Seasonal Strategies in Insects

Few insect species encounter environmental conditions which remain favourable for continuous growth, development, and reproduction throughout the year. Such species are said to be homodynamic: they have life cycles characterised by a lack of seasonally synchronised adaptations. Most species however, inhabit regions in which seasonal variations in environmental conditions impose a corresponding seasonal change in rate of development, altc.rnating between periods of rapid growth and reproduction, and a period of rest or dormancy. Such heterodynamic life cycles are typical of temperate latitudes where seasonal adversities, such as low temperatures often combined nith the absence of food, retard or completely inhibit development during winter. However, even in tropical latitudes, the alternation of a wet ~vith a dry season can also result in specific life cycle adaptations linked temporally to these environmental chaqes. This paper is concerned with those strategies found in insecmt life cycles which enable them to survive unfavourable, recurrent changes in their environment. In general, these comprise 2 basic strategies: a migration, and b dormancy. Migration is well documented for Northern Hemisphere insects. I t may consist simply of flight from emergence site to another place, \\\\.here gro~vth and reproduction can continue; or it may be combined with a dormancy state such as adult (reproductive) diapause..An example of the latter in North,4merica is the monarch butterfly, Danaus plexippus (L.) which migrates and then enters an overwintering adult diapause (Urquhart 1976) ; and in Australia, the bogong moth, Agrotis infusa (Boisd.) which migrates and then enters an aestival (summer) adult diapause (Common 1954). The second major seasonal strategy, that of dormancy, includes the following resting conditions. A Summer dormancies a Heat torpor