Super Weeds And Super Bugs Mean More Pesticide Use

Alan Harman


 The creation of three genetically modified herbicide-tolerant (HT) crops – cotton, soybeans and corn – has actually increased the use of herbicides on the farm.

   Washington State University research professor Charles Benbrook says his counterintuitive finding is based on an exhaustive analysis of publicly available data from the U.S. Department of Agriculture's National Agriculture Statistics Service.

   Benbrook reports in the journal Environmental Sciences Europe that the emergence and spread of glyphosate-resistant weeds is strongly correlated with the upward trajectory in herbicide use.

   Marketed as Roundup and other trade names, glyphosate is a broad-spectrum systemic herbicide used to kill weeds. About 95% of soybean and cotton crops and more than 85% of corn in the U.S. are planted to varieties genetically modified to be herbicide resistant.

   “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%,” Benbrook says.

   Over the first six years of commercial use from 1996-2001, HT and Bt crops reduced pesticide use 31 million pounds, or about 2%, compared to what it likely would have been in the absence of GE crops.

   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 in response to recommendations from entomologists to preserve the efficacy of Bt technology by applying insecticides previously displaced by the planting of Bt crops.

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

   There now are two dozen weeds resistant to glyphosate, the major herbicide used on HT crops, and many of these are spreading rapidly. Millions of acres are infested with more than one glyphosate-resistant weed. The presence of resistant weeds drives up herbicide use by 25% to 50%, and increases farmer-weed control costs by at least as much.

   Benbrook’s analysis shows herbicide-tolerant crops worked extremely well in the first few years of use, but over-reliance may have led to shifts in weed communities and the spread of resistant weeds that force farmers to increase herbicide application rates (especially glyphosate), spray more often, and add new herbicides that work through an alternate mode of action into their spray programs.

   HT crops have increased herbicide use by 527 million pounds over the 16-year period from 1996-2011.

   Today’s major GE crops have increased overall pesticide use by 404 million pounds from 1996 through 2011 – a 527-million-pound increase in herbicides, minus the 123-million-pound decrease in insecticides.

   Overall pesticide use in 2011 was about 20% higher on each acre planted to a GE crop, compared to pesticide use on acres not planted to GE crops.

   Benbrook says the biotechnology-seed-pesticide industry’s primary response to the spread of glyphosate-resistant weeds is development of new HT varieties resistant to multiple herbicides, including 2,4-D and dicamba.

   “These older phenoxy herbicides pose markedly greater human health and environmental risks per acre treated than glyphosate,” he says. “Approval of corn tolerant of 2,4-D is pending, and could lead to an additional 50% increase in herbicide use per acre on 2,4-D HT corn.”


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