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Pests Are Rapidly Developing Resistance to Genetically-Modified Crops

October 11, 2017

A new University of Arizona study bares the frightening truth many had suspected about reliance on GMOs. At least in the case of pest-resistant transgenic crops, it looks like the pests are putting up a very strong fight to thrive.

That conclusion is based on recent research by Bruce Tabashnik and Yves Carrière at the University’s College of Agriculture and Life Sciences.

In 2016 alone, there were more than 240 million acres (98 million hectares) of corn, cotton and soybean crops which had been genetically modified to produce insect-killing proteins based on the bacterium Bacillius thuringiensis, or Bt. Those proteins are toxic to certain beetles and caterpillars which had posed a major threat to such crops in the past, but were seen by many as more-or-less safe for people to consume.

Whether than last part about being safe for human consumption was true, the idea that farmers could plant crops pre-engineered so they would fight off their most serious bug predators was an alluring one. Special GMO seeds embedded with the ability to produce Bt proteins would kill the pests without the need for spraying toxic pesticides on the crops. Eliminating the need to spray saved a costly and time-consuming step in the process, and had the appeal of minimizing further harm of soil and other living things if regular pesticides were used.

The concept – and the Bt transgenic seeds – sold well. But what was always lurking in the background as one of the possible risks of such an investment was about how Mother Nature might react to this. More explicitly, as crop biodiversity suffered and the pests attempted to eat them did battle, there was a big question as to how long it would take for the pests to evolve so they developed partial immunity to the GMO Bt toxins.

There is now an answer. Depending on the crop and the insects attacking it, the pests could evolve in as quickly as five years.

That is part of what researchers Tabashnik and Carrière revealed in their research paper, “Surge in Insect Resistance to Transgenic Crops and Prospects for Sustainability”, as published in the October 2017 issue of Nature Biotechnology.

The pink bollworm, a voracious caterpillar pest, quickly evolved resistance to two Bt proteins produced by biotech cotton in India, but it continues to be suppressed in Arizona after more than 20 years. (Photo: Alex Yelich/UA Department of Entomology. Source: University of Arizona Press Release)

"When Bt crops were first introduced in 1996, no one knew how quickly the pests would adapt," said Tabashnik, a Regents' Professor and head of the UA Department of Entomology. "Now we have a cumulative total of over 2 billion acres of these crops planted during the past two decades and extensive monitoring data, so we can build a scientific understanding of how fast the pests evolve resistance and why."

As reported in the University’s recent press release on the subject, the research was based on 36 cases representing responses of 15 pest species in 10 countries on every continent except Antarctica. The U of A scientists discovered resistance that substantially reduced the efficacy of the Bt crops in the field in 16 cases as of 2016, compared with only three such cases by 2005. In these 16 cases, pests evolved resistance in an average time of just over five years.

'Early Warning of Resistance'

"A silver lining is that in 17 other cases, pests have not evolved resistance to Bt crops," Tabashnik said, adding that some crops continue to remain effective after 20 years. The remaining three cases are classified as "early warning of resistance," where the resistance is statistically significant but not severe enough to have practical consequences.

Fred Gould, Distinguished Professor of Entomology at North Carolina State University and leader of the 2016 National Academy of Sciences study on genetically engineered crops, commented, "This paper provides us with strong evidence that the high-dose/refuge strategy for delaying resistance to Bt crops is really working. This will be critically important information as more crops are engineered to produce Bt toxins."

According to the paper, both the best and worst outcomes support predictions from evolutionary principles. 

"As expected from evolutionary theory, factors favoring sustained efficacy of Bt crops were recessive inheritance of resistance in pests and abundant refuges," Carrière said. 

Refuges consist of standard, non-Bt plants that pests can eat without exposure to Bt toxins. 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. With recessive inheritance, matings between a resistant parent and a susceptible parent yield offspring that are killed by the Bt crop.

"Computer models showed that refuges should be especially good for delaying resistance when inheritance of resistance in the pest is recessive," Carrière explained. The value of refuges has been controversial, and the Environmental Protection Agency has relaxed its requirements for planting refuges in the U.S.

"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. 

In the Southwestern U.S., farmers collaborated with academia, industry, EPA scientists and the U.S. Department of Agriculture to implement an effective refuge strategy. Although India similarly required a refuge strategy, farmer compliance was low. 

"Same pest, same crop, same Bt proteins, but very different outcomes," Tabashnik said.

Cross-Resistance a Factor

The new study revealed that pest resistance to Bt crops is evolving faster now than before, primarily because resistance to some Bt proteins causes cross-resistance to related Bt proteins produced by subsequently introduced crops. 

An encouraging development is the recent commercialization of biotech crops producing a novel type of Bt protein called a vegetative insecticidal protein, or Vip. All other Bt proteins in genetically engineered crops are in another group, called crystalline, or Cry, proteins. Because these two groups of Bt proteins are so different, cross-resistance between them is low or nil, according to the authors of the study.

Yidong Wu, Distinguished Professor in the College of Plant Protection at Nanjing Agricultural University in China, said, "This review provides a timely update on the global status of resistance to Bt crops and unique insights that will help to improve resistance management strategies for more sustainable use of Bt crops."

Although the new report is the most comprehensive evaluation of pest resistance to Bt crops so far, Tabashnik indicated it represents only the beginning of using systematic data analyses to enhance understanding and management of resistance. 

"These plants have been remarkably useful, and resistance has generally evolved slower than most people expected," he 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 resistance to herbicides in weeds and resistance to drugs in cancer cells." 

The study is clearly only a sampling of the full range of research needed to understand precisely how risky the bet on GMOs will turn out to be. Its conclusions that pests – and weeds – are not going to take what is, from their perspective, a ‘GMO invasion’ passively is backed up by other studies. Even Monsanto with its own GMO-crops which rely on the poisonous herbicide glyphosate to work together with it has run into problems, as in recent years new weeds have evolved with some immunity to the glyphosate.

The researchers at the University of Arizona believe the news of their study is an important one, but requires more work to fully quantify how serious the situation may be for farmers insistent on using GMOs to ‘win’.

There will also now be increasing pressure from the GMO providers to keep pushing the latest variant of the GMOs to handle the pest evolution, with the added risk that the rapid turnaround demanded to fight against the pest evolution could result in more toxic GMOs being released into the food supply as the battle wages.

In the particular cases studied by the University of Arizona researchers, the battle is far from over, but it is clear the pests are not going to take this whole Bt GMO thing ‘lying down’.