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Issue No. 24, Article 4/November 3, 2006

Syngenta's Agrisure RW Event Encounters Significant Challenge in University of Illinois Experiment, Urbana

In 2005, Bt corn hybrids were planted on 35% of the U.S. corn acreage (Fernandez-Cornejo and Caswell 2006). The majority of these transgenic Bt corn hybrids were aimed at the European corn borer. However, interest in expanding the Bt corn acreage is escalating as the variant western corn rootworm extends its range and corn continues to be viewed as a more profitable alternative in our corn- and soybean-dominated agroecosystem. In 2003, Bt corn hybrids (MON 863, Cry3Bb1, YieldGard RW) targeted at corn rootworms were commercialized for the first time. In 2006, Herculex RW hybrids (DAS-59122-7, Cry34Ab1/Cry35Ab1) were commercialized. Both of these transgenic corn rootworm events are designed to provide root protection against corn rootworm larvae.

In 2007, Syngenta intends to commercialize a new transgenic corn rootworm event (MIR604, mCry3Aa, Agrisure RW). The U.S. Environmental Protection Agency granted registration approval for this event (MIR604) on October 4, 2006. This new corn rootworm Bt event will be available as a single trait in some hybrids and also in stacks that feature glyphosate tolerance (Agrisure GT). Hybrids with this new event will be marketed and sold under many familiar trade names, including NK Brand, Garst, and Golden Harvest seeds. The number of transgenic corn acres, especially those acres devoted to stacked hybrids (combinations of Bt events and herbicide tolerance or resistance traits), is expected to increase quite significantly over the next several years.

It is imperative that research on the efficacy of these transgenic corn rootworm hybrids be continued by scientists at land-grant universities in multiple locations across the Corn Belt. As we have demonstrated during this technology's infancy, not all transgenic corn rootworm hybrids (even those with the same event) offer the same level of root protection against corn rootworms. Producers should be equipped with as much information as possible in order to make the most informed pest management decisions. As the number of Bt corn acres increases, so, too, will the selection pressure for development of resistance to Bt by multiple insect pests of corn. This underscores the importance of strict adherence to resistance management protocols that have been developed for these pest management tools.


Lodging in MIR604 treatment, Urbana, Illinois, 2006.

Table 2 provides results from the MIR604 experiment we conducted during the 2006 growing season. The planting date for this trial was May 23, 2006. This date is obviously very late by producers' current standards, and interpretation of these data should be viewed accordingly. Had this trial been planted in early April, we wonder what level of root injury would have occurred in these treatments.

Corn rootworm pressure in the untreated check was intense. By July 17, three nodes of roots in the check had been completely destroyed. The mean node-injury ratings of the following treatments were not significantly different at the 0.05 probability level: MIR604 (1.04), MIR604 + Cruiser 5FS (1.33), Force 3G (0.94), and Poncho 1250 (1.05). Both Aztec 2.1G and Lorsban 15G provided greater root protection (mean node-injury ratings of 0.26 and 0.51, respectively) than the MIR604 treatments.

Why was the level of root pruning (more than one node of roots destroyed) so severe by mid-July for this new transgenic event? We have hypothesized previously that the variant western corn rootworm may be able to cause more injury to corn roots than the nonvariant population (worm for worm). At this point, we have circumstantial evidence that this hypothesis is worthy of more careful experimentation.


Corn rootworm pruning on MIR604 treatment, Urbana, Illinois, 2006.

Other interesting observations in our experiments with transgenic rootworm hybrids in 2006 included the very unusual root morphology of our Herculex RW hybrids (Pioneer 34A18, Mycogen 2G777, Mycogen 2P788 hybrids). In many instances the primary roots were very short and densely covered with secondary roots. The injured roots had a characteristic "bottlebrush" appearance. On many root systems, it appeared as if primary roots had been "pinched" off due to intense corn rootworm feeding activity. Evaluating these roots for rootworm larval injury was difficult primarily due to the profusion of secondary roots that were found near corn rootworm injury sites. The root systems had readily visible corn rootworm scarring and some pruning as we reported in the Bulletin (issue no. 21, August 18, 2006). In fact, at the Urbana and Monmouth locations, approximately 1/2 node of roots was pruned on the Herculex XTRA + Poncho 250 (Pioneer 34A18) treatment.


Herculex RW treatment corn rootworm injury, Urbana, Illinois, 2006.

Our experimental trials continue to reinforce that there are no silver bullets for corn rootworm larvae. These new transgenic corn rootworm hybrids offer tremendous potential; however, we will need to continually evaluate the various corn rootworm events at multiple locations. Even with the same transgenic event, we are beginning to learn that corn rootworm protection across hybrids is not always the same. We look forward to receiving your observations regarding the performance of your chosen corn rootworm transgenic hybrid(s) this past season.--Mike Gray and Kevin Steffey

References

Fernandez-Cornejo, J., and M. Caswell. 2006. The first decade of genetically engineered crops in the United States. USDA Economic Research Service, Economic Information Bulletin No. 11.

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.

Authors:
Kevin Steffey
Mike Gray

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