June 11, 2013
Evolution Strikes Back Against Bioengineered Crops
April Flowers for redOrbit.com - Your Universe Online
Since 1996, farmers across the globe have planted more than a billion acres of genetically modified corn and cotton that have been engineered to produce insecticidal proteins from the bacterium Bacillus thuringiensis, or Bt for short. Organic farmers used Bt proteins in sprays for decades to kill a number of devastating pests because the proteins are considered environmentally friendly and harmless to humans. Some scientists, however, feared that widespread use of these proteins in GMO crops would spur the rapid evolution of resistance in pests.
A new study led by researchers at the University of Arizona addresses this concern, attempting to figure out why and how pests became resistant quickly in some cases, but not others. The team scrutinized the available field and laboratory data to test predictions about resistance.
"When Bt crops were first introduced, the main question was how quickly would pests adapt and evolve resistance," said Bruce Tabashnik, head of the UA department of entomology. "And no one really knew, we were just guessing."
"Now, with a billion acres of these crops planted over the past 16 years, and with the data accumulated over that period, we have a better scientific understanding of how fast the insects evolve resistance and why."
The team analyzed data from 77 studies of 13 pest species, covering eight countries on five continents. They 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 found are in the US, where about half of the world´s Bt crop acreage is planted. The findings indicate that resistance evolved in two to three years in the worst cases. In the best cases, the effectiveness of the Bt crops has been sustained more than 15 years. The researchers report that both best and worst outcomes correspond with predictions based on principles of evolutionary theory.
"The factors we found to favor sustained efficacy of Bt crops are in line with what we would expect based on evolutionary theory," said CarriÃ¨re. He further explained that conditions are most favorable if resistance genes are initially rare in pest populations because inheritance of resistance is recessive. This means insects survive on Bt plants only if they have two copies of a resistance gene, one from each parent. The development of resistance is also dependent on abundant refuges, which 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 Yves CarriÃ¨re from the department of entomology at the UA College of Agriculture and Life Sciences.
The chance that two resistant insects will mate with each other is reduced by planting refuges near Bt crops. This makes it more likely the insects will breed with a susceptible mate, creating offspring that are killed by the Bt crop. The value of refuges has been the subject of debate, and in recent years the Environmental Protection Agency (EPA) has relaxed the requirements for planting refuges in the US.
"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," Tabashnik explained. "Same pest, same crop, same Bt protein, but very different outcomes."
In the southwestern US, EPA scientists, academic researchers, industry leaders and the USDA worked together with farmers to develop and implement and effective refuge strategy, according to Tabashnik. The refuge requirement was similar in India, but without the collaborative infrastructure, compliance was low.
The team concluded 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.“¯Setting aside a small area for refuges can delay resistance substantially in such cases, the study found. However, failing to meet one or both criteria creates a higher risk of resistance.
Tabashnik describes the indication of higher risk as a fork in the road. "Either take more stringent measures to delay resistance such as requiring larger refuges, or this pest will probably evolve resistance quickly to this Bt crop."
The findings of this study, published in Nature Biotechnology, are welcomed by other experts in Bt crops. Kongming Wu, director of the Institute for Plant Protection at the Chinese Academy of Agricultural Sciences in Beijing said, "This review paper will be very helpful for understanding insect resistance in agricultural systems and improving strategies to sustain the effectiveness of Bt crops."
Fred Gould, professor of entomology at North Carolina State University, noted: "It's great to have an up-to-date, comprehensive review of what we know about resistance to transgenic insecticidal crops."
Tabashnik cautions that although this is the most comprehensive evaluation of pest resistance to Bt crops so far, it represents only the beginning of using systematic data analyses to enhance understanding and management of resistance.
"These plants have been remarkably useful and in most cases, resistance has evolved slower than expected," Tabashnik said. "I see these crops as an increasingly important part of the future of agriculture. The progress made provides motivation to collect more data and to incorporate it in planning future crop deployments. We've also started exchanging ideas and information with scientists facing related challenges, such as herbicide resistance in weeds and resistance to drugs in bacteria, HIV and cancer."