Monarch Butterfly (Danaus plexippus): Facts and Migration

What does a monarch butterfly look like?

The upper side of the wings is a bright tawny orange crossed by bold black veins. The wing borders are black and dotted with two rows of small white spots, and the black forewing tips carry a few white and orange spots. Underneath, the pattern is similar but paler, with the hindwings a soft yellowish orange.

Males and females are easy to tell apart. A male has a small black spot on a vein in the middle of each hindwing. It is a patch of special scent scales used in courtship. Females lack these spots, have thicker black veins and often look darker overall.

Like other members of the brush-footed butterfly family, Nymphalidae, the monarch walks on four legs; its tiny front pair is held folded against the body. On the Hawaiian island of Oahu, a pale form with white instead of orange wings makes up a noticeable share of the population.

The caterpillar is just as striking. It is banded in yellow, black and white, with a pair of long black filaments at the front and a shorter pair near the rear. The chrysalis is jade green with a rim of tiny gold dots.

Monarch look-alikes

The viceroy (Limenitis archippus) is the classic look-alike. It is smaller and has an extra black line curving across each hindwing. The queen (Danaus gilippus), a close relative found in the southern United States and Mexico, is a deeper chestnut brown with white spots and lacks the monarch’s bold black veins on the upper side. The painted lady is also orange and black but is much smaller, with blotchy rather than veined wings.

How big is a monarch butterfly?

The monarch is one of the larger butterflies in North America. Its wingspan is about 8.6–12.4 cm (3.4–4.9 in), with most adults around 9–10 cm (3.5–4 in). An adult weighs roughly half a gram (0.02 oz).

Size varies with generation and population. Migrating monarchs tend to be larger than summer butterflies, eastern monarchs have bigger, more angular forewings than western ones, and males are on average slightly larger and heavier than females.

Habitat and range

Monarchs breed wherever milkweeds grow in open, sunny country: prairies, meadows, pastures, field edges, roadsides, wetland margins, parks and gardens. In winter, migratory monarchs need very different habitat: cool, sheltered stands of trees such as oyamel fir in Mexico, or eucalyptus, Monterey pine and Monterey cypress groves on the California coast.

The native range runs from southern Canada through the United States, Mexico, Central America and the Caribbean to northern South America. Not all of these monarchs migrate: populations in southern Florida, the Caribbean and Central America breed all year round.

During the 1800s monarchs spread across the Pacific, reaching Hawaii, New Zealand and Australia, where they breed on introduced milkweeds. In the Atlantic they are established in the Azores, Madeira and the Canary Islands, and small populations live along the southern coasts of Spain and Portugal and in Gibraltar. A few monarchs reach Britain and Ireland in some autumns, probably helped across the Atlantic by strong westerly winds.

How far do monarchs migrate?

Each autumn, monarchs east of the Rocky Mountains fly up to about 4,800 km (3,000 mi) to spend the winter in fir forests high in the mountains of central Mexico, mainly in the states of Michoacán and México. Most monarchs west of the Rockies head for groves along the California coast instead. The butterflies that make the trip belong to a special late-summer generation that does not breed until spring.

At the winter sites, millions of monarchs pack together in dense clusters on the trees and spend the cold months resting. In late winter they mate and begin flying north, lay eggs in Texas and nearby states, and die. Their offspring and the following generations complete the journey back to the northern United States and Canada.

Scientists from outside Mexico only located the eastern winter colonies in 1975, although local Mazahua and Otomi communities had known about them for generations. Monarchs find their way with a sun compass corrected by a body clock in their antennae. The full story is in where do monarch butterflies migrate?

What do monarchs eat?

Monarch caterpillars eat only milkweeds, plants of the genus Asclepias and a few close relatives. More than 100 milkweed species are native to North America. Common milkweed (Asclepias syriaca) is especially important: chemical studies suggest that roughly 85–92% of the monarchs wintering in Mexico fed on it as caterpillars.

Milkweed leaves are full of sticky white latex. Older caterpillars often chew through a leaf vein or notch the leaf stalk to drain the latex, then feed beyond the cut.

Adults drink nectar through a long, coiled tongue called a proboscis. They visit many flowers, including milkweeds, goldenrods, asters, blazing stars, Joe-Pye weeds, coneflowers and Mexican sunflowers. Late-blooming flowers are vital fuel during the autumn migration, when monarchs build up fat for the winter. More on butterfly feeding is in what do butterflies eat?

Why are monarchs poisonous?

Milkweeds contain cardenolides, bitter heart poisons related to the digitalis compounds of foxglove. Monarch caterpillars resist them thanks to a few changes in a key protein, the sodium pump, and store the toxins in their bodies. The toxins stay in the adult butterfly, concentrated in the wings and abdomen.

A bird that swallows a toxic monarch usually vomits soon afterward and learns to avoid the orange-and-black pattern, which works as a warning sign. Toxicity varies a great deal, depending on which milkweed the caterpillar ate. The viceroy, long thought to be a harmless copycat, turned out to be distasteful too. Read the details in are monarch butterflies poisonous?

Life cycle and lifespan

Monarchs go through complete metamorphosis: egg, caterpillar, chrysalis and adult. The whole cycle takes from as little as 25 days in warm summer weather to about seven weeks in cool spring conditions.

1. Egg. A female lays her eggs one at a time, usually on the underside of a young milkweed leaf. She typically lays 300–500 eggs, and more than 1,000 have been recorded. Eggs hatch in about 3–8 days.
2. Caterpillar. The caterpillar grows through five stages called instars, shedding its skin between each one, over roughly two weeks. A full-grown caterpillar weighs about 1.5 g (0.05 oz), thousands of times more than when it hatched.
3. Chrysalis. The caterpillar hangs head-down in a “J” from a silk pad, then sheds its skin to reveal the green chrysalis. The adult forms inside in about 8–15 days.
4. Adult. The butterfly emerges, pumps fluid into its crumpled wings and is ready to fly within a few hours.

Summer adults usually live about two to five weeks. The migratory generation lives far longer, about eight months, because shorter days and cooler temperatures in late summer switch off reproduction, a state called reproductive diapause. In eastern North America, the year typically includes about four generations: the long-lived migrants and the two or three short-lived generations that carry the population north again. Fewer than 10% of eggs and caterpillars survive to adulthood.

Predators and threats

Toxins do not protect monarchs from everything. At the Mexican colonies, black-backed orioles pick out the less toxic parts of the butterflies, black-headed grosbeaks tolerate the poisons, and black-eared mice eat monarchs on the ground. Wasps, spiders, assassin bugs, stink bugs and the introduced Asian lady beetle eat eggs and caterpillars, while parasitic tachinid flies lay their eggs on or in the caterpillars.

A single-celled parasite, Ophryocystis elektroscirrha, weakens infected butterflies. Badly infected monarchs may be unable to emerge properly or to complete the migration.

The biggest threats are caused by people: the loss of milkweed and wildflowers from farmland and roadsides, herbicide and insecticide use, building development, logging and forest damage at winter sites, and climate change, which brings drought, heat waves and severe winter storms. Traffic also kills large numbers of migrants on some highways.

Conservation status

The monarch species as a whole is listed as Least Concern on the IUCN Red List (assessed in 2022), because it is widespread across the Americas and on many islands. The migratory North American subspecies, D. p. plexippus, is assessed separately: the IUCN listed it as Endangered in 2022 and reassessed it as Vulnerable in 2023.

In December 2024 the U.S. Fish and Wildlife Service proposed listing the monarch as threatened under the Endangered Species Act. It estimated that the eastern population had fallen by about 80% and the western population by about 95% since the 1980s. The final decision was delayed, and a 2026 court settlement gave the agency until 2030 to decide. Canada listed the monarch as endangered under its Species at Risk Act in 2023.

Most Mexican winter colonies lie inside the Monarch Butterfly Biosphere Reserve, about 56,000 ha (138,000 acres) of mountain forest that became a UNESCO World Heritage Site in 2008. For population trends and threats, see are monarch butterflies endangered?

Monarchs and people

Few insects are as well loved. The monarch is the official state insect or state butterfly of several US states, including Alabama, Idaho, Illinois, Minnesota, Texas, Vermont and West Virginia. In Mexico the butterflies arrive around the Day of the Dead in early November, and in some communities their arrival is linked with the returning souls of the dead.

Monarchs are among the best-studied insects, and volunteers have gathered much of that knowledge. Each autumn, thousands of people place tiny numbered stickers on monarch wings for tagging programs such as Monarch Watch, and the yearly Western Monarch Count has tracked the California population since 1997. Many gardeners now plant native milkweeds and nectar flowers to create “monarch waystations”.

Some scientists caution against rearing large numbers of monarchs indoors for release. Crowded captive butterflies can spread disease, and studies suggest that monarchs raised indoors may navigate less well than wild ones. If you want to see how monarchs differ from their night-flying relatives, read butterfly vs moth.

Myths and facts

Myth: Each monarch flies to Mexico and back. Fact: Only the migratory generation flies south. It begins the return trip in spring, but its children, grandchildren and great-grandchildren complete it.

Myth: Viceroys are harmless mimics of monarchs. Fact: Taste tests with birds published in 1991 showed that viceroys can be just as distasteful, so the two species share the same warning pattern, a relationship called Müllerian mimicry.

Myth: Touching a monarch’s wings means it can never fly again. Fact: Rough handling can rub off scales and damage the wings, but monarchs carefully tagged by researchers routinely fly on to Mexico.