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

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 C-13 to C-12 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, <50% of moths captured in August through October have isotope ratios indicative of larval feeding on C-3 plants. In one October sample, 100% of the moths originated from C-4 hosts even though C-4 crops were harvested at least 1 mo earlier, and no common wild C-4 hosts were available. These findings support other research indicating that many late-season H. zea moths captured in Louisiana and Texas are migrants whose larvae developed on corn in more northern locations. Our isotope data on moths collected in Texas early in the season indicate that the majority of overwintering H. zea do not originate from cotton-feeding larvae and may be migrants from Mexico. Non-Bt corn in Mexico and the U.S. corn belt appears to serve as an important refuge for H. zea.