Pests Mutate to Feed on Genetically Modified Crops

By Chelsea Whyte on June 22, 2012 10:40 PM EDT

pink bollworms
Pink bollworms on a damaged cotton boll. Insects like these are developing resistance to genetically engineered crops, and could bring trouble to cotton farmers around the world. (Photo: Creative Commons)

Corn and cotton biotech crops - those that have been genetically modified to kill plant-eating critters - are fighting a new wave of persistent pests that are adapting to the toxins meant to destroy them.

The resistance of cotton bollworms involves more diverse genetic changes than expected, according to an international research team's findings published in the journal Proceedings of the National Academy of Sciences.

"Scientists expected the insects to adapt, but we're just finding out now how they're becoming resistant in the field," said study co-author Bruce Tabashnik, head of the department of entomology at the University of Arizona College of Agriculture and Life Sciences.

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To avoid having to spray insecticides, which can harm humans and animals other than the targeted pests that plague crops, cotton and corn have been engineered to produce toxins developed from the bacterium Bacillus thuringiensis, or Bt, which kill certain insects but are harmless to most other creatures, including people.

Since 1996, these eco-friendly toxins have been bred into crops by mainstream farmers, but resistance is becoming more common as growing populations of caterpillars adapt to the Bt crops.

Researchers have exposed cotton bollworms to Bt toxins in the lab experiments to study the genetic adaptations one might expect in the field, but some mutations seen in the field differ strikingly from those in lab-bred creatures.

"We found exactly the same mutation in the field that was detected in the lab," said Tabashnik, according to AgProfessional. "But we also found lots of other mutations, most of them in the same gene and one in a completely different gene."

In the lab, recessive mutations - those that require two copies, one provided from each parent - give the bollworms their resistance to Bt toxins. But for the first time, researchers found dominant mutations in the field, which means only one copy of a genetic variant is required to stop the toxins affecting the pests.

"Dominant resistance is more difficult to manage and cannot be readily slowed with refuges, which are especially useful when resistance is recessive," said Tabashnik, according to Futurity.org.

Refuges are unmodified crops planted near those that have the Bt toxin gene with the goal of producing insects susceptible to the toxins that will breed with others and dilute the population of resistant insects.

According to Tabashnik, the refuge strategy worked brilliantly against the pink bollworm in Arizona, where this pest had plagued cotton farmers for a century, but is now scarce.

"As a grower, if you're killing 98 percent of pests with Bt cotton, you wouldn't notice anything. But this study tells us there is trouble on the horizon," he said.

China is the world's top cotton producer, with about 16 billion pounds of cotton per year. India is number two, followed by the U.S., which produces about half as much cotton as China. And the percentage of Bt cotton planted in the U.S. reached 75 percent in 2011, and has exceeded 90 percent since 2004 in northern China where most of the country's cotton is grown. 

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