Grain yield of initial Bt corn hybrid introductions to farmers in the northern corn belt

European corn borer (ECB; Ostrinia nubilalis Hubner), is a major pest of corn (Zea Mays L,) in North America. Recently seed companies have begun to offer control of this pest by introducing synthetic genes derived from Bacillus thuringiensis spp, kurstaki (Bt) into the corn genome, Our objectives were to compare the yield of Bt hybrids with adapted high yielding non-Bt hybrids, and to evaluate Bt hybrid yield under economically significant ECB infestation, Experiments were established in the field at three locations in 1995 and one location during 1996, Three groups of corn hybrids were evaluated: transformed hybrids with the Bt gene, closely related "iso-line" hybrids without the Bt gene, and "standard" high yielding hybrids adapted to these locations. ECB infestation treatments consisted of natural infestation, inoculation four times during the growing season, and insecticide application resulting in an ECB "free" treatment. Grain yield of Bt corn hybrids was not affected by ECB, Yield of isoline hybrids was 10% lower than both standard and Bt hybrids regardless of ECB treatment. Yield of Bt hybrids was 4 to 8% greater than standard hybrids when inoculated with ECB, However, yield of Bt hybrids was 8% less than standard hybrids when an insecticide was applied. Yield of initial Bt hybrid introductions was equivalent to or better than standard hybrids, except in environments with low ECB.