All around the country, farmers are about to finish sowing millions of acres of a genetically altered form of corn that protects itself from pests by producing a toxin in its tissues. But researchers report today that this increasingly popular transgenic plant, thought to be harmless to nonpest insects, produces a wind-borne pollen that can kill monarch butterflies -- a species that claims the Corn Belt as the heart of its breeding range.
Researchers said the laboratory study, conducted by a team from Cornell University, provides the first evidence that pollen from a transgenic plant can harm nonpest species. So the study is likely to become part of the growing debate about whether genetically engineered crops may have unforeseen effects on the environment.
Transgenic crops have proved tremendously popular with American farmers in recent years. This season the new pest-resistant corn, introduced by seed companies three years ago, is being planted on an estimated 10 million to 20 million acres out of an 80-million-acre corn crop nationwide. Known as Bt corn, it carries a gene derived from a bacterium, Bacillus thuringiensis, that produces the Bt toxin, killing corn borer pests that try to eat the plant.
The researchers fed monarch caterpillars leaves of milkweed, their only food, which had been dusted with Bt corn pollen, regular corn pollen or no pollen. Half of those fed Bt corn pollen died within four days, while all those fed regular corn pollen or no pollen survived. The study, published today in the journal Nature, was written by Dr. John E. Losey, an entomologist, Dr. Linda S. Rayor, a behavioral ecologist, and Maureen E. Carter, a biologist.
The Bt toxin itself is already known to be lethal to many butterflies and moths. Researchers said this suggests that butterfly or moth species other than the monarch could be affected by the transgenic plant, particularly those that live on plants like milkweeds that are often found in and around cornfields and could be dusted by Bt corn pollen. But researchers note that the effect of Bt corn pollen on populations of wild insects is unknown.
Academic researchers praised the study as a first step in understanding a previously unsuspected risk.
''Nobody had considered this before,'' said Dr. Fred Gould, an insect ecologist at North Carolina State University. ''Should we be concerned? Yes.''
Dr. John Obrycki, an entomologist at Iowa State University, called the new study ''solid'' and said: ''You now have a novel means of distributing Bt toxins in the environment. This is a technology that's being promoted and we haven't really considered all the consequences.''
Representatives from Novartis Agribusiness Biotechnology, Monsanto and Pioneer Hi-Bred International Inc., the top sellers of Bt corn, challenged the significance of the findings for monarch caterpillars, or larvae, outside the laboratory. Researchers estimate that Bt corn is worth hundreds of millions of dollars annually.
Taking issue with the methods and conclusions of the study, Rich Lotstein, vice president of public affairs for Novartis, said, ''Even if Dr. Losey's results are real, which they could be, the exposure is still minimal, and the impact is extremely small, if any.''
Dr. Lotstein said that as part of the Environmental Protection Agency approval process, predatory insects and honeybees were found not to be harmed by Bt corn.
Researchers, including the authors, say it is still unknown how much of an impact Bt corn pollen is having on wild monarch populations.
''I would be very surprised if there are no monarch larvae being killed,'' Dr. Losey said. But as to how many are being killed, he said, ''That's the big question.''
Researchers say they do know from a study published last year that it is the Corn Belt, states like Iowa, Illinois, Indiana and Ohio, that produces about half of the monarchs that migrate each year to Mexico.
And across that geographic expanse, said Dr. Karen Oberhauser, an ecologist at the University of Minnesota, there was certainly potential for corn pollen and monarch caterpillars to cross paths.
How much milkweed is close enough to cornfields to risk receiving a dusting of pollen is unknown. But as Dr. Marlin Rice, an entomologist at Iowa State University, put it, in many farm states, ''if you're a monarch, odds are you're going to be close to a cornfield.''
Monarchs are not considered endangered, but Dr. Lincoln Brower, monarch biologist at Sweet Briar College in Sweet Briar, Va., said the butterfly faced a growing number of pressures. The No. 1 threat, he said, is still logging in the butterfly's winter resting grounds in Mexico. Other threats include roadside mowing and the use of herbicides on milkweeds.
Whatever level of threat Bt corn pollen turns out to pose, it is almost certainly less damaging to monarchs and insect diversity in general than the spraying of insecticides. But Dr. Obrycki said that in many areas of the country, farmers did not typically spray for corn borer.
Still others viewed the new study as a broader sign of the danger of transgenic crops and the need for tighter regulation.
Dr. Margaret Mellon, director of the agriculture and biotechnology program at the Union of Concerned Scientists, said: ''Why is it that this study was not done before the approval of Bt corn? This is 20 million acres of Bt corn too late. This should serve as a warning that there are more unpleasant surprises ahead.''
Dr. Phillip O. Hutton, chief of the microbial pesticides branch of the Environmental Protection Agency, which regulates the commercial availability of Bt corn, declined to comment on the new study, saying it had not yet gone through the agency's scientific review. In addition to Bt corn, the agency has approved Bt potatoes and Bt cotton.
Despite the potential threat to monarch butterflies, which neither help nor hurt crops, farmers may find it difficult to lay aside Bt corn. Previously, farmers had to scout their crops diligently for signs of corn borers and spray at just the right time in an infestation to kill them. Now they can plant Bt corn and let the internally produced toxins do all the work.
''It's an amazing technology,'' said David Linn, a corn and soybean farmer in Correctionville, Iowa, who plants Bt and regular corn. ''Does it kill more monarchs or not? That's so far down on the list of things we have to decide about.''
Photos: Monarch caterpillars eating a milkweed leaf that was dusted with pollen from corn that had been genetically altered to kill pests. A study says the pollen can kill the butterflies, which breed in the corn belt. (Kent Loeffler/Cornell University)(pg. A25); Photo (Associated Press)(pg. A1)