Scientists find new way to assess where cotton-killing pests develop

December 03, 2002

In a finding that could have broad implications for farmers' ability to stop pests from decimating cotton crops, scientists from North Carolina State University and agricultural research stations in the Cotton Belt have developed a new technique to determine where the larvae of certain agricultural pests develop.

The study, which looks at the characteristics of the moths that the larvae turn into, shows that a large majority of late-season moths in the Cotton Belt - specifically Helicoverpa zea, a major pest of cotton commonly known as the cotton bollworm - are not developing on cotton and soybean plants, as the prevailing theory suggests, but are developing instead on plants like corn.

The research suggests that this non-toxic corn, therefore, provides a major refuge for H. zea moths, and as such is critical to halting the evolution of insecticide-immune pests, perhaps more so than existing small cotton refuges.

The scientists published their work in Proceedings of the National Academy of Sciences.

Although it seems counterintuitive, non-toxic refuges are essential to controlling pests like the cotton bollworm because the pests that come from these refuges have little to no immunity to toxins. It is estimated that about 80 to 95 percent of cotton bollworms are killed by transgenic cotton plants that produce Bt toxin - derived from the bacterium Bacillus thuringiensis - leaving a fair number of potentially immune caterpillars moving about. When these immune pests mate with pests from non-toxic plants, offspring are not immune to the Bt toxin, and are likely susceptible to die a Bt toxin-induced death.

The scientists - including NC State's Dr. Fred Gould, William Neal Reynolds Professor of entomology, and Dr. Neal Blair, professor of marine, earth and atmospheric sciences; representatives from the USDA's Southern Crops Research Lab in College Station, Texas, and the Louisiana Agricultural Experiment Station in Bossier City, La.; and two NC State students -used a novel technique called stable isotope assessment to gauge the origination of moths in late summer. The work was supported by the USDA Biotechnology Risk Assessment Program and the W.M. Keck Center for Behavioral Biology.

The scientists compared the ratios of carbon isotopes 13C and 12C in moths captured over three- and four-year periods from August to October in areas of Louisiana and Texas. These so-called stable isotopes are present in every living organism and in the air. Plants with a certain type of photosynthesis - those with C3 physiology, like cotton - are more depleted in 13C relative to 12C than plants with C4 physiology, like corn. Thus, looking at these ratios in moth wing tissue can provide clues to where the moths grew up, the researchers assert.

And knowing where the moths develop gives researchers clues to how refuges - plants that are not treated with insecticides - are working to put the brakes on the evolution of pests that are genetically resistant to insecticides.

Currently, about 60 percent of cotton grown in the Cotton Belt contains Bt toxin. The Environmental Protection Agency allows the planting of 50 percent Bt corn in cotton-growing areas, as opposed to 80 percent Bt corn in regions where cotton is not grown, Gould says. This policy assumes that H. zea moths migrate from the Cotton Belt to northern Corn Belt states in the summer, but do not return in the fall.

But, since less than 50 percent of late-season moths captured in the study were fed as larvae on cotton, they may be migrating from the Corn Belt, Gould says. This means the traditional assumptions about late-season H. zea moths - that they grow up on cotton, migrate to northern Corn Belt states and then die - seem to be wrong.

"Corn is most likely serving as the predominant alternate C4 host for H. zea," the paper asserts. "Currently, less than 25 percent of U.S. corn produces Bt toxin. If, in the future, most field corn planted in the northern and southern United States remains in non-Bt producing varieties, it could serve as a major H. zea refuge. Maintaining the current limit of 50 percent non-Bt corn in cotton-growing areas therefore seems appropriate for maintaining the long-term utility of Bt cotton."

"In the short term, this is good news for cotton farmers," Gould says. "It shows that corn can provide a refuge so cotton farmers don't have to increase the non-toxic cotton refuge."

Gould says that problems can set in if Bt corn begins becoming ubiquitous in the Corn Belt. Fewer refuge plants - non-Bt corn, in this case - could allow pests' resistance to Bt toxin to evolve more quickly, he says.
-end-
Note to editors: An abstract of the paper follows.

"Bt Resistance Management: Stable Isotope Assessment of Alternate Host Use by Helicoverpa Zea"
Authors: Fred Gould, Neal Blair, Tracy L. Rennie, M. Reid, North Carolina State University; J. Lopez, Southern Crops Research Laboratory; S. Micinski, Louisiana Agricultural Experiment Station Published: Dec. 2, 2002, in Proceedings of the National Academy of Sciences

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 percent of moths captured in August through October have isotope ratios indicative of larval feeding on C3 plants. In one October sample, 100 percent 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 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.

Media Contact:
Dr. Fred Gould, 919-515-1647
Dr. Neal Blair, 919-515-7883
Mick Kulikowski, News Services, 919-515-3470

North Carolina State University

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