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16 Years Of Genetically Modified Crops: The Successes And Failures

Resistance to Biotech Crops Is on the Rise
Image courtesy of Nature Biotechnology

Scientists from the University of Arizona (UA) have reviewed how farming practices have led to the rapid evolution of insecticide-resistant pests and how to avoid future occurrences.

The introduction of Monsanto's banned, 'Roundup Ready' wheat in Oregon has prompted a federal investigation, provoked lawsuits, and reignited the social debate over the use of GM technology.

However, GM crops have been commonplace across the globe since 1996. Over a billion acres of genetically modified corn and cotton — the latter also created by Monsanto — have been planted, the majority in the U.S.

"Bt corn accounted for 67% of corn planted in the United States during 2012 and Bt cotton accounted for 79-95% of cotton planted in Australia, China, India and the United States during 2010 to 2012," wrote the authors.

These crops contain genes from the bacterium Bacillus thuringiensis (Bt), which were introduced to make them impenetrable to insect pests. The bacteria, commonly found in soil, creates a natural toxin for insects, which has been used in liquid spray pesticides since the 1920s. (Bt liquid pesticides are considered environmentally friendly and are often used as sprays in organic farming.)

But, according to the authors of this review, several farms with genetically modified Bt crops have spawned insecticide-resistant bugs. The analysis also suggests Bt resistance has become a more serious problem in recent years with the trait emerging in five major pests between 2005 and 2010 — three of these cases occurred in the U.S. Prior to 2005, only one Bt resistance species was recorded.

"When Bt crops were first introduced, the main question was how quickly would pests adapt and evolve resistance," said lead author Dr. 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."

Resistance to Biotech Crops Is on the Rise

By 2011, the area planted worldwide to genetically engineered Bt corn and Bt cotton increased to 66 million hectares (160 million acres) and the number of pest species with resistance causing reducing efficacy of Bt crops climbed to five. The asterisk indicates the number of resistant pests could be underestimated for 2011 because reports of field-evolved resistance typically are published two or more years after resistance is first detected. (Image courtesy of Nature Biotechnology)
By 2011, the area planted worldwide to genetically engineered Bt corn and Bt cotton increased to 66 million hectares (160 million acres) and the number of pest species with resistance causing reducing efficacy of Bt crops climbed to five. The asterisk indicates the number of resistant pests could be underestimated for 2011 because reports of field-evolved resistance typically are published two or more years after resistance is first detected. (Image courtesy of Nature Biotechnology)

Bruce and his colleagues found that Bt crops must be carefully farmed in order to avoid resistant bugs. They reviewed the data from 77 studies made over two decades, which covered 13 incidences of evolving pests from eight different nations.

"Life Finds A Way" 

Puerto Rico holds the dubious record for the most rapid emergence of Bt crop resistance. Merely three years after Bt corn was introduced on the tiny island, commercial farms began noticing that large swaths of plants were being destroyed by Bt-resistant fall armyworms (Spodoptera frugiperda).

The popular theory is a severe drought-damaged conventional corn in the region, which forced armyworms to retreat to irrigated corn fields that were mainly populated with Bt crops. Rather than dying off, the armyworms adapted to being 'force-fed' a single, pesticide-laced food source.

The armyworms were able to escape, according to the authors, because of two issues with GM farming in the region. First, the Bt dosage in the corn plants was too low. This allowed a small proportion of armyworms that could tolerate moderate levels of Bt. Once the regular armyworms died, this small proportion of insects no longer had competition for food and were able to thrive.

The other major misstep was the lack of refuge crops. Refuge crops are normal, non-GM plants that are seeded alongside GM crops, and while this might seem counter-intuitive, they are the key to eradicating pests with GM farming.

Refuge crops would have ensured that regular armyworms would have remained in the population. These would have greater chance of mating with Bt-resistant worms, reintroducing genetic susceptibility to the insectide back into the menacing population.

"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."

Crop control and illegal planting was another major issue in this example from India, which featured Monsanto's Bt cotton.

Moving forward 

New, second-generation Bt crops — named pyramids — produce more than one variety of the toxin and apply more pressure on pests. Pyramid plants are becoming more popular worldwide, and both the U.S. and Australia replaced their millions of acres of Bt cotton with pyramid varities in just over six years.

While they apply more pressure toward killing pests, two-toxin pyramid crops are also vulnerable if used inappropriately. Resistance can develop faster if pyramid crops are planted next to first generation GM crops that produce only one Bt toxin.

"You're always expecting the pest to adapt. It's almost a given that preventing the evolution of resistance is not possible," said Tabashnik.

Tabashnik and his colleagues concluded that the key to pest management with GM crops is planning and gauging the risk of resistance before Bt crops are commercialized.

Insects from potential farms should be collected and tested for resistance genes prior to seeding fields with GM crops. This would also assure that the GM crops produce a high enough dose of Bt to combat local pests, without setting up an incubator for resistance.

Although the U.S. Environmental Protection Agency has recently relaxed its requirements for refuge farming, Tabashnik and his colleagues are calling for the inclusion of more refuge plants at the future GM farm.

"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," concluded Tabashnik.

 

Source: Tabashnik BE, Brévault T, Carrière Y. Insect resistance to Bt crops: lessons from the first billion acres. Nature Biotechnology. 2013.

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