Principles of Insect Cold-Hardiness

Cold is a relative term which for present purposes will be considered to encompass temperatures too low to support normal development of the in­ sects concerned. The effects of cold on insects may be manifold, but a pri­ mary distinction must be made on the hasis of whether freezing does or does not result. The dividing line, however, is seldom at the freezing point of the tissues but rather at the supercooling point, for insects almost invariably supercool to some extent, usually many degrees. Cooling without freezing has been termed (9), particularly when the emphasis is on temperatures above O°C. Such investigations usually deal with i nsects that do not normally encounter cold, e.g., household pests. Unaccustomed to cold, they tolerate it poorly. On the other hand, insects that hibernate in temperate and colder climates must tolerate a wide range of low temperatures, often for long periods. Some species improve their toler­ ance through processes of cold-acclimation and cold-hardening. All insects supercool to some extent, regardless of need; in fact, some that need to do so least supercool below -40°C. Most hibernating insects do need to supercool, however, to avoid freezing, which would be fatal; the remainder have not this need because freezing is not injurious to them. The former group are called freezing-susceptible and the latter freezing-resistant or freezing-tolerant (34). The principles of insect cold-hardiness will be discussed under three main headings : (a) chilling and cold-acclimation, (b) avoidance of freezing by supercooling, and (c) freezing-tolerance.