Post by Patrea Wilson.
When I was a child, my mom told me that the warm, calm days of early summer were “butterfly days” in Wisconsin, and I believe that to be true. But, this year, during a rare 70⁰ F day in February, I was astounded to see a white butterfly merrily flitting around our yard. Where did that rascal come from? It seemed unlikely that it had just migrated into Wisconsin in the depths of winter, so how had it survived the typically cold days we’d been experiencing before the warm spell? And, I asked myself, how do butterflies in northern climes over-winter or migrate in general?
I began researching this issue, and was delighted when I found a wonderful document titled “Winter Survival Strategies of Common Wisconsin Butterflies” produced by Master Gardener Volunteers of the University of Wisconsin Extension. (http://fyi,uwex.edu/sewmg/files/2011/01/Winter-Survival-Strategies)
I’ve learned that butterflies are described as migratory or non-migratory, depending on the way they protect themselves from harsh winter weather. The best-known of those who migrate are the Monarchs (Danaus plexippus).
Monarchs are the only North American butterflies that make a two-way migration. Adults that emerge from their pupae in September in their northern range fly south and over-winter in central Mexico. In late winter, they return to the southern U.S. where they lay their eggs and die. Their progeny develop into adults that continue the journey north, some reaching the southern Midwest, where they produce another generation. These are the adults that we see each summer in Wisconsin.
The Common Buckeye is one of the northern butterflies that fly south to spend the winter, then simply head north again in spring, looking for good food sources.
Some southern butterflies wander into Wisconsin and other northern states during the summer as they look for nectar plants and caterpillar food plants for laying eggs. Since they can’t survive our winters, the last generation each year dies. New butterflies may then arrive again in the spring. Immigrants include the Variegated Fritillary, Little Yellow, and the Dainty Sulphur.
Butterflies that don’t migrate may survive by entering a type of hibernation called “diapause,” where they produce an internal antifreeze that protects their cells from freezing during the winter. Each butterfly species enters diapause during a particular phase of their growth cycle.
Hibernating as Mature Caterpillars and Pupae
Some Wisconsin butterflies hibernate as mature caterpillars or shed their last skin, forming a pupa and becoming dormant until spring.
The butterfly I’d seen in February was most likely the common Cabbage White, usually the first butterfly to appear in Wisconsin in the spring. It probably had overwintered as a pupa (chrysalis) until those unseasonably-warm days had fooled it into emerging.
Other Wisconsin butterflies that hibernate as mature caterpillars or pupae include the swallowtails.
Most caterpillars go through several stages of growth (called “instars”), shedding their skin between each stage. Some of them, like the Silvery Checkerspot, hibernate by going into diapause during one of the middle stages, resting through the winter to awake and complete their growth in the spring. You may have seen the leaf shelters that some of them form and bind together into a roll using their silk. Others take refuge on the ground under leaves or in seed pods.
The Silver-spotted Skipper also goes into diapause as a caterpillar, mid-way through its growth phases.
A few caterpillars make nests at the base of the food plants where they hatched and hibernate until spring. These include the Aphrodite Fritillary (Speyeria Aphrodite), the Great Spangled Fritillary (Speyeria cybele Cybele) and the Common Wood-nymph (Cercyonis pegala nephele).
Some butterfly eggs laid in the fall simply enter diapause before hatching, waiting for warm spring weather. The tiny caterpillars then emerge to a feast of new leaves. Some butterflies that overwinter in this way are the European Skipper, the Bronze Copper, and several Hairstreaks.
It’s astounding that fragile insects like the Eastern Comma can hibernate through a frigid Wisconsin winter and fly again the next spring. They may find shelter, singly or in groups, in wood piles, beneath loose bark, in hollow trees or logs, or in outbuildings.
According to the North American Butterfly Association, there are over 6,700 species of butterflies in North America. Their excellent website (www.naba.org) provides a searchable database and checklists that can be pared down to manageable regions. For example, I produced a checklist of 57 butterflies verified to have been found in Rock County, Wisconsin. That’s less than 1% of all the butterflies in North America, and seems a reasonable number to try to identify.
All the photographs appearing in this article were taken of butterflies at Rotary Botanical Gardens in Janesville, Wisconsin. We invite you to visit our Gardens with your camera to try your hand at discovering Wisconsin butterflies.
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“All about Monarchs – Their Life and Their Migration”
By Emily and Larry Scheunemann
An evening garden seminar at Rotary Botanical Gardens, Janesville, WI
August 29, 2017, 6:30 – 8:00 p.m.