Overwintering Butterflies

by Christine Hanrahan

Winter may seem an odd time to be thinking of butterflies, but on your winter walk, you may actually be passing a butterfly or two, without knowing it. How can that be, you may ask? Well, our overwintering butterflies frequently find hibernation sites under loose bark of trees or in crevices, cavities, and even caves. I’ve also seen butterflies emerging in spring from the eaves of old buildings.

Many folk find it astonishing that butterflies, which we think of as fragile creatures, can survive our cold winters. Not all butterflies overwinter as adults, of course. In fact, only a few do so in this region, but they fascinate us because of their ability to withstand snow and freezing temperatures. Around Ottawa, Mourning Cloak (Nymphalis antiopa), Eastern Comma (Polygonia comma), Green Comma (Polygonia faunus), Question Mark (Polygonia interrogationis), Milbert’s Tortoiseshell (Nymphalis milberti), and Compton Tortoiseshell (Nymphalis vau-album) overwinter in their adult form. Other species overwinter as adults further south, the most familiar example being the Monarch butterfly which winters in Mexico.

How do these delicate butterflies survive the winter? In a word, diapause. As explained by James and Nunnallee (2011), hibernating adults are dormant “and in a physiological state called diapause, characterized by a lowered metabolic rate and radical biochemical changes. Diapause is different from simple dormancy or inactivity as occurs in butterflies and their immature stages during cool periods in spring and autumn or overnight. It is a rigidly controlled physiological mechanism that is genetically fixed or induced by environmental cues.”

Before hibernating, these adult butterflies fatten up by feeding voraciously, often on rotting fruit and sap. This is no different from what hibernating mammals do (think of bears gorging on food to put on fat for the long winter months).

Warming weather encourages butterflies to become active again, even for just a brief period. During the very early, warm spring of 2010, I found butterflies taking advantage of the warmth in mid-March. Likewise, during the unusual heatwave of mid-March, 2012, when the temperature hit +30, I saw several Mourning Cloaks fluttering around the woods. But when the temperatures cooled again, they vanished until the next warm spell.

The most common species that I see, almost without fail, as soon as we have a few warm days in early spring are Mourning Cloak, Eastern Comma, and Compton Tortoiseshell – sometimes all three in the same general vicinity when walking through woodland sites.

How do other local butterfly species spend the winter? Most do so in the larval (caterpillar) stage, but some overwinter as pupae and some, such as hairstreaks and the European Skipper (Thymelicus lineola) in the egg stage. Just as it seems amazing that adult butterflies can survive our frigid winters, it seems to me equally miraculous that larvae and pupae can withstand the rigours of cold weather.

Last winter (2011/2012), I kept two Black Swallowtail (Papilio polyxenes) in a glass aquarium in my garden shed. I didn’t believe that the seemingly desiccated looking pupae could really become butterflies, but they did. In late April, I brought the aquarium outside into the garden and, on 26 May, a female Black Swallowtail emerged, followed a week later by a male from the second pupa. Like overwintering adults, larvae and pupae also prepare for hibernation. “By changing their physiology, butterflies, or their eggs, larvae, or pupae, are able to survive the winter for months in a state of suspended animation.” (James and Nunnallee, 2011)

Next time you are on a winter walk through a woodland, remember that you may well be passing some overwintering adult butterflies, just waiting for those first warm days of spring to fly once more.

References: James, David G. And Nunnallee, David. 2011. Life histories of Cascadia Butterflies. Oregon State University Press.

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