For more than a century, monarch butterflies (Danaus plexippus) have come under intense scientific scrutiny, mostly because of their migration habits and dwindling population. Monarchs east of the Rockies migrate to Mexico each fall from the United States and Canada. Tens of thousands often land on a single tree on their trek south. But as each year passes, their population declines, mainly because of habitat destruction. Only 20 years ago, a billion monarchs made the trip to Mexico from the U.S. or Canada. Their numbers have now plummeted by more than 90 percent. As a result, scientists have studied the behavior of the orange and black flutterers, their biology and their environs, trying to find ways to bolster their population [source: Gross].
Scientists have been especially concerned by the destruction of milkweed fields on which the monarchs feast. Researchers say that over the past 15 years, the number of milkweed plants in the Midwest has dropped by 58 percent, which correlates with an 81 percent drop of monarch butterflies. Studies also show that the use of glyphosate herbicide is destroying monarch eggs that the butterflies deposit on each milkweed plant. In addition, the increased planting of genetically modified glyphosate-tolerant corn and soybeans is muscling out fields of milkweed [sources: Pleasants and Oberhauser, Gross].