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Issue No. 21, Article 1/August 17, 2007

Preliminary Root Evaluation Ratings for DeKalb, Monmouth, and Urbana Corn Rootworm Product Efficacy Trials

The root evaluations from our corn rootworm product efficacy experiments have been completed at the University of Illinois research and education centers located near DeKalb, Monmouth, Perry, and Urbana. As occurred last year, the level of root injury at the Orr Research and Demonstration Center was very low, so these data will not be reported. It remains unclear to us why we have not been able to generate adequate densities of corn rootworm larvae at this site. Soil type may play a role. Despite the disappointing results thus far, we will continue to establish corn rootworm efficacy trials at this facility in order to share more localized product efficacy data with our western Illinois clientele.

Root rating results from our other trials were strikingly different in many respects (see Table 1). The levels of root injury in our overall check (DKC 61-73) were 2.18 (slightly more than 2 nodes destroyed), 1.14 (slightly more than 1 node destroyed), and 2.74 (nearly 3 nodes of roots destroyed) in DeKalb, Monmouth, and Urbana, respectively. The root protection provided by the soil insecticides varied considerably across locations. In DeKalb, several of the plots treated with granular soil insecticides had approximately 1 node of roots pruned--for example, plots treated with Aztec 2.1G (0.81), Counter 15G (1.0), and Fortress 2.5G (0.96). We suspect that very dry conditions after planting may have contributed to these performance issues. The soil insecticides were applied during planting on May 3. By May 24 (3 weeks later), only 0.31 inch of rain had fallen on these plots. In July, more than 8 inches of rain had fallen at the DeKalb site. The transgenic Bt corn hybrids provided excellent root protection at DeKalb, with root ratings of 0.08, 0.16, and 0.2 for HxXTRA (Pioneer), HxXTRA (Mycogen), and YieldGard VT, respectively. At the Monmouth site, the Bt corn hybrids and the soil insecticides all provided good root protection; however, the overall level of corn rootworm injury was lower than at the other locations. There were no statistically significant differences in average root ratings among the Bt corn hybrids and insecticide treatments.

At the Urbana site, the location with the most severe rootworm injury, the soil insecticides provided better root protection than two of the Bt corn hybrids, HxXTRA (Mycogen 2T787) and YieldGard VT (DKC 61-69). The mean root ratings for these two Bt corn hybrids were 1.04 and 0.84, respectively, and were not statistically different from each other. HxXTRA (Pioneer 33T59) had a mean root injury rating of 0.49 (1/2 node of roots pruned). This root rating mean was not statistically different from the means of the soil insecticide treatments. The two HxXTRA treatment means (0.49 and 1.04) were statistically different from each other.

Pruning to rootworm Bt corn hybrid (HxXTRA Mycogen 2T787) caused by rootworm larvae, Urbana, Illinois, July 9, 2007 (University of Illinois).

Explanations for the variation in product performance of Bt corn hybrids, which have been engineered to provide corn rootworm control, should be explored more fully. Producers typically expect equivalent levels of root protection among Bt corn rootworm hybrids. Our data suggest that these expectations may not always be fulfilled, even though transgenic hybrids with the same Bt event have been used. We have previously shown that root protection provided by YieldGard Rootworm (MON 863, Cry3Bb1) corn varies among hybrids (M.E. Gray, K.L. Steffey, R.E. Estes, and J.B. Schroeder. 2007. Responses of transgenic maize hybrids to variant western corn rootworm larval injury. Journal of Applied Entomology 131[6]: 386-390). Why was root protection offered by some Bt corn hybrids not on par with the root protection provided by soil insecticides at the Urbana site? Why does this phenomenon occur most frequently at our Urbana location? The answer remains elusive. However, we have previously hypothesized that the variant western corn rootworm may be more injurious to Bt corn rootworm hybrids than the nonvariant population of this species. This hypothesis requires further testing.

In Table 2, we provide the mean root ratings from our second evaluations of selected treatments at DeKalb, Monmouth, and Urbana. In general, the differences in root ratings from the first and second evaluations were not very striking. Recall that western corn rootworm emergence was very early this season (mid-June for central Illinois), and by mid-July, we may nearly have reached the maximum level of root pruning that was going to occur. In general, the lack of notable increases in injury among all treatments supports this idea.

In addition to the notable root injury that occurred to the Bt corn hybrids in our trial at Urbana, some lodging and severe pruning was observed by Joe Spencer, an entomologist with the Illinois Natural History Survey, on a Bt corn hybrid (DeKalb 63-74, YieldGard Plus) in his plot north of Urbana. The plot was planted on May 1. Unlike our product efficacy experiments, for which we use trap crops (late-planted corn interplanted with pumpkins) to achieve high densities of corn rootworm larvae, Joe's experiment was with second-year corn. The level of root pruning on some of his plants was excessive--well over 2 nodes of roots destroyed on lodged plants. Again, why did this excessive injury occur? How often is this level of pruning being overlooked in producers' fields? We encourage producers to be more vigilant in their scouting efforts to evaluate Bt corn hybrid performance. More times than not, we receive reports of excessive lodging when producers are harvesting their corn. By then, it's too late to accurately assess injury caused by rootworm larvae.

Lodged Bt rootworm corn plants (DeKalb 63-74, YieldGard Plus) in Joe Spencer's plots near Urbana, Illinois, July 26, 2007 (University of Illinois).

Excessive root pruning on Bt rootworm corn plants (DeKalb 63-74, YieldGard Plus) in Joe Spencer's plots near Urbana, Illinois, July 26, 2007 (University of Illinois).

We also ask that producers not become complacent and assume that Bt corn rootworm hybrids will always outperform soil insecticides when it comes to root protection. This is simply not the case. We recognize that the bottom line is yield, and we intend to report yield data from our trials later this fall. If you are experiencing performance issues (excessive root pruning and/or severe lodging) and you've used a Bt corn rootworm hybrid, let us know. For sure, the "bulletproof" image often portrayed of this technology for corn rootworm control is somewhat misleading.

We thank Ron Estes, manager of the Insect Management and Insecticide Evaluation Program, for his dedicated efforts in ensuring that these data were provided in a timely manner for our University of Illinois Extension clientele.--Mike Gray and Kevin Steffey

Mike Gray
Kevin Steffey

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