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Since the European corn borer has moved rapidly northward from population centers to the south and east into all the corn-producing areas of Minnesota, the effect of winter temperatures in that state on the ability of the borer to continue to survive and to maintain a population level of economic importance has been questioned. One may postulate that the corn borer has become established in the newly invaded areas either because the hibernating larvae are well protected from severe cold by insulation, or because they are sufficiently cold-hardy to withstand the low temperatures of Minnesota winters even in unprotected situations. It has been established by field surveys that there is usually a good survival over winter, even above the snow, in standing corn stalks. However, in the spring of 1948 a high percentage of the borers in the stalks were dead. Thus, we are certain that under some conditions a large proportion of the larvae are unable to overwinter successfully in Minnesota. Insects may be classified into four general categories on the basis of their tolerance to cold. These are: 1) insects which are able to survive freezing and are killed only by long exposure to low temperatures or by one or more sudden changes in temperature; 2) insects that can remain dormant in an undercooled state but do not survive freezing at temperatures below the undercooling point; 3) insects which exhibit very little undereooling and are killed by temperatures near the true freezing point; 4) non-hardy, non-hibernating insects killed by temperatures above freezing. The purpose of this study is to determine whieh of the above categories characterizes the corn borer and consequently the nature of the cold hardiness of this species; and further, to establish the time during the dormant period when natural mortality is most likely to occur. Most of the emphasis has been placed on a study of the importance of the "intensity factor of cold" as indicated by determining the undercooling point and freezing of larvae collected periodically from the field, and of larvae exposed to experimental conditions in the laboratory. Dittman (1943), states that the undercooling of living insects behaves in the same manner as water in a finely divided state. Water particles in this condition, in the absence of ice crystals or other solid particles that may act as nuclei for crystallization in the absence of shock or movement, may be undercooled to a considerable degree.

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