Monarch butterfly caterpillars are easier to raise than most people expect. If you have milkweed growing nearby and a clean container, you can take a tiny egg or newly hatched larva and watch it transform into a full-grown caterpillar over the course of about two weeks. The process is straightforward once you understand what monarchs need at each stage of their growth.
This guide covers everything from finding caterpillars in the wild to feeding them properly, setting up a rearing space, and knowing when to back off and let them do their thing before they form a chrysalis.
Key Takeaways
- Monarch caterpillars eat only milkweed, so you need a reliable supply of fresh leaves before you start raising them.
- They grow through five distinct instar stages, starting at about 2mm and reaching up to 2 inches by the time they are ready to pupate.
- A clean rearing container with good airflow and daily frass removal will prevent the most common causes of caterpillar death in captivity.
- Once a caterpillar starts wandering and stops eating, it is preparing to form a chrysalis and should not be handled or disturbed.
How to Find Monarch Caterpillars
The most reliable way to find monarch caterpillars is to look closely at milkweed plants. Monarchs lay their eggs exclusively on milkweed, and the caterpillars never stray far from their food source. Common milkweed (Asclepias syriaca), butterfly weed (Asclepias tuberosa), and swamp milkweed (Asclepias incarnata) are all good places to search.
Check the underside of leaves first. That is where females usually deposit their eggs, and where small caterpillars tend to feed to avoid being spotted from above. Eggs are tiny, white or pale yellow, and ribbed. They look like a miniature bullet standing upright on the leaf surface.
Signs of caterpillar activity include small round holes chewed in leaves, scattered frass (dark pellet-like droppings), and partially eaten leaf edges. Once you spot these signs, slow down and look more carefully at the surrounding stems and leaf undersides. Caterpillars in the early instars are small enough that a quick glance will miss them entirely.
Timing matters too. In most of North America, monarchs lay eggs from late spring through late summer. Peak caterpillar season tends to fall in July and August when milkweed is lush and monarch populations are at their highest. If your milkweed is covered in aphids, monarchs often avoid it or lay fewer eggs there, so keeping your plants reasonably healthy improves your chances of finding caterpillars.
The 5 Instar Stages
Monarch caterpillars go through five growth stages called instars. Each instar ends when the caterpillar molts, shedding its old exoskeleton to reveal a larger body underneath. The whole process from egg hatch to full size takes roughly 10 to 14 days in warm weather.
The first instar caterpillar is tiny, only about 2 to 6mm long, and nearly translucent with a dark head. It barely makes a dent in the leaf surface and is easy to overlook. This is the most vulnerable stage.
By the second and third instars, the familiar yellow, white, and black banding pattern starts to appear. The caterpillar is now around 6 to 25mm and eating more aggressively. The tentacle-like filaments at the front and rear ends become more prominent and help distinguish monarchs from other caterpillars you might find on milkweed.
The fourth and fifth instars are when growth really accelerates. A fourth instar caterpillar can reach about an inch long, while a fifth instar reaches 1.5 to 2 inches. The fifth instar eats enormous amounts of milkweed in preparation for pupation. You will notice the stripes are bold, the tentacles are long and active, and the caterpillar seems constantly hungry. For a more detailed breakdown of what changes at each stage, the guide on monarch caterpillar stages covers the visual and behavioral differences instar by instar.
At the end of the fifth instar, feeding stops completely. The caterpillar will begin wandering, looking for a safe spot to attach itself and form a chrysalis. This is a sign the caterpillar is healthy and on track, not a sign that something is wrong.
Setting Up a Rearing Container
You do not need anything expensive to raise monarch caterpillars. A mesh popup cage, a large plastic storage container with ventilation holes cut into the lid, or even a simple net enclosure all work well. The key requirements are airflow, enough vertical space for the caterpillar to hang upside down when it forms a chrysalis, and easy access for cleaning.
Avoid glass aquariums or solid plastic containers without modification. Poor airflow leads to bacterial growth and a condition called black death, which spreads quickly through a group of caterpillars. A container at least 12 inches tall gives caterpillars room to hang and form their chrysalis without touching the walls or floor.
Line the bottom with paper towels for easy frass removal. Frass should be cleared out daily because built-up droppings can harbor pathogens. Fresh milkweed cuttings can be placed in a small vase or jar of water inside the container to keep them from wilting. Just make sure small caterpillars cannot fall into the water and drown by covering the opening with foil or cotton balls.
Keep the container out of direct sunlight and away from heat sources. Room temperature, somewhere between 65 and 85 degrees Fahrenheit, works well. Wild monarchs experience temperature variation, but extreme heat in a closed container can stress or kill caterpillars quickly.
Do not crowd the container. A rough guideline is no more than five to ten caterpillars per standard popup cage, fewer if you are using a smaller container. Overcrowding increases the risk of disease spreading and limits food availability.
Feeding and Milkweed Supply
Milkweed is the only food monarch caterpillars will eat. This is not negotiable. They cannot survive on any other plant, so before you bring caterpillars home, you need a steady supply lined up. Running out of milkweed mid-rearing is one of the most common problems people run into.
Fresh milkweed is always better than wilted or dried leaves. Caterpillars will refuse wilted leaves or eat less, which slows their growth. If you are cutting stems from your garden, put them in water immediately and replace them every day or two. Rinse leaves with clean water before offering them, especially if your plants are near roads or have been treated with anything.
The milkweed species you use does not matter much. Monarchs will eat any native milkweed species. Tropical milkweed (Asclepias curassavica) is popular because it is widely available at garden centers and stays green longer, but there are some concerns about its year-round growth in southern climates encouraging monarchs to skip migration. For raising caterpillars over a short period, it works fine. For guidance on growing milkweed and other host plants in your garden, the caterpillar host plants guide has detailed information on which species work best in different regions.
Fifth instar caterpillars eat a remarkable amount. A single large caterpillar can strip a full milkweed leaf in under an hour. Plan to have multiple plants available or a reliable source of cuttings during this final feeding phase. Some people keep a dedicated patch of milkweed in their yard specifically for rearing, which solves the supply problem entirely.
According to the USDA Forest Service, milkweed loss across North America is one of the primary drivers of monarch population decline, which is part of why home rearing with garden-grown milkweed can make a meaningful contribution to local populations.
When to Stop Handling Them
There is a point in every monarch caterpillar’s development when you need to step back and leave it alone. Getting that timing right is one of the most important parts of successful rearing.
When a fifth instar caterpillar stops eating and starts moving around the container in a restless, almost purposeful way, it is entering what is called the wandering stage. It is looking for a spot to attach itself and begin the process of forming a chrysalis. At this point, do not move the caterpillar unless absolutely necessary and do not try to place it somewhere specific. It knows what it is doing.
Eventually the caterpillar will attach itself to the lid, a stick, or the mesh of the cage using a silk pad. It will hang upside down in a J shape for 12 to 24 hours before the skin splits and the chrysalis forms. This J-hang stage is the most delicate moment in the whole process. Vibration, knocking the container, or moving the caterpillar during J-hang can cause it to form a deformed chrysalis or die.
Once the chrysalis has fully hardened, which takes a few hours, it is more resilient. You can carefully move the container if needed, but keep it calm and still as much as possible. The chrysalis stage lasts about 8 to 15 days. When the adult butterfly is close to emerging, the chrysalis will become transparent and you can see the butterfly’s wings through the shell. For everything that happens next, the monarch chrysalis guide walks through the full transformation in detail.
Handling caterpillars at any stage should be minimal. They are not fragile in the way that chrysalises are, but frequent handling causes stress and can expose them to bacteria from your hands. When you do need to move a caterpillar, let it walk onto your finger rather than pinching or gripping it. The National Wildlife Federation recommends keeping interactions brief and washing your hands before and after to reduce disease transmission risk.
Watch for signs of trouble throughout the rearing process. Caterpillars that become lethargic, show dark spots on their body, or stop eating before the fifth instar may be sick. Isolate any caterpillar showing these symptoms right away and check the guide on butterfly diseases and predators for help identifying what might be wrong and whether treatment is possible.
Frequently Asked Questions
How long does it take for a monarch caterpillar to become a butterfly?
From the time an egg hatches, a monarch caterpillar spends about 10 to 14 days growing through its five instars. After that, it forms a chrysalis and remains in that stage for 8 to 15 days before emerging as an adult butterfly. The full process from egg to adult butterfly is roughly 30 days in warm conditions, though cooler temperatures can stretch this timeline.
Can monarch caterpillars eat anything besides milkweed?
No. Monarch caterpillars are obligate milkweed feeders, meaning milkweed is the only plant they can eat and survive on. They have evolved specifically to tolerate the toxic compounds in milkweed that would harm most other insects. These toxins also make the caterpillars and adult butterflies unpalatable to many predators. There is no substitute for milkweed when raising monarch caterpillars.
Why did my monarch caterpillar stop eating?
There are two common explanations. If the caterpillar is a large fifth instar and has been feeding heavily, it may be entering the wandering phase and preparing to pupate. This is normal and healthy. If the caterpillar is in an earlier instar and becomes still and slightly shrunken-looking, it may be about to molt. Caterpillars stop eating and rest for a day or so before each molt. If neither of these fits and the caterpillar looks lethargic or discolored, disease may be the cause.
How do I know if my caterpillar is healthy?
A healthy monarch caterpillar is active, responsive to touch, eating regularly, and has bold, clear yellow and black bands. The body should look plump and firm. Warning signs include dark or black spots on the body, a shriveled appearance, unusual stillness outside of a pre-molt rest, or a caterpillar falling from its perch repeatedly. Any caterpillar showing these signs should be separated from others immediately to prevent any potential disease spread.
Is it okay to raise monarch caterpillars indoors?
Yes, and many people do it successfully every year. Indoor rearing protects caterpillars from predators, parasites, and weather, which can improve survival rates compared to wild populations. The main things to keep in mind are adequate airflow, fresh milkweed every day or two, daily cleaning of frass, and not overcrowding the container. As long as those basics are covered, indoor rearing is a genuinely rewarding way to observe the monarch life cycle up close.
