as Lorsban 15G, had root injury that exceeded the 3.0 root-rating level. This seems a bit curious, considering that Lorsban 15G performed very well in this experiment.
On July 24 and 25, we will continue our corn rootworm insecticide-efficacy plot evaluations in Monmouth and DeKalb, respectively. As soon as data from these experiments are available, we'll share them with you. We are very interested in how soil insecticides are performing in different areas of the state. Give us a call, or send us an e-mail message, if you experience corn rootworm damage that is greater than anticipated.--Mike Gray and Kevin Steffey