Defoliating insects migrate to the foliage of a tree and feed on the leaves. Other chewing insects attack the fruit. Caterpillars and beetles make up the largest proportion of chewing insects. Generally, trees can bounce back from an attack of these defoliators, though repeat infestation will weaken a tree and can eventually kill it by starving it of energy.
Healthier trees are less likely to become infested, so maintaining the strength and vitality of your tree is an important prevention step. Once chewing insects are present, controlling movement up and down the stem with physical barriers can interrupt the lifecycles of many caterpillars. Insecticides can be used to kill the insects.
Boring insects are often the most harmful to trees and if left intreated can cause death. Boring, or tunneling, insects cause damage by boring into the stem, roots, or twigs of a tree. Some lay eggs which then hatch and the larvae burrow more deeply into the wood blocking off the water-conducting tissues of the tree. Boring insects generally feed on the vascular tissues of the tree. If the infestation is serious, the upper leaves are starved of nutrients and moisture and the tree can die. Signs of borer infestation include entry/exit holes in the bark, small mounds of sawdust at the base, and sections of the crown wilting and dying. It is important to regularly monitor a tree's trunk for signs of boring insects to enable early identification and quick treatment.
The key is to prevent infestation by keeping the tree as healthy as possible. This includes proper pruning, watering, mulching and fertilization. Pruning should be done in late fall or winter to avoid attracting insects to open wounds. Dead or fallen wood should be removed immediately. Once borers are present, control becomes extremely difficult, but steps should be taken to prevent further damage and to stop the spread to surrounding trees.
These insects do their damage by sucking out the liquid from leaves and twigs. Many sucking insects (e.g. scale insects) are relatively immobile, living on the outside of a branch and forming a hard protective outer coating while they feed on the plant juices in the twig. Quite often they will excrete a sweet, sticky substance known as honeydew which contains unprocessed plant material. Honeydew can cause sooty mold to form on leaves and can become a nuisance for homeowners. Signs of infestation include scaly formations on branches, die-back of leaves, and honeydew production.
As with other insect infestations, prevention is the best approach. This means maintaining a strong and healthy tree. Once they mature on the tree, sucking insects generally must be killed on contact to prevent reproduction and achieve effective control.