Reliance is placed on naturally occurring parasites and predators and diseases to control insect and mite pests.
Natural control works much of the time.
Sometimes, long-term weather conditions or some other factors that are not obvious allow pests to escape from the natural controls so that they are able to cause widespread, spectacular damage. The elm spanworm, for example, defoliated hundreds of thousands of acres of woodlands in the late 1950's in western North Carolina. Most urban landscapes are not amenable to the basic concept of natural control. Natural control is best suited to wilderness areas or large tracts of mixed woodlands or forests. These areas sometimes attract pine voles and meadow mice which attack the bark of ornamental shrubs and consume flower bulbs.