Life Histories of Cascadia Butterflies

Overwintering, Oversummering, and Diapause

23 Overwintering, oversummering, and diapause is unlikely. Species that pupate upright with a cremaster silk attachment at the base plus a horizontal girdle thread around the middle include the swallowtails and pierids (whites and sulphurs). These species overwinter as pupae; thus the extra girdle support is important for stability during the long diapause, and the diagonal upright posture resembles a thorn or stick for camouflage. Most of our other species (lycaenids and skippers) pupate horizontally, many lightly anchored with a silk girdle and most under some kind of cover (rocks, leaves, sticks, nests, or in holes). The pupae of the Silver-spotted Skipper and some duskywings use an elaborate series of silk strands to suspend themselves inside a leaf shelter in such a manner that they do not touch any solid surface, protecting themselves from mold and perhaps even from small predators. While this is a book about the immature stages of butterflies, one adult life strategy must be mentioned briefly, since it has a profound impact on the occurrence of immature stages. Migratory species extend their range each season into Cascadia, taking advantage of favorable spring and autumn conditions; however, they are unable to withstand winter in the Pacific Northwest and either die out or migrate back toward the south in the fall. Examples of migratory species include the Orange Sulphur, Checkered White, California Tortoiseshell, Painted Lady, West Coast Lady, and Monarch. Other species are elevational migrants, entering the mountains in late spring to spend the summer feeding where the growing season is long, then returning to the lowlands either to lay their eggs (Coronis Fritillary) or to overwinter as adults (e.g., Mourning Cloak, Milbert’s Tortoiseshell). Some multiplebrooded species (Purplish Copper, Anise Swallowtail, Milbert’s Tortoiseshell) move to higher elevations with each successive generation to follow the food supply, while other species remain at lower elevations, choosing instead to seek out other host plants as the season progresses (Becker’s White, Orange Sulphur, Melissa Blue, Common Sootywing). In summary, butterflies and their immature stages are enormously flexible and use a stunning variety of strategies and adaptations to enhance their survival and prosperity. We have touched on a few of these strategies, but much remains to be learned. Overwintering, Oversummering, and Diapause Butterflies and their immature stages, like all insects, are cold blooded and depend on environmental conditions to achieve body temperatures necessary for activity, development, and reproduction. The optimal range of body temperatures for most butterflies and their immature stages lies between 15 and 30 °C. Arctic-alpine species have lower optimal temperatures, and tropical species thrive under higher temperatures. Tropical and subtropical species depend more on ambient air temperatures, whereas butterflies living closer to the poles are generally more dependent on direct sunlight to raise body temperatures to operating levels. When the climate is characterized by temperatures remaining below 5–10 °C with limited, low-angle sunshine, most butterflies and their immature stages are unable to g Celastrina echo (Echo Blue) L4, 14 mm, feeding on nitrogen-rich flowers of Cornus sericea (Red Osier Dogwood). 24 life histories of cascadia butterflies develop or be active. Host-plant and nectar resources are also usually absent or minimal under these conditions, adding to the inhospitable quality of the environment. Winter in Cascadia is cold. In the mountains and lowlands, away from the coastal fringe, temperatures during December–February may remain below 0 °C for days or weeks at a time and dip as low as -35 °C. Clearly, for butterflies, eggs, larvae, or pupae to survive such an extreme environment for 2–4 months or longer, a major modification to normal insect physiology is required. Some butterfly species, faced with an impending inhospitable winter environment, “decide” to leave Cascadia and fly many miles south to find a more forgiving climate. Painted Lady, California Tortoiseshell, and Monarch are familiar examples of migrants that escape the “frozen north” and spend winters in California or Arizona, their descendants returning north the following season. These species are robust, long lived, and powerful fliers, capable of epic journeys; however, most of Cascadia’s butterflies must endure winter in the frozen north. Butterfly hibernation is no less impressive than hibernation of mammals like bears or groundhogs. 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. Hibernating adult butterflies or immature stages are dormant and in a physiological state called diapause, characterized by a lowered metabolic rate and radical...