Less than 20 years ago, a billion butterflies from east of the Rocky Mountains reached the oyamel firs, and more than a million western monarchs migrated to the California coast to winter among its firs and eucalyptuses. Since then, the numbers have dropped by more than 90%, hitting a record low in Mexico last year after a three-year tailspin. Preliminary counts of migrants this fall are encouraging. “But we’re definitely not out of the woods,” said Dr. Satterfield, who studies human effects on migratory behavior. “One good year doesn’t mean we’ve recovered the migration.”
Unlike most migrating species, monarch butterflies employ an improbable strategy that splits their roundtrip migration between generations. So their life cycles must be intricately synchronized with those of the milkweed on which they lay their eggs. But in
the Midwest, which produces half of Mexico’s wintering monarchs, the scores of wild milkweed species among grasslands and farms are fast disappearing.
Monarchs returning from Mexico reach the Southeast soon after native milkweeds appear in spring, producing the first of up to three generations that breed on new milkweed through summer. When the perennials start dying back in the fall, a final generation of
butterflies typically emerges in a sexually immature state. Rather than reproduce when food is scarce and caterpillars might freeze, they fly to Mexico, to wait out the winter.
Scientists claim that droughts caused by climate change and increased use of certain new
pesticides have caused the decline of the Monarch.
Source: New York Times