Well before the introduction of GM plants, some people voiced concerns about the impact these plants could have on the environment. We will consider two major ecological issues associated with genetically modified plants: the appearance of resistance to Bt toxin among target insects and the possible creation of "superweeds."
When insect populations are sprayed with an insecticide, resistance to this chemical invariably occurs. One famous example is DDT. This insecticide has completely lost its effectiveness in many parts of the world because so many insect species (including mosquitoes that carry the malaria parasite and the tsetse fly that carries the parasite causing sleeping sickness) have developed resistance to it. Fears that insects that are pests for cotton would develop resistance to the Bt toxin made by GM cotton have been borne out. Just two years after the introduction of cotton modified with the Bt toxin gene, such resistance has appeared in the field.
We must minimize the rise of resistance if Bt toxin is to remain an effective tool to fight pest damage. One such measure, instituted since the initial introduction of Bt modified crops, is that farmers who plant Bt crops are now required to grow a certain percentage of non-GM crops in close proximity of the GM crop. The idea is to create refuges in which susceptible insects can survive, that is, areas planted with crops that do not contain Bt toxin. This should help to maintain insect populations still susceptible to the toxin. Indeed, if Bt crops were the only food source for insects, only resistant individuals would survive and reproduce. Because susceptible individuals are not killed by Bt toxin in the refuges, availability of the refuges reduces the rise of resistant individuals. The goal of having refuges is to allow many sensitive insects to survive and dilute the effect of rising resistance. We cover more about the rise of resistance in a population in chapter 11. The lesson with Bt-modified crops is that genetic modification of crops with Bt toxin is not a cure-all against insect infestation of crops.
Another potential ecological problem is the transfer of genetically modified traits from GM crops to their weedy relatives. For example, if herbicide resistance in crop plants is transferred to their weedy relatives, it can produce "superweeds." All cultivated plants are derived from plants found in the wild. Many crops still have wild, weedy relatives with which they can occasionally interbreed. If this breeding happened between herbicide-resistant GM crop plants and their weedy relatives, herbicide-resistant weeds would be created, and they could destroy the effectiveness of the herbicides. Another example is that of transfer of viral resistance from genetically modified crops to their weedy relatives. For example, viral resistance in squash may be transferred to their wild cousin that is a pest, especially in the Southern US. The transfer of genetic modification from GM crops to their wild relatives has not yet been documented in the wild. Yet it is clear that a small amount of interbreeding can occur between domesticated crops and their wild counterparts.
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