According to this story, as controversy builds over the safety of genetically modified crops, the evidence so far hasn't pinpointed any specific problems but also can't dispel the doubts An American entomologist publishes a study in Nature showing that pollen from genetically modified corn kills monarch butterfly larvae; two of his colleagues denounce him in a commentary for publishing "preliminary results" and imply that he is spreading rumors. A British food-safety expert writes in Nature that a concept that underlies regulation of genetically modified food in most of the developed world is "pseudoscientific"; an opponent fires back in a letter calling his commentary "a mish-mash of old-hat sociology and poor science." A British scientist announces on television that genetically modified potatoes stunt the growth of rats and damage their immune system; his supervisors suspend him 2 days later. It would be hard, the story says, to find a scientific debate more polarized than the one now being waged about the safety of genetically modified (GM) crops.

But while biotech opponents talk of Frankenfoods and terminator genes and industry groups minimize safety concerns, a small group of investigators has been taking a serious look at GM crops to see what health and environmental risks they might pose. What they are finding is in many cases reassuring - but not always. The plants, most of which have been modified to resist pests or weed-killing herbicides, seem to pose minimal risks to human health, say experts. But environmental concerns such as the possibility that the novel genes might spread to wild plants and produce new strains of weeds, although hard to substantiate, are also proving hard to dispel. The story says that complicating the weighing of risk is the question of how much any potential hazards are offset by the crops' potential benefits, such as reducing the use of chemical pesticides, lowering costs, and improving nutritional value.

Plant pathologist James Cook of Washington State University in Pullman, who chaired an international panel to devise risk assessment methods for GM crops, was cited as saying that part of the problem is that, unlike drugs or pesticides, plants have never been subjected to a risk analysis, adding, "We have to ask what are the safety issues raised by plants, then apply that to crop plants with transgenes."

Food-safety concerns have stirred the most passionate debate among the public, prompting boycotts, bans, and protests. But few accept the conclusions of the report that sparked the furor over GM potatoes in Britain (Science, 22 October, p. 656). And there's little other research that might raise concerns that the transgenic crops now on the market threaten human health. Microbiologist Abigail Salyers of the University of Illinois, Urbana-Champaign was quoted as saying, "There's something wrong with the perception of risk here."

For example, GM food critics worry about plant genetic engineers' practice of attaching the genes they are trying to introduce into plants to an antibiotic-resistance gene. They can then readily select those plants that have acquired the genes by treating them with the antibiotic, which kills any nonresistant plants. The critics charge that the antibiotic-resistance genes, which sometimes remain in the transgenic crops, could spread to pathogens in the body and make antibiotics less effective. But several panels of antibiotic-resistance experts have concluded otherwise. Salyers was quoted as saying, "Unanimously, the verdict has been that the chance of antibiotic-resistance genes getting into intestinal bacteria is minuscule," says. And if they did get in, "the virtually unanimous verdict is that it wouldn't matter" because the same resistance genes are already present in many of the bugs.

A more plausible - though still unlikely - threat to human health from transgenic foods comes from food allergies. An allergic reaction to food can be serious, even life-threatening, if it leads to anaphylactic shock. Food microbiologist Bruce Chassy of the University of Illinois, Urbana-Champaign, a former food-safety adviser to the U.S. Food and Drug Administration, was quoted as saying "That's one you certainly want to worry about."

Indeed, in a study reported in 1996 in The New England Journal of Medicine, Steve Taylor and his colleagues at the University of Nebraska, Lincoln, showed that people allergic to Brazil nuts are also allergic to soybeans that have been engineered to express a Brazil nut protein to make them more nourishing.

The story says that to Chassy, the outcome was reassuring: The results led the producer of the transgenic bean, Pioneer Hi-Bred International, to discontinue the soy line voluntarily before it was commercialized. What's more, the producers of GM foods screen their products for allergenicity, he says. Among other methods, they can check to see if the amino acid sequences of the proteins made by the genes they put into crop plants resemble those of known food allergens.

Critics say that because many proteins that trigger allergic reactions have not yet been sequenced, the sequence comparison test will fail to detect some allergens. Rebecca Goldburg, senior scientist at the Environmental Defense Fund in New York City was quoted as saying, "If you find a match, then you have a problem. If you don't, it doesn't say anything." But Chassy was cited as noting that conventional foods already on the market, such as peanuts and Brazil nuts, pose much higher risks of allergies than GM foods, as do plants produced by classical breeding methods, which introduce many potential allergens into the product, adding, "If a zero-risk standard prevails, we shouldn't put any new food on the market and we should get rid of a lot of old ones."

It's the potential environmental effects of GM crops that stir deeper scientific debates, as was evident at a recent meeting, held near Chicago on 2 November, that examined whether pollen from so-called "Bt corn" corn containing an insecticidal protein from the bacterium Bacillus thuringiensis could harm monarch butterflies in the field. The colorful monarch became the poster child for the anti-GM movement last May, after entomologist John Losey and colleagues at Cornell University published a short laboratory study in Nature showing that Bt corn pollen could kill monarch butterfly caterpillars in the laboratory.

The story says that out of a group of caterpillars that had munched on milkweed leaves - the larvae's only food source - dusted with Bt corn pollen, 44% died within 4 days, while larvae eating leaves dusted with ordinary pollen all survived.

Although perhaps not surprising - researchers had known for years that Bt bacteria, which are themselves widely used as pesticidal sprays, could harm a variety of butterflies and moths - the Losey study was the first one published showing that a Bt plant could directly harm a non target butterfly.

The study attracted widespread media attention and alarmed biotech observers throughout the world.

The story goes on to say that at issue in the new monarch studies is just how far pollen might drift from cornfields, and how toxic it might be to any monarch larvae that eat it. The potential for harm is certainly there, says entomologist John Obrycki of Iowa State University in Ames. Because corn pollen is relatively heavy, it is likely to settle near cornfields. What's more, Obrycki says, "we do find lots of milkweeds growing near cornfields, and they are being used by monarchs."

At a meeting of entomologists held last March, he and graduate student Laura Hanson reported results suggesting that enough pollen might collect on nearby milkweeds to harm the larvae. The work, which has not yet been published, showed that about 20% of monarchs that fed for 2 days on potted milkweed plants left at the edge of Bt cornfields died, compared to 3% of monarchs that fed on plants left near non-Bt cornfields. At the Illinois meeting, the industry- and USDA-funded researchers presented similar results, but their interpretation was more optimistic. In one study, for example, botanist John Pleasants of Iowa State and entomologist Richard Hellmich of the USDA's Agricultural Research Service and Iowa State first determined the levels of Bt pollen that are toxic to monarch larvae, then measured how much pollen they could trap on sticky slides left near cornfields.

The results showed that even milkweeds within 1 meter of the cornfield were unlikely to be dusted with toxic levels of Bt pollen from two of the most widely planted corn varieties, AgrEvo's CBH 351 and Monsanto's Mon810, Hellmich said at the meeting. The researchers did find that pollen from the same line that Obrycki tested, Novartis Seeds' 176, is sufficiently toxic to threaten monarchs feeding on milkweeds up to 2 meters away, thus confirming Obrycki's results. But the more toxic Bt line represents just 2.5% of the corn planted in the United States. Overall, Hellmich says, his team's results and similar data presented by others at the meeting revealed a minimal risk to the monarch. "A lot of the data presented were overwhelmingly positive," he concludes.

Still, monarch experts were not entirely reassured. One problem, says insect ecologist Orley Taylor of the University of Kansas, Lawrence, who directs the conservation group Monarch Watch, is that even if Bt exposure doesn't kill monarchs, it could make them less fit for their long migration to Mexico, where they over winter en masse. At the meeting, Taylor presented a model, based on current Bt corn acreage and the butterfly's migration patterns and reproductive behavior, that predicted a worst case scenario in which 7% of the North American monarch population would die. Although the real effect would undoubtedly be less, he says, "there's plenty of indication that there's going to be an impact. It's a matter of degree."

The story also says that Bt toxins might also threaten beneficial insects indirectly, by entering the food chain. For example, in work published in 1998 and 1999, Angelika Hilbeck and her team at the Swiss Federal Research Station for Agroecology and Agriculture in Zurich, Switzerland, showed that green lacewing caterpillars - a beneficial pest-eating predator - were more likely to die when they ate European corn borer caterpillars that had fed on Bt corn than when the borers had fed on non-Bt corn.

Entomologist Richard Roush of the University of Adelaide in Adelaide, Australinga was quoted as sayi "It's interesting science because of what it says about the toxicology of Bt." But "a lot of us wonder whether it's really important in the field." He and others note that Bt bacteria have been sprayed on farm fields for 3 decades, and that earlier studies had shown that beneficial predator insects were unaffected. Hilbeck argues, however, that because the toxin is expressed at high levels throughout GM crop plants, rather than just sprayed on their surfaces, plant-eating insects could receive a much bigger dose. She has begun field trials, and she says others should monitor the effects of Bt crops on lacewings and other insect-eating predators before a problem develops. The story says that ecologists also worry that genes such as those conferring resistance to herbicides or insect pests might pass from the crops into wild relatives and create so-called superweeds - invasive plants with the potential to lower crop yields and disrupt natural ecosystems. They note that a variety of crops, including canola, squash, sunflower, and sorghum, can outcross with weedy relatives growing nearby.. Plant geneticist Val Giddings, a spokesperson for the Biotechnology Industry Organization, was cited as downplaying the risk, saying that even if such outcrossing allowed a weed to pick up a gene, it would not persist for long in the wild.

A herbicide-resistance gene, for example, would disappear from weeds outside the confines of farm fields because there would be no herbicide to select for plants containing it. Giddings was quoted as saying, "There is abundant literature that demonstrates that in the absence of selection pressure, a neutral trait will be lost over time."

Sometimes, but not always, answers plant ecologist Allison Snow of Ohio State University, Columbus. In a study published in April in Molecular Ecology, Snow, with Rikke Jorgensen and Bente Andersen of the Riso National Laboratory in Roskilde, Denmark, crossed canola plants carrying the gene that encodes resistance to the herbicide glufosinate with a weedy relative called field mustard. They found that the gene persisted in the weed even when no herbicide was applied. What's more, the weed produced equally fit offspring whether or not it had the herbicide-resistance gene. That means that the gene will probably stick around, Snow says.

Another type of gene that might move to weeds are virus-resistance genes, such as those that have been engineered into yellow squash and zucchini, says Alison Power of Cornell University. If populations of the weedy relatives of these crops are kept in check by viruses, a virus-resistant weedy squash could potentially outgrow ordinary plants and become more aggressive. Researchers won't know until someone does field tests to find out, however. "It could be a significant issue," Power says, but "we don't have good information to go on."

The story says that the backers of GM crops say that all this talk of their potential risks overlooks their benefits to consumers, farmers, and the environment. But although the risks remain hypothetical, it's also too early to tell whether GM crops are a proven boon, because only a few independent studies have been conducted, and those show clear benefits for some crops but not for others, agriculture experts say.

Cotton, for example, is notorious for needing heavy doses of pesticides, so Bt cotton should offer substantial savings and environmental benefits. Indeed, by planting modified rather than conventional cotton on 2.3 million U.S. acres (nearly 1 million hectares) in 1998, farmers reduced chemical pesticide use by over a million pounds (450,000 kilograms), according to a report released earlier this year by Leonard Gianessi and Janet Carpenter of the National Center for Food and Agriculture Policy, a think tank in Washington, D.C., that is funded by industry and the USDA. What's more, cotton farmers increased their yields by 85 million pounds (39 million kilograms) and made $92 million dollars more than farmers who did not use the technology.

The report says, however, that not all Bt crops fared as well. Although 14 million acres (5.7 million hectares) of U.S. cornfields about one-fifth of the total corn acreage in the United States were planted with Bt corn in 1998, the increased profits from higher corn yields did not cover the extra cost of the Bt corn seed. In addition, the Bt crop saved only 2 million of those acres (800,000 hectares) from chemical insecticides because most farmers don't bother to spray for corn borers because spraying often doesn't protect the corn. Researchers also worry that pest insects could develop resistance to the Bt toxins over the next several years because the bacteria is now so widespread. That would make Bt sprays ineffective, eliminating one of the few effective pest-control strategies available to organic farmers, who forswear chemical pesticides.

Another recent report takes a look at the pros and cons of Roundup Ready Soybeans a herbicide-resistant line from Monsanto and concludes that the results were mixed. On the plus side, says report author Charles Benbrook, an independent consultant to consumer and environmental groups in Sand Point, Idaho, and a former executive director of the National Research Council's Board on Agriculture, Roundup Ready soybeans allow farmers to substitute Roundup for more hazardous and long-lasting herbicides like acetochlor. And they reduce the need for farmers to till the soil to ward off weeds, which reduces soil erosion.

But Benbrook's findings did not support industry claims that the Roundup Ready beans reduce herbicide use by allowing farmers to kill weeds with one dose of Roundup after the soybean plants have sprouted instead of dosing the fields with a variety of herbicides before and during the growing season. Instead, the Benbrook reported concluded, farmers applied two to five times more herbicides of all kinds to their GM soybean fields than to fields growing conventional soybeans. And in contrast to industry claims, a recent study by agricultural economist Michael Duffy of Iowa State University showed that Roundup Ready beans made Iowa soybean farmers no more money than farmers growing ordinary beans. Despite the increased herbicide usage, applications costs were lower, but so were yields from the GM soybeans.

Even if the technology has yielded few clear-cut benefits in the developed world, agbiotech backers say that in the developing world, new crops in the pipeline could improve yields for farmers and make tremendous strides toward reducing malnutrition and environmental degradation. A genetically engineered line of rice reported earlier this year, for example, can make more vitamin A precursor and accumulate more iron, which could prevent infections, blindness, and anemia in people in the developing world (Science, 13 August, p. 994). Other researchers are developing plant-based vaccines to prevent diarrheal and other diseases in the developing world, says plant biochemist Charles Arntzen, president of the Boyce-Thompson Institute for Plant Research in Ithaca, New York.

And a Cornell group is engineering a virus-resistant papaya plant that could save crops in Brazil, Puerto Rico, and Jamaica. A version of the plant, which resists the papaya ringspot virus, has already revived Hawaii's papaya groves, devastated by the virus in the mid-1990s, says plant pathologist Dennis Gonsalves, who leads the effort. "You should go back and look Now it's beautiful," he says.

But before farmers sow GM crops around the world, researchers and regulators need to do a better job assessing the ecological risks, says Ohio State's Snow: "We shouldn't just be waving our hands. There really are not enough ecologists doing this research," in part because research funds are scarce.

And even biotech backers acknowledge the need for better data. "I would say that the benefits totally outweigh the risks, but we can't ignore the risks," Washington State's Cook says.

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