This article is intended as an introduction to raising caterpillars for people with little or no knowledge or experience of butterflies and moths. If you are not familiar with the life cycle, start here. If you are, you can skip to “Raising Caterpillars.”
The Life Cycle of Butterflies and Moths:
Butterflies and moths, like many other insects, go through a process called complete metamorphosis over the course of their brief lives. They begin life as tiny eggs, which hatch out to become larvae (the plural of larva), commonly known as caterpillars. The larva eats and grows, filling out its stretchy skin until the skin becomes too tight to expand any further and must be shed, revealing a fresh skin underneath. This happens several times (usually four). The caterpillar’s life-stages between sheds are known as instars – you may see descriptions of first through fifth instars – and may look quite different (see
Black Swallowtail for an example), although usually the head size is the most reliable way to determine which instar the caterpillar has reached.
Eventually the caterpillar reaches its final instar, having grown to be hundreds of times as big as when it first hatched. Its final molt reveals the third life stage – the pupa. In butterflies, the pupa is also known as the chrysalis (plural: chrysalides) – this name (from chrysos, the Greek word for gold), refers to the golden color seen on many species’ pupae. In many moth species an enclosure, which may be spun entirely of silk or incorporate leaves, hairs or other debris, is spun around the pupa. This is referred to as a cocoon. Some moth caterpillars dig down into the soil to pupate, and do not form a cocoon. If you have one of these, you should resist the temptation to dig it up after it pupates, as you may damage the pupa and leave it open to dehydration and disease.
After a period of time which can be anywhere from a few days to several months, the adult butterfly or moth emerges from the pupa. This is the reproductive stage. Its goal is to find a mate and thus reproduce. Once mated, the female will lay eggs on a host plant, and the cycle begins again.
Raising Caterpillars:
Caterpillars (and thus butterflies and moths) may be collected and raised from any of the life stages described above. The important thing to consider is that, as a group, caterpillars are extremely particular about what they will eat. Although there are some polyphagous (=eat many different plants) exceptions, in general each butterfly or moth species is restricted to eating a few, often related, plant species. These are referred to as the “host plants.” Caterpillars may not even transfer successfully from one known host plant to another, especially in later instars. If placed on the wrong food plant, a caterpillar will usually not eat – it just does not receive the chemical trigger it requires. Researchers have found that the chemical receptor is in the caterpillar’s jaw – when this receptor was removed in experiments, the caterpillar would eat anything it was given. However, a caterpillar raised on the wrong food plant may not pupate and almost certainly will not become a successfully reproductive adult. ( The exception to this rule is the Painted Lady butterfly, which can be reared on artificial food. That is why this species is commonly sold in kits for rearing in the classroom or at home.)
Because caterpillars are voracious eaters, it is unwise to try to raise one in captivity unless you have access to a plentiful supply of the host plant. If you collect a caterpillar from the wild, you should document what it is eating, as that can be very helpful with identification of the species, and will also enable you to find more of the host plant later.
Please note: neither caterpillars nor host plants should be removed from any land where you do not have the permission of the owner to collect. In general, parks will require you to have a permit for collection of either plants or animals.
When a caterpillar is ready to pupate, it will wander away from the host plant to find a place either to attach itself while it forms a chrysalis, or to spin its cocoon, or to dig into the soil (depending on the species). It is to the caterpillar’s advantage to get far away from the host plant, as the pupa is a very vulnerable stage, having little defense except camouflage to protect it from predators. A host plant loaded with pupae would be a feast for predators, which seek out host plants to find the tasty caterpillars. Often people find caterpillars in this final stage, wandering across sidewalks or up the side of buildings. The advantage of capturing a wandering caterpillar is that there is no need to find food plants – just provide a place for the caterpillar to pupate, either a twig or some loose soil or leaf litter, depending on the species (in general, twigs or at least a vertical surface for butterflies, leaf litter or loose soil for moths).
When the caterpillar has pupated, the waiting begins. Several factors influence the time it will take for the adult to emerge. The most important is temperature. If the pupa is chilled this may trigger diapause, a resting state in which the creature waits out the winter cold in order to emerge in spring when food and mates will be available. If diapause is triggered, it may take several months for the adult to emerge. Some species overwinter as adults or as caterpillars – a little research will usually turn up this information if you can identify your caterpillar to species.
What Can Go Wrong?
Many species of flies and wasps (known as parasitoids – not parasites, because they kill the host, which parasites don’t) lay eggs on caterpillars, which may go on eating and growing but eventually will be consumed on the inside by the developing larvae, and will never successfully pupate. You may not see any outer sign of this until the adult parasitoids emerge. See
Tobacco Hornworm for an example of wasp pupae on a parasitized caterpillar. The only reliable safeguard against this is to rear your caterpillar from the egg stage, entirely inaccessible to these parasitoids (but more work).
Handling the caterpillar puts it at risk for problems. You may have insect repellent or other chemicals on your hands. Even household soaps may irritate the caterpillar’s skin (ever hear of insecticidal soap?) – so it’s always best to handle them as little as possible. Soft paintbrushes can be useful for transporting or nudging caterpillars to where you want them to be.
Your home may be too dry for the creature’s needs – in winter, heating, and in summer, air-conditioning may dry out the atmosphere. Better to keep your caterpillars in an unheated garage or outdoor protected location, if you can, or find a way of introducing more humidity into their habitat (this can be as simple as a moistened paper towel).
The relatively constant temperature of your home may not give the caterpillar the signals it needs to complete its natural cycle. Again, an outdoor or unheated enclosure may be preferable. This is mostly a concern in fall when caterpillars need to prepare to overwinter. Alternatively, pupae may be overwintered in a refrigerator and then allowed to warm up in spring.
Simple Home-Rearing Containers
Ideally, try to reproduce the natural living conditions for your caterpillar to feel at home. At a minimum, good air circulation, and a frequently replenished supply of the host plant until the caterpillar pupates are desirable. There are many ways to do this with whatever containers you have on hand. You don’t even need the host plant to be enclosed until the caterpillar is close to pupating, as it will not wander away from a plentiful food source until then. An easy container may be made of a tube of window screening wired into a cylinder shape, held at the top and bottom by a cake pan (see
here for a photo). This has the advantage of easy disassembly for cleaning. Caterpillars eat a lot and they defecate a lot, and keeping them enclosed with their poop, technically known as frass, is unhealthy. The host plant should ideally be a living containerized plant (but then you have to provide the right growing conditions for the plant – more complicated) or have its stem in a water source to keep it fresh. Beware: caterpillars have a tendency to fall into and drown in an open water-container. Cheap florists' picks, sold for keeping stems fresh in bouquets, are a convenient solution, but you may use any kind of water-holding container as long as you cover any large openings to prevent caterpillars falling in (foil, paper towels, cotton balls or cling-wrap work fine for this purpose). In the Miami Blue rearing program at the University of Florida, Dr. Jaret Daniels raises larvae in two paper cups nested together, with the water in the bottom cup and a hole punched in the top one to poke the host plant stems through.
For moth species that usually pupate in the ground, it is preferable to provide a few inches of loose soil or leaf litter, but if this can’t be managed, they will pupate without it eventually if you make sure they can’t escape. For most butterfly caterpillars, any vertical surface will serve for them to attach to and form their chrysalis. Again, make sure they are properly enclosed or the wandering caterpillar may travel far from its host plant and pupate somewhere inconvenient for you both. If this happens, and you find a pupa somewhere around your house, gently moisten the silk which attaches it and you should be able to pull it free without damaging it. At this point you can very carefully attach the silk to a twig or other surface (use the warm setting of a hot-glue gun), making sure you keep it the same way up.
When the butterfly or moth is about to emerge from its pupa, which may be several days or several months later, generally this will be signaled by a color change a few hours ahead of time. The emergence is very rapid (turn away and you may miss it), but afterwards the adult will spend a couple of hours stretching out and drying its wings. Amazingly, the adult does this by pumping blood through the veins in the wings, but then sucks the blood back into its body so the veins become hollow air-filled tubes – that’s why butterfly wings don’t bleed when they get torn. It will need something to crawl up and hang from during this process so the wings can fully expand, or it may be crippled. you should provide a twig or other rough-textured vertical surface suitable for climbing – at a pinch a crumpled paper towel in the container can provide enough clearance.
Watching the creature emerge from its pupa is the most magical part of the whole experience. Release it to the wild when it is ready to fly, and with luck it will find a mate so the cycle can begin again.
Online Resources
If you know the host plant and are trying to ID the caterpillar, the British Natural History Museum's
HOSTS database provides a list of caterpillars known to feed on any given plant. Be sure to narrow your search by location (Nearctic, which includes the USA and Canada, will return the most relevant reports for BugGuide users) for best results.
If you don't have a copy of Wagner's book
(1), the online version of
Caterpillars of Eastern Forests, through the USGS, is a good resource. It includes a section on
Rearing Caterpillars.
There is a similar USGS publication for the Pacific Northwest -
Caterpillars of Pacific Northwest Forests and Woodlands
Books
Caterpillars of Eastern North America by David L. Wagner
(1) has color photos of nearly 700 species, and lots of information about finding and rearing them.
