Bacillus thuringiensis-toxin resistance management: Stable isotope assessment of alternate host use by Helicoverpazea

Data have been lacking on the proportion of Helicovera zea larvae that develop on noncotton host plants that can serve as a refuge from selection pressure for adaptation to transgenic cotton varieties that produce a toxin from the bacterium Bacillus thuringiensis . We found that individual H. zea moths that develop as larvae on cotton and other plants with C 3 physiology have a different ratio of 13 C to 12 C than moths that develop on plants with C 4 physiology, such as corn. We used this finding in determining the minimum percentage of moths that developed on noncotton hosts in two cotton-growing areas. Our results indicate that local corn can serve as a refuge for H. zea in midsummer. Our results contrast dramatically with the prevailing hypothesis that the large majority of late-season moths are produced from larvae feeding on cotton, soybean, and other C 3 plants. Typically,