The Monarch Butterfly Migration

The monarch’s journey is the greatest of all insect migrations and is the only known butterfly migration on Earth. Every fall, millions of monarchs travel from Canada and the United States to overwinter in Mexico because they can’t survive the freezing winters of central and eastern North America. Even though not a single one of these butterflies has ever been to Mexico, they “return” to the same small groves of trees where their ancestors overwintered one year and at least three generations earlier.

How they accurately navigate to a place they have never been is a great mystery. Lincoln Brower, a leading entomologist who has studied monarch butterflies since the mid-1950s and helped discover the monarch migration, has been investigating the possible role of magnetic fields or magnetite crystals in the earth, as there is evidence that minerals carried in the monarchs’ bodies provide them with an ability to navigate to a specific point on the planet.

After spending four months hibernating on their host trees, the majority of monarchs returning north migrate to Louisiana or Texas, where they reproduce and die. Newly hatched monarchs head toward the Great Lakes where they, too, lay eggs before death. The next generation of butterflies makes their way past the Appalachians and along the East Coast, then heads south to the Gulf Coast and veers west, finally reaching their winter home. To make this long journey possible, the migrating generation lives 4 to 6 months, instead of the 2- to 6-week lifespan of nonmigrating adults.

Sometimes it takes two entire generations before northbound monarchs reach the Great Lakes; thus, it is possible that it is not until the fourth generation that the butterflies finally come full circle and make it back to Mexico. Gliding on southbound breezes at speeds of up to 35 miles per hour, most make the trip in about six weeks, sometimes covering more than 250 miles a day.

Even with the discovery of the winter hibernation groves, there was still a question as to whether all monarchs in North America migrated to the same place.

For many years, researchers interested in tracking monarchs relied on tagging the wings of butterflies and hoping someone will find those individual insects amongst the clouds of millions in the mountains of central Mexico, thousands of miles away. Obviously, this is a difficult method, as the chances of re-catching a tagged butterfly are very slim, as are the odds of any individual monarch surviving the journey to Mexico.

In the mid-1980s, an incredible discovery aided the study of monarch distribution. Scientists learned the compounds in milkweed plants differ from region to region and are retained in the butterfly’s body throughout its growth. With this new information, scientists were able to discern whether a monarch had recently migrated from southern, northern, eastern or western parts of the United States, gauged by the composition of the milkweed contained in its system.

Brower has been able to establish the migration of the eastern monarch butterfly through the technology of milkweed tracing. He has also tracked successive generations, as they migrate in a relatively fixed path across the eastern side of the country. We now know that for the most part, monarchs that breed east of the Rocky Mountains migrate to Mexico, and those that mate west of the Rockies migrate to a stretch of the California coast near Monterey Bay.

Header Credit: Mike Bruscia

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