ARIZONA, June 22, 2013 - Researchers at the University of Arizona say a new global assessment they released this month helps scientists explain why genetically modified crops have suppressed some pests for more than a decade, while other pests have adapted in just a few years.
The research team says that since 1996, farmers worldwide have planted more than a billion acres of genetically modified corn and cotton that produce insecticidal proteins from the bacterium Bacillus thuringiensis, or Bt. The Bt proteins, used for decades in sprays by organic farmers, kill some 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.
Bruce Tabashnik and Yves Carrière in the university’s department of entomology at the College of Agriculture and Life Sciences, along with visiting scholar Thierry Brévault from the Center for Agricultural Research for Development (CIRAD) in France, scrutinized the available field and laboratory data to test predictions about resistance. Their results are published in the journal Nature Biotechnology.
Analyzing data from 77 studies of 13 pest species in eight countries spanning 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 United States, 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, Bt crops have remained effective for more than 15 years.
“The factors we found to favor sustained efficacy of Bt crops are in line with what we would expect based on evolutionary theory,” said Carrière, explaining that conditions are most favorable if resistance genes are initially rare in pest populations; inheritance of resistance is recessive – meaning insects survive on Bt plants only if they 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,” said Carrière.
Planting refuges near Bt crops reduces the chances that two resistant insects will mate with each other, making it more likely that pests will yield offspring that are killed by the Bt crop. The value of refuges has been controversial, and in recent years, the EPA has relaxed its requirements for planting refuges in the United States.
“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 U.S.,” Tabashnik said. “Same pest, same crop, same Bt protein, but very different outcomes.”
He said that in the southwestern United States, scientists from the EPA, academia, industry and USDA worked with growers to craft and implement an effective refuge strategy. In India, on the other hand, the refuge requirement was similar, but without the collaborative infrastructure, compliance was low.
One of the paper’s main conclusions is that evaluating two factors can help to gauge the risk of resistance before Bt crops are commercialized.
“If the data indicate that the pest’s resistance is likely to be recessive and resistance is rare initially, the risk of rapid resistance evolution is low,” Tabashnik said. In such cases, setting aside a relatively small area of land for refuges can delay resistance substantially. Conversely, failure to meet one or both of these criteria carries a higher risk of resistance.
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