Immobile and vulnerable all winter, the Giant Swallowtail waits. It has many adaptations for survival. During the winter wait, it is too cold to do anything but wait. Enclosed in a chrysalis it transforms from caterpillar to winged jewel. It transforms within its last larval skin during the fall from a full-grown caterpillar to a chrysalis in a hidden location.
Inside the jewel of a chrysalis, body tissues of the caterpillar dissolve to an apparent liquid. Primordial cells actively work to rebuild an adult body through most miraculous changes. Cool or cold weather delay completion of the work for weeks or months. In the more southern states the wait between caterpillar and adult is shortest. Warm weather arrives sooner allowing the transformation process to continue.
In Michigan it is thought that the process never completes and the butterfly freezes to death in the chrysalis before spring arrives. The result is that we have no Giant Swallowtails until some immigrate north to colonize the state with a new population.
I disagree in part with such thoughts. I have seen Giant Swallowtails year after year at Ody Brook where the host plant for the caterpillar is abundant and have found them in repeated years in other patches of Xanthoxylem americanum. In still other patches of the prickly ash, I have been unable to find the butterfly. That led me to the scientific hypothesis that some swallowtails survive in isolated patches to provide an ongoing legacy.
To further support the hypothesis, I discovered that reliable patches containing the butterfly year after year had none after a very cold winter with subzero temperatures. It took a few years before those patches supported the species again. My idea is that it took a few years for immigrants to find populations of the host plant.
When host plants are found by an adult swallowtail, it lays eggs that develop into bird turd looking caterpillars that fed on the leaves. Caterpillars pupate and develop into adults by fall to provide many beauties in landscapes with host plants. Those pupating in fall wait the long winter. Many freeze but a few might survive to maintain a population in isolated patches of prickly ash. This is still not scientifically proven.
To scientific prove my hypothesis, it is necessary to provide evidence that chrysalises survived the winter in the wild. I have not looked for or found supporting evidence. That does not mean they do not survive. It means there is no supporting evidence that some survive winter in this climate. This skeptical process is what makes scientific method so valuable and self-correcting. It dispels unsupported information that we often choose to believe because we want to or because it appears logical.
Through scientific process we learned the sun does not rise in the east and set in the west. Instead we discovered the Earth rotates to provide the illusion of sunrise and sunset.
For now, science supports that Giant Swallowtails do not survive Michigan winters. I expect any Giant Swallowtail chrysalises found this winter will be frozen. Immigrants will be essential if we are to see the species this summer. The winter provided chilling subzero temperatures in local nature niches. Maybe the deep snow covered and protected a few precious winged jewels.
Natural history questions or topic suggestions can be directed to Ranger Steve (Mueller) at firstname.lastname@example.org Ody Brook, 13010 Northland Dr, Cedar Springs, MI 49319-8433. 616-696-1753.