Evaluation of Transgenic, Bt -Containing Corn Hybrids

Abstract

European corn borer [ECB; Ostrinia nubilalis (Hübner]) is a major economic pest of corn (Zea mays L.) throughout the Corn Belt. Transgenic corn hybrids containing a modified Bacillus thuringiensis (Bt) gene were evaluated at four Corn Belt environments as a novel method of managing ECB. Manual infestation was done to insure the presence of ECB, and Bt-based insecticides were used to control natural infestations. First generation ECB damage, estimated by leaf damage ratings, and second generation ECB damage, estimated by amount of stalk tunneling, were reduced or eliminated with the use of the Bt hybrids. Grain yield, stalk lodging, and test weight of the transgenic hybrids were not affected by first or second generation ECB infestation. Grain yields of six nontransgenic hybrids were not affected by first generation ECB infestation, but second generation ECB infestation damage, even though relatively low in the environments tested, reduced grain yield in these hybrids by 4.0 to 6.6%, slightly increased stalk lodging, and slightly decreased test weight. In the absence of ECB pressure, the transgenic hybrids performed the same as their nontransgenic counterparts. It is clear that the incorporation of the Bt gene into corn hybrids provides a high level of protection against ECB, with little if any effect on agronomic performance. Research Question European corn borer (ECB) is one of the major insect pests of corn in much of the Corn Belt and causes significant yield losses in many years. Transgenic corn hybrids containing a gene from the bacterium Bacillus thuringiensis (Bt), coding for the production of an insecticidal protein toxic to ECB, have been produced and may protect corn plants from damage and yield loss. This study measured the amount of yield protection and decreased damage afforded by two Bt hybrids under field conditions. Literature Summary Yield losses in hybrid corn due to ECB infestation are often estimated at 5.0%/borer per plant for first generation and 3.0%/borer per plant for second generation. Until recently, the major means of managing ECB was through the use of chemical insecticides, although control for first and second generation infestations may only be 50 to 70% and 75%, respectively. One of the strategies used in integrated pest management (IPM) is plant resistance, and Bt hybrids can be used in an IPM strategy to control ECB. Study Description Field trials were planted in 1994 at four locations: Crawfordsville, IN; St. Joseph, IL; Clinton, IL; and Washington, IA. Eight hybrids were planted: four with varying resistance to first and second generation ECB, two Bt hybrids, and their isogenic counterparts. Laboratory-reared, neonate ECB larvae were applied according to four infestation schemes: first generation only, second generation only, first and second generation, and no infestation. Those plots not receiving an infestation treatment were treated with a Bt-based insecticide to control natural infestation of ECB. Leaf ratings (from 1 = no damage to a maximum of 9) were taken to estimate damage from first generation ECB and inches of stalk tunneling were measured to determine damage from second generation ECB. Grain yield, stalk lodging, ear droppage, test weight, and grain moisture were measured. Applied Questions Did the Bt hybrids protect corn against damage from first and second generation ECB infestation? Both first generation leaf ratings and amount of stalk tunneling were greatly reduced in the Bt hybrids. Leaf rating scores of the Bt hybrids averaged 1.05 (1.0 = no damage), while hybrids lacking the Bt gene had an average rating of about 3.5, indicating moderate injury. Stalk tunneling was also much decreased in the Bt hybrids: nontransgenic hybrids receiving second generation ECB infestation averaged about 2 in. of tunneling/plant, and Bt hybrids with the same infestation treatment had only 2 to 3% as much tunneling. Did the Bt hybrids provide yield protection from ECB infestation? Yes. Yields of the Bt hybrids were not affected by any of the ECB infestation treatments. This finding agrees with the ECB damage results, and indicated that the addition of the Bt gene into the plant genome enabled the plants to avoid yield loss due to ECB infestation. Yields of the nontransgenic hybrids were not significantly affected by first generation ECB infestation, but were reduced 6.3% by second generation infestation. Did the addition of the Bt gene affect yield or other agronomic performance by hybrids? No. Comparison of the hybrids with Bt to their counterparts without the gene showed that, when there was no ECB present, yield, test weight, lodging, and ear droppage were not different. This is a small sample of hybrids, however, and these results should not be used to predict what will be the case with all transgenic hybrids. Are there economic implications regarding the use of Bt? Insect pressure was relatively low at all of the locations at which this work was conducted. Even so, transgenic hybrids yielded about 11 bu/acre more than their nontransgenic counterparts. This yield increase is enough to pay for the technology, which “costs ” about 5 bu/acre at present prices. Still, one of the nontransgenic hybrids in the study yielded nearly as much under ECB pressure as the transgenic hybrids did, even though it sustained injury. The decision about using Bt hybrids clearly has to be studied carefully, taking into account not only the expected size of loss to be prevented, but also how all available hybrids perform under ECB pressure.