Migrating Monarchs

As summer comes to a close, the most interesting part of a monarch butterfly's life is beginning.

What they are: By any measure, monarchs are extraordinary insects.

The strikingly beautiful adult is, perhaps, the world’s most well recognized butterfly. But it’s just as interesting to look at in its other phases of life.

As a larva – the caterpillar stage – are bright yellow and black. The chrysalis it wraps itself in during its pupa phase is a brilliant green flecked with shining gold.

The captivating adult butterfly may look lovely to us, but those bright orange and black markings are a dire warning to its potential predators. Monarchs are toxic, thanks to their exclusive diet of milkweed leaves early in life.

Females lay eggs exclusively on milkweeds, unassuming, weedy-looking plants with plain purple blooms. The milky sap contains a substance poisonous to most vertebrates, and over countless generations, the monarch’s likely predators – birds, lizards and the like – have learned to avoid the beautiful butterfly.

But monarch’s most interesting characteristic may be the fact that they undertake an amazing migration each year. They are the only butterflies to do so.

After munching milkweed all summer and transforming into their adult forms, the entire population of monarchs in the eastern U.S. embarks on a two-month migration to the evergreen forests of south-central Mexico. (Western individuals end up in southern California and western Mexico.)

There, millions of monarchs drape native fir trees in living curtains throughout the winter. It’s an amazing spectacle.

For such a delicate-seeming organism, the journey itself is extraordinary. But when you dive into the particulars, it’s even more fascinating. Butterfly migration is not like bird migration. Because the round-trip travel time to Mexico and back is far longer than the average individual’s lifespan, the butterflies that fly south are not the ones that return to us in the spring.

Instead, several generations hatch and die during the journey, making it a migration of a species, rather than one of individuals.

Where to find them: Now is the time of year to start looking out for migrating monarchs.

The best way to spot them is to grab a pair of binoculars and look up on a warm, sunny day with clear blue skies. You may see small groups of them fluttering high above you in a purposeful way, all heading in the same direction – south.

If you want more monarchs in your life, plant milkweed and wait. You’ll likely play host to some babies next spring.

Why bother: Why monarchs migrate is still an entomological mystery. Scientists don’t fully understand why they never evolved to hibernate instead, or how individuals know precisely when to leave and where to go.

What we do know is that they’re highly dependent on Mexico’s evergreen forests. The butterflies cling to the trees and to each other to stay warm (the climate may be tropical, but in the higher elevations where the butterflies overwinter, temperatures can get chilly).

In deforested areas, the monarchs die in droves, forming carpets of dead butterflies six feet deep or more. As a result, there’s increasing pressure to preserve the forests where the monarchs migrate year after year.