Killing natural enemies often results in secondary outbreaks of insects and mites. For example, spider mites are often present on plants at low densities but become excessively abundant and cause damage when pesticides applied against other species kill the natural enemies of the spider mites. Here a pesticide applied to kill aphids (Pest A) not only killed aphids but also killed predaceous green lacewing larvae and predatory mites, leading to a secondary outbreak of spider mites (Pest B). Insecticides applied during hot weather appear to have the greatest effect on mites, sometimes causing dramatic mite outbreaks within a few days after spraying.

Illustration by V. Winemiller.