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. Research QuestionLosses due to European corn borer (ECB) exceed $1 billion in the USA annually. Feeding disrupts physiological processes in the plant, leading to lower plant yield as well as plant lodging and ear drop. In 1993, Ciba-Geigy reported transformation of corn inbreds with genes derived from Bacillus thuringiensis spp. kurstaki (Bt). Since then seed companies have incorporated Bt genes into elite corn hybrids. Little information is available on the grain yield of Bt hybrids in relation to current commercially grown hybrids. Our objectives were to compare Bt hybrid yield with adapted high yielding non-Bt hybrids, and to evaluate Bt control of ECB under economically significant infestations. Literature SummaryControl of ECB has traditionally been achieved through genetic, chemical insecticides and biological methods. Plant breeders have concentrated on feeding resistance to plant tissue, DIMBOA concentration, and general standability. Effective use of insecticides to control ECB requires scouting and knowledge of treatment thresholds, and immediate application in a relatively short period of time during crop development. Biological methods for controlling ECB include use of various Bt sprays and granules and the release of natural insect enemies. A new tool for controlling ECB is the successful development of corn transformed with genes derived from Bt. Study DescriptionExperiments were established at three locations in 1995 and one in 1996. Three groups of corn hybrids were evaluated: Bt hybrids that had been transformed to contain the Bt gene, closely related “isoline” hybrids that did not have the Bt gene, and “standard”hybrids that have historically yielded well at these locations. ECB infestation treatments consisted of inoculation four times during the growing season with 100 ECB eggs or larvae per plant, natural ECB infestation, and insecticide application protecting plots from infestation by feral moths. Applied QuestionsHow does the yield of initial Bt hybrid introductions compare with adapted commercial hybrids? Yield of Bt hybrids was unaffected by ECB infestation. Under natural infestation, Bt hybrids yielded similarly to standard hybrids (Fig. 1). Inoculation of standard hybrids with ECB eggs or larvae decreased yield 4% to 154 bu/acre compared with Bt hybrids. Application of an insecticide to the standard hybrids increased yield to 173 bu/acre, an 8% increase over that of Bt hybrids grown under natural infestation. Fig. 1Grain yield of standard and Bt corn hybrids infested with ECB during 1996 at Arlington, Wl. When and where should Bt corn be used? The current cost per acre for Bt hybrid seed is equivalent to the cost of applying an insecticide, however, growers have to consider the long-term potential for economic infestations of ECB. The benefit of Bt hybrids is realized during seasons or in areas of consistently high ECB damage. There are good reasons not to grow Bt hybrids in areas where they are not of potential benefit. First the cost of the seed premium will not economically justify the use of Bt hybrids. Since standard non-Bt hybrids yielded as well as or better than Bt hybrids in the absence of infestation, Bt seed may not be worth the additional cost where there is a low probability of infestation. Second, the widespread use of Bt hybrids raises concerns about ECB developing resistance to Bt. How should Bt corn hybrids be ‘selected’ by farmers? Bt hybrids do not necessarily have the most yield potential in every location. Our results indicate that yields of initial Bt hybrid introductions were lower than contemporary elite adapted hybrids in the absence of ECB. Breeding programs will most probably improve the yield of Bt hybrids as new Bt genes are incorporated into the breeding cycles of elite inbreds earlier in the breeding cycle. Hybrid selection by farmers should still be based upon consistent yield performance over an area of wide adaptation and many management practices. RecommendationBt hybrids show significant potential to protect yield under heavy ECB damage and are recommended for use in situations where such infestations may occur. Economic considerations and concerns about the development of resistance dictate that the hybrids be used only in situations where high yield loss is expected, and used in accordance with strategies designed to deter resistance development. We do not recommend use of Bt hybrids in situations where ECB damage is not expected. Unnecessary use of these hybrids will foster development of resistance shortening the useful life of this product, an important production tool. Seed companies, educators and farmers must carefully communicate and deploy strategies to minimize resistance development so that the Bt technology is successful over a long time.