Many people are surprised to learn that the adult (butterfly or moth) stage is usually the shortest, lasting from 7 days (for moth species that don't feed as an adult) to two weeks for many butterfly species*. For most species, their longest stage is as a pupae, often as a chrysalis (butterflies) or enclosed in a cocoon (silk moths), which is how they survive the cold winter season.

Rearing caterpillars is the most labor intensive part of raising butterflies -- providing enough food, as well as protecting them from predators and disease -- but approximating winter conditions can be equally tricky. As the days get shorter, we keep the pupae in a closed off cooler room of the house with a window cracked open. This serves dual purposes of exposing them to the natural decreasing daylight cycle of autumn and more natural humidity levels.

Once outside temperatures are consistently cold (around Thanksgiving), we move the pupae to the refrigerator until spring. Maintaining proper humidity in the refrigerator is critical, so we store them in sealed containers. This prevents the pupae from desiccating, or drying out.  We also add some paper towel cushion, as it helps to collect residual moisture.

During the winter months while the pupae are stored in the refrigerator, there are still some maintenance pieces that require attention:

  • First, we make sure that the refrigerator maintains a temperature around 38F. Thermometers are easily found at a local hardware store, and they are generally helpful even when not storing bugs.
  • Second, we vent the containers once each month and check for any visible condensation inside the container. Like any living thing, pupae are still breathing and releasing gasses, including water vapor. We open up the container, wipe out any moisture, and put things right back in the fridge.
  • Third, during the monthly check we look for signs of mold growth on pupae.  If this is occurring, the pupae affected need to be cleaned and checked for proper color and weight.  Typically this indicates a dead or dying pupae.  However, it's also normal for small amounts of mold to grow on cocoon silk, which is fine. We dry those cocoons (with pupae inside) with mold out for a few hours then place them back in the fridge.

Here in Oregon, it requires about 90 days of cold treatment in the fridge before pupae will eclose (hatch) when removed.  However, with some species, increasing daylight cycle in itself is enough to cause pupae to hatch.  For this reason, the absolute latest for putting pupae into the fridge is the week of Winter Solstice.  I get many emails each year asking what to do with moths and butterflies which have hatched in the middle of January because they were kept indoors.  The pupae senses the increasing daylight cycle combined with a nice warm home and decides that it's spring.  Unfortunately, there's  little that can be done at this point.

Once days warm up and lengthen, we move pupae out of the fridge and back to a pop-up tent, exposing them to the natural light cycle again, along with warmer inside temperatures. This triggers their development into an adult.  Misting pupae daily creates a more humid environment, as inside humidity is much lower than outdoors. (Some species, such as Two-Tailed Swallowtails only need to be misted every other day.)

We try to remove pupae from cold storage about three weeks prior to when the species naturally fly well in the area. For example, in our region Ceanothus Silkmoths are typically at peak flight about the first week of June, so three weeks prior to that we remove that species from cold storage.  This allows us to outbreed our reared female moths with wild males, keeping the genetics strong.  Doing this with all of our moth and butterfly broods also protects wild populations.  Should our stock be wild released by us or any of our customers in Oregon, the genetics of the broods are strong and will compliment, rather than hinder, the species in the wild.

Shipping pupae in the winter:

We ship a lot of pupae to customers in across the US.  Shipping pupae in diapause is fine.  We remove them from cold storage, pack them up, and mail them out.  Pupae will stay in diapause, and just need to be put back into cold storage within a week of being removed.  This works great, save rare cases where a package is excessively delayed.  

Sometimes temperature during shipment can get down into the teens, or even below zero F.  This is fine, and pupae will not be hurt.   Pupae have built up antifreeze inside to survive the winter, and also those extreme temperatures that are found in nature.  We store pupae at 38F in the fridge because it not only works well for the pupae over a long period of time, but it also works for other items in the fridge.

So now you've read through this and are wondering...is there a simpler way to overwinter?  Here's the TLDR:

If a butterfly or moth pupae hasn't hatched within six weeks of pupation, it will typically overwinter. Store it in a tent inside your house with a natural light cycle, mist it every couple of days, and refrigerate it in a sealed container after Thanksgiving.  Check the container monthly, remove moisture, seal back up and continue to refrigerate.  Take the pupae out in spring and mist daily in same pop-up tent and they'll hatch out in 3-6 weeks.

Questions?  Please email.  We're happy to answer them as our schedule permits (some parts of the season are very busy so it may take an extra day or two to respond).

*There are exceptions, such as the Mourning Cloak, which overwinters as an adult butterfly.  Some Mourning Cloaks spend 10 to 11 months as an adult butterfly.