No. 22 Article 6/September 2, 2005

An Addendum to the Preliminary Root Rating Results for DeKalb and Urbana, 2005

As our readers of this Bulletin may recall, in 2004, we observed greater than expected late-season pruning of brace roots in the YieldGard rootworm treatment in our standard corn rootworm product trial located near Urbana. This summer, we decided that it might be prudent to evaluate roots from several treatments (soil insecticides, a seed treatment, and the YieldGard rootworm treatment) somewhat later in the summer to determine if this observation would occur again for perhaps many products. Roots were rated (Table 5) on early and later evaluation dates for five treatments selected from our standard corn rootworm product experiments (DeKalb and Urbana). In a later issue of this Bulletin, we will report on the first and second root rating evaluations from our Monmouth experiment.

As might be expected, the root rating averages increased from the first to the second evaluation dates at each location for all products. The separations in timing between the first and second root evaluations for DeKalb and Urbana were approximately 3 and 4 weeks, respectively. This suggests that some corn rootworm larvae were continuing to feed from late July through early August. However, care should be exercised when looking at these numerical differences in root injury for each product between the two "digs." For example, please note that the root injury in the check at DeKalb appears to go down slightly during the second evaluation. Obviously, root injury does not go away over time. Also, keep in mind, we randomly select and rate five roots from each treatment by replicate (four replicates) combination. This translates into 20 root rating scores (20 plants) for each treatment average. Expected variation in root injury within a treatment alone may account for the smaller numerical differences in averages that were observed from the first to the second dig for some products.

Later this season, we will report the results of our full analysis concerning the influence of late-season corn rootworm injury on product performance.

--Mike Gray and Ron Estes

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