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 reproduct ive and fall migrant cohorts of the monarch butterfly in southwest Ohi o in 1994. Reproductive and migrant butterflies were distinguished on the basis of lipid content and whether the female had mated. We compar ed crystallization temperatures (the temperature at which ice forms in side the body) and the capacity to resist chilling injury (i.e. injury due to subzero chilling in animals which supercooled but did not free ze) between reproductives and migrants. Very low crystallization tempe ratures (< -10 degrees C) were found only in the migrants. Among repro ductives there was a significant positive correlation between lipid co ntent and crystallization temperature. Chilling injury occurred freque ntly among reproductives; in contrast to migrants which survived subze ro exposure until ice began to form in their tissues. Records of micro habitat and ambient temperatures from field data loggers and meteorolo gical stations indicate that the monarch migrants in this region infre quently encounter temperatures low enough to promote extensive cold ac climation, and that subzero temperatures are very rare during the time we observed migrants. Data from previous studies of monarchs at the o verwintering site or acclimated in a laboratory cold room indicate tha t this species is capable of greater cold hardiness than we observed i n freshly-caught field specimens. We conclude that the increased cold hardiness we observed in migrants was due almost entirely to physiolog ical changes accompanying reproductive diapause and migration, and tha t cold acclimation played little if any role. Copyright (C) 1996 Elsev ier Science Ltd