Magnificent Monarch Migrations

As winter’s cold arrives in Massachusetts every year, all the butterflies seem to disappear, but really they just become harder to find.  Many familiar garden species, like the black swallowtail and tiger swallowtail, spend the winter as a well-camouflaged chrysalis.  Others, like many of the small hairstreaks and blues, spend the winter as tiny caterpillars at the base of their food plants.  Mourning cloaks and related species spend the winter as adults, generally hiding in a well protected place, sheltered from the worst cold and snow.  Only the Monarch butterfly has a more bird-like strategy for surviving the winter – they fly south to escape winter conditions that are just too cold for them in any life stage.

Unlike the spring and mid-summer generations before them, the Monarch butterflies that emerge from their chrysalides in late summer or early autumn are not interested in finding a mate or laying eggs.  Instead, each is motivated to find as much nectar as it can to increase the fat levels in their body.  As day length decreases and evening temperatures get cooler, they begin their journey south.

Many birds gather in flocks for the journey south, while Monarch butterflies are apparently acting individually, but since they all are heading in the same direction, it can sometimes seem like their migration is more coordinated than it probably is in reality.  They can often be found in large groups, especially on the northern side of a large body of water, waiting for just the right conditions to cross when the wind will be assisting their flight rather than hindering it. 

Tagging experiments have demonstrated that most Monarch butterflies from the eastern side of the Rocky Mountains end up at a small number of forested hillsides in central Mexico, northwest of Mexico City.  Those from the western United States fly to a small number of wooded areas along the central California coast.  What makes these locations well suited for overwintering?  Unlike their summer homes in Massachusetts and across North America, it rarely freezes or snows at these overwintering grounds.  The weather remains warm enough that the Monarchs don’t freeze, yet cold enough that they are able to stay inactive most of the time to conserve their limited body fat energy reserves, and humid enough that they don’t desiccate. 

Next spring when the weather warms up and the milkweed plants begin to grow again across the continent, the Monarchs will find a mate and then leave their overwintering sites to travel north.  As they reach the southern United States, they will lay their eggs on the young milkweed plants and then die.  Their caterpillars will feed on the milkweed, form their chrysalides, and then upon emerging continue the journey north that their parents started, laying their eggs on milkweed plants in the central and northern states and into Canada.  What makes this migration most impressive and mysterious is that the butterflies flying south to spend the winter in Mexico in autumn were not the same ones that flew north the previous spring.  At least three but as many as six generations of Monarchs will be born and die during the summer, such that those flying south for the winter have no direct knowledge or experience telling them where their great-great-grandparents spent the previous winter.  They are genetically encoded with the information necessary to find these small overwintering sites year after year.

Much about Monarch butterfly migration remains unknown, but scientists and amateur butterfly watchers continue to gather more data every year with the hope to better understand how the migration works and at the same time protect this unique phenomenon.  The migration is threatened by the destruction of their overwintering grounds due to development in both California and Mexico.  Government-supported preserves have been created in an effort to protect many of these sites, but the Monarchs are also threatened by the loss of summer habitat and destruction of caterpillar host plants in the north.  You can help by planting milkweeds for caterpillars to feed on, and nectar-rich flowering plants to feed the adults throughout the summer and as they fuel up in preparation for their migration south in the fall. 

To learn more about Monarch butterflies and their fascinating natural history, visit the University of Kansas’s Monarch Watch website at http://www.monarchwatch.com/.   Members of Monarch Watch help with tagging research and by creating butterfly-friendly habitats around the USA and Canada.  The Journey North website at http://www.journeynorth.org/ is also a good place to learn more and get involved in Monarch butterfly research and conservation projects.  These websites also include information on where to go in California to see tens of thousands of overwintering Monarchs, as well as sites in Mexico where literally MILLIONS of these butterflies gather.  A journey to one of these overwintering grounds will be an awe inspiring visit that you will not soon forget. 

Michael J. Weissmann, Ph.D., Entomologist

Kallima Consultants, Inc.

Northglenn, Colorado, USA