Species Spotlight: Monarch Butterfly
Long before the monarch butterfly became the state insect of Illinois it had already piqued human curiosity. C.V. Riley, Missouri's first state entomologist, noted as early as 1878 that midwestern populations of monarchs underwent birdlike migrations each autumn. Where they went, however, remained a mystery for many years. Finally, using a butterfly tagging system that involved thousands of collaborators, researchers discovered in January 1975 that monarchs from eastern North America overwinter south of the Tropic of Cancer in the mountains of central Mexico on Oyamel fir trees (also called sacred fir). Their numbers were so dense the branches literally sagged with the weight. Fir trunks were so densely clad with bright orange monarchs that they mimicked the shinglelike scaling found on each individual's wings. How these multimillion monarchs come to this place each year is perhaps the best-known feature of this remarkable creature, but it is only one aspect of the fascinating tale of the life of the monarch!
This story begins with an egg. A female monarch is an excellent, if somewhat narrowly focused, botanist and chooses only milkweeds on which to trust her young. These common plants of roadsides, fields, and prairies throughout North America are not so common chemically, and most are laced with toxic compounds called cardenolides or heart poisons. If eaten, these chemicals can cause irregular heartbeats, but have a somewhat more obvious effect on unsuspecting ingesters--they cause vomiting. Monarch caterpillars don't seem to mind the poisons and incorporate them into their bodies as a potent defense against predators. Even adults that emerge after five caterpillar molts and pupation retain the toxins. Both monarch caterpillars and adults advertise their distastefulness to the world. The caterpillars are colorfully banded with alternating white, yellow, and black stripes, while the bright orange and black adult coloration is worthy of any highway traffic warning sign! Entomologists have a name for this--aposematic coloration.
The monarch butterfly, Danaus plexippus.
The story of migration begins with the last generation of monarchs produced in the Midwest. During most Illinois summers three generations or broods of monarchs are produced, but the last one is unique because it does not reproduce (called reproductive diapause). When the cool days of September and October approach, monarchs begin to congregate together and head in a southwesterly direction. Monarchs usually stop their flights as dusk approaches and form temporary clusters on trees or shrubs. These groups may break up the next morning or last for a few days, depending on the weather. The journey can take 75 days and individuals may average at least 50 km per day to reach the high altitude forests of the Sierra Madre, where they will spend the winter.
Midwestern monarchs are adapted to the Mexican montane forests. The cold allows them to lower their metabolic rates and activity from mid-November to mid-March while they rest quietly in the familiar dense clusters on the firs. They must conserve their fat reserves if they are to make the return flight. As the winter proceeds, mating frequency increases in preparation for the return home. After the spring equinox, monarchs return to Gulf Coast states and lay eggs on southern milkweeds to produce the first generation of new adults. These migrate northward, laying eggs as they go, as far as southern Canada. Up to three generations are produced in the upper Midwest with two produced in the south. The final, nonreproducing generation, often the great-great-great-grandchildren of the spring migrants, must then make the long return flight as the fascinating life cyle continues.
Michael Jeffords and Susan Post, Center for Economic Entomology