Causes of insect outbreaks and plant injury

By The Old House Web

Causes of insect outbreaks and plant injury

Trees and shrubs are often planted in unfavorable environments that predispose them to insect damage. For example, the pine needle scale is seldom destructive in the forest, where it is found in low population densities, but in nursery and landscape situations, the same insect often reaches outbreak proportions and can ruin the aesthetic value of the plants and even kill them. The unnatural conditions in the man-made environment induce unusually high and destructive insect populations.

Despite quarantine regulations, many foreign insect species have been accidentally introduced into the United States. Away from their natural environment, many of these become pests. The black vine weevil, the European pine shoot moth, the gypsy moth and the mimosa webworm are examples of introduced species that have become serious problems to ornamental plants. In this country, these insects became pests when they found an abundant food supply, a favorable climate and few or no effective natural enemies.

Many ornamental trees and shrubs are chosen for their beauty and planted without proper consideration of their suitability for a particular site. Some trees and shrubs planted in the north central states are native to more southern climates and become severely stressed because of winter injury. Examples of these plants include southern dogwood and azaleas. Winter injury can make these plants susceptible to attack by insects.

Most of the commonly used shade trees are native species from the forest that have been moved into the city. These trees, while visually healthy, are often in a low state of vigor because of exposure to full sunlight and moisture stress. Insect pests of native species always abound in neighboring woodlots, but there they are usually only secondary problems. In weakened shade trees, the same insects may build up to damaging levels.

Many of our popular, widely used ornamental trees and shrubs are asexually propagated varieties and clones of identical genetic composition. Strains of insects can quickly develop and adapt to such genetically homogeneous cultivars. For example, the Sunburst honey locust clone is more susceptible to attack by the mimosa webworm than other commonly planted clones of the same species.

Drought stress or nutrient stress may also weaken trees and shrubs and make them more susceptible to insect attack. Birch trees adequately watered and fertilized are less likely than stressed trees to be attacked by bronze birch borer. Proper watering and fertilizing of Euonymus and Taxus plants allow them to compensate for root pruning by root weevils, while stressed shrubs may be seriously injured by the same amount of root pruning.

Sudden outbreaks of aphids, spider mites and scale insects may occur when pesticides wipe out their natural enemies. Ladybird beetles, lacewings, predaceous mites, parasitic wasps and other natural enemies devour or parasitize aphids, scales and mites, keeping populations under control. Some insects are more tolerant of insecticides than their natural enemies are. Applications of insecticides may stimulate a rapid outbreak or a delayed resurgence of mites and aphids, depending on the relative toxicity of the pesticide applied. Outbreaks of scale insects, leaf miners and other insects protected inside galls or under waxy secretions are frequently associated with pesticide application because the pests are physically protected from insecticides but their natural enemies are not. (Natural enemies of all plant feeding insects are adversely affected by pesticides. Aphids, scale insects and mites have been used as examples because they reproduce rapidly and have the most obvious outbreaks associated with pesticides.)

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