The Monarch Migration miracle

by Rod Green

Few natural events are more fascinating than the annual migration of monarchs. Since I can remember, I have marveled at the grace and beauty of the brightly colored black and orange butterflies, fluttering through summer fields and roadsides to probe nectar from sunflowers, ironweed, and other wildflowers. I have wondered at the insects’ delicate, seemingly fragile, condition upon which they draw strength to perform great flying feats.

I have even studied them as they flocked together in a late summer evening migration roost – thousands of the feather-light insects hanging together from the same tree. There they rested the night before resuming their southward journey to some distant destination. Clinging in clusters of numbers beyond counting, packed together like masses of living colored leaves, they usually hung motionless, their wings folded above. New butterfly arrivals were greeted with a gentle mass flutter of wings, resembling fall leaves quaking in a soft breeze. While the numbers of bodies and wings were so great that the collective fluttering produced a soft whir, the total weight barely caused the lightest branches to bow.

Several generations of monarchs are reared in Missouri each summer. Only those that develop late in the season will be equipped to migrate to winter in Mexico. Late summer and early fall fields offer places of rest and nectar for refueling. To stay beyond the first hard frost would mean death to the fragile butterflies. Newly hatched early fall monarchs — several generations removed from those relatives that arrived from the south in the spring — look like others, except that they lack developed reproductive organs. Stimulated by lowering temperatures and decreasing sunlight of the changing season, the monarchs take to the air, adding to the swell of birds and insects that migrate in the autumn rush.

The monarch is a miracle. The fragile nectar-sipper with wings of gossamer and weight of only half a gram (about 1/50 ounce) annually takes a navigator’s trip. Monarchs from southeastern Canada and the eastern United States migrate a 2,000-mile autumn trip to Mexico. Incredibly, some 200 million of these insects (most of the entire late summer North American population) travel alone or in large groups to eventually reach a small alpine fir forest in the Sierra Transvolcanica Mountains some 75 miles west of Mexico City. A similar though smaller migration occurs on the west coast, some monarchs flying as far as southern Alaska to winter at the southern tip of Monterey Bay. There they await spring and a return journey.

No one knows how the insects are guided on their journey to one tiny spot on the globe. Almost all the travelers are some five generations removed from those that made the last trek. Yet, some instinct must provide the mystical knowledge that guides their primitive being over new and uncharted country from the milkweeds of their birth.

The central Mexico mountain wintering grounds provides special climatic conditions necessary for survival. Temperatures remain just above freezing, allowing the monarchs to remain in a semi dormant state. A few degrees drop and the butterflies would freeze (such a climatic event killed some 2.5 million adults in 1981). Warmer conditions would activate the insects, causing them to burn up stored energy reserves needed for the spring trip north.

Nature has provided excellently for this fragile living form. Unlike most American butterflies that live only a few weeks, the migrating monarchs typically live up to eight or ten months. Added to this longevity, the monarch is given immunity to attack by predators. Few birds or other insect-eaters prey upon the distasteful monarch during migration or on the wintering grounds.

A remarkable biological phenomenon is responsible in part for the monarch’s survival. Milkweeds are the only plants that the monarch larva, caterpillars, will eat. Thus females lay their eggs on or near milkweed plants. The milky juice of the plants contains a toxic acid to which the caterpillars are immune. In fact, they consume it with gusto, converting the plant material into animal tissue. The acidic juice imparts a bitter taste to larva and ultimately the molted adult butterfly. One episode with a foul-tasting monarch and would-be predators learn to avoid the black and orange patterned butterflies (ironically, the viceroy butterfly — a very palatable creature — is similarly colored and marked; it enjoys the mimicry protection from predators that have tasted the monarch).

The increasing sunlight of spring causes the wintering monarchs to grow restless. Sometime between mid-February and late March, they depart on a northern flight of no return. The northward migration is an unhurried individual event that usually goes unnoticed by most human eyes. Before dying the females lay eggs, ensuring a new generation of their kind. Some six months later, the offspring, a few generations removed, once again head south of the border with early fall.

Monarchs usually don’t show in Missouri until frost-free spring. Some early sightings may be one of the mass that went south last fall, but most are of newly hatched generations from eggs laid on the way up north.

The annual monarch mass migration time is likely in its locally waning hours. The monarch miracle is a marvel as it unfolds…Outdoors in Northwest Missouri.