Conservationists say they want tougher rules limiting the use of a common weed killer to save America's dwindling population of monarch butterflies. The Natural Resources Defense Council, in a petition to the Environmental Protection Agency, said current levels of use of glyphosate -- first marketed under the brand name Roundup -- are wiping out milkweed, the only plant upon which monarch caterpillars feed. That's having a significant negative effect on the life cycle of the beloved orange-and-black butterflies, which migrate back and forth across the United States, Canada and Mexico, it said. Because each monarch lives only a few weeks in the summer, it takes several generations to make the long migratory round trip, making the availability of milkweed all the more important, council senior scientist Sylvia Fallon said. "The tenfold increase in the amount of glyphosate being used corresponds with huge losses of milkweed and the staggering decline of the monarch," Fallon told the Los Angeles Times. "We are seeking new safeguards desperately needed to allow enough milkweed to grow." The EPA is scheduled to complete a new review of glyphosate rules in 2015, but "given the rapid decline in monarch numbers, the EPA should take immediate steps to review and restrict glyphosate's uses," the council's petition says.