European Union environmental officials have determined that two kinds of genetically modified corn could harm butterflies, affect food chains and disturb life in rivers and streams, and they have proposed a ban on the sale of the seeds, which are made by DuPont Pioneer, Dow Agrosciences and Syngenta. The environment commissioner, Stavros Dimas, contends that the genetically modified corn, or maize could affect certain butterfly species, specifically the monarch, and other beneficial insects. For instance, research this year indicates that larvae of the monarch butterfly exposed to the genetically modified corn behave differently than other larvae." In the decision concerning the corn seeds produced by Dow and Pioneer, Mr. Dimas calls "potential damage on the environment irreversible." In the decision on Syngenta's corn, he says that "the level of risk generated by the cultivation of this product for the environment is unacceptable." Barbara Helfferich, a spokeswoman for Mr. Dimas ... said that the European Union was within its rights to make decisions based on the "precautionary principle" even when scientists had found no definitive evidence proving products can cause harm. "The commission has the authority to be a risk manager when it comes to the safety and science of genetically modified crops," Ms. Helfferich said. In the decisions, Mr. Dimas cited recent research showing that consumption of genetically modified "corn byproducts reduced growth and increased mortality of nontarget stream insects" and that these insects "are important prey for aquatic and riparian predators" and that this could have "unexpected ecosystem-scale consequences."