Pesticides are not the only way
a recent study could have broad implications for farmers' ability to stop pests from becoming resistant to pesticides. Scientists from the us-based North Carolina State University have found that in the us, cotton pests mature feeding on corn all summer before moving towards south to munch cotton in the autumn. Therefore, researchers say, if large quantities of pesticides are used in the corn fields, pests would move to the more commercially significant cotton fields armed with resistance to these pesticides.
A large amount of cotton grown in the us is genetically modified to contain the Bt gene from the bacterium Bacillus thuringiensis. This enables the plant to produce a toxin that is lethal to many pests. It is estimated that about 80-95 per cent of cotton bollworms (the predominant cotton pests) are killed by the Bt toxin. However, long-term exposure to Bt can make the pests resistant to it. Therefore, farmers grow 'refuges' or non-engineered crops between fields of genetically modified plants. When the pesticide-resistant pests mate with pests feeding on 'refuges', their offspring are not immune to the Bt toxin.
The researchers used a technique called stable isotope assessment to gauge the development patterns of pest moths during summer. They compared the ratios of carbon isotopes c13 and c12 in moths captured from Louisiana and Texas during August to October. Dicot plants like cotton have less of c13 as compared to monocot plants like corn (which have more of c12). Pests feeding on a particular plant would have the same carbon isotope ratio as the plant.
The researchers found that a large majority of pests moths, specifically that of cotton bollworm, had more c12 than c13. This means they are maturing not on cotton and soybean plants, as the prevailing theory suggests, but on corn.
Knowing where the moths developed was essential for the researchers to analyse how well 'refuges' function to put the brakes on the evolution of pests that are resistant to insecticides.
Currently, less than 25 per cent of the us corn is genetically modified to produce Bt toxin. If most corn grown in the southern parts of the us remains non-Bt, it could serve as a major cotton pests 'refuge'. Maintaining the current limit of 75 per cent non-Bt corn in cotton-growing areas is a must for maintaining the long-term utility of Bt cotton, the researchers assert. "Our finding shows that corn can provide a refuge, so cotton farmers don't have to increase the non-toxic cotton refuge," says Fred Gould, the lead researcher. This could work for other 'cotton belts' of the world also, he opines.
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