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The Monarchian Odyssey
| Article
# : |
15893 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
1 / 1989 |
1,646 Words |
| Author
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Stephen B. Malcolm
Stephen B. Malcolm is a research fellow with the Department of
Zoology at the University of Florida. He has recently edited
a book, Biology and Conservation of the Monarch Butterfly, to
be published by the Los Angeles Country Natural History
Museum, Contributions in Science. |
With the onset of autumn each year in the northeastern United States and southern Canada, Danaus plexippus, more commonly known as the monarch butterfly, leaves its senescing food plants and the increasingly cold northern temperatures to begin a remarkable journey. The autumn migration of the monarch butterfly takes this extraordinary insect to tiny, remote areas in high altitude tropical forests in Mexico, where they spend the cold winter months clustered among cool conifers. The following spring the same butterflies remigrate in search of their newly appearing food plants. The key to this most dramatic of insect migrations lies with the monarchs preferred food plant--the milkweed.
As long ago as 1753, the great Swedish systematist Linnaeus assigned a group of medicinally important plants to the genus Asclepias, naming them appropriately after the Greek god of medicine. Although Linnaeus could not have known that the milky latex of these handsome and diverse plants contains special bitter-tasting steroids called cardenolides, he probably did know that "milkweeds" had long been of value in heart therapy and as rather drastic purgatives and emetics. However, long before Linnaeus acknowledged the medicinal value of milkweeds, a large orange and black butterfly knew all about them and their bitter-tasting chemicals. The larvae of monarch butterflies feed exclusively on milkweeds, and it is probably this fact more than any other that is responsible for the most extraordinary insect migration yet known.
Cardenolide Protection
Cardenolides are used by milkweed plants as a toxic chemical defense against a wide variety of herbiroves. Monarchs are one of a small number of species that have effectively broken through this defense. And not only have the larvae of monarch butterflies beaten the plant, they have incorporated the plant's chemical defenses in their own poisonous defense against predators, such as birds, mice, large insects, and spiders. Thus, monarchs contain large amounts of milkweed-derived cardeonlides, which explains why the adult butterflies are splendidly adorned in orange and black. Normally, bright colors act as a warning to would-be predators that the prey is potentially dangerous or unpalatable. It is this highly effective defense that allows monarchs to fly unmolested each spring as they migrate in search of their milkweed hosts.
Each year, monarchs reappear in the southern Gulf states from Texas to Florida during the last days of March. The timing of their arrival is always remarkably consistent.
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