Even though it’s been a pretty mild winter, we have had some snow and cold weather. It’s been months since I’ve seen a butterfly outside – yet I’m quite confident that as the weather warms next spring, there will again be butterflies here in northeastern Illinois. So where are the butterflies now? Did they migrate off someplace else? Are they hibernating? As it turns out, the answer varies from species to species.
Some butterflies do spend the winter elsewhere. The most familiar example is the Monarch, which spends the winters in the highlands of Michoacán in Mexico. It’s the only local species that makes an annual round-trip migration.
About a dozen other species spend the winter in the desert southwest or along the Gulf Coast in the Deep South. These include species such as the Buckeye, Painted Lady, and Little Yellow. They don’t seem to have much of an organized southward migration; they simply die off in more northern locales as the weather cools in the fall. Each spring they begin dispersing northward as the weather warms, though it may take several generations to arrive here.
Although it may be hard to believe, especially on a really cold day in the middle of winter, some species of butterflies hibernate and spend the entire winter here. Each species has one particular life stage that hibernates. There are examples of all four species being used. Species such as the Purplish Copper overwinter as eggs. These are laid on twigs or leaves, where they remain for the entire winter. Many species, including Baltimore Checkerspots, hibernate as caterpillars. The caterpillars burrow into the leaf litter at the base of their host plants as fall approaches. Many swallowtail butterflies spend the winter as chrysalises. About a half dozen Illinois species, such as Mourning Cloaks, even overwinter as adults. They spend the winter tucked into crevices in logs, or underneath loose bark on trees. These are the species that can be seen flying on the very first warm days of spring, and occasionally even during warm spells in January or February.
How do the hibernating butterflies survive? As cold-blooded animals, their body temperatures drop to that of their surroundings. The secret turns out to be in their chemistry. As the days shorten during the autumn, they begin secreting natural antifreezes into their body fluids. The natural antifreezes are necessary no matter which life stage overwinters. If ice crystals form they rupture cells, which is fatal to eggs, caterpillars, chrysalises and adult butterflies alike. The natural antifreezes are small molecules such as glycerol. Glycerol shares many chemical properties with the antifreeze that is used in car engines. Although the body temperature of a hibernating butterfly may drop to well below zero, the glycerol in its body fluids prevents the formation of ice crystals. The butterfly can therefore survive the very low temperatures, become active again when the weather warms in the spring and complete the life cycle. Next time you are taking a walk in midwinter, consider that there are thousands of butterflies tucked away in warm spots, waiting to fly next summer.