New Scientist recently published a news story (second item below), based on a study part-funded by Monsanto, which reported that to "the surprise of many researchers, genetically modified cotton plants are continuing to fend off the pink bollworm" when it was "expected that the pest would have evolved resistance to a toxin produced by the GM cotton in as little as three years".
A letter in response to the article, published in the current issue of New Scientist, suggests that any celebrations may be premature.
Liz Wright Leeds, UK
New Scientist, issue 2526, 19 November 2005, page 24
The lack of developed resistance in genetically modified Bt cotton crops in Arizona is indeed surprising, especially considering what has been found elsewhere (29 October, p 4). Australian researchers recently obtained "unequivocal evidence" that a strain of the cotton bollworm Helicoverpa armigera was resistant to the Cry1Ac toxin. Unlike the Monsanto-funded study in Arizona, where the inheritance of resistance was recessive, the resistance mechanism was found to be semi-dominant, raising serious concerns about how such resistance might be managed.
There are growing concerns that inconsistent levels of Cry toxin both within the plant and during the growing season could lead to the development of resistance. Such variations in toxin levels have been observed in Bt cotton in several studies in India, China and the US.
Pest-killing GM cotton still potent after all these years
New Scientist, issue 2523 , 29 October 2005
TO THE surprise of many researchers, genetically modified cotton plants are continuing to fend off the pink bollworm after eight seasons. It was expected that the pest would have evolved resistance to a toxin produced by the GM cotton in as little as three years.
"The Bt crops produce toxin all the time. So in terms of generating resistance, we thought it would be like spraying with insecticides very often, if not more or less continuously," says Bruce Tabashnik, an entomologist at the University of Arizona, in Tuscon.
But instead of developing genetic tricks to evade the Bt toxin, pink bollworms (Pectinophora gossypiella) continue to be killed by it. Since cultivation of Bt cotton began in 1997, Tabashnik and his colleagues tested an average of more than 2500 larvae per year from up to 17 cotton fields in Arizona. They found that only about 1 in 50,000 of the larvae exhibited resistance (Proceedings of the National Academy of Sciences, vol 102, p 15389). In the meantime, insecticide use in the state is down 60 per cent and so are losses to the pink bollworm.
The resistance gene the team did see gave only partial protection, and the insects need two copies of the gene to express the trait. Even this modest resistance seems to come at a cost: when the pests eat unmodified cotton they do worse than their non-resistant cousins. Planting refuge areas of unmodified cotton plants near to the Bt crop seems to have diluted the pressure for resistance genes to be selected.
The study was part-funded by Monsanto. But Tabashnik says the biotech company played no part in designing or publishing the study.
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