A new study suggests that the increased use of genetically modified (GM) crops across the Midwestern U.S. may be causing a decline in monarch butterfly populations. From 1999 to 2010, a period when GM crops became more common on U.S. farms, the number of monarch eggs in the Midwest declined by 81 percent, according to researchers from the University of Minnesota and Iowa State University. The reason, according to the study, is the near-disappearance of milkweed, an important host plant for monarch eggs and caterpillars. The researchers attribute sharp declines in milkweed to widespread use of genetically modified corn and soybeans that are resistant to the herbicide, Roundup, which is then sprayed on fields, killing milkweed.