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GENETICALLY MODIFIED FOODS
Are genetically modified crops an environmental dream come-true or a disaster in the making Scientists are looking for answers.
The world seems increasingly divided into those who favor genetically modified (GM) foods and those who fear them Advocates assert that growing genetically altered crops can be kinder to the environment and that eating foods from those plants is perfectly safe. And, they say genetic engineering which can induce plants to grow in poor soils or to produce more nutritious foods will soon become an essential tool for helping to feed the world’s burgeoning population. Skeptics contend that GM crops could pose unique risks to the environment and to health risks too troubling to accept placidly, Taking that view, many European countries are restricting the planting and importation of GM agricultural products. Much of the debate hinges on perceptions of safety. But what exactly does recent scientific research say about the hazards The answers, too often lost in reports on the controversy, are served up in the pages that follow.
Two years ago in Edinburgh, Scotland, eco-vandals stormed a field, crushing canola plants. Last year in Maine, midnight raiders hacked down more than 3000 experimental poplar trees. And in Sun Diego, protesters smashed sorghum and sprayed paint over greenhouse walls.
This far-flung outrage took aim at genetically modified crops. But the protests backfired: all the destroyed plants were conventionally bred. In each case, activists mistook ordinary plants for GM varieties.
It’s easy to understand why. In a way, GM crops—now on some 109 million acres of farmland worldwide—are invisible. You can’t see, taste or touch a gene inserted into a plant or sense its effects on the environment. You can’t tell, just by looking, whether pollen containing a foreign gene can poison butterflies or fertilize plants miles away. That’ invisibility is precisely what worries people. How, exactly, will GM crops affect the environment-and when will we notice
Advocates of GM, or transgenic, crops say the plants will benefit the environment by requiting fewer toxic pesticides than conventional crops. But critics fear the potential risks and wonder how big the benefits really are. "We have so many questions about these plants," remarks Guenther Stotzky, a soil microbiologist at New York University. "There’s a lot we don’t know and need to find out."
As GM crops multiply in the landscape, unprecedented numbers of researchers have started fanning into the fields to get the missing information. Some of their recent findings are reassuring; others suggest a need for vigilance.
Fewer Poisons in the Soil
Every year U.S. growers shower crops with an estimated 971 million pounds of pesticides, mostly to kill insects, weeds and fungi. But pesticide residues linger on crops and the surrounding soil, leaching into groundwater, running into streams and getting gobbled up by wildlife. The constant chemical trickle is an old worry for environmentalists.
In the mid-1990s agribusinesses began advertising GM seeds that promised to reduce a farmer’s use of toxic pesticides. Today most GM crops-mainly soybean, com, cotton and canola-contain genes enabling them to either resist insect pests or tolerate weed-killing herbicides. The insect-resistant varieties make their own insecticide, a property meant to reduce the need for chemical sprays. The herbicidetolerant types survive when exposed to broad-spectrum weed killers, potentially allowing farmers to forgo more poisonous chemicals that target specific weed species. Farmers to limit the use of more hazardous pesticides when they can, but GM crops also hold appeal because they simplify operations ( reducing the frequency and complexity of pesticide applications) and, in some cases, increase yields.
But confirming environmental benefit is tricky. Virtually no peer-reviewed papers have addressed such advantages, which would be expected to vary from plant to plant and place to place. Some information is available, however. According to the U.S. Department of Agriculture, farmers who plant herbicidetolerant crops do not necessarily use fewer sprays, but they do apply a more benign mix of chemicals. For instance, those who grow herbicide-tolerant soybeans typically avoid the most noxious weed killer, turning instead to glyphosate herbicides (苷磷除草剂), which are less to toxic and degrade more quickly.
Insect-resistant crops also bring mixed benefits. To date. insect resistance has been provided by a gene from the soil bacterium Bacillus thuringiensis (Bt) (杆菌苏立菌) This gene directs cells to manufacture a Crystalline protein that is toxic to certain insects-especially caterpillars and beetles that gnaw on crops-but does not harm other organisms. The toxin gene m different strains of B. thuringiensis can affect different mixes of insects, so seed makers can select the version that seems best suited to a particular crop.
Of all the crops carrying Bt genes, cotton has brought the biggest drop in pesticide use. According to the Environmental Protection Agency, in 1999 growers in states using high amounts of Bt cotton sprayed 21 percent less insecticide than usual on the crop. That’s a "dramatic and impressive" reduction, says Stephen Johnson, an administrator in the EPA’s Office of Pesticide Programs. Typically, Johnson says, a farmer might spray insecticides on a cotton field 7 to 14 times during a single growing season. "If you choose a Bt cotton product, you may have little or no use for these pretty harsh chemicals," he notes. Growers of Bt com and potatoes report less of a pesticide reduction, partly because those plants normally require fewer pesticides and face fluctuating numbers of pests.
Defining the environmental risks of GM crops seems even harder than calculating their benefits. At the moment, public attention is most trained on Bt crops, thanks to several negative studies. Regulators, too, are surveying the risks intensely. This spring or summer the EPA is expected to issue major new guidelines for Bt crops, ordering seed producers to show more thoroughly that the crops can be planted safely and monitored in farm fields.
At What Cost to Wildlife
In 1998 a Swiss study provoked widespread worry that Bt plants can inadvertently harm unlucky creatures. In this laboratory experiment, green lacewing(草蛉) caterpillars proved more likely to die after eating European corn-borer caterpillars that had fed on Bt com instead of regular corn. The flames of fear erupted again a year later, when Cornell University entomologist John Losey and his colleagues reported that riley had fed milkweed (乳草属植物) leaves dusted with Bt corn pollen to monarch butterfly larvae in the lab and that those larvae, too, had died.
"That was the straw that broke the camel’s back." says David Pimentel, also an entomologist at Cornell. Suddenly, all eyes turned to the organisms munching GM plant leaves, nipping modified pollen or wriggling around in the soil below the plants-organisms that play vital roles in sustaining plant populations. Another alarming study relating to monarch but-terries appeared last August.
But the lab bench is not a farm field, and many scientists question the usefulness of these early experiments. The lab insects, they note, consumed far higher doses of Bt toxin than they would outside, in the real world. So researchers have headed into nature themselves, measuring the toxin in pollen from plots of GM com, estimating how much of it drifts onto plants such as milkweed and, finally, determining the exposure of butterfly and moth larvae to the protein. Much of that work, done during the 2000 growing season, is slated to be reported to the EPA shortly.
According to the agency, however, preliminary studies evaluating the two most common Bt corn plants (from Novartis and Monsanto) already indicate that monarch larvae encounter Bt corn pollen on milkweed plants but at levels too low to be toxic. What is toxic The EPA estimates that the insects face no observable harm when consuming milkweed leaves laden with up to 150 corn pollen grains per square centimeter of leaf surface. Recent studies of milkweed plants in and around the cornfields of Maryland, Nebraska and Ontario report far lower levels of Bt pollen, ranging from just 6 to 78 grains of Bt corn pollen per square centimeter of milkweed leaf surface. "The weight of the evidence suggests Bt corn pollen in the field does not pose a hazard to monarch larvae," concludes EPA scientist Zigfridas Vaituzis, who heads the agency’s team studying the ecological effects of Bt crops.
But the jury is still out. "There’s not much evidence to weigh." notes Jane Rissler of the Union of Concerned Scientists. "This issue of nontarget effects is just a black hole. and EPA has very little good data at this point to conclude whether the monarch butterfly problem is real, particularly in the long term."
In an EPA meeting on GM crops last fall. Vaituzis acknowledged the lack of long-term data on Bt crops and insect populations. Such studies "require more time than has been available since the registration of Bt crops," Vaituzis remarked. The EPA, he added, continues to collect Bt crop data but so far without evidence of "unreasonable adverse effects" on insects in the field.
Rice is one of the major GM crops containing genes which help to either resist insect or tolerate weed - killing herbicides.