Monarch Butterflies 101

A butterfly cocoon, and an orange and black butterfly on a purple flower.

The monarch’s epic journey

If you live in Chicago or other parts of the Midwestern and Eastern United States, you might see monarch butterflies passing through not once, but twice a year. These iconic orange and black butterflies migrate north through Illinois during the summer months before turning south again in the fall to spend the winter in Mexico. Monarchs (Danaus plexippus) are known for their long-distance migration—up to 3,000 miles spanning three countries!—and their large winter gatherings in Mexico and California. And their epic travels still remain largely a mystery to scientists: the trip from the Northern US and Canada to Mexico and back again requires up to five generations of individual butterflies. While we don’t know exactly how they find their way, it’s clear that Illinois and other Midwestern states are a pivotal pit stop on this journey.

A monarch’s life

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A small, translucent egg-shaped object on a fuzzy green surface.
Monarch egg. Photo: John Balaban.

These butterflies start out as tiny eggs—very tiny, around one millimeter long, or about the size of a pencil tip. Female monarchs only lay eggs on milkweed leaves, and they attach the eggs with a glue-like substance. After a few days to a week, a larva, or caterpillar, emerges from the egg. Monarch caterpillars sport white, black, and bright yellow stripes. As it grows, the monarch larva sheds its skin. After around 12 to 14 days, something crazy happens: the caterpillar hangs upside down and sheds its skin to reveal a harder casing—the chrysalis—underneath. It’s in this pupal stage that metamorphosis occurs, when the ensconced caterpillar finishes its transformation into an adult. A butterfly emerges after about a week or two, with its recognizable orange and black markings.

Monarch or mimic?

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Two butterflies that are orange with black stripes, in green foliage
Top: monarch. Bottom: viceroy. Both photos by John Balaban via the monarch habitat Field Guide. 

So, how do you know if you’ve spotted a monarch? Monarchs are visually striking butterflies, but they have a sneaky double. The viceroy (Limenitis archippus) also has bright orange coloring and bold black lines. One key difference in appearance is that viceroys have a black line across their hind wings. Also, monarchs tend to be much larger in size and stronger fliers than viceroys, which rapidly flutter their wings. Scientists are still debating the reason for this mimicry. A popular theory is that viceroys developed this resemblance to protect themselves: predators like birds will avoid eating monarchs because they know they’re toxic—a result of noshing on milkweed as young caterpillars. If viceroys are mistaken for monarchs, birds might avoid them, too. While viceroy adults eat milkweed nectar, they enjoy a wider variety of plants and flowers, and the caterpillars feed on trees in the willow family.

Why monarchs need milkweed

Monarch numbers have been dropping dramatically since the 1990s: it’s estimated that the population that spends winters in Mexico has declined 80 percent. Many environmental and human factors play into this decline, including increased stress from climate change and reductions in milkweed and other plants that provide nectar.

Monarchs rely on milkweeds during the northern part of their migration: the caterpillars won’t eat anything except for the leaves of milkweed plants (so if you see holes in milkweed leaves, that’s actually a good sign!). 

If you live in a city, you might feel like you can’t help out with spring plantings. But a few milkweed plants in your backyard or community garden could help add to the larger landscape. Most milkweeds even do well in containers on a balcony or in a shared outdoor space like a patio or front walk. Learn how to plant in containers, with Field Museum ecologists giving key tips for success.

Find out more about how to get involved in monarch conservation