Researchers in Mexico say the use of genetically modified crops has fueled an historic decline in the population of migrating monarch butterflies. While monarchs once occupied 50 acres of of Mexican forest, this winter they occupied less than three acres. That’s in part because herbicide-tolerant crops have allowed U.S. farmers to decimate the monarchs’ food supply of milkweed. Researchers said drought and record-shattering heat last spring also played a key role in the decline.