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Pesticides and resistance are like summer and winter, one following the other with inevitability. When genetic engineering was invented, it gave the farmer some respite from the perpetual cycle of pesticide use and pesticide resistance in insects. After more than a decade of widespread use, genetically engineered crops seemed to be holding their own against the insects, despite some stray reports to the contrary. Now a US researcher says that resistance is indeed developing in some insects, with important ramifications for Indian cotton farmers.
Bruce Tabashnik, professor at the department of entomology at the University of Arizona, analysed all the field data from the last 10 years in four countries: Australia, Spain, China and the US. He focused on one gene, the so-called Cry1Ac. This gene produces a protein that is toxic to some pests that attack cotton and corn. Tabashnik found that resistance has increased substantially in one pest, but not in others.
“It is the natural process of evolution,†says Tabashnik. The development of resistance in other pests was delayed because of other tactics to control them.
The gene Cry1Ac is derived from a bacterium called Bacillus thuringiensis. This bacterium was discovered in the early 20th century. It has several genes that code for proteins that are toxic to insects, and Cry1Ac is only one of them. The bacterium was being used as sprays to kill pests in the 1960s.
By the 1980s, when pesticide resistance became a problem and genetic engineering techniques were developed, agri-biotech companies transferred the toxic genes to crops like cotton, corn and potato. If the larvae of the pests ate this plant, they would die immediately. The first genetically engineered cotton variety was planted in the US in 1996.
Since 1996, Bt cotton and corn have been grown in 162 million hectares around the world. Such large areas over 10 years provide enough opportunity for insects to grow resistant, unless measures are taken to slow its development. Farmers who grow genetically engineered, insect-resistant crops are asked to maintain a refuge: a separate region where they grow crops that are susceptible to insects. In all developed countries, farmers maintain a refuge. However, such a practice is not strictly adhered to in India, thus giving the pests more opportunities to develop resistance.
Laboratory studies have shown that pests indeed develop resistance quickly to Bt genes. However, lab experiments do not prove much beyond the principle of natural selection. “It is very easy for pests to evolve resistance in the lab,†says Tabashnik. Evolution of resistance in the field, while also being inevitable in the long run, is a more serious matter because farmers practise methods that delay the development of resistance.
Tabashnik and his colleagues did not do any field experiment for this study. Instead they analysed data from peer reviewed journals, provided they met some criteria. The most important of these criteria is the inclusion of data about concurrently tested susceptible strains of pests. Data from India were ignored for this reason; it did not include concurrent data about susceptible strains.
Tabashnik analysed the emergence of resistance in six major strains of pests. Of these, he found resistance developing in only one strain, Helicoverpa zea. Resistance to the Cry1Ac gene has developed in this strain of pest in the US, particularly in the states of Arkansas and Mississippi. However, this resistance has not caused any crop losses for two reasons. One, farmers had used some pesticide as well to control pests. Second, the crop had killed 40-60 per cent of the pests even when they were resistant.
What do these results mean for India? “The data for India were ambiguous,†says Tabashnik. This does not mean that resistance to the Bt gene has not developed among cotton pests in India.
“Indian farmers do not strictly adhere to the methods to be employed while cultivating transgenic (with genes from another species) crops,†says K.K. Narayanan, managing director of Metahelix, an agri-biotech company in Bangalore.
US farmers escaped damage from resistant pests because of good pest control practices. Are Indian farmers listening?
Sources: The Telegraph (Kolkata, India)
