Biological communities: Epidemics of insects on forest trees in New Zealand

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In the conventional treatment of insect epidemics it is usual to recognise a number of development stages which may be follows:
1. The biotic balance; this is the normal state, previous to the epidemic, in which a balance is maintained at approximately the normal level.
2. The preparatory stage; in which there is an increase in insect population but no visible damage.
3. The prodromal stage; in which damage is visible but not of economic importance; there is a definite increase in insect numbers.
4. The eruptive stage; in which the population rapidly attains epidemic proportions and severe damage may be caused.
5. The crisis; at which point the epidemic breaks down.
6. The decline; in which the population falls to or below the normal level. The biotic balance is then restored.