The butterfly that outlives its parents by months

Every autumn, orange-and-black butterflies weighing less than a gram set off across North America. The eastern population is heading for a handful of mountain forests in central Mexico, thousands of kilometres away. None of the butterflies making the trip has ever been there before. Their parents never went. Neither did their grandparents.

This is the migration of the monarch butterfly, and it works in a way no single butterfly could manage on its own.

A relay across generations

According to the USDA Forest Service, monarchs produce up to four generations each summer, and each one travels a little further north than the last. These summer adults live only about two to five weeks. In that short time they mate, the females lay the next generation’s eggs — between 100 and 300 eggs over a lifetime — and then they die.

The eggs are laid on milkweed. When the butterflies return north in spring, the U.S. National Park Service notes, they lay their eggs on certain types of milkweed plants along the way. The caterpillars feed on the leaves, grow, form a chrysalis and emerge as the next adults, who carry the journey further.

So the northward trip is not one flight but a relay: a new generation picks up where the old one stopped.

The super generation

At the end of summer, something changes. The last generation of the year does not breed straight away. Instead, it flies south.

This generation is different in a striking way. While summer adults live for weeks, the migrating adults can live as long as nine months, according to both the Forest Service and the Park Service. That is many times the lifespan of their parents. It has to be: these butterflies must fly south, survive the winter and then begin the journey north again in spring.

The distance is enormous for an insect. The Forest Service states that migrating monarchs fly between 2,000 and 3,000 miles — roughly 3,200 to 4,800 kilometres — to their overwintering sites in Mexico.

East and west

North America has two main migrating groups, divided roughly by the Rocky Mountains. The Park Service describes how eastern monarchs spend the winter in the oyamel fir forests of Mexico, while western monarchs make similar multi-generation journeys over shorter distances and overwinter at sites along the California coast.

The eastern population is special. The Park Service notes that it is the only population of monarchs in the world known to make a journey spanning so many generations.

A winter in the mountains

The overwintering sites are few and far between. One of the best-known sites is the El Rosario Sanctuary in the Mexican state of Michoacán, which the Forest Service features in its material on monarch biology. The butterflies stay there through the winter, and in spring the cycle begins again: the long-lived migrants head north, lay eggs on milkweed, and a new chain of short-lived summer generations takes over.

What monarchs need

A migration like this depends on a long chain of places. Monarchs need milkweed for their caterpillars all along the route north. They need nectar from flowers to fuel their flights. And they need their wintering forests to remain intact.

That is why gardeners, farmers and land managers across North America plant milkweed and native nectar flowers. A single garden is a small link in the chain — but the chain is made of nothing else.

Why it matters

The monarch shows that a migration does not have to be remembered by an individual to be repeated faithfully. It is passed on through the generations, and it can only continue if every part of the route — from a roadside milkweed to a mountain forest in Mexico — still offers what the butterflies need.