Summary of Major Findings and Definitions of Important Terms

“Impacts of genetically engineered crops on pesticide use in the U.S. – the first sixteen years” by Charles M. Benbrook

Published in Environmental Sciences Europe, Vol. 24:24 doi:10.1186/2190-4715-24-24, 28 September 2012. Available online at

For more information, contact the study’s author: Charles Benbrook, research professor, WSU Center for Sustaining Agriculture and Natural Resources, 541-828-7918,

This document supplements the press release located at Download this document as a PDF.

Major Findings

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

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

Bt crops have reduced insecticide use by 10-12 million pounds annually over the last decade. From 1996-2011, Bt crops have reduced insecticide use on the three crops by 123 million pounds, or about 28%.

The annual per acre reduction in insecticide use on acres planted to Bt corn and cotton has trended downward since 1996, because of the shift toward lower-dose insecticides and the expansion of Bt corn onto acres that would not likely be treated with an insecticide.

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.

Herbicide-tolerant crops worked extremely well in the first few years of use, but over-reliance led to shifts in weed communities and the emergence of resistant weeds that have, together, forced farmers to incrementally –

  • 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.

Each of these responses has, and will continue to contribute to the steady rise in the volume of herbicides applied per acre of HT corn, cotton, and soybeans.

HT crops have increased herbicide use by 527 million pounds over the 16-year period (1996-2011). The incremental increase per year has grown steadily from 1.5 million pounds in 1999, to 18 million five years later in 2003, and 79 million pounds in 2009. In 2011, about 90 million more pounds of herbicides were applied than likely in the absence of HT, or about 24% of total herbicide use on the three crops in 2011.

Today’s major GE crops have increased overall pesticide use by 404 million pounds from 1996 through 2011 (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.

There are now 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.

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. 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.

Substantial volumes of Bt toxins are produced per acre planted to Bt corn and cotton. The volumes of these toxins produced by the plants on an acre exceed in nearly all cases the volume of insecticides displaced by the planting of a Bt cultivar. For example, Bt corn targeting the corn rootworm and related soil insects expresses one to two pounds of Bt toxins per acre, while displacing about 0.19 pound of insecticide per acre. The first GE crop expressing eight traits, so-called SmartStax corn, produces 3.7 pounds of Bt toxins per acre and displaces around 0.3 pounds of insecticides.

Reductions in pesticide-related environmental and human health risks have been among the benefits thought to be associated with the shift to glyphosate-based HT crops and Bt corn and cotton. Over the last 16 years, there has been dramatic growth in the volumes of both Bt toxins and glyphosate required to bring crops to harvest. The levels of glyphosate and Bt in the ambient environment, animal feed, and food have markedly increased, creating a myriad of new exposure pathways.

Much new research will be required to translate emerging data on higher exposures to glyphosate and Bt toxins into estimates of human, farm and companion animal, and environmental risks.

Important Terms and Definitions

“Pesticide” is the term used by the U.S. EPA, and pest control experts and scientists, to describe any chemical sprayed or applied to control insects, weeds, plant disease, and rodents. “Pesticide” encompasses herbicides, insecticides, fungicides, rodenticides, and fumigants. “Pesticide use” on a given crop refers to the volume of pesticides applied during a production season, either per acre/hectare or across all acres/hectares planted to the crop. Pesticide use is typically measured as the sum of the pounds of herbicides, insecticides, fungicides, and other types of pesticides applied.

“Genetically-engineered (GE) crops” have been transformed to express a novel trait using the tools of molecular biology. The new traits in GE crops are derived from a foreign species that is not sexually compatible with the transformed crop (e.g., a bacterium, a fish, an animal, a tree).

“Herbicide-tolerant (HT) crops” are genetically engineered to withstand the application of specific herbicides over the top of the crop, killing or stunting weed growth, while leaving the crop unharmed.

“Herbicide-resistant weeds” have developed the capacity to withstand or overcome applications of herbicides that once killed or controlled the weed.

“Bt-transgenic (Bt) crops” refer to varieties of corn and cotton genetically engineered to biosynthesize in plant cells one or more protein endotoxins produced by subspecies of the bacterium Bacillus thuriengiensis.