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Transgenic Corn Rootworm Hybrids: A Promising IPM Tool, Yet Important Concerns Linger

March 20, 2003

On February 25, 2003, Monsanto Company announced it had received a registration from the U.S. Environmental Protection Agency for the sale of its YieldGard Rootworm-protected corn. The USDA and the Food and Drug Administration had previously finished their reviews of this transgenic event (MON 863, Cry3Bb1). This completed the regulatory review process for both the United States and Japan. The current registration is for 1 year; however, Monsanto Company anticipates extending this registration for several more years. For this planting season, YieldGard Rootworm hybrids will be available through Monsanto's seed businesses (DeKalb and Asgrow) and also will include licensed independent seed companies. The following twelve North Central states have granted approvals for the sale of YieldGard Rootworm hybrids: Illinois (2/26/03), Indiana (2/25/03), Iowa (3/03/03), Kansas (3/04/03), Kentucky (2/26/03), Michigan (3/07/03), Missouri (2/26/03), Nebraska (2/26/03), North Dakota (3/10/03), Ohio (3/06/03), South Dakota (2/26/03), and Wisconsin (3/07/03).

Similarly to other unapproved transgenic crops in Europe, YieldGard Rootworm grain and/or processed products will need to be channeled away from other grain intended for shipment to customers in Europe. Serious concerns have been raised by several key farm organizations regarding the potential erosion of the important U.S. corn gluten export market because of continuing European concerns about this technology. On November 11, 2002, the Illinois Corn Growers Association devoted a full one-page notice (FarmWeek) to Illinois corn producers explaining potential producer benefits and losses regarding the adoption of YieldGard Rootworm hybrids. In this article, the Illinois Corn Growers Association offered a sobering statement: "Due to the high percentage of Illinois corn going into the corn wet mill industry, it is extremely likely the corn refining industry will not be able to deliver a corn gluten feed product that meets the 1% tolerance of European customers." They also pointed out that Illinois and Iowa account for roughly 67% of the corn gluten feed in the United States. Because of the importance of the corn gluten export market for U.S. corn producers (particularly Illinois and Iowa producers), the Illinois Farm Bureau Board of Directors (November 2002) has urged producers not to plant corn hybrids unless they are approved for the European Union or other major markets.

In spite of these important concerns, a Scientific Advisory Panel that met in Arlington, Virginia, during August 28 and 29, 2002, to offer advice and counsel to the U.S. EPA as part of the registration process as outlined by the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA), outlined several important benefits to producers who choose to use YieldGard Rootworm hybrids: "(1) equivalent to or better than soil insecticides in terms of plant damage; (2) reduced applicator, handler, and farmer worker exposure to insecticides; (3) a narrow spectrum of activity could possibly eliminate or greatly reduce the environmental concerns generated by broader spectrum insecticides; (4) the technology is easy to use and does not delay planting; (5) the technology does not require special application equipment, the need for calibration, or the disposal/return of containers; and (5) performance consistency is improved since each plant is protected and this protection is relatively unaffected by weather." The panel was divided regarding the most appropriate resistance management plan to recommend to the U.S. EPA for implementation by American producers. Some panel members urged the EPA to recommend a 50% refuge; other members believed that a 20% refuge was more reasonable and would accomplish the goal of prolonging the usefulness of this technology. The North Central Region Technical Research Committee on Corn Rootworms and Soil Insects (NCR-46) recommended to the EPA that a 20% refuge should be utilized.

Because of important biological differences between corn rootworms (mating is much more localized, less mixing of adults from different fields) and European corn borers, the resistance management plan for YieldGard Rootworm hybrids has some unique features and includes the following elements: (1) growers will be required to sign stewardship agreements if they purchase YieldGard Rootworm hybrids; (2) growers will be required to plant a structured refuge of at least 20% non-Cry3Bb1 (MON 863) Bt corn; (3) the refuge may be treated with insecticides to control corn rootworm larvae; (4) refuge acres should be planted as blocks adjacent to MON 863 cornfields or as in-field strips; (5) refuges planted as strips must be at least six rows wide, preferably 12 consecutive rows wide; and (6) insecticides labeled for control of corn rootworm adults cannot be applied while adults are present in the refuge unless the YieldGard Rootworm field is treated in a similar manner. This last guideline (number 6) will no doubt be debated among entomologists regarding its "fit" within the IPM paradigm.

Growers who plant YieldGard Rootworm hybrids should anticipate finding adult corn rootworms in their YieldGard Rootworm fields. These adults will include immigrants as well as those that have emerged directly from YieldGard Rootworm fields. Results from University of Illinois experiments that were conducted in 2001 indicated that more than 27,000 western corn rootworm adults (males and females) per acre were capable of emerging from a MON 863 hybrid. Finding this many western corn rootworm adults may surprise some growers who are familiar with the highly lethal effect of Bt hybrids used for European corn borers. Although the YieldGard Rootworm hybrids do protect root systems very effectively against larval injury, they are low to moderate in their expression of the Cry3Bb1 protein. In fact, the Scientific Advisory Panel that met to discuss this technology made the following observation: ". . . the use of SS (homozygous susceptible) survival rates was sufficient to demonstrate that MON 863 is not high dose, because SS survival is so much higher than that expected at 25H the LC99 (lethal concentration to 99% of a population)." Registration of future corn rootworm events may be higher dose, and entomologists, ecologists, and modelers will have to decide if one resistance management plan "fits all" for this insect pest.

All YieldGard Rootworm hybrids will be treated with Gaucho (imidacloprid, 0.16 milligrams of active ingredient per kernel) in an effort to achieve control of some secondary insect pests such as wireworms and seedcorn maggots. With respect to the use of soil insecticides and seed treatments, the panel offered the following cautionary statement: "It was the consensus of the Panel that soil insecticides and seed treatments targeted toward corn rootworms could be used in the refuge if significant numbers of adult beetles are still produced. This is the case with currently registered soil insecticides. However, if a highly efficacious insecticide that prevented significant adult emergence were to be used, this could have a major detrimental effect on IRM." We intend to conduct investigations over time regarding the influence of seed treatments on adult emergence patterns of western and northern corn rootworms. For additional information on the Scientific Advisory Panel recommendations regarding the use of transgenic hybrids for corn rootworms, please visit the EPA web site.

Because the great majority of producers have made their seed selection decisions and the availability of YieldGard Rootworm hybrids will be limited in 2003, soil insecticide use will remain very high on continuous and rotated corn acres in many areas of the Corn Belt. With crop rotation not performing adequately as a pest management tool for western corn rootworms in the eastern Corn Belt and resistance to methyl-parathion and carbaryl confirmed in Nebraska, producers are eager to utilize a new pest management tool for this significant insect pest. We suspect that interest in this transgenic technology will continue to grow. Unlike the use of transgenic insecticidal cultivars for European corn borer management, the use of transgenic hybrids for corn rootworms could work in concert with existing scouting programs and established economic thresholds. By monitoring their fields for corn rootworm adults in late summer, farmers could base their decision to use transgenic rootworm hybrids the following spring on scouting input and knowledge of thresholds. Crop consultants and other professionals in the agribusiness sector could take a very active role in this decision-making process.--Mike Gray and Kevin Steffey

Author: Mike Gray Kevin Steffey

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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