By now, you've probably heard the bad news. This winter's eastern monarch population hit a record low, occupying a scant 1.19 hectares of oyamel forest in Mexico. Since 1975, when the monarch overwintering habitat was discovered, scientists have never recorded numbers this low. Chip Taylor of Monarch Watch notes that this represents a 59% decline over the previous year. And 2011-2012 was not a good year, either.
While much of our attention over the past three decades has been focused on saving the monarchs' winter habitat in Mexico, we've done very little to preserve their breeding habitat here in the U.S. Monarch caterpillars are specialist feeders that feed only on milkweeds (genus Asclepias). As milkweed goes, so go the monarchs.
By some estimates, a full 70% of monarch butterflies fed on milkweeds growing in farm fields in the past. But thanks to Monsanto, an agricultural biotechnology corporation and the world's leading producer of the herbicide Roundup, most of those farm fields no longer sprout milkweed. In the past 20 years, Monsanto introduced Roundup Ready corn and soybeans, and farmers were quick to adopt these genetically-modified crops. Instead of tilling their fields to control weeds, farmers spray them with Roundup. The Roundup Ready crops continue to grow, while the weeds - including the milkweeds - die back. Milkweed can tolerate tilling, but not Roundup.
Roundup isn't the only factor contributing to the monarch decline, of course, but it certainly isn't helping. That's why Roundup Ready crops top my list of 10 threats to monarch migration.