Monarch butterflies are famous for their annual migrations. Some of these insects travel thousands of miles each fall -- an astonishing distance for such fragile creatures. Yet few people realize that the Monarchs we see in the spring are not necessarily the same ones that fluttered past in the fall.
Beginning in late September, the skies along the Gulf Coast of Texas slowly become filled with meandering groups of Monarchs. Their flight, while not hurried, is purposeful, moving southwest toward a small forest in the highlands of Central Mexico. These butterflies travel from southern Canada and the northern United States at a rate of approximately 50 miles per day. They'll spend the winter in a few small groves of evergreen trees, with each grove containing as many as 20 million butterflies. Sheltered from the wind and snow, the butterflies conserve energy, for they still have a lot of work ahead of them.
The Monarchs become active again in February. Mating begins, and the air fills with swirling masses of copulating pairs. The first warm days of late March trigger their northward flight. A close look at these butterflies, now eight months old, reveals that their wings are faded and tattered. Still, the Monarchs fan out across the southern United States, looking for Milkweed plants on which to deposit their eggs.
Four days later, the eggs hatch, producing small caterpillars that immediately begin to feed on the Milkweed leaves. Ten to fifteen days later, each caterpillar stops feeding and forms its chrysalis -- a beautiful soft green jewel flecked with gold. In another ten to fifteen days the chrysalis splits open, and a new Monarch emerges.
This generation of butterflies mates, lays eggs, and dies within the span of a few weeks. During this time it moves north, following the progress of spring and the emergence of Milkweed. By the end of summer, two more of these short-lived generations will have repeated the process, ultimately coming to inhabit the Milkweed patches in the far north latitudes.
Thus the Monarchs born in the Northeast and Canada in September are the great great grandchildren of the last Monarchs to inhabit the area. These are the ones that will head to Mexico. They're significantly larger than the three generations that preceded them and still sexually immature. Rather than mate and lay eggs, they seek out nectar-producing flowers. The nectar serves two purposes: some of it fuels the southward migration, and some of it is converted to fat reserves that sustain the butterflies through the winter.
This incredible annual cycle applies to all Monarchs east of the Rockies. The populations in the West follow a similar pattern, though their migratory path is westward, from the Great Basin to overwintering sites along the Pacific Coast.