Thanks for your question. The study you are referring to discusses insect-resistant (IR) cotton and herbicide-tolerant (HT) soybeans. You might be interested in two recent responses that answer similar questions.
Plant genetics expert Daniela Brioschi responded to a question about insect-resistant crops and the recent worm infestation in Brazil. An excerpt is below:
“The attack of insects, from planting to close to harvest, is a limiting factor for crop productivity, regardless of the production system used. Under favorable environmental conditions and/or in the absence of the use of appropriate management practices, the proliferation of insects can reach levels of economic damage and cause losses.
“In the 2012–2013 crop season, there were reports on the occurrence of worm infestations in Brazil, the damages of which were observed in cotton and soybean crops in western Bahia, south of Maranhão and Piauí. In the Cerrado region, there are also reports of attacks on corn, cotton, beans, millet, sorghum and soybeans. The worms referred to belong to the species Helicoverpa spp., Heliothis virescens and Spodoptera spp.
“Brazilian farmers have opted for the use of genetically modified (GM) seeds containing the Bt technology as an alternative to control pests. This technology is widely used in the country because of its efficiency. Moreover, it presents other advantages, such as: no adverse effects to non–target organisms, maintenance of biological control agents in nature, as well as the rational use of insecticides, resulting in social and environmental benefit due to the decreased risk of contamination by undue exposure.
“With the increased adoption of this technology in corn and cotton, and given the occurrence of insect attacks to Bt plants, the effectiveness of transgenic crops has been questioned. Therefore, some clarifications are needed to elucidate the reasons for the infestation, confirm the effectiveness of the technology and reinforce the recommendation of control measures…”
The full response is available here: http://gmoanswers.com/clarifications-pest-infestations-brazil-0.
You also might be interested in a response from Graham Brookes, agricultural economist at PG Economics Ltd, UK, which answers a question about the use of herbicides and pesticides with GM crops:
“…The GM insect-resistant (GM IR) technology provides a form of protection against pests and often replaces insecticides as a form of control. In corn and cotton, the use of GM insect-resistant technology has resulted in major reductions in the usage of insecticides that have been traditionally used to control the pests the GM technology now controls. For example, between 1996 and 2011, the use of insecticides on these crops in the countries using the technology has fallen by nearly 240 million kg of insecticide active ingredient.
“The GM herbicide-tolerant (GM HT) technology allows farmers to simplify and improve their weed control through the use of one or two herbicides that are effective against a broad range of weeds, instead of having to often rely on the use of a larger number of herbicides that are more selective in their ability to control weeds. In other words, the adoption of this GM technology has resulted in a change in the profile of herbicides used in many countries. In some (mostly developing) countries, the GM HT technology has also enabled farmers to significantly improve their weed control by replacing hand weeding, which is unpopular and difficult to find people willing to do. Not surprisingly, the impact of adoption of this technology on herbicide usage varies by crop, country and time. Using the USA as an example, in the early years of adoption across all crops, GM HT technology use resulted in significant aggregate reductions in the volume of (weight of active ingredient) herbicides used in crops such as corn and canola. However, there were differences among the crops, and in some, such as soybeans, the average amount of herbicide active ingredient applied remained largely unaltered, or increased, in the case of sugar beet.
“Since the mid-2000s, in the main crops of corn, cotton and soybeans in the USA, the average amount of herbicide applied to crops has tended to increase. The main reason for this has been an increasing incidence of weed species becoming resistant to the main herbicide used with GM HT crops, glyphosate, and increasing recognition among farmers, coupled with both public- and private-sector weed scientist recommendations, that weed management programmes should diversify and not rely on a single herbicide for total weed control. Farmers have therefore increasingly incorporated one or two other herbicides, in addition to glyphosate, into their weed-management programmes.
“The development of weed species being resistant to herbicides should, however, be placed in context. Nearly all weed species have the potential to develop resistance to herbicides, and there are hundreds of resistant weed species confirmed in the International Survey of Herbicide Resistant Weeds (www.weedscience.org). Reports of herbicide-resistant weeds predate the use of GM HT crops by decades. The development of weeds resistant to herbicides is therefore a problem faced by all farmers, not just those using GM HT technology. In fact, GM HT technology offered a solution to controlling some weeds that had developed resistance to mainstream herbicides used in soybeans in the mid-1990s. The use of herbicides on conventional (non-GM) arable crops in the USA is equally affected by weed-resistance issues, and herbicide use patterns on conventional crops have followed the upward trends that have occurred in GM HT crops.”
The full response is available here: http://gmoanswers.com/ask/why-are-genetically-modified-crops-increasing-use-herbicides-and-pesticides-i-thought-claim-was. If you have any additional questions, please ask.