Unlike mammals and birds, butterflies and moths rely mainly on external sources of heat to warm their bodies so that they can be active. Although many are adept at increasing their body temperature way above ambient air temperature by basking in sunshine or shivering (vibrating their flight muscles), when their surroundings are really cold, most butterflies and moths are forced to remain inactive.
So, not surprisingly, winter poses a problem for butterflies and (most) moths in temperate climates such as ours. It is difficult for them to get warm and, therefore, be active and so they have evolved ways of dealing with this unfavourable season. Most species enter a dormant phase. This can be as an egg, larva, pupa or adult insect, dependent upon species. The majority of butterflies and moths overwinter in the larval stage, with pupae being the next most common choice, followed by eggs and adults. A few are capable of overwintering in more than one stage. The Speckled Wood butterfly for example can overwinter as a caterpillar (larva) or a pupa.
An alternative evolutionary strategy, employed by Painted Lady butterflies and Silver Y moths, is to avoid winter conditions completely by migrating to warmer parts of the world.
Note that I’m talking about dormancy and overwintering rather than hibernation. Strictly speaking insects don’t hibernate, according to the precise scientific meaning of the term. However, as most people regard hibernation as akin to “sleeping through the winter”, it is often applied to butterflies and moths.
In midwinter, the butterflies and moths you are most likely to encounter are those that are either active as adults or those that are dormant as adults. Eggs, larvae & pupae tend to be hidden away, though you may find Large White pupae attached to the walls of your house. A small group of incredibly hardly moths have their main period of adult activity in wintertime, including the Winter Moth, December Moth and Spring Usher, but that’s another story.
Those species that overwinter as dormant adults include the butterflies Brimstone, Small Tortoiseshell, Peacock and Comma and moths such as the Twenty-plume Moth, Red-green Carpet, Tissue, Sword-grass, Herald and Bloxworth Snout.
The Red Admiral, which has become a common sight in British winters of late, doesn’t enter a proper dormancy but becomes active on any suitable days.
Among the butterflies, it is only the Small Tortoiseshell and Peacock that regularly overwinter inside houses. They come in during late summer/early autumn, when it is still warm outside and our houses appear to provide suitably cool, sheltered, dry conditions. However, come Christmas, when the central heating is cranked up, such butterflies may be awoken prematurely by high indoor temperatures. This presents a major problem for the butterfly as the outside weather conditions may be very hostile and there is little nectar available in gardens.
It is a problem for the concerned householder too. How best to help these poor confused butterflies unwittingly tricked into thinking spring has come early. The best solution is to rehouse the butterfly into a suitable location. Catch the butterfly carefully and place it into a cardboard box or similar, in a cool place for half and hour or so to see if it will calm down.
Once calmed down you might be able to gently encourage the sleepy butterfly out onto the wall or ceiling of an unheated room or building such as a shed, porch, garage or outhouse. Just remember that the butterfly will need to be able to escape when it awakens in early spring.
If you have no options at all for suitable hibernation places, then it would be best to keep the butterfly as cool as possible, to minimise activity, and then to release it outside during a spell of nice weather.