Modification of adverse conditions by insects

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Many insects modify their environments directly, rather than merely choosing available sites that are already favourable. The modifications are carried out by making excavations in soil and other substrates, constructing feeding or resting shelters, inducing plant responses such as galls, aggregating, building colonial nests, and through parental actions. Such environmental modifications are briefly reviewed and related to the conditions that they modify. Some of the modifications offset physical factors such as dryness or flooding and cool or freezing temperatures. Others reduce the effects of natural enemies or enhance food resources. These effects have seldom been quantified and much of the evidence is anecdotal, but preliminary generalizations are made from existing information. Although potential roles often overlap, excavations and shelters protect especially against physical factors, while aggregations, colonies and parental actions more often influence the acquisition of resources. How modifications affect the impact of natural enemies differs among different kinds of enemies and is especially difficult to test. In any event, adaptive local modifications of the environment by insects are shown to be widely distributed and important. However, their specific roles have often been assumed rather than tested, or have been overlooked along with the potential interdependence of different effects. Therefore, environmental modifications should be considered explicitly and examined with greater rigour during the study of insect life cycles.