Plant Guide

Dannaus plexippus

Family: Nymphalidae - Danainae
Common Name: Monarch Butterfly


Description of Species:
The monarch butterfly is undoubtedly one of the most recognized animal species in the world. This is partially due to its wide distribution, but probably more due to its beautiful, striking coloration.

The monarch is a medium sized butterfly, measuring about 3 inches from wingtip to wingtip. Its body is about one inch long. Its four wings are generally a field of yellow, orange or gold, with veins of black running through them. A band of black, thickest at the front, rings the wings, and the body is black as well. This black band is usually speckled with white spots, larger at the front and smaller at the back.

The Monarch Butterfly (Dannaus plexippus) has a rich natural history that has been studied extensively by entomologists and biologists. Despite this scrutiny, new discoveries are still emerging about this beautiful insect. It has an extensive home range, but specific habitat needs. Its mating habits are in some ways the opposite of what one would expect, and its complex adaptations continue to cause argument amongst researchers to this day.
The ecology and the home range of the monarch butterfly are closely intertwined, as with most species. Put simply, it is dependent upon milkweed plants, belonging to the family Asclepiadaceae, of which about 2,400 species exist (Urquhart, 1987). The distribution of the monarch is controlled by the distribution of milkweed, it regulates their density in a given area, and it is for this plant that the monarchs migrate for long distances every year. So dependent upon milkweed is the monarch that where one finds the monarch, one will also find milkweed.

Milkweed is the host plant for most  of  the monarchs life cycle. Eggs are deposited and hatch on the underside of  leaves of the milkweed plant. Upon hatching, the larva will feed upon the fine hairs on the leaves of this plant, and stay on this same plant throughout its five molting stages. After molting, the larva will leave the milkweed and construct its chrysalis somewhere else. However, once an adult monarch emerges from the chrysalis, it will soon head back to a milkweed plant for foraging and shelter (Urquhart,1987).

The monarchs will spend their summer either in the New England-Great Lakes area, or else in the canyons of the eastern Rocky Mountains. The Great Lakes population will migrate southwest in the fall, and spend the winter in the Sierra Madre mountains  of central Mexico. Despite having been studied for so long, it was not until 1976 that their overwintering ground was discovered (Urquhart,1987). That population which spends its summers in the Rocky Mountains will migrate to California and spend the winter in a variety of roosting sites as far north as Monterey, CA. Although their overwintering grounds do not necessarily depend upon the presence of milkweed, they will always return to areas rich in this plant to deposit their eggs.

The monarch butterfly is a fascinating animal that has earned its nickname "The International Traveler" (Urquahart 1987). Although it is one of the most successful species of animal on the planet, the population in its original range, North America , is threatened by deforestation of its overwintering areas in the Sierra Nevada of Mexico.  It is further threatened by  the fragmentation of  its summering habitat in  the northern United States. Due to its wide range and somewhat predictable nature, it is a prime indicator species for the ecological  health of a very large area.


Source: David Munro San Francisco State University student 1999

Some great Monarch sites:

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