The U.S. EPA put new restrictions Friday on the growing of genetically modified corn, responding belatedly to concerns that genetically modified crops may be causing ecological disruptions. Under the new rules, farmers who cultivate genetically engineered Bt corn, which produces its own insecticide, will be required to plant at least 20 percent of their acreage in conventional corn, an attempt to make sure that Bt corn does not speed the evolution of “superbugs,” insects resistant to standard insecticides. The rules also require biotech seed producers and farmers to monitor insect populations for signs of insecticide resistance; if such resistance is detected, sales of the offending seed varieties will be halted. Enviros said the new regulations are a step in the right direction, though not nearly strong enough.