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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.