Efficacy and risk efficiency of sweet corn hybrids expressing a Bacillus thuringiensis toxin for Lepidopteran pest management in the Midwestern US

Field studies were done in 1997 and 1998 to assess the efficacy of sweet corn hybrids expressing the Bacillus thuringiensis (Bt) toxin, Cry 1Ab, for control of Ostrinia nubilalis (Hübner), Helicoverpa zea (Boddie), and Spodoptera frugiperda (J.E. Smith). Trials were performed over multiple planting dates and locations in Illinois, Minnesota, and Wisconsin, USA. The Bt hybrids tested are based on the BT-11 event (Novartis Seeds, Inc.) and express the Bt toxin in green and reproductive tissues. In both years, except when pest densities were low, Bt hybrids provided significant reductions in larval densities for each pest species, often 100% control for O. nubilalis, when compared to non-Bt isolines. This resulted in significant increases in percent marketable ears, which ranged from 70-100% and 90-100% marketability for fresh-market sale and processing, respectively. Expected utility analysis, based on all data sets, was conducted by multiplying each marketability outcome by its frequency of occurrence and averaged for each produce type, either fresh-market or processing. Expected utility results indicated that the Bt hybrids provided 75-99% and 94-99% marketable ears for fresh-market and processing, respectively. By contrast, the non-Bt hybrids provided only 14-34% and 41-56% expected marketable ears for fresh-market and processing, respectively. Moreover, variability (risk) in marketability, when compared with the non-Bt isolines, was lower for the Bt hybrids. Coefficients of variation (CV), for all three states combined, averaged 1.17 and 0.84 for non-Bt fresh-market and processing, respectively; however, CVs for Bt fresh-market and processing were only 0.17 and 0.05, respectively. These results suggest that Bt hybrids can provide consistent performance, regardless of the pest complex, pest density, or geographic location in the Midwestern US. Additional analyses, including comparisons with insecticide control, and the costs associated with the use of insecticides versus Bt sweet corn, are necessary to assess the full benefit of using Bt sweet corn.