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Genetically modified corn, harvested here, helps nearby farmers who planted conventional varieties. The engineered crops keeps damaging insects away from surrounding fields as well as their own.
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Fifty years of records from cornfields in five Midwestern states show that genetically modified corn has dramatically reduced populations of a common corn-eating worm, the European corn borer. That has put more money in farmers' pockets, but paradoxically most of the financial benefit went to farmers who did not grow those new kinds of corn.
Why? The fields of conventional corn also had the benefit of fewer worms because the fields of genetically engineered corn nearby had killed off the insects. But the conventional corn was cheaper to grow because the seed costs less. It's similar to the situation of people who decide not to get a flu shot -- if everyone around them gets the shot, they're still less likely to get the flu. They get the benefit without the pain.
Bt corn stalks (left) are resistant to the European corn borer insects. Non-Bt corn stalks show damage from the hungry pests.
The European corn borer hitched a ride to the New World on a ship from Europe in 1917. It loves corn and quickly spread across corn-growing areas of the Midwest. Every year scientists from the U.S. Department of Agriculture went into cornfields and counted those worms. The new study, published in the journal Science, is based on those records.
A Pattern Of Infestation
"Amazingly, these data sets were just sitting in filing cabinets," says William Hutchinson, a specialist on insects at the University of Minnesota.
Hutchinson and a group of colleagues pulled out those files and took a closer look. For most of the past 50 years, he says, there was a regular pattern to the corn borer infestation. "About every six to seven years there would be an outbreak, and then they would naturally decline."
But starting 14 years ago, corn borer populations started falling and the occasional big outbreaks stopped. Hutchinson says there's a simple reason: Also 14 years ago, farmers started growing corn that contained a gene inserted in the laboratory. This genetically engineered corn is poison to corn borers, so when the insect larvae start feeding, they die.
The gene came from a kind of bacteria called Bacillus thuringiensis, or Bt, so the corn is known as Bt corn. It has become extremely popular among farmers.
- The United States grows about 88 million acres — more farmland than any other crop
- The largest share of the corn harvest becomes chicken, hog, and cattle feed. About 30 percent goes into making ethanol.
- Genetically engineered corn was first sold in 1996, and since then, scientists have introduced a variety of new genes. They include genes that allow the plant to withstand popular weed killers, as well as Bt genes that produce toxins that kill the European corn borer and another pest, the corn root worm.
- Farmers who grow Bt corn are required to plant at least 10 percent of their fields with conventional corn. This "refuge" is intended to delay or prevent the emergence of insects that are resistant to the effects of Bt corn.
Here's how: In the vast population of corn borers, a few individual insects may possess a genetic mutation that gives them resistance to Bt. If 100 percent of the corn crop is toxic to corn borers, it is more likely that so many susceptible insects will be killed off that the rare, resistant corn borers will find each other and mate, creating many offspring that carry this resistance. If this happens, the population of resistant insects will explode. The hope is that the refuges of non-Bt corn will sustain a population of insects that remain susceptible to Bt.
"We're up to about 63 percent of the total U.S. [corn] acres," Hutchinson says. "Fifty-five million out of 88 million acres are now Bt corn."
Hutchinson and his colleagues estimate that farmers made $7 billion in extra profits over the past 14 years because fewer worms were eating their corn. And two-thirds of that added profit came from fields of conventional corn.
Advantages Of Conventional Corn
Hutchinson and his colleagues are highlighting the financial rewards of growing conventional corn for their own reasons and they want farmers to keep planting it. If farmers plant only genetically engineered corn, it increases the risk that a new kind of corn borer will emerge -- one that is resistant to the effects of Bt.
Bruce Tabashnik, an entomologist at the University of Arizona, says it's not just a theoretical danger. "There are some other situations where insects have evolved resistance to Bt crops, including Bt corn," he says. For instance, in Puerto Rico, farmers once grew Bt corn almost exclusively, and insects called fall armyworms used to die when they ate it. Now, they don't.
To help keep the same thing from happening here, farmers in the U.S. are supposed to plant conventional corn on at least 10 percent of their acres. These are so-called "refuges" where corn borers can live in peace. But some farmers don't do it.
Tabashnik, who was not involved in the new study, says this research could help convince more farmers that it's not just a good idea to plant conventional corn -- it's also profitable.
"It's a wonderful result," he says. "This example is sort of the poster child for a case where resistance has not evolved. We can see what wonderful benefits there are, and hopefully some of the lessons from this case can be exported and encourage growers more generally, globally, to adopt the refuge strategy."
Some farmers, in fact, seem already to have figured out that conventional corn can be more profitable. There are reports that demand for Bt corn has leveled off.
But even when farmers want to move away from Bt corn, it's not always easy for them to do it. Seed companies such as Monsanto and DuPont now sell seeds that contain lots of different traits. Sometimes, if farmers want one of those genes -- one that kills the corn rootworm, for instance -- they also get others that they may not want, such as the Bt gene that kills the European corn borer.