The Environmental Protection Agency has placed new restrictions on the cultivation of genetically modified corn, a response to concerns that gene-altered crops may be causing ecological disruptions.

The new restrictions, which were released late Friday and are effective immediately, make unprecedented demands on the producers of biotech seeds and on farmers who wish to plant so-called Bt corn, which has been endowed with a gene that allows the corn to make its own insecticide.

Among the new restrictions is a requirement that farmers plant 20 percent to 50 percent of their acreage in conventional corn, which some farmers have said would be burdensome and some experts said could lead to a decline in plantings of the high-tech seeds.

Bt corn has enjoyed a meteoric rise in popularity among farmers since it was approved for sale in 1996, and was planted on more than one-third of U.S. corn acres last year.

But some experts have warned that large-scale plantings of Bt corn may be speeding the evolution of "superbugs"--insects resistant to standard insecticides.

Then, last summer, Cornell University scientists presented preliminary evidence from laboratory studies that pollen from Bt corn could blow onto milkweed plants and kill monarch butterfly caterpillars. Although field studies aimed at measuring the true ecological impact of Bt corn on monarchs are not yet complete, the EPA suggested Friday that farmers voluntarily plant their conventional cornfields upwind of their biotech fields so the Bt corn pollen won't blow onto these refuges.

Milkweed, the only plant on which monarch butterflies lay their eggs, grows around cornfields.

Environmentalists praised the new regulations, which the EPA negotiated with the biotechnology industry, as a step in the right direction, if not as strong as they might have liked.

"Many of the companies and industries have gone to great lengths to belittle concerns about toxic pollen on butterflies and the development of resistance in insects," said Rebecca Goldburg, a scientist at the Environmental Defense Fund in New York and a member of a National Academy of Sciences panel that is preparing a report on the environmental impact of gene-altered corn. "What EPA has done is to confirm that there are some serious environmental problems concerning the widespread planting of Bt corn."

Several varieties of genetically modified corn have been rejected by European consumers and others because of environmental and health concerns, costing U.S. farmers more than $200 million in exports last year. With trade tensions rising over the crops, and insect populations holding at modest levels in many parts of the American corn belt, some experts were already predicting that sales of engineered corn might decline this spring for the first time.

A straw poll of 400 farmers conducted by Reuters last week at the annual meeting of the American Farm Bureau Federation found that some farmers are planning to call it quits with biotech varieties. Farmers said demands by U.S. consumers that engineered food products be labeled, and ongoing European rejection of the crops, could depress the prices farmers will get at harvest for the costly new varieties.

The poll results predict a 24 percent decline in plantings of Bt corn compared with last year, and a 26 percent decline in plantings of Bt cotton. They also predict a 15 percent decline in RoundUp Ready soybeans--a gene-altered variety of soy that protects the plants against the popular weed killer made by St. Louis-based Monsanto Co. and was planted on more than half of all U.S. soy acres last year. And it predicts a 22 percent drop in RoundUp Ready corn.

Representatives from major producers of biotech seeds could not be reached for comment, but a spokesman for Monsanto told Reuters last week that farmers have been pleased with the new varieties and that it's too soon to say what farmers will do in the spring.

The new EPA restrictions, described in letters to biotech seed producers from Janet L. Andersen, director of EPA's biopesticides and pollution prevention division, could influence those decisions for corn.

They demand that farmers plant large "refuges" of conventional corn near their Bt corn to reduce Bt pressures on insects and delay the evolution of resistance in pest populations. Farmers will not be allowed to spray refuges with conventional insecticides unless they can prove that pests have exceeded certain levels.

And biotech seed producers and farmers will have to monitor insect populations for the emergence of insecticide resistance. At the first sign that such resistance is occurring, sales of the new seed varieties must be halted.

The rules also demand that seed producers develop grower agreements that farmers must sign or produce educational materials and programs such as workshops and publications to ensure compliance with the rules. Companies must submit details of those plans to the EPA for approval by Jan. 31.

CAPTION: Monarch butterflies emerge from cocoons. Preliminary evidence suggests that pollen from gene-altered corn could kill monarch caterpillars.