As recently as a decade ago, farms in the Midwest [commonly contained] unruly patches of milkweed amid the neat rows of emerging corn or soybeans. Not anymore. Fields are now planted with genetically modified corn and soybeans resistant to the herbicide Roundup, allowing farmers to spray the chemical to eradicate weeds, including milkweed. A growing number of scientists fear it is imperiling the monarch butterfly, whose spectacular migrations make it one of the most beloved of insects. Monarchs lay their eggs on milkweed, and their larvae eat it. Experts like Chip Taylor say the growing use of genetically modified crops is threatening the orange-and-black butterfly by depriving it of habitat. This milkweed has disappeared from at least 100 million acres of these row crops," said Dr. Taylor, an insect ecologist at the University of Kansas and director of the research and conservation program Monarch Watch. "Your milkweed is virtually gone." About five times as much of the weed killer was used on farmland in 2007 as in 1997, a year after the Roundup Ready crops were introduced, and roughly 10 times as much as in 1993. "It kills everything," said Lincoln P. Brower, an entomologist at Sweet Briar College who is also an author of the paper documenting the decline of monarch winter populations in Mexico. "It's like absolute Armageddon for biodiversity over a huge area." A spokesman for Monsanto, the inventor of the Roundup Ready crops and the manufacturer of Roundup, agreed.