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Bt and the "Beasts"

Bt-enhanced field corn fends off an insect and disease pest complex, but is the price right?

The second year of a project comparing Bt (Bacillus thuringiensis) and non-Bt corn hybrids confirmed what was learned last year. This naturally occurring insecticide, when added to the genetic material of corn, provides protection for that corn from the insect European corn borer and the disease anthracnose stalk rot. Detectable European corn borer feeding injury was near zero for both of the Bt hybrids planted this year. Anthracnose stalk rot injury was also minimal.

Silage yields were somewhat higher for both of the Bt hybrids than for their non-Bt counterparts, but the highest yield came from a well-adapted non-Bt commercial hybrid. Yield differences were due to factors other than Bt this year. European corn borer populations were so low in 1997 that there was little risk of injury and little call for the built-in protection provided by the Bt.

Counterbalancing the two years of data are two important reasons to either limit or postpone adoption of this form of biological control: cost and the development of resistance. Economic benefit from planting Bt hybrids is not a sure thing. Why? Because the European corn borer is a variable pest. It does not show up every year. Or it shows up in such small numbers--as in 1997--that it isn't a pest worth attending to. The Bt hybrids, on the other hand, invariably cost more than their non-Bt counterparts.

The build-up of resistance is a common concern in the arena of pest management. Often new materials are introduced in a system and are used for a time to ward off pests and then are rendered impotent by the pests' adapting to them. Bt is not immune to this problem.

The data suggest that growers considering the use of Bt hybrids should first compare them to other well-adapted commercial hybrids. If they decide to plant Bt hybrids, they should be aware of recommendations about maintaining portions of their corn fields as non-Bt "refuges," places where corn borers will not come in contact with Bt and will not develop resistance to it.