No resistant pests
Despite extensive cultivation of Bt plants in the USA, it appears that no pests with resistance to the Bt toxin have yet emerged. This was the surprising finding of scientific investigations commissioned by the US Department of Agriculture.
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Genetically modified maize and cotton plants which are resistant to specific insect pests have been grown extensively in the USA for over eight years. This resistance trait stems from the Bt toxin , the gene for which was transferred to the plants from a soil bacterium. It was anticipated that sooner or later the pests concerned, such as the European corn borer or the cotton bollworm, would develop resistance and so become unsusceptible to the Bt toxin. This could then render the Bt strategy ineffective. However, a newly published study conducted under the direction of Bruce Tabashnik at the University of Arizona in Tucson and Cornell University (Geneva, New York) failed to confirm these expectations: not a single resistant pest was found. Even Tabashnik seemed surprised. According to the entomologist, nobody had expected that there would not be even a minimal rise in resistance. Current studies in China paint a similar picture. For the past five growing years scientists there have been examining whether resistant forms of the cotton bollworm are emerging as a result of growing Bt cotton. They too have found no evidence of this. These findings conflict with various reports on the emergence of insects resistant to Bt plants, which have yet to be scientifically validated. So far, increased resistance to Bt toxins in field conditions has been found only in the diamond-back moth – caused by spraying with conventional Bt preparations, which have been used as a biological insecticide in organic farming for over fifty years. Resistant forms of this pest were first spotted in the Philippines in the early nineties. They are now regularly found in North America and Asia. This implies that Bt crops can be used for longer than anticipated. When the first Bt plants appeared on fields in 1996, it was assumed that resistant populations of the pests concerned would have evolved within the next five to eight years. This would have severely limited the effectiveness of the Bt concept. But even organic farming, which uses conventional Bt preparations in the form of sprays, would have lost one of its main methods of pest control, with very few alternatives available. |
Refuge zones: Few chances for resistant pests
Farmers in the USA are obliged to plant fields with non-Bt crops in the vicinity of Bt fields. The creation of these refuges is regarded there as the most important strategy for avoiding the development of Bt-resistant pests and limiting their spread. These zones ensure that "normal", non-resistant pests survive and can then mate with resistant pests, if there are any, in the adjacent Bt maize field.
However, the refuge strategy only works if inheritance of the resistance genes is recessive, rather than dominant: insect pests would then be resistant to Bt toxin only if two of these recessive resistance genes – one from the father and one from the mother – were present in the DNA. If a resistant insect (with two resistance genes) from the Bt field mates with a non-resistant insect (with no resistance gene) from the refuge, all progeny will have one resistance gene. These remain susceptible to the Bt toxin and die when they eat the Bt maize. The number of resistant insects therefore remains low over a long period of time.The assumptions on which the refuge strategy is based have proved to be correct: all cases of strong resistance found so far in insect pests in the laboratory have been based on recessive genes.
As an additional safety precaution, Bt plants should contain a far greater quantity of the Bt toxin than would be required to kill non-resistant insects. This ensures that at least moderately resistant insects still die and are unable to develop into a pest population.
Als weitere Sicherungsmaßnahme sollen Bt-Pflanzen das Bt-Toxin in weitaus höheren Mengen enthalten, als es zum Abtöten nicht-resistenter Insekten erforderlich wäre. Auf diese Weise wird sichergestellt, dass zumindest auch die moderat resistenten Tiere noch sterben und sich in einer Schädlingspopulation nicht anreichern können.
Resistance genes - a handicap rather than an advantage
Another key to the long-term success of the refuge strategy is that the resistance genes present a "handicap" for insects living in conventional fields without Bt crops. For instance resistance genes could reduce the rate of reproduction of the carriers or increase their susceptibility to disease. This type of reduced fitness causes insects with resistance genes in conventional fields and refuges to breed less successfully than members of the same species without resistance genes. This requirement too seems to have been met in many cases. Tabashnik has been able to show that Bt resistance genes cause significantly reduced fitness in the diamond-back moth for instance.
The findings of the study prompt Fred Gould, entomologist at North Carolina State University, to ask: "The main question is whether we don't see resistance because the EPA has instituted the high-dose, refuge approach or whether we never needed a resistance management approach in the first place."
New concepts to combat resistance development
But it still seems too early to abandon the present concept. Resistance development is a fundamental and virtually unavoidable process. Insects are extremely adaptable. Sooner or later resistances develop to almost every active substance which has been used in the past to control them. Today we know of more than five hundred species of insect with resistance to one or more conventional insecticides.
Laboratory experiments give an indication of how quickly this can happen even with Bt toxins: Sixteen different insect species have evolved resistance to it in a relatively short time: ten different butterfly species, two beetle species and four fly species. This included resistance to dreaded cotton and maize pests like the cotton bollworm and the European corn borer.
Tabashnik too continues to regard the resistance problem as a genuine risk, despite the positive experiences of growing Bt plants. This is why new strategies for avoiding resistance development are currently being considered.
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In future, Bt plants could produce Bt toxins for a limited period of time or only in specific plant parts. This would significantly reduce selection pressure on the insects. In Bt plants grown today the toxin is still produced in all parts of the plant and throughout the entire growing season.
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Bt plants could produce several different Bt toxins or a toxin from a completely different toxin class in addition to a Bt toxin. It is highly unlikely that an insect would develop resistance to two or three different active substances simultaneously. The agrobiotechnology company Monsanto is therefore developing Bt cotton plants which contain two different Bt genes (cry IAC and cry 2Ab).
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