Abstract
Diapausing larvae of Aphidoletes aphidimyza (Diptera: Cecidomyiidae) had relatively low supercooling points (SCP) ranging from -19.0 to -26.4°C. None of the specimens that froze at this temperature survived. A high survival rate (up to 87%) at -10°C for 10 days was observed in supercooled larvae. Such features are characteristic for insects that use a chill-tolerance strategy of cold hardiness. However, the cocoons formed by the diapausing larvae were penetrable by external ice crystals and the larvae showed a relatively high survival rate (23 - 34%) at -10°C for 10 days also in the frozen state caused by inoculation by external ice at high subzero temperatures. Such a duality with respect to cold hardiness strategies seems to be ecologically relevant to overwintering in soil habitats where there may be unpredictable contact with external ice.
Highlights
The overwintering strategies of insects are broadly divided in two main categories, freeze-intolerance (-susceptibility) and freeze-tolerance
We show in this paper that the diapausing larvae may survive exposure to subzero tem peratures either in a supercooled or frozen state depending on the moisture conditions in their microhabitat
Some of the larvae froze shortly after addition of an ice crystal at tem peratures between -1 and -5°C (Fig. 1, stippled area a); the exact numbers of frozen larvae could not be directly obtained from the record because the freezing exoterms were densely packed and mixed with the exoterms produced by freezing of wet pieces of cellulose
Summary
The overwintering strategies of insects are broadly divided in two main categories, freeze-intolerance (-susceptibility) and freeze-tolerance. Freeze-intolerant insects are killed by freezing and, for winter survival, rely on a (stabilized) supercooled state at which the temperature of crystallization of body fluids (super cooling point, SCP) is depressed, usually to about -25°C but sometimes much lower (about -50°C) in species/populations which are evolutionarily adapted to extreme conditions. A striking characteristic of most freeze-tolerant insects is their relatively high SCP ranging typically from -6 to -10°C (for recent reviews on insect cold hardiness see Lee & Denlinger, 1991; Bale, 1996; Block, 1996; Danks, 1996; Somme, 1999; Sinclair, 1999). Each of the two categories was subdivided into a few more or less distinct classes based on the relationship between the tempera ture of crystallization and the limits of cold survival (Bale, 1996; Sinclair, 1999)
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