August 11, 2011 | 638
By the turn of the century, the number of people on Earth is expected to increase from the current 6.7 billion to 10 billion. How can we feed the growing population without further degrading the environment?
Because the amount of land and water is limited, it is no longer possible to simply expand farmland to produce more food. Instead, increased food production must largely take place on the same land area, while using less water. Compounding the challenges facing agricultural production are the predicted effects of climate change: flooding in some places, droughts in others and new pests and disease outbreaks.
Thus, an important goal for the US and other countries is to develop more effective land and water use policies, improved integrated pest management approaches, reduce harmful inputs, and create new crop varieties tolerant of diverse stresses.
These strategies must be evaluated in light of their environmental, economic, and social impacts—the three pillars of sustainable agriculture (Committee on the Impact of Biotechnology on Farm-Level Economics and Sustainability and National Research Council 2010).
WHAT ARE GENETICALLY ENGINEERED CROPS?
Genetic engineering differs from conventional methods of genetic modiﬁcation in two major ways: (1) genetic engineering introduces one or a few well-characterized genes into a plant species and (2) genetic engineering can introduce genes from any species into a plant. In contrast, most conventional methods of genetic modiﬁcation used to create new varieties (e.g., artiﬁcial selection, forced interspeciﬁc transfer, random mutagenesis, marker-assisted selection, and grafting of two species, etc.) introduce many uncharacterized genes into the same species. Conventional modiﬁcation can in some cases transfer genes between species, such as wheat and rye or barley and rye.
In 2008, the most recent year for which statistics are available, 30 genetically engineered crops were grown on almost 300 million acres in 25 countries (nearly the size of the state of Alaska), 15 of which were developing countries (James 2009). By 2015, 120 genetically engineered crops (including potato and rice) are expected to be cultivated worldwide (Stein and Rodriguez-Cerezo 2009). Half of the increase will be crops designed for domestic markets from national technology providers in Asia and Latin America.
SAFETY ASSESSMENT OF GENETICALLY ENGINEERED CROPS
There is broad scientiﬁc consensus that genetically engineered crops currently on the market are safe to eat. After 14 years of cultivation and a cumulative total of 2 billion acres planted, no adverse health or environmental effects have resulted from commercialization of genetically engineered crops (Board on Agriculture and Natural Resources, Committee on Environmental Impacts Associated with Commercialization of Transgenic Plants, National Research Council and Division on Earth and Life Studies 2002). Both the U.S. National Research Council and the Joint Research Centre (the European Union’s scientiﬁc and technical research laboratory and an integral part of the European Commission) have concluded that there is a comprehensive body of knowledge that adequately addresses the food safety issue of genetically engineered crops (Committee on Identifying and Assessing Unintended Effects of Genetically Engineered Foods on Human Health and National Research Council 2004; European Commission Joint Research Centre 2008).
These and other recent reports conclude that the processes of genetic engineering and conventional breeding are no different in terms of unintended consequences to human health and the environment (European Commission Directorate-General for Research and Innovation 2010). This is not to say that every new variety will be as benign as the crops currently on the market. This is because each new plant variety (whether it is developed through genetic engineering or conventional approaches of genetic modiﬁcation) carries a risk of unintended consequences. Whereas each new genetically engineered crop variety is assessed on a case-bycase basis by three governmental agencies, conventional crops are not regulated by these agencies.
Still, to date, compounds with harmful effects on humans or animals have been documented only in foods developed through conventional breeding approaches. For example, conventional breeders selected a celery variety with relatively high amounts of psoralens to deter insect predators that damage the plant. Some farm workers who harvested such celery developed a severe skin rash—an unintended consequence of this breeding strategy (Committee on Identifying and Assessing Unintended Effects of Genetically Engineered Foods on Human Health and National Research Council 2004).
“A truly extraordinary variety of alternatives to the chemical control of insects is available. Some are already in use and have achieved brilliant success. Others are in the stage of laboratory testing. Still others are little more than ideas in the minds of imaginative scientists, waiting for the opportunity to put them to the test. All have this in common: they are biological solutions, based on the understanding of the living organisms they seek to control and of the whole fabric of life to which these organisms belong. Specialists representing various areas of the vast ﬁeld of biology are contributing—entomologists, pathologists, geneticists, physiologists, biochemists, ecologists—all pouring their knowledge and their creative inspirations into the formation of a new science of biotic controls.” (Carson 1962, p. 278)
In the 1960s, the biologist Rachel Carson brought the harmful environmental and human health impacts resulting from overuse or misuse of some insecticides to the attention of the wider public. Even today, thousands of pesticide poisonings are reported each year (1200 illnesses related to pesticide poisoning in California, 300,000 pesticide-related deaths globally).
This is one reason some of the ﬁrst genetically engineered crops were designed to reduce reliance on sprays of broad-spectrum insecticides for pest control. Corn and cotton have been genetically engineered to produce proteins from the soil bacteria Bacillus thuringiensis (Bt) that kill some key caterpillar and beetle pests of these crops. Bt toxins cause little or no harm to most beneﬁcial insects, wildlife, and people (Mendelsohn et al. 2003).
Bt toxins kill susceptible insects when they eat Bt crops. This means that Bt crops are especially useful for controlling pests that feed inside plants and that cannot be killed readily by sprays, such as the European corn borer (Ostrinia nubilalis), which bores into stems, and the pink bollworm (Pectinophora gossypiella), which bores into bolls of cotton.
First commercialized in 1996, Bt crops are the second most widely planted type of transgenic crop. Bt toxins in sprayable formulations were used for insect control long before Bt crops were developed and are still used extensively by organic growers and others. The long-term history of the use of Bt sprays allowed the Environmental Protection Agency and the Food and Drug Administration to consider decades of human exposure in assessing human safety before approving Bt crops for commercial use. In addition, numerous toxicity and allergenicity tests were conducted on many different kinds of naturally occurring Bt toxins. These tests and the history of spraying Bt toxins on food crops led to the conclusion that Bt corn is as safe as its conventional counterpart and therefore would not adversely affect human and animal health or the environment (European Food Safety Authority 2004).
Planting of Bt crops has resulted in the application of fewer pounds of chemical insecticides and thereby has provided environmental and economic beneﬁts that are key to sustainable agricultural production. In Arizona, where an integrated pest management program for Bt cotton continues to be effective, growers reduced insecticide use by 70% and saved .$200 million from 1996 to 2008 (Naranjo and Ellsworth 2009).
A recent study indicates that the economic beneﬁts resulting from Bt corn are not limited to growers of the genetically engineered crop (Hutchison et al. 2010). In 2009, Bt corn was planted on .22.2 million hectares, constituting 63% of the U.S. crop. For growers of corn in Illinois, Minnesota, and Wisconsin, cumulative beneﬁts over 14 years are an estimated $3.2 billion. Importantly, $2.4 billion of this total beneﬁt accrued to non-Bt corn (Hutchison et al. 2010). This is because area-wide suppression of the primary pest, O. nubilalis, reduced damage to non-Bt corn. Comparable estimates for Iowa and Nebraska are $3.6 billion in total, with $1.9 billion for non-Bt corn. These data conﬁrm the trend seen in some earlier studies indicating that communal beneﬁts are sometimes associated with planting of Bt crops (Carriere et al. 2003; Wu et al. 2008; Tabashnik 2010).
Planting of Bt crops has also supported another important goal of sustainable agriculture: increased biological diversity. An analysis of 42 ﬁeld experiments indicates that nontarget invertebrates (i.e., insects, spiders, mites, and related species that are not pests targeted by Bt crops) were more abundant in Bt cotton and Bt corn ﬁelds than in conventional ﬁelds managed with insecticides (Marvier et al. 2007). The conclusion that growing Bt crops promotes biodiversity assumes a baseline condition of insecticide treatments, which applies to 23% of corn acreage and 71% of cotton acreage in the United States in 2005 (Marvier et al. 2007).
Beneﬁts of Bt crops have also been well-documented in less-developed countries. For example, Chinese and Indian farmers growing genetically engineered cotton or rice were able to dramatically reduce their use of insecticides (Huang et al. 2002, 2005; Qaim and Zilberman 2003; Bennett et al. 2006). In a study of precommercialization use of genetically engineered rice in China, these reductions were accompanied by a decrease in insecticide-related injuries (Huang et al. 2005).
Although Bt cotton is effective in reducing cotton bollworm outbreaks in China other pests that are not killed by Bt cotton are increasingly problematic (Wu Review 13et al. 2008; Lu et al. 2010). These results conﬁrm the need to integrate Bt crops with other pest control tactics (Tabashnik et al.
2010). In Arizona, such an integrated pest management (IPM) approach has been implemented (Naranjo and Ellsworth 2009). In Arizona’s cotton IPM system, key pests not controlled by Bt cotton are managed with limited use of narrow-spectrum insecticides that promote conservation of beneﬁcial insects (Naranjo and Ellsworth 2009). Mirids such as the Lygus bug (Lygus hesperus) are controlled with a feeding inhibitor, and the sweet potato whiteﬂy (Bemisia tabaci) is controlled with insect growth regulators (Naranjo and Ellsworth 2009).
One limitation of using any insecticide, whether it is organic, synthetic, or genetically engineered, is that insects can evolve resistance to it. For example, one crop pest, the diamondback moth (Plutella xylostella), has evolved resistance to Bt toxins. This resistance occurred in response to repeated sprays of Bt toxins to control this pest on conventional (nongenetically engineered) vegetable crops (Tabashnik 1994).
These results underscore a well-known paradigm in agriculture: pest resistance will evolve is the selection pressure is high. Why then, have most Bt crops remained effective against most pests for more than a decade (Tabashnik et al. 2008; Carriere et al. 2010)? The answer is genetic diversity. The inclusion in farmers fields of crop plants that do not make Bt toxins has helped to delay evolution of pest resistance to Bt crops (Carriere et al. 2010).
In cases where insect resistance to Bt crops has evolved, one or more conditions of this crop diversity strategy have not been met. For example, failure to provide adequate refuges of non-Bt cotton appears to have hastened resistance of pink bollworm in India (Bagla 2010). In contrast, Arizona cotton growers complied with this strategy from 1996 to 2005, and no increase in pink bollworm resistance occurred (Tabashnik et al. 2010).
In the United States, Bt cotton producing only Cry1Ac is no longer registered and has been replaced primarily by Bt cotton that produces two toxins (Carriere et al. 2010). More generally, most newer cultivars of Bt cotton and Bt corn produce two or more toxins. These multitoxin Bt crops are designed to help delay resistance an to kill a broader spectrum of insect pests (Carriere et al. 2010). For example, a new type of Bt corn produces ﬁve Bt toxins—three that kill caterpillars and two that kill beetles (Dow Agrosciences 2009).
Despite the success of the crop diversity strategy in delaying insect resistance to Bt crops, this approach has limitations, including the fact that not all farmers will comply. An alternative strategy entails release of sterile insects to mate with resistant insects (Tabashnik et al. 2010). Incorporation of this strategy in a multi-tactic eradication program in Arizona from 2006 to 2009 reduced pink bollworm abundance by 99%, while eliminating insecticide sprays against this pest. The success of such creative multidisciplinary integrated approaches, involving entomologists, geneticists, physiologists, biochemists, and ecologists, provides a roadmap for the future of agricultural production and attests to the foresight of Rachel Carson.
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