Early this month when I was at Ball Mill Resurgence Conservation Area, I noticed that there were a few butterflies around. This one was cooperative enough to land on a branch in the sun and let me photograph it. I didn’t know what kind it was, but I was surprised to see how worn it looked. Clearly this butterfly didn’t just emerge from a pupa.
I looked through my butterfly books, but couldn’t find it; I later realized that was because they’re much yellower when freshly emerged, fading to more white later. I posted it to the Butterflies of the Eastern US Facebook group, and within about 10 seconds I had a response; “Mourning Cloak”. And someone added, “they overwinter as adults.”
I was intrigued. In fact, I was shocked. I realize that insects have to get through the winter somehow; beetles have larvae — grubs — that burrow in the ground, maybe some survive as eggs, that sort of thing.* But an adult butterfly? Surely it would freeze. Even here it gets down into the single digits Fahrenheit — over 10 degrees C below freezing. How can they do it?
Not content to just wonder, I hit the literature. First I read about “freezing-intolerant” and “freezing-tolerant” insects. These were the accepted categories for overwintering insects at least into the early 1990s. What I got from work by W. Block and K.B. Storey was that the former use antifreeze compounds to lower the freezing point of their cell contents, and survive as long as the temp doesn’t get below their supercooling point (SCP). The latter use nucleation compounds in their extracellular hemolymph to allow it to freeze while keeping the intracellular material from freezing.
But it turns out that it’s more complex than that. More recent work, summarized in 1999 by Brent Sinclair, puts all of these on a continuum. Most insects use both antifreeze compounds and ice-nucleating compounds in some combination. They lower their freezing temperature, but then they may also be able to survive below the SCP when actually frozen.
The butterfly in the photo, the Mourning Cloak – Nymphalis antiopa, is a member of the family Nymphalidae. While most Lepidoptera overwinter as larvae or in some cases eggs, many Nymphalids overwinter as adults. They crawl under leaf litter or under loose bark and wait for spring.
The Mourning Cloak doesn’t freeze until -20 C, and it can survive temps down to -34 C — well below any recorded temp in this area. To me it’s pretty astounding. These guys sit there and freeze solid in a cold winter. Then when it warms up, they thaw out, crawl out of where they were hiding, and fly away.
So which of the butterflies you’re seeing sat out the winter as adults? “Hibernal Diapause of North American Papilionoidea and Hesperioidea,”** a 1979 review by James Scott, includes a handy table listing each species’ hibernal diapause stage as E, L, P, or A (egg, larva, pupa, adult), with references to back them up. For me in Southeast Missouri, the take-home message of the table is that Red-Spotted Purples and most Fritillaries overwinter as larvae, Swallowtails overwinter as pupae, and Brushfoots mostly overwinter as adults. So the adults that overwinter that I’m likely to see include the Mourning Cloak; genus Polygonia – the Comma and the Question Mark; and genus Vanessa – Painted Lady, American Lady, and Red Admiral.
Polygonia interrogationis, the Question Mark
Polygonia comma, Eastern Comma
Vanessa cardui, Painted Lady
*The process of getting through a cold winter by slowing down body processes and waiting somewhere is loosely called “hibernation” in insects. More properly, it’s referred to as “hibernal diapause”. Some insects have a quiescent stage when it gets too hot as well, which is “estival diapause”.
**The Papilionoidea superfamily includes all the butterflies except the skippers, which are in the Hesperioidea. So together those superfamilies constitute “butterflies”.
NOTE: One more local butterfly that overwinters as an adult is the Sleepy Orange, Eurema nicippe.