Insects and disease may be generally grouped together because they are natural biological enemies of trees, but they should be examined separately. No one knows how many different species of insects there are, but estimates run from 600,000 up to several millions. This is more than all plant and other animal species combined. About 80,000 species have been identified in North America. Scientific classification, however, is based on the configuration of the adult body form.
All insects originate from eggs and develop by identifiable stages into the eventual adult form.
This system of change is called metamorphosis. When this system of change requires four steps, it is called complete metamorphosis. Butterflies and moths develop through complete metamorphosis. A three-step maturation process is typical of grasshoppers and roaches. The insect egg hatches into a nymph which usually resembles a mature adult.