Diapause is a state of suppressed development which occurs in response to certain levels of environmental factors, usually the temperature and/or duration of daylight. Used as indicators of approaching adverse conditions, these factors trigger a number of physiological and biochemical changes. The optimum environmental conditions for development during diapause are not the same as those for normal development (in winter, diapause development may proceed faster at lower temperatures), thus ensuring that normal development does not resume prematurely (by the time diapause is over, the environmental conditions are outside the range at which normal development can proceed). Besides helping the insects survive a period of adverse conditions, diapause regulates their development, synchronizing maturation. Depending on the species, diapause may occur during any stage of development - egg, larva (immature stage), pupa (transitional stage) or adult - and in some species may occur in more than one stage. Insects that experience diapause as adults postpone their sexual maturation until the conditions for reproduction are favorable.
In areas where the climate is favorable throughout the year diapause generations may be absent. In other areas, species which can develop quickly might alternate diapause and non-diapause generations. In this case, eggs laid in the spring develop into reproductive adults which lay eggs in the fall. These eggs might experience diapause and hatch in the spring when the larvae would develop quickly. Alternately, they might hatch in the fall and the larvae develop to a certain stage before entering diapause, completing their development in the spring. These spring reproductive adults lay non-diapause eggs which produce the fall reproductive adults. Since different species have different environmental requirements, diapause is a good strategy for surviving adverse conditions which are predictable.
When the weather is cool many insects appear sluggish or less energetic. This effect, called torpor or quiescence, results from a slowing of the metabolism and differs from diapause in that it is reversible once conditions improve.
Ron Lyons (volunteer 1990-1999)