For butterflies and skippers, surviving winter demands a period of dormancy known as hibernal diapause. This multi-step process suspends development or reproduction at the onset of cold weather, and is initiated by a hormonal response to changing environmental conditions, particularly the length of day and temperature. These same conditions trigger the end of diapause the following spring, as daylight lengthens and warmer temperatures return. At such time, development and reproductive processes return to normal.
Species that overwinter as adults must build up energy reserves by converting nectar into fat. Next, they must secure a location in which to pass the winter, such as in a shed or unheated outbuilding, under loose bark, in a hollow tree or in a crevice. The cryptic colouring of the underside of their wings helps them to remain hidden from predators. At this time, both the metabolic and respiratory rates slow down, feeding ceases and egg production is put on hold. The blood is thickened with antifreeze compounds called cryoprotectants. These compounds lower the temperature at which a butterfly will freeze. It has been shown that the Mourning Cloak can withstand temperatures as low as -60 degrees Celcius in this way. Compton and Milbert's tortoiseshells and the anglewings, or commas, also hibernate in the adult stage. Mild early spring days of +10 degrees Celcius or more may induce these butterflies to break diapause but they must again find suitable shelter if conditions worsen.
Spending the winter as an egg is another strategy, one used by our hairstreaks and coppers. The eggs may be laid on litter at the base of the host plant or on twigs, or sometimes laid rather haphazardly on debris in the vicinity of the host plant. An insulating layer of snow helps to prevent desiccation from the cold air. These butterflies emerge in summer.
Other species enter this period of inactivity as larvae (caterpillar). Larvae can be anywhere from young to fully grown, depending on species. This is the most common method of hibernation. After having put on a layer of fat, larvae will move down into the leaf litter at the base of the food plant to spend the winter. Antifreeze compounds in the blood protect against sub-zero temperatures. Some larvae enter diapause without feeding. The Jutta Arctic requires two years to reach adulthood and therefor the larva overwinters in two different stages. Dreamy Duskywing caterpillars construct a hibernaculum in which to await spring, as do Viceroy and White Admiral. Emergence of the adults depends upon the stage in which the larvae entered diapause, with mature caterpillars becoming adults before those that entered diapause as young to partially grown larvae.
Large Marble, Eastern Pine Elfin and Spring Azure are some who experience diapause in the pupal stage (chrysalis). This is of course the preparatory stage to becoming an adult. The pupae are usually nestled in leaf litter at the base of the host plant but in some cases can be suspended from the food plant. They are all early-season flyers, owing to the advanced stage in which they entered diapause.
The simplest way to survive winter is not to survive winter at all, and to migrate before conditions become harsh. Species such as Painted Lady, Red Admiral and Monarch cannot survive our winters in any of the above life stages. Instead, they move south in autumn and subsequent generations move northward in spring to repopulate our region.