In the decisions, 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.”