By Carey Gillam

(Reuters) - U.S. farmers are using more hazardous pesticides to fight weeds and insects due largely to heavy adoption of genetically modified crop technologies that are sparking a rise of "superweeds" and hard-to-kill insects, according to a newly released study.

Genetically engineered crops have led to an increase in overall pesticide use, by 404 million pounds from the time they were introduced in 1996 through 2011, according to the report by Charles Benbrook, a research professor at the Center for Sustaining Agriculture and Natural Resources at Washington State University.

Of that total, herbicide use increased over the 16-year period by 527 million pounds while insecticide use decreased by 123 million pounds.

Benbrook's paper -- published in the peer-reviewed journal Environmental Sciences Europe over the weekend and announced on Monday -- undermines the value of both herbicide-tolerant crops and insect-protected crops, which were aimed at making it easier for farmers to kill weeds in their fields and protect crops from harmful pests, said Benbrook.

Herbicide-tolerant crops were the first genetically modified crops introduced to world, rolled out by Monsanto Co. in 1996, first in "Roundup Ready" soybeans and then in corn, cotton and other crops. Roundup Ready crops are engineered through transgenic modification to tolerate dousings of Monsanto's Roundup herbicide.

The crops were a hit with farmers who found they could easily kill weed populations without damaging their crops. But in recent years, more than two dozen weed species have become resistant to Roundup's chief ingredient glyphosate, causing farmers to use increasing amounts both of glyphosate and other weedkilling chemicals to try to control the so-called "superweeds."

"Resistant weeds have become a major problem for many farmers reliant on GE crops, and are now driving up the volume of herbicide needed each year by about 25 percent," Benbrook said.

Monsanto officials had no immediate comment.

"We're looking at this. Our experts haven't been able to access the supporting data as yet," said Monsanto spokesman Thomas Helscher.

Benbrook said the annual increase in the herbicides required to deal with tougher-to-control weeds on cropland planted to genetically modified crops has grown from 1.5 million pounds in 1999 to about 90 million pounds in 2011.

Similarly, the introduction of "Bt" corn and cotton crops engineered to be toxic to certain insects is triggering the rise of insects resistant to the crop toxin, according to Benbrook.

Insecticide use did drop substantially - 28 percent from 1996 to 2011 - but is now on the rise, he said.

"The relatively recent emergence and spread of insect populations resistant to the Bt toxins expressed in Bt corn and cotton has started to increase insecticide use, and will continue to do so," he said.

Herbicide-tolerant and Bt-transgenic crops now dominate U.S. agriculture, accounting for about one in every two acres of harvested cropland, and around 95 percent of soybean and cotton acres, and over 85 percent of corn acres.

"Things are getting worse, fast," said Benbrook in an interview. "In order to deal with rapidly spreading resistant weeds, farmers are being forced to expand use of older, higher-risk herbicides. To stop corn and cotton insects from developing resistance to Bt, farmers planting Bt crops are being asked to spray the insecticides that Bt corn and cotton were designed to displace."

(Reporting By Carey Gillam; Editing by Ken Wills)

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