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

Author
item GOULD, FRED - NC STATE UNIV
item BLAIR, NEAL - NC STATE UNIV
item REID, M - NC STATE UNIV
item RENNIE, T - NC STATE UNIV
item Lopez, Juan De Dios
item MICINSKI, S - LA AG EXP STATION

Submitted to: Proceedings of the National Academy of Sciences (PNAS)
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/26/2002
Publication Date: 12/24/2002
Citation: Gould, F., Blair, N.E., Reid, M., Lopez, J., Micinski, S. 2002. Bacillus thuringiensis-toxin resistance management: Stable isotope assessment of alternate host use by Helicoverpa zea. Proceedings of the National Academy of Sciences. 99:16581-16586.

Interpretive Summary: Federal regulations require producers to plant refuges of conventional cotton when growing genetically modified cotton cultivars that produce insecticidal proteins called Bt toxins. These refuges are expected to slow the development of resistance in pest populations by producing susceptible pest individuals that subsequently mate with any resistant individuals that develop on Bt cotton. Cotton pests such as bollworms feed on non-cotton plants that may serve as alternate refuges, but the proportion that do so is not known. Our research showed that it is possible to distinguish between bollworm moths that developed on cotton and those that developed on corn based on the ratio of two carbon isotopes detected in their wings. Results indicated that a high proportion of male bollworm moths captured in LA and TX during mid-summer developed from corn-like plants, but the proportion that developed on cotton-like plants during early and late season (when corn was not available) was not as high as expected. Apparently, moths are migrating into LA and TX from corn-growing areas in Mexico during the early season and from more northern corn-growing areas in the U.S. during the late season. Thus, non-cotton hosts in Mexico and the Corn Belt apparently are important natural refuges for bollworms and will remain as such only if Bt corn is not planted extensively in these areas. It is important that producers realize the complexity that is involved in the insect resistance management program to ensure continued effectiveness of Bt technology.

Technical Abstract: Data have been lacking on the proportion of H. zea larvae that develop on non-cotton host plants that can serve as a refuge from selection pressure for adaptation to Bt-cotton. We found that individual H. zea moths that develop as larvae on cotton and other plants with C3 physiology have a different ratio of 13C to 12C than moths that develop on plants with C4 physiology, such as corn. We used this finding in determining the minimum percentage of moths that developed on non-cotton hosts in 2 cotton-growing areas. Our results indicate that local corn can serve as a refuge for H. zea in mid-summer. 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 C3 plants. Typically, less than 50% of moths captured in August through October have isotope ratios indicative of larval feeding on C3 plants. In one October sample, 100% of the moths originated from C4 hosts even though C4 crops were harvested at least one month earlier, and no common wild C4 hosts were available. These findings support other research indicating that many late-season H. zea moths captured in LA and TX are migrants whose larvae developed on corn in more northern locations. Our isotope data on moths collected in TX 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 US cornbelt, appears to serve as an important refuge for H. zea.