A peer-reviewed study, authored by Professor Charles Benbrook at Washington State University, negates the claims by agricultural corporations, Monsanto and Dupont, that GM crops are beneficial to both the environment and human health by reducing pesticide use.
The study was published in the peer-reviewed journal Environmental Sciences Europe and shows that overall pesticide use in the US has gone up by 7 % from 1996 to 2011. This rise is directly linked to genetically modified (GM) crop cultivation . Herbicide-tolerant (HT) crops, mainly those tolerant to Monsanto’s glyphosate-based herbicide Roundup, have increased herbicide use by 239 million kilograms, while GM crops expressing the insecticidal toxins Bacillus thuringiensis (Bt) have reduced insecticide use by 59 million kilograms. But the emergence of Bt-resistant pests means that insecticide use is expected to rise in the near future.
Drawing upon publicly available USDA data, the study applied a model to quantify the impacts of the 6 major GM crops on the kilograms of pesticides applied per hectare and across all GM hectares. The 6 crops were glyphosate-tolerant corn, soybean, and cotton; Bt corn targeting the European corn borer; Bt corn targeting corn rootworms; and Bt cotton targeting Lepidopteron insects. Non-GM and GM crop data were compared to assess the impact GM crops have on pesticide application rates. Of the extra 239 million kgs (527 million pounds) of herbicide applied in the 1996–2011 period, soybean accounted for 70 % of the total increase across the three HT crops, mostly from reliance on glyphosate. Furthermore, the data show that although pesticide use decreased from 1996-2002, it then increased since 2002 as the result of two major factors.
The first is a reduction of herbicides applied to non-HT crops, which can be explained by the increasing progress made by the pesticide industry in discovering more potent herbicidal active ingredients effective at progressively lower levels. Between 1996 and 2006, the number of registered soybean herbicides applied at levels below 0.11 kg/ha increased from nine to 17. As a result, the amount of herbicides applied to conventional crops has steadily fallen since 1996. In contrast, glyphosate is a relatively high-dose herbicide usually applied at between 0.67 to 0.9 kg/ha.
The second major factor for the increase in pesticide use due to GM crops is the spread of glyphosate-resistant weeds. That, the authors suggest, is by far the most important factor in driving up herbicide use over the last decade.
Since the emergence of glyphosate-resistant ryegrass in Australia in 1996, there has been an additional 22 species of resistant species that have forced some farmers to abandon their crops due to the severity and resilience of the so-called “super-weeds” (see  Monsanto Defeated by Roundup-Resistant Weeds SiS 53). The majority of resistant species have been documented since 2005, reflecting the extreme selection pressure placed on plants as a result of wide-scale glyphosate application.
Pest resistance to Bt toxins has only been recently documented and does not therefore parallel the impacts seen with glyphosate-resistant weeds. However, with populations of corn rootworm resistant to Cry3Bb1 toxin  (see  Bt Resistant Rootworm Spreads, SiS 52), reduced insecticide use is in jeopardy, particularly when companies advise farmers to revert back to soil-based insecticides as a management tool. Not only is this increasing the pesticide burden, but completely negates the purpose of Bt crops in the first place.
An important point brought up by the study is the notion of counting Bt toxins produced by GM crops as “insecticides applied”. When Bt toxins are applied exogenously, they would be included in insecticide use counts. Although there are differences between exogenous Bt and that produced by GM plants, the environmental burden may indeed be the same. Further, GM Bt crops increases the likelihood of ingestion of the insecticide by animals and humans, posing more risk than Bt sprays which can be washed off and break down in sunlight. Bt crops have been associated with health problems including respiratory problems, allergies and immunogenicity, internal organ damage, as well as skin and eye damage (see  Bt Crops, Failures and Hazards, SiS 53).
With the proposed deregulation of GM crops tolerant to the herbicide 2.4-D, this herbicide was projected to increase an estimated 30-fold from 2010 levels if 55 % of HT corn is 2,4-D resistant coupled with 2.3 applications at 0.94 kg/ha on average. The herbicide has been linked to many health problems including congenital birth defects, so this dramatic rise in 2,4-D application would be a huge public health concern.
Benbrook, C. Impacts of genetically engineered crops on pesticide use in the U.S. — the first sixteen years.Environmental Sciences Europe 2012, 24:24