Based on evidence accumulated from more than a decade of short- and long-term studies carried out in different geographical areas, Bt crops are a novel and safe pest control technology that will improve agro-ecosystems because the spectrum of activity of these crops against insects and other nontarget invertebrates is so much narrower than that of synthetic chemical insecticides. To date, no significant or long-term detrimental impacts have been found on nontarget insect populations or those of other invertebrates under operational growing conditions in Bt cotton or Bt corn fields, other than for obligate parasites such as parasitic wasps dependent on the target pests as hosts. Studies employing high doses of Cry toxins have identified some negative effects of Bt crops on nontarget insects under laboratory conditions, but subsequent field studies have shown that the risk to these populations is negligible. In addition, feeding studies conducted in the laboratory against a range of nontarget vertebrates have shown no detrimental effects.
Although the public remains concerned about the safety of Bt crops owing to negative reports about these in the popular press, there is no evidence that Bt insecticides and Bt crops pose risks for humans any greater than those that result from eating non-genetically engineered crops. Thus, as former President Jimmy Carter125 wrote almost 10 years ago, the panic over genetically modified plants is completely uncalled-for. Similar views have been expressed by many scientific organizations with expertise about Bt crops, as well as by the (U.S.) National Academy of Sciences, the American Society for Microbiology, and many other scientific societies. Some organizations such as the Union of Concerned Scientists, and several other predominantly lay groups such as the Environmental Defense Fund and Friends of the Earth, remain strongly opposed to Bt crops on the basis that the long-term safety of these crops has not been demonstrated for either nontarget organisms or humans. Although there is always the possibility that these crops will have some detrimental effects, none has been identified at this time. Moreover, risk/benefit analyses conducted by the EPA and other agencies show that the benefits of using Bt crops far outweigh the risks. As noted above, plantings of Bt crops in the United States have already resulted in annual reductions of millions of pounds of chemical insecticide use, which also reduces soil erosion because the use of equipment to apply these has decreased. As far as direct benefits to human health, this has led to significant reductions in human pesticide poisonings, which in 1996 were estimated to be more than 100,000 per year.126 More extensive deployment of this new technology will significantly reduce the use of synthetic chemical insecticides worldwide, thereby further reducing human insecticide poisonings and deaths due to these, while at the same time benefiting nontarget arthropod populations.
While the long-term field studies cited above were under way, several researchers (including Marvier127) called for long-term studies with greater statistical power. These suggestions were merited given the short-term nature and small plots evaluated in the first field studies of transgenic crops.101103 However, it has recently been suggested that even more detailed and more rigorous ecological studies be conducted prior to the release of new Bt crops. A paper with the peculiar title "Science-Based Risk Assessment for Non-Target Effects of Transgenic Crops"128 (which implies that earlier studies of risk assessment were not based on scientific principles), recommends that purified transgene products be used to evaluate long-term effects on selected nontarget species, which would be selected based on their functional roles in ecological webs. Any effects detected would be followed by studies involving whole transgene plants. All developmental stages of the nontarget insect would be studied. The tests would include evaluations of tritrophic interactions. Although from an ecological standpoint these types of studies could produce interesting data, there is no reason to think that the overall outcome of such studies would provide more meaningful findings on the effects of Bt crops on nontarget organisms than the assessments obtained in the long-term studies cited above. Moreover, a paradox here is that no such studies are required for synthetic chemical insecticides, although it is well known that these have a much greater impact on nontarget populations. The point is that because the transgene products in Bt crops are biodegradable Cry proteins, current long-term studies in which several years of data are collected on a variety of nontarget species will reveal any significant nontarget effects, and these studies will be much more informative than those required for chemical insecticides.
Given the high level of concern for nontarget arthropods by scientists,129 governmental agencies, and groups opposed to Bt crops, it is worthwhile to remember that agro-ecosystems are not natural ecosystems. They have a much greater abundance of certain individual species of insects and lower species diversity than occurs in natural ecosystems in the same geographical areas prior to the advent of agriculture. Thus, even if there were significant nontarget effects in, for example, a corn ecosystem, the species affected most likely occurred in even lower populations year after year in the original natural ecosystem. A very appropriate example given the controversy it inspired is the Monarch butterfly. Because milkweed grows in open and only partially shaded fields, this plant was uncommon within forests that dominated much of the midwestern and eastern region of the U.S. corn belt. Therefore, Monarch populations were likely much lower prior to the clearing of these forests and planting of various field crops. For example, in Indiana, a major corn-growing state, it is estimated that forests covered 85% of the state in 1800. By 1860, approximately 50% of the forests had been cleared, with much of the cleared land being devoted to farming.130 Indiana is typical of other states in the corn belt, and as we know that the milkweed on which Monarch larvae feed is commonly found around and even in corn fields, milkweed and associated Monarch populations expanded significantly, perhaps by as much as two-fold, as the forests were cleared for agriculture. Studies have shown that Bt corn is not likely to have any major impact on Monarch populations.105,106 Nevertheless, for the sake of argument, even if Monarch populations were reduced by 20 percent and stable but lower population resulted, in all probability this population would be much larger than that which existed during the early nineteenth century. Such an analysis applies to most of the corn belt, not to mention areas outside this major U.S. region.
The point to emphasize here is that the yearly diversity and variations in the size of nontarget arthropod populations in agro-ecosystems are artifacts created by human activity — they are not natural to begin with. Though they are contrived and unbalanced ecosystems, we want to maintain their peculiar ecological distortions and do this in a "green" or environmentally compatible manner. Bt crops are better at doing this than any other routinely applied pest control technology we have developed for large-scale agriculture, upon which we depend so much, over the past century.
Despite the clear environmental and human health benefits of Bt crops (the latter including reductions in mycotoxin levels in addition to chemical insecticide usage), numerous articles continue to appear in the popular press creating on concern (if not fear) about these crops. A recent example was an editorial by Deborah Rich of The Providence Journal.13 With respect to Bt crops, although she acknowledged that
Americans have been eating "thousands" of products containing foodstuffs derived from Bt corn, she cited studies of effects on rat kidneys and intestinal cells disputed in the scientific community years ago, noting that these should "give us pause." 131,132 Articles like these, which routinely and significantly misinterpret and distort the scientific literature on Bt crop safety, are rarer than before, yet they nevertheless wind up causing alarm among the public.
In summary, Bt crops are arguably the most significant advance in insect crop protection technology of the latter half of the twentieth century due to their unparalleled high degree of efficacy and safety. Bacterial insecticides that employ the same proteins as active ingredients have been used for more than 40 years, with few if any health effects on humans or other vertebrates or on nontarget invertebrates, with the exception of species closely related to target insect pests. Evidence for Bt crop safety comes from studies of the effects of Bt and Cry proteins on nontarget organisms tested in the laboratory and field, as well as from knowledge of the Cry protein mode of action. Although it has been suggested that Bt insecticides may be a cause of food poisoning, this has not been shown in a single case, and a variety of studies make this possibility highly unlikely. Over the past decade, Bt cotton and Bt corn have been widely adopted by farmers in the United States, with acreage approaching 50% of the area planted with cotton or corn in 2006. Recent multiyear studies of the effects of Bt crops on nontarget invertebrates under operational growing conditions have demonstrated significant environmental and economic benefits, including substantial reductions in the use of synthetic chemical insecticides, preservation of natural enemies of insect pests, reduction in mycotoxin levels, and increases in crop yields. These beneficial results are expanding the use of Bt crops in Australia, China, India, and several other countries. Over the next few decades, the rapidly evolving technology of controlling plant pests and pathogens directly through the plant will, with appropriate diligence, likely result in a variety of new pest management programs that are much safer for the environment and our food supply.
In closing, opponents of Bt crops will eventually have to accept the reality that this pest control technology is here to stay and will continue to expand worldwide. This still-new technology is simply too powerful and offers too many benefits to humanity to not be further developed and deployed.
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