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 six 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.

Seeding Super-weeds?

Worries about the flow of genes from the original plant to others also surround GM crops. Unwitting insects or the right wind might carry GM crop pollen to weedy plant relatives, fertilizing them. And if that happens, the newly endowed plants could break ecological rank, becoming “super-weeds” that are unusually resistant to eradication by natural predators or pesticides. Scientists have stopped asking if such gene flow is possible. “In many cases,” says Cornell ecologist Allison Power, “we know gene flow will occur. The question now is, what will the consequences be?”

So far no scientific studies have found evidence of GM crops causing super-weeds, and a 10-year study reported in Nature in February found no weed-like behavior by GM potatoes, beets, corn or canola planted in England. But worrisome anecdotes have appeared. Canadian farmers, in particular, have described GM canola escaping from farm fields and invading wheat crops like a weed. This canola also resisted pesticide sprays.

Power's studies of gene flow from virus-resistant GM plants give further reason for precaution. For now, virus-resistant crops stake a small share of the GM landscape, but they are likely to become more prevalent, particularly in the developing world. Power investigates gene flow in cultivated grain crops- wheat, barley and oats-engineered to contain genes that make the plants resistant to the barley yellow dwarf virus (which damages some 100 grass species). These GM grain crops could be on the market within the next decade.

Power's work, carried out in the laboratory, indicates that wild oats-a weedy relative of cultivated oats-can “catch” the genes conferring resistance to barley yellow dwarf virus. If that happened in the field, she says, wild oats might run amok in the western U.S., outcompeting native grasses with kudzu-like intensity. Every GM crop, Power cautions, brings its own environmental personality and its own risks.

In the U.S., at least, landscape logistics make it rather unlikely that herbicide-tolerant or Bt crops will spread their biotech genes to weeds. That's because the GM crops sown in this country have no close relatives in the regions where they grow; most plants can pollinate others only if the recipients and the donors have certain features in common, such as the same chromosome number, life cycle or preferred habitat. A known exception to the “no relatives” rule in the U.S. is wild cotton growing in Hawaii and southern Florida, which, by virtue of its unusual similarity to GM cotton, can accept the GM pollen. To separate the wild and biotech plants from each other, the EPA has ordered companies not to sell GM cotton south of Florida's Interstate 60 or in Hawaii.

But it may prove harder to avoid creating super-weeds outside North America, where weedy relatives of cultivated crops are common. Wild cotton, for instance, creeps past the Florida Keys, across the Gulf of Mexico and into Mexico. In South America, a weedy corn relative, teosinte, dresses the edges of domesticated cornfields. Either plant would readily accept the pollen from a GM relative. Indeed, scientists say, GM crops in many countries could end up growing near their ancestral plants-and sharing more than the sunshine overhead. “Almost every crop has weedy relatives somewhere in the world,” says Stephen Duke, a USDA plant physiologist in Oxford, Miss. “How do you keep GM crops out of places where they're not supposed to be?”

Taking Refuge

Finally, one risk follows GM crops wherever they're planted: evolution. Over time, insect pests and weeds can become resistant to killing by routine chemical sprays. The same is bound to happen in the biotech age: eventually, impervious insects will munch away on GM insect-resistant plants, and the weeds surrounding herbicide-tolerant crops will shrug off the herbicide of choice. “Agriculture is an evolutionary arms race between plant protections and pests,” comments botanist Jonathan Wendel of Iowa State University. “And GM crops are just one more way that we're trying to outsmart pests-temporarily.”

To keep weeds vulnerable to herbicides, Monsanto and other companies urge growers to use the sprays responsibly, only when necessary. To slow insect resistance to the Bt toxin, the EPA requires Bt crop growers to set aside some part of their farmland for crops that have not been genetically modified. These “refuges” may be a corner of a field outside a Bt crop, for instance, or rows of standard plants that break up a Bt plot. Inside the refuges, insects that have acquired some Bt resistance breed with those that have not, diluting the resistance trait.

After five years of commercial Bt crop use, no reports of insect resistance to the crops have emerged, according to Monsanto. The company contends that roughly 90 percent of Bt corn and cotton growers comply with refuge requirements.

But some environmentalists question that rosy scenario and also argue that non-Bt refuges are either too small or too poorly designed to keep insect resistance at bay for long. “At the EPA meeting last fall, scientists seemed to agree that bigger, better refuges were the way to go but that cotton farmers would never agree to big refuges,” says Rebecca Goldburg, a senior scientist at Environmental Defense, a nonprofit organization based in New York City. More broadly, Goldburg questions how much GM crops really do for the environment. “In however many years,” she says, “we'll lose Bt as an effective control against insects, and then we'll be on to another chemical control. Many of us view this current generation of biotech crops as a kind of diversion, rather than a substantive gain for agriculture.” She favors sustainable agriculture alternatives, including careful crop rotation and organic farming methods, over pesticides sprayed on or engineered into plants.