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Insects Find Crack In Biotech Corn’s Armor

Filed by KOSU News in Science.
December 5, 2011

Hidden in the soil of Illinois and Iowa, a new generation of worms appears to be munching happily on the roots of genetically engineered corn, according to scientists. It’s bad news for corn farmers, who paid extra money for this line of corn, counting on the power of its inserted genes to kill those worms. It’s also bad news for the biotech company Monsanto, which inserted the worm-killing gene in the first place.

In fact, the gene’s apparent failure, as reported in the journal PLoS One, may be the most serious threat to a genetically modified crop in the U.S. since farmers first started growing them 15 years ago. The economic impact could be “huge,” says the University of Arizona’s Bruce Tabashnik, one of the country’s top experts on the adaptation of insects to genetically engineered crops. Billions of dollars are at stake.

The story of how this happened is long and complicated, but the details are important, so let’s start at the beginning.

Almost the entire agricultural biotech industry has been built on just two genetic traits, and our story involves one of them.

The gene (actually a family of genes) in this story — the first pillar of the industry — was copied from a worm-killing bacterium called Bacillus thuringiensis, or Bt. In the 1980s, scientists managed to insert a Bt gene into plants, and voila, the plant cells started manufacturing the same worm-killing toxin as the bacteria. (The other big gene for the agricultural biotech industry allows a plant to survive doses of the popular herbicide glyphosate, widely known by Monsanto’s trade name, Roundup.)

So-called Bt corn went on sale in the late 1990s. It was astonishingly effective against the European corn borer, a common pest.

But from the beginning, scientists worried that biotech companies were overusing Bt and increasing the chances that it would eventually stop working. Why? The key word is resistance.

The more widely you spray any insecticide, the more likely you are to uncover and promote the growth of a new strain of insects that’s resistant to your insect killer. It has happened with one insecticide after another over the decades. Eventually, scientists said, the same thing would happen to a crop that carries its own insecticide. Covering fields with Bt crops would lead to a strain of insects that the crops didn’t kill.

So federal regulators came up with a strategy to preserve Bt’s effectiveness. First of all, they said Bt crops (mainly corn and cotton) had to be extremely effective. Ideally, they would kill 99.99 percent of all the target insects that fed on them.

And for those rare insects that survived, regulators came up with a second line of defense, to prevent resistant insects from mating and producing lots of resistant offspring. Farmers who grew Bt corn (or cotton) were required to grow non-Bt crops on some of their farm, as a “refuge” for normal insects. That way, the rare, surviving, resistant insects would probably find non-resistant mates, instead of each other, and their offspring still would (likely) be killed by the Bt corn.

To the surprise of some environmentalists, the strategy has worked. There’s no evidence that the European corn borer has evolved resistance to the Bt toxin. The same goes for insects that feed on cotton — at least in the United States.

Yet the danger is real, as shown by something that happened in India. Tabashnik says farmers there ignored rules that required them to plant refuges of non-Bt cotton. As feared, a resistant strain of pink bollworm emerged in 2009 to prey on cotton fields, and farmers are back to spraying insecticides.

Here in the U.S., though, the Bt train rolled on. After its success against the European corn borer and various cotton pests, scientists realized that Bt genes come in many flavors. They found another version of the Bt gene that kills the corn rootworm. This is an important scourge of corn fields; at the time, farmers reportedly spent $1 billion a year on insecticides to fight it.

This new Bt gene was not, however, quite as lethal as its predecessors. It did not produce a “high dose” that killed 99.99 percent of all rootworms. Instead, it was what you’d call a “moderate dose,” says Tabashnik. He says it’s the worst situation: “The fastest way to get resistance.”

So a scientific advisory panel urged the Environmental Protection Agency to strength the second line of defense against resistance, and demand large refuges on non-Bt corn. They proposed that farmers be allowed to plant this new gene on no more than half of their corn acres.

Monsanto argued that such a large refuge wasn’t necessary, and the EPA agreed. In 2003, the agency decided to allow farmers to plant this new product on 80 percent of their farms.

The scientists who called for caution now are saying “I told you so,” because there are signs that a new strain of resistant rootworms is emerging. In eastern Iowa, northwestern Illinois, and parts of Minnesota and Nebraska, rows of Bt corn have toppled over, their roots eaten by worms. Entomologist Aaron Gassman at Iowa State University, who authored the PLoS One paper, collected insects from some of these fields and found many with a greater-than-expected ability to tolerate Bt.

Monsanto says other factors may be causing this. Because corn is so profitable right now, many farmers are growing corn year after year, causing a boom in insects that feed on corn.

But a committee of experts at the EPA is now recommending that biotech companies put into action, for the first time, a “remedial action plan” aimed at stopping the spread of such resistant insects. The agency’s experts want farmers in areas where such damage has been observed to stop planting this kind of Bt corn altogether. Instead, those farmers will have to use other methods, such as spraying chemical insecticides, to control the rootworm. Some may simply plant soybeans or other crops instead.

The EPA’s experts also are suggesting that the agency reconsider its approval of a new kind of rootworm-killing corn, which Monsanto calls SmartStax. This new version of Bt corn includes two different Bt genes that are supposed to kill the rootworm in different ways.

This should help prevent resistance from emerging, and the EPA is allowing farmers to plant it on up to 95 percent of their farms. But if one of those genes is already compromised, Tabashnik says, such small refuges will rapidly produce insects that are resistant to the second one, too. [Copyright 2011 National Public Radio]

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