Genetically modified cotton stops one bug but fosters others

The widespread planting of a genetically engineered crop designed to withstand a menacing pest has had the unanticipated consequence of transforming benign bugs into agricultural predators, according to a new study.

In findings that drive home the difficulty of trying to stay one step ahead of nature, scientists explain how farmers of bioengineered cotton in northern China were able to drastically reduce their insecticide use for more than a decade, only to find themselves spraying a crop that wasn’t supposed to need such measures.

The genetically engineered plants were designed to withstand attacks from the cotton bollworm by growing their own pesticide — a deadly toxin that was originally discovered in a soil bacterium called Bacillus thuringiensis, or Bt. Splicing the Bt genes into the cotton plants’ DNA has kept the bollworm at bay.

Opponents of genetically engineered crops had warned that insects like the bollworm would inevitably breed resistance to the Bt toxin. So far, that hasn’t happened. Instead, the crops effectively created a new category of pests called mirid bugs.


Researchers from the Chinese Academy of Agricultural Sciences and the National Agro-Technical Extension and Service Center in Beijing documented that as adoption of Bt cotton rose — and pesticide use declined — mirid bugs did more damage to cotton crops. What’s more, the growing population of hungry critters also devoured crops of Chinese dates, grapes, apples, peaches and pears.

In essence, the introduction of genetically engineered cotton transformed the fields into a habitat that enabled mirid bugs to thrive and spread, the researchers reported Thursday in the journal Science.

Researchers from Cornell University in Ithaca, N.Y., first noticed the problem in 2004, when they surveyed 481 farmers in five Chinese provinces. They suspected something was amiss when they discovered that Bt cotton farmers were using more pesticides than farmers planting conventional cotton.

“That made no sense to us,” said Per Pinstrup-Andersen, a professor of food, nutrition and public policy at Cornell who oversaw the survey. Then they realized that mirid bugs had emerged as secondary pests.

“They had had a field day because farmers were using very little pesticide, since they didn’t have to spray for the bollworm,” said Pinstrup-Andersen, who published his findings with colleagues. “Over time, the farmers had to go in and use pesticide again.”

Historically, mirid bugs were considered minor pests in China. Insecticides sprayed on conventional cotton took care of the bugs, and the need for additional measures arose “only sporadically,” according to the study team.

That began to change in 1997, when genetically modified cotton was approved for use in China. The plants use genes from the Bt bacterium to make a toxin that lodges in the cell walls of an insect’s digestive tract. That causes cells to swell and break apart, with lethal results. The toxin is not harmful to higher animals, including humans.

By killing off insect larvae, Bt cotton also benefits conventional corn, peanut, soybean and vegetable crops, the researchers found in a previous study. Today, 95% of the cotton grown in China is genetically engineered.


Cotton farmers in Arizona faced the same dilemma after they began planting Bt cotton in 1996 and the absence of pesticides made fields safe for lygus bugs, a type of mirid bug. Farmers there dealt with the problem by using targeted pesticides that spared insects like ladybugs that feed on lygus bugs, said University of Arizona entomologist Bruce Tabashnik.

“The lesson here is that Bt cotton is not a silver bullet,” said Tabashnik, who nonetheless endorses the crop for its role in reducing the need for anti-bollworm pesticides.

There’s no reason to think that other types of genetically engineered crops would be immune to this type of problem, Pinstrup-Andersen said. Ultimately, he said, the solution is to develop genetically modified plants that are resistant to a variety of insects, but that will be a continuous process.

“We have to constantly stay ahead of those things,” he said.


Times staff writer Amina Khan contributed to this report.