Bollworm pest remains beaten
Genetic study confirms insects are still not resistant to Bt toxin.
Cotton that has been genetically engineered to be toxic to pests remains effective after nearly a decade in the field, scientists have announced, defying predictions that insects would evolve to tolerate them. Widespread planting of genetically modified (GM) cotton across the southern United States has not increased the incidence of resistance in the major insect pest, pink bollworm.
Cotton varieties genetically modified to produce a natural insecticide, the Bt toxin, borrowed from the bacterium Bacillus thuringiensis, have been planted commercially in the United States since 1996. Many experts predicted that pests would evolve to resist the toxin within a few years.
But research published in Proceedings of the National Academy of Sciences today shows that this has not yet happened1. A survey of pink bollworm in Arizona, where Bt cotton makes up more than half of the cotton grown, shows that genes for Bt resistance have not become more common since 1997.
"Ten years ago many experts were predicting resistance within three years," says Bruce Tabashnik, lead author of the new research, which was partially funded by the company Monsanto. "If I had made a prediction, I would have said it would be maybe four to eight years until resistance evolved." Tabashnik and colleagues were surprised in 2003 when they found little evidence of resistance in pests2 (see ' Resistance to Bt toxin surprisingly absent from pests'). Two years on, their genetic study confirms those earlier results.
But there is no question of Bt cotton being indefinitely effective, Tabashnik adds. "I would say we might see resistance in maybe five additional years," he says.
The researchers used a mathematical model in an attempt to explain how and why the insects have not developed resistance.
Farming practices are one reason, they conclude. The US Department of Agriculture requires that farmers growing Bt cotton plant at least 5% of their crop interspersed with 'refuge' zones of non-Bt cotton. Mixing of insects from within the refuges, which do not gain an advantage from the resistance genes, with insects from the GM crops, is intended to dilute the concentration of resistance genes. The latest research shows that this strategy seems to work.
But there are additional factors at play. Bt-resistant insects do not survive as well in refuges as non-resistant ones, the researchers note. And the genes for resistance are recessive, so that only insects carrying two copies have the trait.
Not over yet
Despite the findings, researchers continue to be concerned that resistance will one day be a problem. "Compared to conventional insecticides, Bt crops have done extremely well, but it's inevitable that resistance will evolve," says Graham Moores, a senior research biochemist at Rothamsted Research in Harpenden, UK.
Moores recently contributed to work that found a novel kind of Bt resistance in cotton bollworm bugs from GM fields in Australia3. If these insects have a different kind of resistance mechanism, then others could too, he says; so there could be genes for resistance out there that behave differently from the one studied in Arizona.
"The existing management strategies have been successful, but we cannot rest on our laurels," says Moores. Bt cotton is grown on a large scale in China, South Africa and Australia.
One hope that the evolution of pests resistant to Bt cotton can be delayed further lies with the second generation of the GM crop. These plants carry another bacterial gene and so produce two Bt toxins. In Australia, the government allows only this variety to be grown, and it is also becoming more popular in the United States. These crops are safer because it is less likely that pests could evolve to be resistant to both toxins. "Two-toxin cotton should last longer, at the very least twice as long, and it could be much more," says Tabashnik.
- Tabashnik B.E., et al. Proceedings of the National Academy of Sciences of the United States of America http://www.pnas.org/cgi/doi/10.1073/pnas.0507857102
- Carrière Y., et al. Proceedings of the National Academy of Sciences of the United States of America, 100. 1519 - 1523 doi:10.1073/pnas.0436708100 (2003).
- Gunning R.V., et al. Applied and Environmental Microbiology, 71. 2558 - 2563 doi:10.1128/AEM.71.5.2558-2563.2005 (2005).