Butterflies in Snow

Black Swallowtail caterpillar feeding on dill.

At 5 degrees below zero, butterflies were the last things on my mind as I brushed the fluffy snow from the porch.  But as I swept away the last flakes along the railing, I noticed a small, brown sack about the size of a tootsie roll attached to the wood.  It was firmly held in place by fine threads.  Back inside with a warm cup of bird-friendly coffee, I identified it as a chrysalis of a Black Swallowtail butterfly.  Many Black Swallowtail caterpillars fed on my dill plants in the nearby butterfly garden for most of the summer.  It was almost certainly one of them.  Incredibly, this chrysalis will remain in place until spring returns, signaling the butterfly to emerge from its winter home and fly away.

All butterflies develop from egg to larva (caterpillar) to pupa (chrysalis), and finally to the winged adult, which we see fluttering around.  Each species has evolved a strategy that allows them to successfully pass the winter in one of these four life stages.  For example, swallowtails pass the winter in the pupae stage like the Black Swallowtail I found.  Skippers, quick little butterflies whose identification can challenge even avid butterfly enthusiasts, spend the winter as a caterpillar. The beautiful little coppers and blues remain as eggs through the winter.  Monarchs glide to more hospitable temperatures in the south.  And some, like the Mourning Cloak, hunker down and spend the winter as an adult!

Most temperate-zone butterflies survive the deep snows and frigid temperatures of New England in a stage called winter diapause.  Metabolic and respiratory rates are low and slow during diapause.  The cold itself is not a direct hazard to the butterflies. However, the formation of ice crystals in body tissue is quickly lethal. To keep from freezing, butterflies reduce the amount of water in their blood (White Admiral caterpillars reduce the amount of water in their body by 30 percent) and thickened it with glycerol, sorbitol or other antifreeze agents.  These chemicals function much like the antifreeze we pour into our car radiators.  Mourning Cloaks can withstand temperatures down to minus eighty degrees.  But it takes cold weather to trigger them to produce these antifreeze agents.  If you put a Mourning Cloak in the freezer on a warm summer day, it will quickly die because it lacks any antifreeze.

Some species of butterflies produce several generations each summer with the last generation of the summer entering diapause for the winter.  The number of daylight hours, and to a lesser extent temperature, controls onset of diapause.  When the summer or fall days reach a certain daylight length, the individual is genetically programmed to begin diapause at a certain time later in the season, either in its current stage or a later stage of its life cycle.  Because on any given day of spring or summer there are more hours of sunlight in the northern latitudes than in the more southern areas, butterflies go into diapause earlier the farther north an individual is located to avoid the earlier onset of winter.  Viceroy larvae are programmed to enter diapause after the individual receives less than 13 hours of daylight in Maryland, 13.5 hours of daylight in Vermont, and 15 hours in Newfoundland.

Overwintering eggs employ two strategies.  Eggs are either laid on the twigs of host plants, where they remain until new leaves develop in the spring and the larvae emerge and feed, or the eggs are laid on the leaf litter at the base of the plant (predominantly leafy plants that are destroyed by frost), where they remain until the plant sprouts from the ground in the spring.  The main difference between the two is host plant type.  Species which feed on herbs place their eggs on the ground and species that feed on trees or shrubs place them on the woody portions.  Some, like the endangered Karner Blue, rely on an insulating blanket of snow to protect them from harsher weather.  When there is little snow, eggs can become damaged by dry, cold air.

Adults that overwinter, such as the Mourning Cloak or the Milbert’s Tortoiseshell, store fat in there bodies in the fall.  Before entering diapause they find a place to hide, such as hollow trees or logs, cracks in rocks, or inside old buildings.  In these protected and somewhat insulated hideouts, they enter diapause until the longer and warmer days of spring bring them forth to mate and lay eggs for the next generation.  These early spring adults may have wings that are very tattered and ragged from their relatively long life of 8 to 10 months.

To break diapause in the spring an individual must pass through a long period of cold weather and into a longer daylight period.  The cold period must be months long to trigger the end of diapause.  If it were shorter the individual might end diapause during a short warm spell only to be clobbered by the next arctic front.

The beautiful butterflies are still around us, despite the snow.  Their special adaptations will allow them to emerge during the warm days of spring.  I’ll be watching the little Black Swallowtail chrysalis on my porch throughout the winter and the day it bursts from its case and spreads its wings, I’ll know it is truly time to put the snow shovel away and sharpen the garden spade.