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How to Keep a Caterpillar Until It Becomes a Butterfly

Caterpillars need the exact host plant they were found on and nothing else. Container, fresh leaves daily, and the mistakes that kill a chrysalis.

By Zakaria 5 min read Save to Pinterest — opens Pinterest in a new tab
A striped monarch caterpillar feeding on the edge of a milkweed leaf
Photo: U.S. Fish and Wildlife Service - Midwest Region, Public domain, via Wikimedia Commons
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The single thing that decides whether this works: feed it leaves from the exact plant you found it on. Most caterpillars can eat one plant family and will starve beside anything else.

Host plants — the part that matters

Common caterpillars and the only thing they eat
CaterpillarHost plant
MonarchMilkweed only (Asclepias)
Black swallowtailParsley, dill, fennel, carrot tops, rue
Eastern tiger swallowtailWild cherry, tulip tree, ash, birch
Painted ladyThistle, mallow, hollyhock
Red admiralNettle
Cabbage whiteBrassicas — cabbage, kale, nasturtium
Peacock, small tortoiseshellNettle
Luna, polyphemus mothBirch, walnut, hickory, oak

If you did not note the plant, put it back

A caterpillar removed from an unidentified plant is very likely to die, because you cannot guess your way to the right leaf. Returning it to roughly where it was found is far kinder than a week in a jar refusing everything you offer.

And never substitute lettuce or fruit. It is not that they prefer their host plant — most physically cannot digest anything else.

The setup

  • Container: tall rather than wide, with real ventilation — a mesh enclosure, or a jar with a muslin or fine-mesh lid held with an elastic band. Not a sealed lid with a few holes.
  • Headroom: at least three times the caterpillar’s length, and more is better. The emerging butterfly has to hang and expand its wings, and a short container produces a deformed adult.
  • Floor: paper towel, changed daily. It makes cleaning droppings trivial.
  • A twig or rough stick leaning against the side, plus mesh at the top — pupation needs something to grip.
  • Position: indirect light, room temperature. Never a sunny windowsill or near a radiator; a closed container in sun cooks in minutes.
  • No water dish. They get moisture from leaves, and a dish is a drowning hazard. Mist the leaves lightly if the air is very dry.

Daily routine

  1. Fresh leaves every day. Wilted leaves are refused and mouldy ones are dangerous.
  2. Remove droppings — “frass” — every day. This is the main source of mould.
  3. Move the caterpillar with the old leaf, or nudge it with a soft paintbrush. Never pull one off a surface; the feet grip hard enough that pulling causes injury.
  4. Keep leaf stems in a little water in a covered container if you like, but seal the opening around the stem so the caterpillar cannot fall in.
  5. Watch for moulting. A caterpillar that sits motionless for a day, sometimes with a slightly dull look, is preparing to shed its skin. Do not disturb it, and do not assume it is dying.

Wash shop-bought leaves, and be careful where you pick

Parsley or fennel from a supermarket or garden centre very often carries systemic insecticide, which no amount of washing removes. Nursery plants are frequently treated too.

Pick from a garden you know is unsprayed, or grow the host plant yourself. Roadside verges are also worth avoiding for spray drift.

The chrysalis stage

  • The caterpillar stops eating, wanders restlessly looking for a site, then attaches
  • It hangs in a J shape for a day or so before pupating — this is normal, not distress
  • Do not move or touch the chrysalis. If one detaches, it can sometimes be rehung with a dab of glue on the silk pad, but success is mixed
  • Expect one to three weeks, temperature-dependent. Monarchs are usually 9 to 14 days in warm weather
  • The chrysalis often turns dark and transparent a day before emergence, and the wing pattern becomes visible through it. That is imminent success, not death

If nothing happens for weeks

Some species overwinter as a pupa and will not emerge until spring. Indoors, warmth can trick one into emerging in midwinter with nothing to feed on.

If it is late in the season and the chrysalis has gone quiet, move it somewhere cold, sheltered and unheated — an unheated shed or garage, in a ventilated container safe from mice — and let it come out in spring.

Emergence and release

This is where most preventable damage happens.

  • The butterfly emerges with small crumpled wings and hangs, pumping fluid into them
  • It needs several hours, undisturbed, hanging freely
  • Do not touch it, move it, or offer it food during this
  • A butterfly that cannot hang — or that is handled — gets permanently deformed wings

Release once the wings are fully expanded, dry and held together:

  • A warm, dry day, ideally morning
  • Near flowers, so it can feed immediately
  • Outdoors, close to where the caterpillar was found
  • Not in rain, cold, wind or darkness — keep it another day with a slice of orange or a sugar-water soaked cotton pad if the weather is bad

The honest caveat

Wild survival is low, which is what makes this worth watching. But rearing at scale is discouraged by conservation groups — it can spread disease such as OE in monarchs and can produce weaker adults than wild-reared ones.

One or two caterpillars, watched properly by a child, is a good thing to do. Planting host plants and leaving them alone does far more for butterfly numbers than any amount of rearing.

Common questions

How do I keep a caterpillar until it becomes a butterfly?

Keep it in a tall ventilated container with fresh leaves of the exact plant you found it on, replaced daily, and clean out the droppings every day. Add a twig or rough stick for it to pupate on and give at least three times the caterpillar's length in headroom so it can hang and expand its wings. Keep it out of direct sun and away from radiators. Most species spend one to two weeks eating, then one to three weeks as a chrysalis, then need several hours to dry their wings before release.

What do I feed a caterpillar?

Leaves of the exact plant you found it eating, and nothing else. Most caterpillars are specialists that can only digest one plant family — monarchs eat milkweed only, black swallowtails eat carrot-family plants such as parsley, dill and fennel, and a caterpillar offered the wrong leaf will starve beside food. If you did not note the plant, put the caterpillar back where you found it rather than guessing. Never offer lettuce or fruit as a substitute.

How long does a caterpillar take to become a butterfly?

Usually three to five weeks in total for common garden species: one to two weeks feeding and growing through several moults, then one to three weeks in the chrysalis. Monarchs typically take 9 to 14 days as a chrysalis in warm weather. Temperature drives all of it, so a cool spell stretches the timetable. Some species overwinter as a pupa instead, in which case nothing will happen until spring and the pupa needs to be kept somewhere cold and sheltered rather than indoors.

Why did my caterpillar die?

The common causes are the wrong host plant, wilted or mouldy leaves, a sealed container with no ventilation, droppings left to build up and grow mould, pesticide residue on shop-bought leaves, or a parasitoid wasp or fly whose eggs were already inside the caterpillar when it was collected. Parasitism is more common than people expect and there is nothing to be done about it — it is a normal part of how caterpillar populations work rather than a mistake you made.

What do I do when the butterfly emerges?

Leave it alone for several hours. A newly emerged butterfly hangs with crumpled wings and pumps fluid into them, and it must hang undisturbed to do it — a butterfly that cannot hang, or that is handled during this stage, ends up with permanently deformed wings. Once the wings are fully expanded, dry and held together, release it outdoors on a warm dry day near flowers, ideally in the morning. Do not release in rain, cold or darkness.

Is it better to leave caterpillars outside?

Usually yes. Survival is genuinely low in the wild, which is why raising one is such a satisfying project, but rearing indoors at scale is discouraged by conservation groups because it can spread disease such as OE in monarchs and can produce weaker adults. One or two caterpillars for a child to watch is a fine thing to do. Planting host plants and leaving them to it does far more for butterfly numbers than rearing ever will.

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