Phenology of Cold Hardiness in Reproductive and Migrant Monarch Butterflies (Danaus plexippus) in Southwest Ohio

We examined seasonal changes in the cold hardiness of summer reproductive and fall migrant cohorts of the monarch butterfly in southwest Ohio in 1994. Reproductive and migrant butterfl ies were distinguished on the basis of l ipid content and whether the female had mated. We compared crystall ization temperatures (the temperature at which ice forms inside the body) and the capacity to resist chil l ing injury (i.e. injury due to subzero chil l ing in animals which supercooled but did not freeze) between reproductives and migrants. Very low crystall ization temperatures (<-10"C) were found only in the migrants. Among reproductives there was a significant positive correlation between lipid content and crystall ization temperature. Chil l ing injury occurred frequently among reproductives, in contrast to migrants which survived subzero exposure unti l ice began to form in their t issues. Records of microhabitat and ambient temperatures from field data loggers and meteorological stations indicate that the monarch migrants in this region infrequently encounter temperatures low enough to promote extensive cold acclimation, and that subzero temperatures are very rare during the time we observed migrants. Data from previous studies of monarchs at the overwintering site or acclimated in a laboratory cold room indicate that this species is capable of greater cold hardiness than we observed in freshly-caught field specimens. We conclude that the increased cold hardiness we observed in migrants was due almost entirely to physiological changes accompanying reproductive diapause and migration, and that cold acclimation played litt le if any role. Copyright O 1996 Elsevier Science Ltd