New scientific studies on the impact of genetically engineered corn on monarch butterflies say the corn is having virtually no effect. The papers also make the provocative claim that earlier studies raising the question might have been seriously flawed.

But the debate seems far from ended.

The earlier papers reported that many monarch caterpillars died after ingesting pollen from the genetically modified corn. The new papers say the pollen used in those experiments appeared to be mixed with other parts of the genetically modified plants and that it is those plant parts, not the pollen, that actually killed the caterpillars.

Defenders of the earlier work said it was the new results that were open to question. The other plant parts, they say, might also be a natural part of the caterpillar diet and that the new studies, which look only at pollen, could be ignoring important effects.

"It's part of what's naturally deposited in the field," said John Obrycki, a professor of entomology at Iowa State University and an author of one of the earlier reports.

While critics of genetically modified food have not yet seen the papers - which were released on Friday night to the press but not yet to the public - they are already pointing to this pollen issue. They also note that the studies addressed the short-term impact of the crop, which is called BT corn, but not the longer- term impacts of low levels of exposure to the pollen.

"The results suggest that the major BT corn varieties on the market are not immediately lethal to monarch butterfly caterpillars," said Rebecca Goldburg, senior scientist at Environmental Defense, an environmental group. "They don't take a very hard look at what might be called sublethal effects long term."

Even some of the authors of the new papers were hesitant to say that the question is completely settled, though concerns are certainly less than they were before. "I don't think there's a near and present danger," said May R. Berenbaum, a professor of entomology at the University of Illinois who edited the six new papers, which will be published on line this week by the Proceedings of the National Academy of Sciences.

BT corn contains a bacterial gene that causes it to produce a toxin that kills pests that eat the plant. In 1999, scientists at Cornell showed in a laboratory experiment that monarch butterfly caterpillars - which live on milkweed plants often found in or near cornfields - could be killed by eating milkweed leaves dusted with pollen from the engineered corn. A study last year at Iowa State showed that these toxic effects could be seen at pollen levels normally observed on the leaves in and near cornfields.

The new research is a combination of laboratory and field studies. All six papers conclude that caterpillars are not likely to be exposed to levels of pollen high enough to be harmful, except from one type of the corn that has a particularly high level of the toxin in its pollen. But that corn is not grown that much and is being phased out. One of the papers estimated, for instance, that only 0.4 percent of the monarch population in Iowa would be expected to encounter a high enough concentration of pollen to be harmful.

Scientists found in laboratory tests that for the two most common types of BT corn, there would need to be at least 1,000 pollen grains per square centimeter on the milkweed leaves to affect the caterpillars. But the typical level found in field studies was much lower. "It's a rare event for them to come in contact with large amounts of pollen," said Mark K. Sears, a professor of environmental biology at the University of Guelph in Ontario, a lead author on one of the papers.

Dr. Sears said the scientists could not duplicate the earlier findings of harm, even with pollen piled on the leaves. He called the earlier work "deficient" and said it should be reevaluated.

But he and his colleagues were using pure pollen. The older Iowa State study showed signs of harm involved pollen that contained pieces of anthers, the part of the plant that produces the pollen. These plant parts have higher levels of toxin than the pollen. It is not clear what level of pollen was used in the Cornell study and whether it contained the plant pieces. When scientists doing the new work left these plant parts in the pollen, some caterpillars were killed. That suggests that it was these plant parts that killed the caterpillars in the earlier studies.

But Dr. Obrycki of Iowa State said anthers tend to be shed by the plant along with the pollen and are found on milkweed leaves in cornfields. So the newer studies could be underestimating the effects. "They are missing part of the story by concentrating solely on pollen and emphasizing pure pollen tests to the exclusion of anther and pollen mixtures, which is more representative," he said. ** NOTICE: In accordance with Title 17 U.S.C. Section 107, this material is distributed without profit to those who have expressed a prior interest in receiving the included information for research and educational purposes. **