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Issue No. 4, Article 4/April 16, 2004

What Is Bt Corn?

This article is not intended to be academic, with a detailed discussion about Bacillus thuringiensis (Bt), the soil-dwelling bacterium whose genes are used to create transgenic corn that expresses toxic proteins to kill certain insects. Rather, I want to clarify our use of the term "Bt corn" so that confusion can minimized.

The term "Bt corn" originated when transgenic corn for control of Lepidoptera pests (primarily European and southwestern corn borers) was being developed. The first Bt corn (from Ciba-Geigy) was registered in 1996, and YieldGard hybrids (from Monsanto Company) were relatively widely available in 1997. And for several years, YieldGard hybrids were the only Bt corn hybrids on the market. The term "non-Bt corn" originated at the same time, and for a while, non-Bt corn meant any hybrid that was not a YieldGard hybrid. The insect resistance management requirements included 20% non-Bt corn for the refuge.

However, the term "Bt corn" now is generic and can be used to describe at least four types of corn hybrids--Herculex 1 Insect Protection (from Dow AgroSciences), YieldGard Corn Borer, YieldGard Plus, and YieldGard Rootworm. Three of these products express different proteins from B. thuringiensis--Cry1F (Herculex), Cry1Ab (YieldGard Corn Borer), and Cry3Bb1 (YieldGard Rootworm). YieldGard Plus expresses both the Cry1Ab and Cry3Bb1 proteins for control of corn borers and rootworm, respectively. To distinguish among their own products, Monsanto added "descriptors" behind their trademarked YieldGard. Consequently, the terms "Bt corn" and "non-Bt corn" are not as descriptive as they used to be.

Using the terms "Bt corn" and "non-Bt corn" with respect to insect resistance management could cause some confusion. So it is important to make certain that we are more specific about these transgenic products and their insect resistance management requirements. For example, if a corn producer plants a YieldGard Rootworm hybrid (expressing the Cry3Bb1 protein), indicating that a non-Bt hybrid must be planted as a refuge is not entirely correct. The producer should plant a non-Cry3Bb1-expressing hybrid in the refuge, but the refuge hybrid can be a Herculex (Cry1F) or YieldGard Corn Borer (Cry1Ab) hybrid, and both of these are Bt corn. The refuge must be specific to the target insect of the Bt corn being planted.

We recently received the following question: "Can I plant half of a field with a YieldGard Rootworm hybrid and plant the other half of the field with a YieldGard Corn Borer hybrid and still comply with insect resistance management requirements?" The answer is yes. In fact, this individual is planting a 50% refuge for each of his types of Bt corn. The YieldGard Rootworm hybrid will serve as the refuge for the YieldGard Corn Borer hybrid, and vice versa.

The only type of Bt corn that does not fit the aforementioned scenario is YieldGard Plus, which expresses proteins for control of both corn borers and corn rootworms. If a grower plants YieldGard Plus, the refuge for insect resistance management must truly be a non-Bt hybrid. Also, we can't consider using Herculex and YieldGard Corn Borer hybrids as refuges for each other. Although the proteins are different, the target insects for both types of hybrids are the same. Remember, the concept of a refuge is to encourage the survival of insects that have not been exposed to Bt.

To Bt or not to Bt, that is the question. (Sorry for the sad play on words.) Understand the concept, principles, and requirements for insect resistance management, and use the Bt corn products accordingly. Complying with the requirements for insect resistance management should enable us to use transgenic Bt corn well into the future.--Kevin Steffey

Kevin Steffey

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