Many people know that monarch butterflies (Danaus plexippus) perform annual migrations that cover vast amounts of land and sea. They move south in late summer, and return again in spring. However, many do not know that no single monarch has ever completed even an entire leg of the migration. It takes several generations of the insects to reach their overwintering grounds and then return to the United States and Canada for summer.
Description of Monarch Butterflies
Monarch butterflies are milkweed butterflies, which are butterflies that lay their eggs on milkweed plants, which their caterpillars feed on until maturity. There are two significant populations of these insects in the United States, one of which is east of the Rocky Mountains, and the other is found west of the range. The eastern butterflies travel to southern Mexico for the winter months. Some monarchs have year round populations in islands in the Caribbean Sea, and in Bermuda, and on East Atlantic islands, such as the Azores. Monarchs are also found in South America and tropical and subtropical parts of the eastern hemisphere. These butterflies are colored mostly burnt orange, although they have black wing highlights and lines, and white spots in the black outlines of their wings. They usually have wingspans between 3.5 and 4 inches.
North American Annual Monarch Butterfly Migration
In late summer (usually in late August), monarch butterflies begin to move toward the warmer climates that they will live in through the winter. Monarch butterflies from every state east of the Rocky Mountains, as well as those from every Canadian province that borders the continental United States, begin a journey to the south in late summer. They all head to the same place, which has been designated as the Monarch Butterfly Biosphere Reserve (Mariposa Monarca Biosphere Reserve) in the eastern part of the Mexican state of Michoacan. Although these insects constantly reproduce as they begin moving south, the last generation, which is usually born sometime on the way south, enters a non-reproductive phase, and completes the journey south.
Last Generation of Monarch Butterflies in Migration
Often it will take two or three generations of monarchs to reach Michoacan, Mexico, after the movement south begins in the summer. However, the final generation of the journey which completes the trip (and does most of the traveling, as monarchs are still lingering around in the Southern United States in early fall) enters a non-reproductive state known as the diapause, and wintering butterflies in this state can live for over half of a year, and begin the migration back north in the spring. Usually it is in late February that the monarchs leave the biosphere and head back north, and at this time, they leave the diapause phase, and the butterflies reproduce and die early in the trip north. The following generations of these insects are the ones that complete further sections of the trip, and later generations yet are the ones that return to their summer grounds in the northern states and southern parts of Canada.
The population of butterflies west of the Rockies does not perform such an incredible migration, as this population winters in California. But the eastern population undertakes a greater migration than that of any other butterfly in the world. And what is truly amazing about it is that the annual migration of monarch butterflies brings them all to the exact same wintering ground in a remote part of Mexico, even though it takes many generations to get there and back. People may track and view reports of the annual monarch migration here.
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