In issue no. 22 (September 5, 2003) of the Bulletin, we provided root ratings from the three rootworm control trials we conducted in 2003. In this issue, we provide the root ratings again, as well as yields from all three locations (Urbana--Table 1; Monmouth--Table 2; DeKalb--Table 3). However, you might notice a slight difference in the statistical significance of the root ratings in these tables. The averages have not changed, but the letters indicating differences among means according to Duncan's New Multiple Range Test are different from those presented in Table 4 in issue no. 22 of the Bulletin. As we explained in the accompanying article, two untreated check plots were included at all three sites--a DeKalb (DK) hybrid and a Northrup King (NK) hybrid. The DeKalb hybrid was the nontransgenic isoline of the DeKalb YieldGard Rootworm hybrid. The Northrup King hybrid was the hybrid planted in all other plots at all three sites. Consequently, Ron Estes, manager of the Insect Management and Insecticide Efficacy program in the Department of Crop Sciences, re-analyzed the data with the DeKalb hybrids not included. However, we provide both the average root ratings and yields for the two DeKalb hybrids, without letters associated with Duncan's New Multiple Range Test.
I'm not certain how much the yield data tell us. The root ratings were relatively easy to explain, as we wrote in the article in issue no. 22 of the Bulletin. However, as you can determine for yourself, the yield data do not necessarily correlate well with the root-rating data. This is particularly true for the data from the Urbana site. We had excellent growing conditions at that site in 2003, and the Northrup King hybrid in the untreated check compensated for the root damage quite well. At the Monmouth site, where the growing conditions were not quite as good as they were at the Urbana site, the untreated check plots took serious hits in yield, demonstrating the potential for rootworm larvae to cause significant yield losses. As you may recall, the trial at the DeKalb site was planted in late May. At all three sites, the YieldGard Rootworm hybrid had significantly higher yields than its nontransgenic isolines.
These root-rating and yield data demonstrate very well the interaction of rootworm damage, corn hybrids, and environmental conditions. As we learned in a study we conducted in the early 1990s, under certain growing conditions, root damage that might otherwise be considered economic (a root rating >3.0) does not have a dramatic effect on yield. On the other hand, when growing conditions are poor, root damage that might otherwise be considered noneconomic (a root rating <3.0) can result in yield loss. Consequently, neither root ratings nor yield should be considered in isolation.--Kevin Steffey and Mike Gray