Monarchs face threats at every phase of their journey. In 1983, the International Union for Conservation of Nature listed monarch migration as an “endangered phenomenon.” The butterflies themselves, while not officially endangered, are in decline.
Climate change is a major threat to monarchs, says Oberhauser. Scientific models predict that as the planet warms, storms that can slow migration and kill overwintering populations will happen more often. Milkweed is the only plant monarch caterpillars eat. The plant’s range is expected to move northward into Canada. If that occurs, the monarchs will need to travel farther north to their summer breeding grounds. In turn, their offspring—the migratory generation—will have to fly greater distances to reach the sacred fir forests in Mexico. There, a drought and warming climate are already affecting the sacred fir trees.
Another huge threat is loss of milkweed and other flowering plants, which adult butterflies depend on to fuel their flight. Growing caterpillars need a lot of milkweed. For every 30 milkweed plants, one monarch lives to adulthood. Unfortunately, modern agriculture is not friendly to the plant, which is considered a weed. Milkweed once grew in corn and soybean fields. But in the late 1990s, people began planting crops genetically modified to tolerate herbicides. “Farmers could spray their fields with herbicide that killed off the milkweed,” Oberhauser says.
Development, like building houses, roads, and farms, also destroys monarch habitat. In Mexico, illegal logging is shrinking the butterflies’ winter habitats.