Defoliators are insects that feed on the foliage (leaves or needles) of forest trees. These insects harm trees by removing the chlorophyll containing tissues that are responsible for photosynthesis, the process by which light energy is captured by plants to produce food and energy. While many trees can survive defoliation that is minor or infrequent, defoliation can stress trees and make them more susceptible to attack from other insects and diseases, and repeated defoliation can kill them.

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Insect prevention and control begin with using good forest management practices to promote vigorous and healthy trees. Healthy trees are less likely to be attacked by insect pests than weakened or sick trees. Some insect pests that attack forest tree are caterpillars (moth species) and bark borers.