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Issue No. 20, Article 2/August 5, 2005

Late-Season Root Protection of YieldGard Hybrids: Evaluating Performance

As some of our readers may recall, by early August 2004, we began to notice greater levels of root pruning and lodging in our YieldGard treatment in corn rootworm evaluation trials located near DeKalb and Urbana. In addition to the results that we reported from our 2004 experiments, some growers in Illinois reported greater levels of injury and lodging in their fields with various YieldGard hybrids. In part, some of these lodging observations from producers resulted from a very widespread and severe storm in mid-July characterized by torrential rains and winds. The late-season root injury we observed in our trials last summer raised many questions about the expression of the Cry3Bb1 protein in YieldGard hybrids later in the season (late July, early August). Did protein levels decline sufficiently by late July and early August to enable corn rootworm larvae to prune roots at potentially damaging levels?

In a peer-reviewed journal article that was published in Crop Science (March 28, 2005), several scientists documented a decline in the expression of the Cry3Bb1 protein from V4 to V9 stages across five YieldGard hybrids. We summarized these findings in an earlier article in the Bulletin. We also reported that corn rootworm development may be delayed when larvae feed on hybrids expressing the Cry3Bb1 protein (Journal of Economic Entomology, April 2005, pp. 534-551). Because of these documented characteristics of some YieldGard hybrids, we indicated that in 2005 we would pay particular attention to levels of late-season root feeding. We also encouraged producers to make similar observations. Thus far, the performance of YieldGard hybrids has looked quite exceptional in many producers' fields across the state based on numerous testimonials. In addition to general field observations, thus far, various YieldGard treatments in our experiments have looked very good this summer despite punishing levels of corn rootworms.

However, once again, we are beginning to notice greater-than-expected late-season pruning in our YieldGard treatment in some of our experiments. We will continue to watch this development closely over the next several weeks. What are the consequences of late-season root pruning? If the pruning is extensive, lodging could reduce yields and complicate the harvest operation. However, if the pruning is sufficiently delayed and enables plants to move through the pollination phase of corn development, late-season pruning may be less detrimental from a physiological perspective. Please alert us to any late-season lodging that occurs unexpectedly in your fields with any of the corn rootworm-control products that you have selected.

Root pruning on a YieldGard rootworm hybrid, University of Illinois DeKalb Research and Education Center, August 2, 2005.

Tuesday afternoon, Mike enjoyed a very warm field day at the University of Illinois Research and Education Center located near Shabbona, Illinois. He took some sweep net samples within a soybean field and found "swarms" of western and northern corn rootworm adults emerging from the net. This is probably the peak egg-laying period for variant western corn rootworms in many soybean fields across central and northern Illinois. Egg laying will most likely occur very deeply within drought cracks this summer. Females will seek out moist soils to lay their eggs. Because of this deeper egg laying, we may have a late hatch next spring, particularly if the spring is cool. Time will tell. We will continue to provide some updates on the unfolding story of late-season corn rootworm injury.--Mike Gray and Kevin Steffey

Kevin Steffey
Mike Gray

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