Research released on Tuesday has examined the development of resistance amongst insect pests to insecticidal genetically modified crops grown around the World.

One of the most persistent criticisms levelled at deeply controversial GM crops is the issue of resistance. Although their existence was initially denied by the companies developing GM crops, insect pests resistant to modified crops, which have been engineered to produce insecticidal 'Bt' toxins, have been discovered in a number of countries since the early 2000s.

Biotech seed companies have blamed farmers for resistance, claiming certain growers had failed to follow instructions intended to keep the technology effective, such as growing 'refuges' of conventional crops; according to an international study published this week in Nature Biotechnology, their claims might carry some weight.

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Using computer modelling and analysis of earlier research, the study's authors found that in areas where best practice is followed, insect pests find it more difficult to develop resistance to genetically modified crops. However, critics suggest that in the U.S. Environmental Protection Agency experts and seed manufacturers had disagreed on the size of refuge needed for effective control of resistance, and the EPA eventually accepted more lax recommendations.

Following their assessment of the issue, the scientists behind the Nature Biotechnology study maintained that some genetically modified crops have effectively suppressed pests for over a decade in certain regions, while in others insects adapted to the crops in just a few years.

Scientists study evidence on pest resistance

Since 1996, farmers worldwide have planted more than a billion acres (400 million hectares) of GM maize and cotton that produce insecticidal proteins from the bacterium Bacillus thuringiensis, known as Bt. Bruce Tabashnik and Yves Carrière, two experts from the Univeristy of Arizona, working with Thierry Brévault from the French Centre for Agricultural Research and Development (CIRAD), examined field and laboratory data to assess the current understanding of resistance and develop their own hypotheses.

At the outset, Professor Tabashnik admitted that he believes resistance amongst crops pests is inevitable; he said, "When Bt crops were first introduced, the main question was how quickly would pests adapt and evolve resistance… 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 international team analysed data from 77 studies of 13 pest species in eight countries, spread across five continents. They found well-documented cases of field-evolved resistance to Bt crops in five major pests by 2010, whereas in 2005 only one such pest existed (resistance in pests is deemed to occur when over 50 percent of individuals are resistant in a local population).

Three of the five identified cases are in the United States, where farmers have planted about half of the world's Bt crop acreage. The others are in South Africa and India. According to the researchers, some areas saw resistance develop within two to three years, though in other regions, Bt crops are still effective. Giving insight into the scientists' hypotheses, Carrière said both the best and worst outcomes correspond with predictions based on evolutionary principles.

He explained that inheritance of resistance is recessive – meaning insects only survive on Bt plants if they have two copies of a resistance gene, one from each parent. According to computer models, if resistant genes are rare within insect populations to begin with, and 'refuges' of conventional crops that pests can eat without ingesting Bt toxins are present, then resistance should remain low.

Carrière added that planting refuges near Bt crops reduces the chances that two resistant insects will mate, but claimed that questions over the value of refuges, rather than the corporates' desire to keep them small in order to boost profitability, has led to a relaxation of EPA requirements for planting refuges.

Commenting on the subject, Prof Tabashnik added, "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: same pest, same crop, same Bt protein, but very different outcomes." He said that, whereas growers in the Southwestern U.S. had access to USDA support and developed a refuge strategy, those in India did not get such treatment, and although refuges were required, few farmers complied. 

Reflecting on the team's research, Tabashnik said that, although he supports use of GM crops, he doesn't believe resistance will ever be controlled altogether. He explained, "You're always expecting the pest to adapt. It's almost a given that preventing the evolution of resistance is not possible."

In addition to the problem of resistance in insect pests, crops engineered to resist applications of glyphosate have been hit by weed species which can survive applications of the herbicide. In response to this, industry has developed seeds resistant to multiple agrochemicals, though these have been strongly opposed by environmentalists, and plans have been called in by USDA.

Criticising a declaration of open support for GM crops by Defra secretary Owen Paterson, despite the absence of a clear scientific consensus either for or against the technology, Soil Association policy director Peter Melchett said late last year, "US Government figures show that overall pesticide use has increased since GM crops have been grown there, because as scientists opposed to GM predicted, superweeds and resistant insects have multiplied." 

The Soil Association director was referring to Professor Charles Benbrook, who, working for US Department of Agriculture, came to the conclusion that 'superweeds' and resistant insect pests are an inevitable part of the GM package; he also found that the technology's rise to prominence in the U.S. has coincided with increased use of more hazardous chemicals.

Melchett added, "This is one example of why GM crops don't offer a real solution. Not only have these GM technologies failed to deliver on their fundamental promises, they have made the problem they were designed to solve even worse and locked farmers further into depending on costly inputs from a handful of powerful chemical companies."

Meanwhile in Europe the deadlock over approvals of new crops continued this week. EU officials in the Standing Committee on Food Chain and Animal Health failed to reach an agreement over the approval of three genetically modified maize varieties for import into Europe for use in food and feed.

Only one GM crop, Monsanto's MON810 maize is licensed for cultivation in the bloc, and the license renewal process for this GM maize variety has dragged on since 2007. However, over 50 GM crops have been cleared for import into the bloc, mostly for use in animal feed.