In the United States, several hundred insect species cause damage to trees, shrubs and bushes. Additionally, at least as many pathogen pests attack trees by causing damage to leaves, shoots, trunk and roots, leading sometimes to the demise of the tree. The general category of pest can be determined by identifying the type of damage it inflicts. Exact identification of the pest in question is best for determining the proper approach to pest management.
Leafminers are a class of insects whose larvae eat the internal tissue of leaves, giving affected foliage a splotched or ragged appearance. The term "leafminer" is applied to the burrowing young of several types of insects, including moths, beetles, sawflies and flies. Leafminer damage is rarely lethal to the tree, but the resulting poor appearance can severely affect the value of commercial growers' nursery stock. Eggs are deposited on the undersides of leaves, and larvae burrow into leaves after hatching, feeding on the soft inner tissue until they pupate and become adults. Natural predators are the best and main control of leafminer outbreaks in trees, though topical and systemic pesticides are also available for control of severe infestations.
One of the most serious groups of tree pests, borers are a major threat to most types of fruit tree, especially those which have already been weakened by disease or poor growing conditions. Other species commonly affected by borers include aspen, fir, birch, ash, juniper, lilac, pine, spruce and cottonwood. The winged adults, often no larger than 1/4 of an inch, lay eggs either on or just beneath the surface of a tree's bark, and when the larvae hatch they immediately begin feeding on the soft transport tissues located just beneath the bark, rapidly impairing a tree's ability to take up nutrients and water. After pupating, new adults emerge from the tree by boring a hole to the surface to lay eggs and begin the cycle anew. Though larvae damage usually goes unnoticed, the tell-tale bore hole and accompanying sawdust or oozing sap are the main indicators of a borer infestation. Heavily infested trees are difficult to save, though newly attacked trees are best treated with systemic pesticides over several growing seasons.
Weevils and Beetles
These hard-winged insects are responsible for a large range of damage to trees, some lethal and some merely disfiguring. Weevils and their larvae can attack roots, young shoots, leaves and nut fruits, while beetles and their grubs frequently burrow beneath tree bark and cause severe damage similar to that of borer species. Weevils are shorter and stubbier than beetles, generally with long narrow heads; most species take more than a year to complete their life cycle and will overwinter in leaf litter or underground. Beetles are larger, rounded insects with small, almost inconspicuous heads and occasionally, very long and conspicuous antennae. Severe infestations are usually treated by applying systemic pesticide to a tree's root zone or by drenching the root zone and affected tree parts with a persistent pesticide to break the insect's reproductive cycle.
Aphids and Psyllids
Related to each other, aphids and psyllids are common pests of many ornamental and vegetable plants as well as trees. Feeding on the sap of leaves and other soft plant parts by piercing them with sharp mouth parts, aphids and psyllids are usually found clustered together in great numbers. Pear-shaped and soft-bodied, the lethargic aphid is easy to hand-pick and kill once discovered, but is also a favorite food of many predator insect species. Psyllids are powerful jumpers, and adults have clear wings; their presence is readily identified by shoot or tip dieback or deformity. Both insect species cause collateral damage by encouraging the growth of sooty mold, which proliferates on the honeydew excreted by feeding. Aphids are best controlled through use of natural predators, while psyllids can be trapped by setting up bright yellow sticky strips; the insect is strongly attracted to the color.
Mites and Scale
Mites and scale are two other types of insect which feed on soft plant parts using piercing mouthparts, sucking the sap from the plant and leaving behind excrement which encourages the growth of mold and mildew. Mites, which are generally too small to be seen with the naked eye, are usually identified by the rusty or silvery sheen they leave on the undersides of leaves, though spider mites spin weak, slender webs on leaves and in leaf nodes. Scale insects, as their name implies, are flat and disk-shaped, usually closely adhering to the leaf or stem. Actually a soft-bodied insect, scale insects excrete a waxy shield to cover their bodies as they feed. Both mites and scale are a favorite of many common garden predators, including ladybugs and parasitic wasps; failing biological control, applications of horticultural oil, insecticidal soap or pesticides such as rotenone, pyrethrin, malathion and sulfur in the early spring can help control outbreaks. Keeping trees healthy and well-watered is the main preventive control against these pests.