Introduction

As seen in Chapters 2 and 5, a large number of insect species is usually associated with each tree species, but only a few of them become serious pests. For example, out of over 174 species of insects that can feed on the living teak tree, only three have become pests. Usually insects do not cause serious damage to trees unless the number of individuals, i.e. the population size, becomes large. Under what circumstances do insect populations increase to damaging levels? And why do some insects build up in large numbers while others do not? Our ability to control pests depends on the answers to these questions.

A group of individuals of a species living together in a defined area is called a population, and the study of the changes in the size or density of populations over time is known as population dynamics. It tries to predict these changes and explain the causes. A population has certain group characteristics, in addition to those possessed by the individuals constituting the group. It has a genetic composition, sex ratio, age structure, density and dispersion (clumped, random, etc.), each of which influences its behaviour. It is obvious that a pest problem is essentially a population dynamics problem. So we shall examine in some detail the circumstances under which insect population densities change.

Characteristics of population growth

Under ideal conditions insects, like other organisms, have the capacity to increase exponentially.