The use of parasitoids, predators, and pathogens to reduce injurious pest insect populations and consequently the damage they cause. Viruses and bacteria are the most commonly used pathogens, but fungi, protozoa, and nematodes may also be important biological control agents.

Three ecological assumptions underlie biological control. First, natural enemies are among the prime factors responsible for the regulation, or control, of pest populations. Second, the influence exerted by parasitoids, predators, and pathogens is density-dependent. Density dependence means that the killing power of the natural enemy increases as the prey or host density increases. Conversely, the mortality induced by density-dependent natural enemies decreases as host density increases. In the dominant, or classical, form of biological control the third assumption is found: when an insect species escapes into a new area without its natural enemies, it reaches outbreak levels and becomes a pest. Biological-control practitioners believe regulation can be reestablished by importing the natural enemies of the pest from its area of origin.

In classical biological control, all efforts are typically directed toward establishing the natural enemies that were left behind in the area of origin. Classical biological control is by far the most frequently used form, assuming one excludes the use of resistant plant varieties as biological control.

Conservation involves manipulation of the environment in order to favor survival, reproduction, or any other aspect of the natural enemy's biology that affects its function as a biological control agent.

Aspects of research on and application of biological control may provide new or improved approaches. The improvement of biological control agents through selection, hybridization, or genetic engineering techniques may play an important role. A major strategy for control of pest insects may involve the use of genetic engineering to introduce traits into natural enemies that enhance their performance, or mortality-causing traits of natural enemies, such as insect pathogens, into plants. Breeding (plant) Genetic engineering