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Over the Garden Fence

Keeping Insect Cocoons and Pupae Cases Over the Winter

Caterpillars are frequently collected by students for science classes, parents to show their little ones the amazing change from caterpillar to butterfly or moth. It is how they are handled after being collected that makes the difference in how successful your project is.

Most of our caterpillars are found this time of year, late summer and early fall, being nearly full grown out in the yard. They are big enough to be easily spotted in the garden or shrub bed. The caterpillars should be brought indoors, placed with the foliage from the plants you found them with into a terrarium or a large canning jar. Since they are full grown or nearly so, they feed for a short time before beginning their journey from caterpillar to adult butterfly or moth. Children will be able to seem them transform into an over wintering stage called a pupa. With many species the pupae are inside of what we call a cocoon, but not always. If you miss the adult caterpillar, you can still find their cocoons already attached to twigs, so take in the cocoon, twig and all. Pupae without a cocoon covering are much harder to spot.

Typically, the pupae are a reddish brown structure and will not feed anymore. This is how it is going to look for several months until nature signals the emergence of the adult next spring. While it looks like nothing is going on, there are lots of changes occurring inside transforming the caterpillar.

So far all this goes on at room temperature similar to our outdoor temperatures. From this point forward, those pupae encased in a silken cocoon or not need to experience the colder temperatures of our winter in order to make the change.

Cocoons and pupa should be placed in containers that small mice, shrews and other small animals, that would otherwise find them a good choice for dinner, cannot get into. Those canning jars with a lid and ring work well with a few small holes punched in the lid. The container should be placed outside at the base of shrubs or perennials covered with leaf litter to experience the needed cold weather.

Once spring arrives, retrieve the containers and place the cocoons and pupae back into a larger container with some branches. Within a few more weeks, both cocoons and pupae will have emerging butterflies and moths. They will need those branches you placed in that larger container or terrarium to climb onto to allow expansion and drying of their wings. This timeframe should coincide with what is happening outdoors so after a few hours of being amazed at the transition, it is best to release the butterflies and moths back into the backyard.

About the author: Richard Hentschel’s expertise extends across several subject areas with specialties in lawn care, fruit tree production, woody ornamentals, and home and community gardening. During his 45-year career in horticulture and agriculture, Hentschel became a well-known and respected expert for commercial and homeowner audiences, industry organizations, and media. He retired from University of Illinois Extension in April 2022 with nearly 30 years of service as a Horticulture Specialist and Educator in northern Illinois.