It isn't the boll weevil that Leadbelly crooned about, but the bollworm that is eating up cotton crops today despite the best efforts of science.
The genetically modified cotton that was designed to make the Bt (that's short for Bacillus thuringiensis, a bacterium) toxins that kill boll weevils has been thwarted by the bugs, according to new research to be published in Nature Biotechnology. The bollworm is the first pest to evolve resistance in the field to so-called Bt crops, according to University of Arizona entomologists.
Bt-resistant populations of bollworm (Helicoverpa zea) were found in more than a dozen crop fields in Mississippi and Arkansas between 2003 and 2006. The first resistant worms showed up just seven years after the genetically modified crops were introduced, in 1996, according to the analysis of monitoring studies.
"What we're seeing is evolution in action," said lead researcher Bruce Tabashnik.
The bollworm is a major cotton pest in the southeastern U.S. and Texas, but not in Arizona. The major caterpillar pest of cotton in Arizona is a different species, known as pink bollworm (Pectinophora gossypiella), which has remained susceptible to the Bt toxin in biotech cotton.
U.S. farmers planned to plant 13.2 million acres of cotton in 2007, according to the National Cotton Council of America. And more and more of that crop now 87% in 2007 is grown genetically modified seeds, according to the USDA. Worldwide, 400 million aces of Bt corn have been planted since 1996.
Pesticide resistance is a common problem, but this is the first time a pesticide manufactured by the plant itself rather than sprayed on crops by farmers has inspired the evolution of resistance in a pest. Scientists had expected the bollworm to develop resistance because Bt resistance is a dominant trait, so even hybrid resistant/nonresistant offspring are resistant to the Bt toxin; to stave off the development of resistance, farmers planted plots of non-Bt cotton to minimize the selective pressure for resistant bollworms.
The researchers, whose research was funded by the Department of Agriculture, did not see their results as a condemnation of genetically modified crops. The bollworm is the only one of six caterpillars studied that showed resistance, and the length of time it took to develop resistance seven or more years "refutes some experts' worst-case scenarios," according to the researchers. Further, they said the use of bt cotton reduced the spraying of pesticides, which kill additional insects besides the bollworm and related pests.
"The resistance occurred in one particular pest in one part of the U.S.," Tabashnik said. "The other major pests attacking Bt crops have not evolved resistance. And even most bollworm populations have not evolved resistance."
Still, as with other pesticides that prompted resistance in pests, the solution to this problem that geneticists have hit upon is to up the ante with increased toxicity. Instead of a single Bt toxin, cotton is now being engineered to produce two.
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