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Issue No. 25, Article 4/December 7, 2007

Yield Data from the University of Illinois Standard Corn Rootworm Product Efficacy Trials

We have conducted "standard" rootworm control efficacy trials for many years at University of Illinois research and education centers near DeKalb (northern Illinois), Monmouth (northwestern Illinois), and Urbana (east-central Illinois). In 2006 and 2007, we also conducted a standard corn rootworm product efficacy trial near Perry (the Orr Agricultural Research and Demonstration Center). These standard efficacy trials have included rootworm Bt corn hybrids, granular and liquid soil insecticides, and insecticidal seed treatments. The data generated from these trials include ratings to assess rootworm larval injury and yields. The node-injury ratings for selected treatments tested in three of the trials in 2007 were presented in issue No. 21 (August 17, 2007) of the Bulletin. (Data from the Perry site were not, and they are not presented here because there was little rootworm larval feeding pressure in the untreated checks.) In this article we focus on yield data, first by providing some comments about our trials in 2006 (yield data were not published), then by focusing on 2007 data.

In our 2006 corn rootworm product efficacy trial at Urbana, almost every soil insecticide we tested prevented significant rootworm larval injury, despite very heavy corn rootworm pressure in the untreated check plots (average node-injury rating of 2.95). The mean node-injury rating for the YieldGard RW hybrid (DKC61-68) in the Urbana trial was 0.96, indicating almost one node of roots pruned. Despite this level of rootworm larval injury, however, the YieldGard RW hybrid had the largest yieldby a large marginin the trial (unpublished data). The yield of YieldGard RW corn at Urbana was significantly larger than the yields of all plots (DKC61-72, the non-Bt isoline) treated with soil insecticides, about 56 bushels per acre greater than the average yield of four plots treated with granular soil insecticides. Although this lopsided yield advantage in favor of the YieldGard RW hybrid also was apparent at the Monmouth site, the yield disparity between Bt corn and plots treated with soil insecticides was not apparent at our DeKalb site in 2006.

As the yield data from 2007 attest, mean node-injury ratings may not have been the best predictors of yield once again. Although the mean yields from all plots with rootworm control products were significantly larger than the mean yields from the untreated check plots in DeKalb (Table 1) and Urbana (Table 2), the mean yields and node-injury ratings among rootworm control products did not necessarily line up.

For example, the mean yield for Counter 15G at DeKalb was 236.88 bushels per acre, despite a mean node-injury rating of 1.0. Comparatively, the mean yield for Fortress 2.5G was significantly lower than the mean yield of Counter 15G, despite similar mean node-injury ratings (0.94 and 1.0, respectively). The mean yield of YGVT was 225.53 bushels per acre and the mean yield of HxXTRA (Pioneer) was 216.18, despite comparable mean node-injury ratings of 0.2 and 0.08, respectively. And despite a mean node-injury rating of 0.84 at Urbana, the mean yield for YGVT was 205.17, significantly larger than the mean yield of HxXTRA (Pioneer; 153.79 bushels per acre) with a mean node-injury rating of 0.49.

In some ways, yield data from Monmouth (Table 3) are even more difficult to explain. The level of rootworm larval feeding injury in the untreated check plots was relatively low in 2007, with average of only about one node of roots destroyed. However, yields ranged from 206.8 to 244.81 bushels per acre, with yields in two of the untreated checks (DKC61-73 and Pioneer 33T57) not statistically different from the yields in all but five of the rootworm-protected plotsYGVT + Counter, Fortress 2.5G, YGVT, Aztec 4.67G, and Force CS. The yields of the two HxXTRA products (Mycogen 2T787 and Pioneer 33T59) and the Counter 15G treatment, all of which produced extremely low node-injury ratings, were not statistically different from the yields of the untreated checks.

These and others' findings raise significant questions about the utility of root ratings as the sole determinant of rootworm larval injury and their relationship to yield. We asked the following questions in issue No. 15 (July 6, 2007) of the Bulletin, and they are even more relevant now that the data have been collected and analyzed:

  • Why have large yield differences occurred between rootworm Bt corn and plots treated with soil insecticides when the node-injury ratings were essentially equivalent? In fact, why would rootworm Bt corn outyield isolines treated with soil insecticides even when the rootworm Bt corn hybrid had more rootworm injury?
  • Are we comparing apples with oranges when we use the same rootworm-injury scale for roots treated with soil insecticides and for rootworm Bt corn hybrids?
  • Is the current 0-to-3 node-injury scale the most appropriate one for comparing rootworm injury to corn treated with soil insecticides and rootworm injury to rootworm Bt corn hybrids?
  • Does it make sense to use any rootworm-injury scale to compare the performance of soil insecticides and rootworm Bt corn hybrids?
  • Should we develop a new rootworm injury rating scale for rootworm Bt corn hybrids?
  • Are rootworm-injury rating scales relevant for rootworm Bt corn hybrids?

    We can only initiate discussion with the data generated thus far. However, the relationship between rootworm larval injury and corn yields deserves considerable attention as increasingly more acres are devoted to production of transgenic Bt corn hybrids. Both the 1-to-6 root rating scale (Hills and Peters 1971) and the more recent node-injury scale (Oleson et al. 2005) were developed while soil insecticides were the primary tools being used for control of corn rootworms. Now that we have entered a new era of corn rootworm management, a new standard seems to be justified.

    References

    Hills, T.M., and D.C. Peters. 1971. A method of evaluating postplanting insecticide treatments for control of western corn rootworm larvae. Journal of Economic Entomology 64: 764-765.

    Oleson, J.D., Y.L. Park, T.M. Nowatzki, and J.J. Tollefson. 2005. Node-injury scale to evaluate root injury by corn rootworms (Coleoptera: Chrysomelidae). Journal of Economic Entomology 98: 1-8.

    --Kevin Steffey, Mike Gray, and Ron Estes

    Authors:
    Kevin Steffey
    Mike Gray
    Ron Estes

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