The Ultimate Relay Race

Published October 29, 2014

Butterflies are known for one biological marvel in particular: their ability to metamorphose from caterpillars into beautiful, brightly colored, winged creatures. 


But monarch butterflies possess another mind-boggling skill that receives less acclaim: Without any guidance, these insects inherently know how, when and where to migrate across continents—and it takes four generations to make the yearlong trek. 

The Starting Line 

The arrival of spring marks the kickoff of the monarch butterfly’s migration, with the first generation making its debut into the world. Born in March and April, these tiny insects pick up where their predecessors left off, traveling farther north on a generational journey that totals 1,200 to 2,500 miles. That’s quite the feat for creatures with wing spans of only 3.5 to 4 inches! 

Not all will reach their destination in the eastern, western and midwestern United States—some travel as far as central and eastern Canada—but those that don’t will lay eggs along the way. Their contribution to the migratory marathon is much shorter—though no less significant—than that of their parents. This first generation usually doesn’t live longer than two months, spending a large part of their lives eating, metamorphosing and laying eggs for the next phase of the race.

Keeping Pace 

The eggs laid by the first generation hatch into baby caterpillars after about four days, typically entering the world in the warm months of May and June. The next two weeks are spent eating and growing until they’re ready to attach themselves to stems or leaves and transform into a chrysalis. Ten days later, stunning butterflies emerge and fly away, carrying on the course and stopping for snacks on the “milkweed highway.” This generation lays eggs for the next, which is born in July and August. Like the first, both the second and third generations have only two to six weeks to complete their leg of the trip. 

The Anchor Leg

It’s the fourth generation, born in September and October, that leads the longest and most exciting life. Unlike their parents, grandparents and great-grandparents, these butterflies don’t die after two to six weeks—there’s too much to do before they pass the torch. Instead, they live upward of eight months, spending their extra time traveling the continent to ensure their species’ survival. 

With cold weather just around the corner, these butterflies instinctually know it’s time to head south. Monarchs that spend their summer vacation in western North America fly to the southern coast of California. About 60 million to 1 billion more migrate from eastern North America to the Sierra Madre Mountains of central Mexico. These frequent flyers sometimes rack up about 80 miles a day, spreading out their journey across two months. 

Monarch Map

Despite the overwhelming number of monarchs traveling to Mexico, there are only about a dozen overwintering sites for them, and not all of these are protected from logging. The Monarch Butterfly Biosphere Reserve, a national protected area and nature preserve in central Mexico, hosts the majority of wintering monarchs from the eastern United States and Canada. 

In late October, the tops of the reserve’s trees are enveloped in a blanket of orange and black. The monarchs huddle together to fight the cool night air and use the surrounding trees to shield themselves from the wind, snow, rain and hail. The hibernating butterfly clusters can become so dense that tree branches occasionally bend and snap beneath their weight. It’s truly a sight to see—and it drives a good deal of tourism, with 80,000 sightseers visiting Mexico every year just to catch a glimpse of the phenomenon. 

The monarch butterflies remain in the treetops for the next five months, until the cycle begins all over again in February and March, when they desert their southern homes for the promise of milkweed thousands of miles north.

The Fall of Monarchs

Monarch butterflies have been beautifying backyards across North America for centuries, their brilliant orange and black wings swooping in for snacks and delighting all who catch a glimpse. In the past 20 years, however, these seasonal visitors have become much more difficult to spot. The species is facing a 90 percent population decline, prompting at least three major conservation groups to call for its designation as threatened. 

Part of the problem lies in the lack of milkweed. A perennial flowering plant, milkweed is critical to the butterflies’ survival. It provides nectar along the migration route and is the only plant on which these insects can lay their eggs. However, it’s quickly disappearing from the Midwest as farmers cultivate more land for agricultural use. 

Individuals can help by promoting protective legislation through online petitions, such as the one at Equally easy: Plant milkweed and other nectar plants to create a “monarch way station” around your own home—or recommend the planting of these monarch food sources in other areas of the community. Your contribution can help ensure the preservation of the species and the continuation of the incredible insect’s migration throughout North America.

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