Are Monarch Butterflies Poisonous? Milkweed Toxins Explained

Where does the monarch’s poison come from?

The poison comes from the caterpillar’s only food, milkweed. Milkweeds (Asclepias) protect themselves with sticky white latex and with cardenolides, also called cardiac glycosides. These compounds block the sodium-potassium pump, a protein that animal cells, especially heart and nerve cells, need in order to work. They are chemical relatives of the digitalis compounds found in foxglove.

Most plant-eating insects cannot eat milkweed at all. Monarchs evolved a handful of changes in their own sodium pump that make it resistant. In a 2019 experiment, scientists gave fruit flies the same three key changes using gene editing, and the flies became resistant to these toxins too.

A monarch is not poisonous because of anything it makes itself. It borrows its defense from its food, a strategy called sequestration. Monarchs reared on plants without cardenolides, such as cabbage, are eaten by birds without harm.

How do caterpillars cope with milkweed?

Milkweed latex can glue a small caterpillar’s jaws shut. Young caterpillars often chew a circle in the leaf to cut off the flow of latex before feeding inside it. Older caterpillars bite through a leaf vein or notch the leaf stalk so that the latex drains away, then eat the leaf beyond the cut.

Toxin levels vary widely among milkweed species. Common milkweed has moderate levels, tropical milkweed (Asclepias curassavica) is rich in cardenolides, and butterfly weed (Asclepias tuberosa) has very little. Too much toxin can slow a caterpillar’s growth, and egg-laying females often prefer plants with moderate levels.

The blue jay experiments

The monarch’s defense was proven in the 1960s by the American ecologist Lincoln Brower and his colleagues. They offered caged blue jays monarchs raised on a toxic milkweed. The jays ate them and soon vomited. After one bad experience, many birds refused to touch another monarch.

The key test came next. Monarchs raised on cabbage, or on a milkweed relative without cardenolides, were eaten by the same jays with no ill effects. This showed that the butterfly’s poison comes entirely from the plant its caterpillar ate.

Later work showed a wide “palatability spectrum” in wild monarchs, from highly toxic to almost harmless, depending on the milkweed eaten. The less toxic butterflies are still protected, because birds cannot tell them apart from the toxic ones, a pattern called automimicry. Learn more about this clever bird in our blue jay profile.

Warning colors: how monarchs advertise

Animals that are dangerous to eat often wear bold, contrasting colors, an adaptation called aposematism. The monarch caterpillar’s yellow, black and white bands and the adult’s orange and black wings work like a warning sign that predators learn quickly and remember.

The toxins are not spread evenly. Adults carry high levels in their wings, so a bird that grabs a wing gets a bitter taste and may let go, and some of the most potent compounds are stored in the abdomen.

The viceroy and other look-alikes

The viceroy (Limenitis archippus) looks so much like a monarch that it was long the textbook example of Batesian mimicry, where a harmless species copies a dangerous one. In 1991, however, taste tests with red-winged blackbirds showed that viceroys can be as distasteful as monarchs. The two butterflies are Müllerian mimics: both are unpleasant, and sharing one pattern teaches predators faster.

Viceroy caterpillars eat willows and poplars, not milkweed, and their bad taste may come partly from the willow’s chemical defenses. In Florida, where the monarch’s chestnut-brown relative the queen (Danaus gilippus) is common, viceroys are darker and look more like queens.

Predators that eat monarchs anyway

The defense is strong but not perfect. At the winter colonies in Mexico, black-backed orioles taste the butterflies and tear out the less toxic parts, while black-headed grosbeaks can tolerate the poisons. Black-eared mice eat monarchs that fall to the forest floor. As the winter goes on, monarchs slowly lose toxins and become easier prey.

Many insects and spiders also eat monarch eggs and caterpillars, including wasps, assassin bugs, stink bugs and the introduced Asian lady beetle. Chinese mantises have been seen discarding a caterpillar’s toxic gut before eating the rest. Parasitic flies and wasps develop inside caterpillars and pupae.

Are monarch butterflies dangerous to people?

No. Monarchs are not a danger to people. They cannot bite or sting, and their toxins only matter to an animal that swallows them, so touching a monarch or a monarch caterpillar is harmless. As with any wild insect, they should never be eaten.

The best way to enjoy monarchs is to watch them. Their wings are covered in tiny, loosely attached scales, and rough handling can damage them. Poisonous animals harm others only when eaten or touched, unlike venomous animals that inject toxins; see our list of the most poisonous animals and are ladybugs poisonous? for other examples. The monarch butterfly profile covers the rest of its life.