Time as a Factor in the Freezing of undercooled Insects.

Undercooling points are shown to be unreliable as a measure of insect cold-hardiness. Insects held in an undercooled state freeze at irregular intervals, often over long periods of time. Freezing, which is fatal to most insects, is initiated by the formation of an ice-crystal nucleus, and the probability of such formation is dependent upon the extent of undercooling (temperature), cold-hardiness (a complex resulting from previous treatment), and time. For a specified degree of cold-hardiness, the probability of freezing is dependent upon temperature and time. If the temperature is fixed, the probability of freezing can be expressed in units of time. Freezing can take place on a rising temperature gradient as well as on a falling one.Unless an overwintering population is adequately protected by its environment and a high degree of cold-hardiness, losses from freezing will take place by degrees during the entire winter.