Monarch Butterfly Overview

Monarch butterflies are among the most beautiful and familiar butterflies in North America. Through the years, adults as well as children have been fascinated and awed by their spectacular transformation from caterpillar into adult. The monarch, along with the viceroy butterfly, also offers one of the best examples of mimicry in nature. And, as if this were not enough, this butterfly, like many birds, participates in the annual mystery of migration in order to perpetuate the species.

The monarch butterfly is primarily an American butterfly occurring from Canada to South America, although absent from Alaska and much of the Pacific Northwest. It has also spread somewhat to Europe and Australia. The adult monarch has a wingspread of anywhere from 3 3/4 to 4 inches with an orange-brown to reddish upper surface, and veins broadly outlined in black. The outer wing margins have wide black borders with two rows of white spots. The male can be distinguished from the female by a prominent black enlargement alongside the third vein from the inner margin of the hind wing.

This butterfly is generally found in association with the milkweed plant, particularly in rich meadows, weedy fields and near waterways. The monarch lays its pale green eggs on the underside of milkweed leaves, or on the flowers. After hatching, the young caterpillar first eats the protein rich egg shells, and then begins to feed on the milkweed plants themselves. The richly striped black, white and yellow caterpillars, with 2 pairs of black filaments near the head and tail, will remain on the plant for two to three weeks. Another 10 to 12 days are spent in the beautiful chrysalis(cocoon). This one-inch structure produced by the larvae is pale, jade green and studded with glistening gold trim. A few hours before the adult emerges, the chrysalis becomes thin, transparent and brittle. Within minutes the monarch’s wings are fully expanded, but it will be unable to fly for at least another hour.

Common among insects, the phenomena known as mimicry is the resemblance of one species, the mimic, to the model, a species avoided by predators because of a protective feature. The monarch butterfly serves as a model for its mimic, the viceroy butterfly. Monarchs are unpalatable to birds, as most of the milkweed plants upon which they feed contain cardiac glycosids, or heart poisons. Because of this, birds have learned to avoid monarchs, and, since both species have similar color patterns, they also avoid viceroys. Curiously, females contain more of the glycosids than males, and more of the northeastern U.S. adults are poisonous than southern U.S. and California adults.

The monarch butterfly is the only butterfly that annually migrates north and south. Adult monarchs start to move south from their northern breeding sites in July, and large congregations can be seen on the move by September. They apparently move by daylight, stopping to feed along the way. For many years, individual butterflies have been marked with tiny wing tags and released for study. From these studies it has been found that monarchs regularly cover distances in excess of 1,180 miles, at speeds of up to 80 miles per day (although 20 miles is more typical for a day’s flight). The longest recorded flight by one butterfly is just under 1,875 miles in a period of 130 days. However, opinions differ as to how far most individuals fly and how much the migration may be made up of successive short trips by different individuals. Although butterflies seem to be capable of directional movement, they are also influenced by the effect of wind and other air currents and may be blown off course. Many scientists believe that they possess an internal “compass” to guide them along.

At night and when weather conditions are bad, monarchs roost in trees. The butterflies finally reach their destinations of either southern Florida, California or Mexico in late November. Along the way, however, an estimated more than half of those beginning the long migration will have perished due to wind or water damage, predators and a host of other causes. Those who do survive the arduous journey tend to collect at only a few sites, and in some cases on particular trees. For instance, almost the entire population of monarch butterflies from the central United States can be found clustered together in large masses in a small area west of Mexico City. Luckily for the butterflies, in 1986 the Mexican government formally designated this area as a permanently protected sanctuary. Those in California may not be so fortunate, as nearly all of the roosting sites face development.

The monarchs are somewhat sluggish in winter and do not reproduce but will venture out on warm days in search of nectar. With the arrival of spring they begin to head north. The egg carrying females will lay roughly 400 eggs along the way. But few survive to return to their original starting point. This task is left to the summer’s first and second generations born along the way.