Monarch Butterfly

The monarch is the most famous butterfly in the world, and for good reason: a fragile half-gram insect completes an annual migration across a continent, navigating to a specific patch of mountain forest it has never seen, using a compass built into its head.

Overview

Monarchs live throughout the Americas and have spread to Hawaii, Australia, and parts of Europe. What made them famous is the North American migration. Each autumn, monarchs from as far north as Canada funnel down to a handful of overwintering sites — high-altitude oyamel fir forests in central Mexico for the eastern population, and groves along the California coast for the smaller western one — clustering in their millions on the same trees year after year.

The multi-generation journey

The migration is stranger than a simple round trip, because no individual butterfly does the whole thing:

1. In spring, overwintered monarchs leave Mexico, fly north, breed on the first milkweed they find, and die.
2. Their offspring continue north and breed again. This repeats over three or four short-lived generations through the summer, spreading monarchs across the eastern US and southern Canada.
3. In late summer, environmental cues trigger a “super generation.” These butterflies do not breed. They shift into migration mode, live six to eight times longer than a summer monarch, and fly up to 4,000 km south to the exact forests their ancestors left.
4. They overwinter in a semi-dormant state, then start the cycle again the next spring.

Navigation

To hold a southwest course, a migrating monarch uses a time-compensated sun compass. Its compound eyes track the sun’s position, and a circadian clock housed in the antennae tells it the time of day, so it can adjust for the sun’s movement across the sky. On overcast days it falls back on an ability to sense the Earth’s magnetic field. Cool overnight temperatures reset it each morning.

Milkweed and defence

The monarch’s life is bound to one group of plants. Female monarchs lay eggs only on milkweed, and the caterpillars eat nothing else. Milkweed is laced with heart-affecting toxins called cardenolides; monarch caterpillars tolerate them and store them, so the caterpillar and the butterfly it becomes are toxic and foul-tasting to most birds. The vivid orange, black, and white pattern is a billboard advertising that fact — and several harmless butterflies mimic it to borrow the protection.

Threats and conservation

The migratory monarch is in serious trouble:

• Loss of milkweed. Herbicide-tolerant crops let farmers spray fields clean of the milkweed that once grew between rows across the US Midwest — the monarch’s core breeding ground.
• Degradation of overwintering forests in Mexico through illegal logging and, increasingly, drought and unseasonal storms that can kill millions of clustered butterflies in a night.
• Pesticides, climate change, and roadside habitat loss along the migration route.

Recovery efforts centre on planting native milkweed and nectar plants across the migratory corridor — in farms, roadsides, parks, schoolyards, and gardens — and on protecting the Mexican forest reserves. It is one of the few endangered animals almost anyone can directly help by changing what grows in their yard.