Insects evolve levels of cold hardiness that are sufficient to meet the severity of thermal conditions in their hibernacula. This study examined freeze tolerance and related features in caterpillars of Hypercompe scribonia, which is the overwintering stage for this species, to compare with existing data for more northerly distributed species in Arctiidae that also overwinter as caterpillars. All specimens were collected from western Pennsylvania during mid-October 2004 and 2006 and eventually acclimated to 3°C for ≥4 wk. All caterpillars initially survived freezing at −3°C, which converted ≈45% of their body water into ice, and one third of them developed into moths. In contrast, freezing at −10°C was invariably lethal to caterpillars, even in the short term. Body composition (hemolymph osmolality, hemolymph glycerol, body mass, and water content) of caterpillars was stable between the 2 yr, with glycerol accounting for 3–5% of their solute. Thawed caterpillars showed an initial decline in routine aerobic metabolism that persisted through the first 6 h of the recovery period. H. scribonia caterpillars have a level of freeze tolerance that is adequate for hibernaculum conditions in western Pennsylvania; however, it is limited with respect to the tolerance levels of other arctiid caterpillars whose ranges extend northward well into colder regions of North America.
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