Most eggs and larvae will either dessicate (insufficient humidity) or mold (too much humidity/insufficient airflow) if kept in the refrigerator for a period of time. If you live in a temperate area with cold winters, the best technique to overwinter eggs and larvae of brushfoot butterflies, coppers, hairstreaks, sulphurs, pine whites, etc. is to use a wine cooler or place them outside in a protected environment where they naturally have humidity, airflow and protection from predation. 

Using the refrigerator is an effective overwintering technique for pupae of swallowtails, whites, orangetips, some skippers and some hairstreaks and blues that naturally overwinter as pupa in temperate areas of the world. The amount of time you should overwinter your pupae depends upon the altitude and latitude of where your butterfly flies.

Because I live in Northern Utah, USA, my general rule of thumb is to overwinter my pupae for four months.  This applies to collectors who are not releasing butterflies back into the habitat.  However, if you are overwintering pupae with the intent of releasing it back into its natural habitat at an appropriate time, it is advisable to bring your pupae out of refrigeration two weeks before the butterfly is in mid flight or 1 week before the butterfly starts to fly.  This may require a longer overwintering cycle than four months, which is fine. 

You can place your pupae inside your refrigerator at any time as long as your pupa has been in hibernation for roughly one month.  In the Northern Hemisphere, it is advisable to place your pupae in the refrigerator as early as late summer or as late as Thanksgiving.  The goal should be to emerge the butterfly when its species is naturally in mid flight and you can release it into the right habitat at the right time.  Butterfly flight times can vary from year to year or from season to season depending upon winter precipitation and/or the advancement of spring.

Misting pupae in the refrigerator is advised during the last 60 days of your overwintering cycle to simulate moist springs to encourage pupae to break hibernation and emerge.  Some desert species of swallowtails, whites, and orangetips will extend hibernation over multiple years depending upon the severity of low annual precipitation. If your pupa does not emerge within one month after bringing it out, either it has died or has not broken hibernation.  If it is alive, you can leave it at room temperature for another month and repeat the overwintering cycle. 

The following photographs demonstrate how to overwinter pupae.

 

Wine coolers are especially helpful at overwintering butterfly eggs and caterpillars that are susceptible to desiccation (drying up) under artificial conditions because of the humidity that they need and wine coolers can provide.

These sensitive early stages include eggs, unfed first instar caterpillars, half-grown caterpillars, mature caterpillars, and adult butterflies; the last of which is THE most difficult to overwinter and not advisable unless you know what you're doing.

At the same time, shelf space in wine coolers comes at a premium as compared to using refrigerator space or your own backyard which are much easier and less expensive to provide.  Therefore, it is overkill to overwinter chrysalids of those butterflies that naturally overwinter as pupa (including non-Parnassius swallowtails, some blues, some skippers, most pierids, etc.) in a wine cooler unless you already have plenty of extra space.

The immature stage of pupa is the least sensitive to desiccation of any other immature stage where the refrigerator technique works just fine and cannot desiccate a pupa unless the pupa was exposed to pathogens and became sick as a caterpillar.  Also, it is easy to expose pupae to intermittent humidity using a refrigerator or outside as explained in their subsections.

Because there are four stages that complete butterfly metamorphosis, there are a possibility of four stages that temperate butterflies, based upon genus, overwinter which includes egg, caterpillar, chrysalis, and adult.  The list below shows which butterfly groups (in some cases, genera) overwinter in these sensitive stages where a wine cooler can be extremely handy.

Eggs:
- Most species of copper butterflies (Genus: Lycaena)
- Some genera of hairstreak butterflies (Genus: Hypaurotis and Satyrium)
- Pine white butterflies (Neophasia menapia and Neophasia terlooti)
- Shasta blue butterfly (Plebejus shasta)

Unfed first instar caterpillars:
- Species of larger fritillaries (Genus: Speyeria)
- Species of wood nymph butterflies (Genus: Cercyonis)
- Wyoming satyr (Neominois wyomingo)
- Shasta blue butterfly (Plebejus shasta)

Half-grown caterpillars:
- Temperate zone checkerspots (Genera: Euphydryas, Chlosyne, Poladryas, etc.)
- Temperate zone crescentspots (Genera: Phyciodes, Anthanassa, etc.)
- Some checkered skippers (Pyrgus communis and Pyrgus albescens)
- Species of emperor butterflies (Genus: Asterocampa)
- Species of viceroy, true admiral, and purple hibernacula (Genus: Limenitis)
- Some species of copper butterflies including Lycaena cupreus and Lycaena phlaeus)
- Subspecies of the boisduval's blue( Plebejus icarioides)

Mature caterpillars:
- Temperate zone arctic and alpine butterflies (Genera: Oeneis, Erebia, Neominois, etc.)
- Spreadwing skippers (Genera: Pholisora and Hesperopsis)
- Grass feeding skippers (Genus: Amblyscirtes)
- Giant skippers (Genus: Megathymus)
- Mormon metalmark complex (Apodemia spp.)
- Subspecies of the boisduval's blue( Plebejus icarioides)

If you’re interested in overwintering butterfly early stages on your own property, you need to make sure that your setup has some degree of consistent humidity, protection from predators, including mice, and a reasonable amount of airflow. The photographs below show containers placed inside glass terrariums with screen lids with the terrariums themselves protected by a wooden box and lid. This setup offers less airflow than the Alpine Method of overwintering; but is suitable for those stages of immatures which are less sensitive to dessication and mold as a result of decreased airflow. Those immature stages include pupae as well as mature fifth instars. This technique does not work as well with Nymphalid half-grown larvae, ova, and especially unfed first instar Cercyonis and Speyeria larvae. This technique works only where cold winter temperatures exist. If you live in the sunbelt, using your refrigerator to winter immatures is recommended.

Utah butterfly collector Jacque Wolfe devised the alpine overwintering technique which satisfies the three important criteria of overwintering—that butterfly eggs, caterpillars, and pupae be provided airflow, humidity, and protection. Immatures are placed inside squat tubs that have tens of thousands of needle holes. These "holy containers" are placed inside either a screen-riveted five gallon bucket or a heavy duty screen cage. This setup is found in a high elevation Utah mountain bog where plenty of winter snow provides more or less constant humidity.

This method especially accommodates sensitive immatures such as Lycaenid ova, Limenitis hibernacula, Melitaeinae half-grown larvae, and especially unfed Speyeria and Cercyonis first instar larvae.

Please remember that this technique is more principle-driven than method-driven. In other words, Jacque was innovative in deciding to overwinter sensitive immatures in holy squat tubs, hanging in screened five gallon buckets over alpine bogs in the tops of the mountains of Utah. However, you yourself can create a setup that satisfies the same criteria with a completely different methodology. Simply remember the three main criteria, that immatures need humidity (to avoid dessication), airflow (to avoid mold), and protection from predators.

Overwintering caterpillars of the Red Spotted Purple (Limenitis a. astyanax), Viceroy (Limenitis archippus), White Admiral (Limenitis arthemis), Weidemeyer's Admiral (Limenitis weidemeyeri), and/or Lorquin's Admiral (Limenitis lorquini) requires that you create a safe environment from predation where hibernacula are exposed to sufficient humidity and airflow.

If you live in the midwestern or Eastern U.S., this can be accomplished by pinning hibernacula to the branches of its host plants which include black cherry (Prunus serotina), willows (Salix spp.) , or deerberry (Vaccinium stamineum) so that the larvae can crawl out in the spring to resume feeding.  Watch youtube video.

Once hibernacula are pinned to these trees, it is strongly suggested that you place a caterpillar sleeve around the hibernaculum to protect it.  (These can be purchased or homemade.)

The greater fritillaries (Speyeria spp.) and some lesser fritillaris (Boloria spp.) overwinter as unfed first instars and can die easily during the winter. This occurs because these small larvae are extremely susceptible to desiccation on the one hand (insufficient humidity or airflow) or mold on the other hand (too much humidity with insufficient airflow.)

The following presentation provides a step-by-step tutorial of one successful strategy of how to keep Speyeria unfed first instar larvae alive for up to 180 days in the refrigerator.

Special thanks to Abe Homan for providing this detailed information as well as photographs.