Fact Sheets: Keeping Live Invertebrates

How to over-winter butterfly and moth pupae

Many butterflies and moths spend the winter in the pupal stage. Throughout almost all of North America, except perhaps southern Florida and extreme southern Texas, a cold period is desirable or essential for emergence of healthy adults at the proper time of year. Although many pupae will emerge successfully if kept at room temperature, their biological clocks run out and they emerge much too early (late February through March in Alberta). This page is devoted to providing the information needed to successfully over-winter those species that need this cold period. Although this write-up is aimed primarily at people who live in Alberta, the information is provided in such a way that it can be used by anyone in North America.

Canadian Tiger Swallowtail, caterpillar

Why do it? Scientists raise caterpillars through to adult moths to determine what species they are. Many species of caterpillars change their appearances when they molt from one instar to another (instars are the periods between molts). For this reason it is possible for the caterpillar of a particular species of butterfly or moth to look quite different at various stages of its life. Documenting these appearances is essential to our better understanding of each species. Many species of moths in particular still do not have all larval stages properly documented.

For the average person, raising caterpillars to adulthood is fun and certainly educational. More and more people seem to be doing this and many of these people are home schoolers. This is a great way to teach your children about life cycles.

White-lined Sphinx Moth, caterpillar

Which species need a cold winter period? Many caterpillars that are found in late summer or early fall are likely to over-winter in the pupal stage. Caution must be taken though, as there are many moths that have a late generation that will emerge as adults around this time and over-winter in the adult stage, or even lay eggs that over-winter. These will normally emerge from the pupae within three weeks of pupating. The species that most people are likely to want to try over-wintering are the big silk moths and the sphinx moths, but others such as woolly bears and some butterflies such as swallowtails are often tried. The photographs on this web site should allow the reader to identify some of these caterpillars.

When are the caterpillars ready to pupate? If you get a caterpillar before it is ready to pupate, you must continue to feed it until it is ready. If you do this it is essential that you feed it

Waved Sphinx Moth, caterpillar

the right kind of food. The easiest way to ensure that you are providing the right food is to feed it leaves from the plant you found it on. Taking a caterpillar off a plant without making note of what species of plant it was on can spell the death of the caterpillar. If you found your caterpillar on the ground, it is most likely that it is ready to pupate and is looking for an appropriate place to do so. In this case it does not need to be fed.

Columbia Silkmoth, caterpillar

Polyphemus Moth, caterpillar

Getting them to pupate? Providing the right material for caterpillars to pupate is essential to their survival. The problem is that this varies depending on the type of caterpillar. The caterpillars of most sphinx moths pupate in loose soil. It is best to provide about 7 cm (3 in.) of soil for these caterpillars. Any loose soil will do and a jar such as a peanut butter jar will suffice as a container. Some of the smaller sphinx moths, such as the clearwings, pupate on the surface in leaf litter. If you are not sure what species you have, you might want to put some dried leaves on top of the soil. Caterpillars of the large silk moths such as the Polyphemus, Cecropia or Columbia Silkmoth, often pupate in bushes or trees and incorporate leaves into the pupal casing. For this purpose any branch with leaves on it will do. Woolly Bears will simply build a pupal case out of silk in any protected location and need nothing more than the jar they are in. Caterpillars of butterflies should be provided with sticks to attach themselves to. The problem is determining what type of caterpillar you have. The photos on this site should help for some species. Others can often be identified from library books or even on the internet.

How to over-winter the pupae As stated earlier, in most of North America, over-wintering pupae need a cold period. As well, unless you want the adult butterfly or moth as a specimen, you need to ensure that the adult will emerge at the same time that all other individuals of that species emerge. The easiest way to be sure that that happens is to keep them outside for the winter. Most people, however, have their pupae in glass or plastic jars. Exposing these to the direct rays of the sun can cause the air in the jar to overheat and this can easily kill the pupa. For this reason the jar needs to be put in a protected area, preferably on the north side of a building. A north-facing porch or a basement window well will do quite fine. Another possibility is an unheated garage. If you use a garage, it is best to take the jar out of the garage in the spring and place it outside where it will not get any direct sunlight. The reason for this is that a garage often stays colder during the day at this time of year and this may affect the timing of emergence.

For those people who do not have suitable outdoor conditions, or an unheated garage, the other option is to put the jar in the refrigerator. Because most fridges are self-defrosting, one must ensure that the pupa does not desiccate. To do this, put another smaller jar in the large one. This jar should have a small mouth, or a lid with a small hole in it (1/2 in. in diameter). Fill the jar with water and put cotton in the hole to act as a wick. Make sure that the cotton is always in contact with the water to wick the water off.

Observing the emergence You will want to bring the jar indoors at some time so that everyone can watch the moth or butterfly emerge, or at least see it soon after it does so. Timing is critical for this and all I can do is give some guidelines. If you bring the jar inside too early, emergence may take place too early and you may be depriving the moth or butterfly of the opportunity to mate and lay eggs. As a rule of thumb, if you bring the jar inside just after most of the trees start to leaf out, this will probably be just right for most moths and butterflies. The pupae of all butterflies and some moths (those with exposed pupae) will give a careful observer a one or two day warning before they emerge. The wing pads will change colour and often one can detect the actual wing pattern. Butterflies often emerge in the morning, between sunrise and noon, whereas most moths emerge in the evening and well into the night.

I do recommend that, unless you want the specimen, freshly emerged adults should be released within a day of emerging. Keep in mind that the giant silk moths do not have mouthparts as adults and thus cannot feed. They only live for about five days, so every day is critical for reproduction.

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