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. This information is for educational purposes only. References to commercial products or trade names does not imply endorsement by MSU Extension or bias against those not mentioned. This information becomes public property upon publication and may be printed verbatim with credit to MSU Extension.