What Do Monarch Butterflies Eat? Milkweed and Nectar
What do monarch butterflies eat? The answer changes completely across their life cycle: caterpillars eat only milkweed, while adult butterflies drink flower nectar through a long, coiled proboscis. That single diet switch shapes almost everything about monarch biology, from the toxins that protect them to the migration routes they fly each fall.
Four Life Stages, Two Very Different Diets
A monarch (Danaus plexippus) passes through four stages: egg, larva, pupa, and adult. Only two of those stages actually eat. Eggs live off the yolk sealed inside them, and the pupa inside the chrysalis doesn't feed at all while its body is rebuilt from the inside out. Feeding happens during the caterpillar stage, on milkweed exclusively, and again as an adult, on nectar from dozens of flower species.
Eggs: No Feeding Yet
Females lay single eggs on the undersides of milkweed leaves, usually one per leaf. The egg itself supplies all the nutrients the embryo needs for the 3 to 5 days it takes to hatch. The female's choice of plant matters more than anything else in the whole life cycle: whatever she lays on is the only food the caterpillar will eat.
Caterpillars: Milkweed Only
Newly hatched larvae eat their own eggshell first, then move straight to milkweed leaves. Monarch caterpillars feed exclusively on milkweed leaves, and they will starve rather than switch to another plant. Over about two weeks and five growth stages (instars), a caterpillar increases its body mass dramatically before it's ready to pupate.
Why Milkweed and Nothing Else
Milkweed (genus Asclepias) produces a milky latex sap loaded with cardenolides, bitter steroid compounds that are toxic to most vertebrates. Milkweed contains a cardiac poison that is toxic to most vertebrates, and monarch caterpillars have evolved resistance to it. They store the cardenolides in their own tissue instead of breaking them down, which carries the toxin through the chrysalis and into the adult butterfly. Birds that try eating a monarch typically vomit soon after and learn to avoid the species' orange-and-black pattern. Species like common milkweed (Asclepias syriaca), swamp milkweed (Asclepias incarnata), and butterfly weed (Asclepias tuberosa) all work, though cardenolide concentration varies by species and growing conditions.
The Chrysalis: A Fasting Stage
When a fifth-instar caterpillar is done growing, it wanders away from its host plant, spins a silk pad, and hangs in a J-shape before molting into a jade-green chrysalis. No feeding happens during this stage. The pupa or chrysalis stage lasts 10 to 14 days, with the exact length depending mostly on temperature, before an adult butterfly splits the case open.
Adult Monarchs: Nectar and Pollen
Once its wings dry and harden, an adult monarch starts looking for flowers rather than milkweed leaves. Adult monarchs drink nectar from a variety of flowers, uncoiling and extending a long proboscis to sip food. That proboscis works like a drinking straw: coiled tight when not in use, extended into a flower's throat to draw out sugar water, then coiled again before the butterfly moves on.
Which Flowers Monarchs Visit
Monarchs aren't picky about species, but they favor flowers with an open, flat structure that puts nectar within reach of a relatively short proboscis. Common choices include:
- Milkweed blooms: adults still return to milkweed flowers for nectar, even though the leaves are strictly caterpillar food.
- Goldenrod: a major fall nectar source along the migration corridor, blooming right as monarchs head south.
- Asters: another late-season bloomer that overlaps with peak migration timing.
- Coneflowers and butterfly bush: reliable garden sources through mid to late summer.
Pollen sticks to a feeding monarch's body, legs, and proboscis and gets carried to the next flower, so nectar feeding doubles as pollination. Monarchs pick up small amounts of pollen protein this way too, though nectar sugar remains the bulk of the adult diet.
Feeding for Migration
The generation of monarchs that emerges in late summer behaves differently from earlier generations: instead of mating right away, it delays reproduction and feeds heavily on fall nectar to build fat reserves. That fat has to carry the butterfly thousands of miles to overwintering sites in central Mexico or coastal California and sustain it through winter with little or no feeding. A monarch that emerges from a nectar-poor landscape in September has a harder migration than one that fed on abundant goldenrod and aster along the way.
Milkweed and Nectar Loss
Monarch numbers have dropped sharply over the past few decades, and the two biggest drivers tie straight back to diet: herbicide-tolerant crops that eliminated milkweed from millions of acres of Midwest farmland, and development that removes wildflower meadows monarchs rely on for fall nectar. Losing either food source breaks a different link in the chain, host plant for caterpillars or nectar for adults, and both losses show up in population counts.
What Actually Helps
- Plant regionally native milkweed, not just any Asclepias species; local ecotypes match local bloom timing and climate.
- Add fall-blooming nectar plants like goldenrod and native asters so migrating adults have fuel when they need it most.
- Skip pesticides on host plants, since many common insecticides kill caterpillars along with the pests they target.
Milkweed gets caterpillars to the chrysalis. Nectar gets adults through migration. A garden or restoration project that supplies only one half of that equation still leaves monarchs short.
