In 1996, orange-and-black-winged monarch butterflies dotted 45 acres of forest in Mexico—the equivalent of 34 football fields. That year saw the largest-ever migration of the insects between the woods of the Sierra Madre and stretches of the United States and Canada. The same year, Monsanto released its first Roundup Ready crop, soybeans, which drastically altered the way farmers could apply herbicides across the Midwest—an important breeding ground for the butterflies.
The population in Mexico covered just 1.65 acres this winter, and that precipitous drop over the past 18 years has environmentalists increasingly worried about corn and soy that’s been genetically modified to withstand glyphosate, the herbicide Monsanto markets as Roundup, and the fate of a plant called milkweed. The weed is the only place where monarchs lay their eggs, and the near eradication of milkweed from agricultural land at the hands of glyphosate-spraying farmers is widely believed to be the reason for the plummeting butterfly numbers.