Monarch butterflies cannot survive a long cold winter. Instead, they spend the winter in warm roosting spots. No butterfly migrates like the Monarchs of North America. They travel much farther than all other tropical butterflies, up to 3,000 miles. They are the only butterflies to make such a long, two-way migration every year. Amazingly, they fly in masses to the same winter roosts, often to the exact same trees. Individuals only make the round-trip once. It is 3-5 generations later that return south the following fall.
In the fall, Monarchs west of the Rocky Mountains travel to small groves of trees along the California coast. Eucalyptus trees, Monterey pines, and Monterey cypress trees shelter them from wind and storms. Those east of the Rocky Mountains fly farther south to a mountain region in central Mexico. They choose spots that are close to but not quite freezing. When they are cool, they donít use up their energy reserves. They cluster together, covering whole tree trunks and branches. There they remain in hibernation until the following spring.
As winter ends and the days grow longer, the Monarchs become more active. They leave their Mexican roosts during the second week of March, flying north and east looking for milkweed plants on which to lay their eggs. These Monarchs have already survived a long southward flight in the fall and all of the hazards along the way as well as in the winter roosting spots, and are the only Monarchs left that can produce a new generation. If they return too early, before the milkweed is up in the spring, they will not be able to lay their eggs and continue the cycle.
Before they die, the migrating females lay eggs on the milkweed plants they find throughout the southern United States. When these caterpillars hatch and turn into adults, they in turn continue the journey northward. Summer Monarchs live a much briefer life than the over-wintering generation; their adult lifespan is only three to five weeks compared to eight or nine months for the over-wintering adults. Over the summer, there are three or four generations of Monarch butterflies. Since each female lays hundreds of eggs, the total number of Monarch butterflies increases throughout the summer. Before the summer ends, there are once again millions of Monarchs all over the United States.
Even though the Monarch population grows to millions each year, these amazing butterflies are seriously threatened by human activities. In the United States, Monarchs face direct habitat destruction caused by humans. New roads, housing developments, and agricultural expansions all destroy natural landscapes that provide food and protection. Additionally, milkweed, the plant the Monarch larvae feed on exclusively, is considered a weed by some people, which means it is often destroyed. In some areas across North America, milkweed plants are also being severely damaged by ozone. Both milkweed and adult nectar plants are also vulnerable to herbicides used by many landscapers and gardeners. Many pesticides can kill monarchs themselves. Please use herbicides and pesticides sparingly, if at all, and plant milkweeds to attract Monarchs to your garden so you may enjoy these beautiful, mysterious creatures.