Pests and diseases are rarely detected during the incipient stages of damage or infection. Tree destruction or mortality is usually first noticed when very obvious and in some instances the pest may have already disappeared. Several pests may occur at the site, and it is difficult to identify the primary pathogen(s). Another problem is identifying the particular pest since many species are undescribed. A number of insects and caterpillars feed on the foliage of trees; if totally or partially defoliated, a tree will be weakened and more susceptible to attack by disease. Insect infestations of the twig, leaf, and bark may lead to localized necrosis, but in susceptible species the whole tree or even complete stands may be destroyed. The nun moth, for example, has caused extensive damage in European forests, and in North America forests have been virtually wiped out over vast areas by bark beetles and by the spruce budworm. Sucking insects, such as aphids, mealy bugs and scale insects, which are parasitic on trees, severely weaken and reduce the vitality of their hosts. Nearly all tropical forest insect pests belong to one of five orders: Coleoptera, Hemiptera, Isoptera, Lepidoptera and Orthoptera. Forest insect damage may be classified into four broad categories: defoliation (direct or indirect) caused largely by larvae of Lepidoptera and by a few Coleoptera and Orthoptera; boring or mining inside seeds, bark, wood and shoots, mainly due to species of Coleoptera and Isoptera and to a few Lepidoptera; chewing of bark and wood largely by Coleoptera; and necrosis and wilting due to chemicals secreted by some Hemiptera.
Numerous caterpillars defoliate forest trees: in Europe, those of the nun moth on spruce, winter moth and mottled umber moths on broad-leaved trees and pine looper and pine shoot moth; in North America, various tent caterpillars, and those of the gypsy moth, brown-tail moth, spruce budworm and larch case-bearer. The North America fall webworm is a major pest of shade trees in Europe. The Japanese beetle has infested a large area in the USA, defoliating shade and fruit trees. Weevils attack the growing shoots of pines. Wood-boring beetles cause a great deal of damage to park, woodland and forest trees. Indian forests suffer extensive damage from the sal borer.
Other insects, which do little primary damage to trees, are responsible for considerable secondary damage because they are vectors of tree diseases. For example, the Dutch elm disease fungus is introduced into the vascular system of healthy trees by the elm bark beetle. Most of the important tree diseases are fungal, such as chestnut blight, a canker disease which, since its introduction into North America in 1904, has practically annihilated the native chestnut. A similar scourge is white pine blister rust, which spreads rapidly through the tissues of an infected tree, and from one tree to another, invariably killing its hosts. Fungal diseases can take a number of forms of which the most important are wilts, cankers, rusts, heart rots and leaf spots.