Five of 13 major pest species have evolved resistance to genetically modified corn and cotton - crops that carry the insecticidal Bt proteins - as of 2010, a new study undertaken in eight countries including India, has found. In India, the pink bollworm caterpillar quickly evolved resistance to genetically modified cotton, but not in the southwestern US where a coordinated resistance management programme has been in place since the biotech crop was introduced in 1996, researchers said. Analysing data from 77 studies of 13 pest species in eight countries on five continents, the researchers found well-documented cases of field-evolved resistance to Bt crops in five major pests as of 2010, compared with only one such case in 2005. Three of the five cases are in the US, where farmers have planted about half of the world's Bt crop acreage. Their report indicates that in the worst cases, resistance evolved in 2 to 3 years; but in the best cases, effectiveness of Bt crops has been sustained more than 15 years. Genetically modified corn and cotton produce insecticidal proteins from the bacterium Bacillus thuringiensis, or Bt for short.
Bt proteins kill devastating pests but are considered environmentally friendly and harmless to people. However, some scientists feared that widespread use of these proteins in genetically modified crops would spur rapid evolution of resistance in pests. "The factors we found to favour sustained efficacy of Bt crops are in line with what we would expect based on evolutionary theory," said Yves Carriere, from the University of Arizona. Carriere explained that conditions are most favourable if resistance genes are initially rare in pest populations; inheritance of resistance is recessive - meaning insects survive on Bt plants only if have two copies of a resistance gene, one from each parent - and abundant refuges are present. Refuges consist of standard, non-Bt plants that pests can eat without ingesting Bt toxins. "Computer models showed that refuges should be especially good for delaying resistance when inheritance of resistance in the pest is recessive," explained Carriere. Planting refuges near Bt crops reduces the chances that two resistant insects will mate with each other, making it more likely they will breed with a susceptible mate, yielding offspring that are killed by the Bt crop. "Perhaps the most compelling evidence that refuges work comes from the pink bollworm, which evolved resistance rapidly to Bt cotton in India, but not in the US. Same pest, same crop, same Bt protein, but very different outcomes," said Bruce Tabashnik, also from the University of Arizona. He explained that in the southwestern US, an effective refuge strategy was implemented by scientists and growers. In India, on the other hand, the refuge requirement was similar, but without the collaborative infrastructure, compliance was low, Tabashnik said.