Scientists close to a trial run of their `biological' pesticide. GENETICS IN THE CORN FIELD

A microorganism genetically altered to contain a toxin may be released into the environment for the first time this spring. The Environmental Protection Agency has given the proposal an initial nod of approval. The toxin is a naturally produced pesticide that protects corn from insects. The proposed field test could be an important step toward marketing an alternative to chemical pesticides. Many agricultural scientists call the process ``elegant.'' But is it safe?

Genetically altered microorganisms have been legally released in three other field tests. But this microbe, patented by Crop Genetics International, raises new questions because it was engineered to produce a protein that causes disease in some insects.

The firm's bacterium is being used as a kind of Trojan horse. It is a vehicle for getting the natural pesticide into corn plants. The obvious choice for the vehicle was a microorganism that grows inside plants - an endophyte.

The endophyte selected by Crop Genetics does not naturally produce the pesticide. Toxin-making genes taken from another bacterium must be inserted into its DNA, or genetic blueprint.

These designer endophytes are forced into tiny cracks in dried corn seed with a pressure system that has also been patented by Crop Genetics. As the seed grows, the endophytes colonize the plants' water veins. When a corn borer - a moth caterpillar - attacks, it gets a mouthful of the toxic microbes.

This technique is the idea of Peter Carlson, chief scientific officer for the firm. Instead of spraying the corn or changing its own genetics, he decided to ``vaccinate'' it with the toxin-bearing endophyte.

The questions whirling around the proposed release of these endophytes generally fall into two categories. What can they harm? And how far can they spread?

Scientists agree that there is no evidence that the toxin produced inside the bacterium is harmful to humans, mammals, birds, or fish. According to John Henry, chairman of Crop Genetics, the toxin in their endophyte has been sold commercially for 50 years and is the dominant biopesticide on the market.

``There has been a fair amount of toxicity testing with it, and all its [harmful] activity seems to be against insects,'' says Rebecca Goldburg, a biologist with the Environmental Defense Fund. Dr. Goldburg adds that, even among insects, the bacterium's harmful effects appear to be limited to butterflies, moths, and their larvae.

The stickier questions have to do with how the altered endophyte might spread beyond its intended area. The Audubon Society, the National Wildlife Federation, and the Environmental Defense Fund have not protested the small-scale field test which would be carried out in Beltsville, Md., under tightly controlled conditions. They are all enthusiastic about finding a safe alternative to chemical pesticides.

But they have also expressed concerns about wide-scale use of the Crop Genetics technology. Goldburg says that ``very little is known about the organism that Crop Genetics has engineered.'' She is not convinced that the endophyte couldn't be spread by farm machinery, although the studies that suggest this possibility were done under exaggerated conditions.

One speculation is that if the endophytes move into the environment outside the corn plant, they may multiply inside other plants. If such a plant was the primary food source for a butterfly species, for instance, there is at least a small chance that a beneficial species could be depleted.

``People are taking several hypotheticals and compounding them,'' says Dr. Carlson. ``It's fair that we respond, though.'' He says the data complied by Crop Genetics over several years indicate that, in the unlikely event that some spread did occur, the endophyte wouldn't be able to be passed to successive generations.

An independent panel of scientists will address some of these issues and make a recommendation to EPA about the field test before the agency makes a final decision in May.