"ANGANGUEO, Mexico — On a high mountain slope in central Mexico, a patch of fir trees looks dusted in orange and black. In fact, millions of monarch butterflies cloak the trees. The forest murmurs with the whir of their flapping wings.
Every year, hundreds of millions of monarch butterflies _ each so light that 50 together weigh barely an ounce _ find their way on what may be the world’s longest insect migration, traveling the length of North America to pass the winter in central Mexico.
Yet the great monarch migration is in peril, a victim of rampant herbicide use in faraway corn and soybean fields, extreme weather, a tiny microbial pathogen and deforestation. Monarch butterfly populations are plummeting. The dense colonies of butterflies on central Mexican peaks were far smaller this year than ever before."