COLD TOLERANCE OF THE PUPAE IN RELATION TO THE DISTRIBUTION OF SWALLOWTAIL BUTTERFLIES

A steep decline in the diversity of swallowtail butterfly species at high latitudes could be due to limited cold tolerance of overwintering pupae. If this is so, species with unusually northerly distributions should be unusually cold tolerant. We compared the northerly distributed Papilio canadensis with its southern relative, P. glaucus. Pupae were exposed for 2-5 months to four acclimatization treatments: outdoors in Alaska, outdoors in Michigan constant 5-degrees-C. Field temperatures encountered by pupae in Alaska were lower than in Michigan. The supercooling point of P. glaucus pupae was unaffected by acclimatization (mean +/- SE = -23.5 +/- 0.52-degrees-C). The supercooling point of P. canadensis pupae did not differ from that of P. glaucus pupae, except following acclimatization in Alaska, when it dropped to -27.0 +/- 0.55-degrees-C. Survival of pupae in Michigan was high for all populations (70-90%); in Alaska, survival of P. canadensis was just as high, but survival of P. glaucus dropped to 14%. Freezing was usually fatal in both species, but death was not immediate. No pupae survived 6 weeks at -25-degrees-C. Trehalose was the most conspicuous metabolite revealed by nuclear magnetic resonance spectroscopy of live pupae and hemolymph. Labelled glucose was metabolized differently by the two species, which may underly the difference in acclimation potential and cold tolerance. The results support the hypothesis that winter temperatures limit swallowtail distributions.