The advent of autumn brings with it one of the greatest migration stories ever told. This migration tale does not start with the wildebeests of Africa nor the birds flying south for the winter, but the Eastern monarch butterfly.
Through September and October, this majestic butterfly will begin its epic journey from as far north as Canada, all the way to Mexico. Often cited as one of the longest insect migrations recorded, the monarch will travel upwards of 3000 miles to reach its winter home. However, unlike the monarch’s migratory counterparts, they will not return. In many ways how it actually happens is still a mystery.
This migration is not a “round trip”, in fact it will take 4 to 5 generations of monarchs to complete the entire journey. First, it is important to understand the difference between the summer and fall generations of monarchs.
The summer generation of monarch has a relatively short life span, between six and eight weeks. After overwintering in Mexico, monarchs begin to mate in spring, marking the first leg of their northern journey. Before her life cycle ends, the female monarch must find a milkweed plant so she can lay her eggs.
Milkweed is the only plant that will support a monarch caterpillar. She will lay one to three eggs on the underside of a milkweed leaf and repeat this process until she lays several hundred eggs.
Three to five days later the egg will hatch into a caterpillar. For two weeks the caterpillar will eat nothing but the milkweed leaves before entering the pupal stage. A hard shell called a chrysalis forms around the pupa for protection. It will remain in this stage for 10-14 days before emerging as an adult monarch butterfly. After 4-5 days this new monarch becomes sexually mature, and the cycle begins again.
This new monarch will take to the skies and continue its journey north in search of food and milkweed. This process will continue for several generations until the monarch’s northern destination is reached and the fall generation is born.
The fall or migratory generation is very different from the previous generations, in fact it is a subspecies. After emerging from the chrysalis, it enters a phase called diapause where the body will temporarily pause reproduction until the following spring. This particular monarch will live up to nine months. It is this generation that will be responsible for the longest leg of the journey, a seemingly impossible feat, all the way to Mexico!
Just a few mountains in Central Mexico provide this “super generation” of monarch a safe place to spend the winter. They huddle together by the millions on the branches of oyamel fir trees, known locally as sacred firs. The number of monarchs is so vast they create their own microclimate which protects them. Then as spring rolls around, this generation flies north towards Texas where they breed, thus finishing their lifecycle and the story begins anew.
There is one lingering question; how this generation of monarchs know how to find the sacred firs in this remote mountain range in Mexico. Remember, they are at least four generations removed from the last monarchs there. Scientists are only now beginning to understand.