Biological control, less formally called biocontrol, is a group of methods used for controlling pests (including weeds). All these methods have in common the use of living organisms, by people, to control the pests. It includes the use of commercial products (biopesticides) made from pathogens (fungi, bacteria, viruses, etc.). It includes the commercial sale of insectary-reared minute parasitoid wasps for control of greenhouse whitefly. It includes habitat manipulations by farmers and growers (such as deliberate provision of preferred nectar-producing plants) to allow build-up of populations of parasitoids that attack pests.
Biological control uses pathogens, predators, or parasitoids. Predators are animals (including insects) that eat several prey during their lives. An example is the Vedalia beetle (Rodolia cardinalis), a species of ladybird beetle that was imported into California in the last century to control cottonycushion scale on citrus. The beetle larvae, while growing, eat many scales, and the adult beetles also eat many scales. In contrast, a parasitoid destroys only one host during its larval development. An example is a parasitoid fly that lays an egg on a caterpillar, and the fly larva (maggot) that hatches from the egg burrows into the caterpillar and consumes it during a period of many days or weeks; the fully-grown maggot turns into a pupa, and an adult fly emerges from the pupa - the adult fly feeds on plant nectar. Predators, parasitoids, and pathogens as a group are called natural enemies, for want of a better expression. Biological control agent is an expression used for such a natural enemy when it is used in biological control. The pest against which the action is taken is called the target.
Perhaps the best-known form of biological control is classical biological control, which is sometimes called inoculative biological control. It has been used against many pests of foreign origin. It has been seen that some insects (and other organisms) of foreign origin are not or are rarely pests in their homelands, but become major pests where they have immigrated. In such instances, careful search in the homeland has often shown the existence of specialist parasitoids or predators which seem to hold populations in check. However, pests that have arrived from abroad often come initially in very small numbers (for example as contaminants of cargoes) without the specialist natural enemies. Classical biological control tries to find such natural enemies in their homelands and import them to the place to which the pest has immigrated - and this approach has worked spectacularly in many instances. The imported natural enemies may establish populations, spread to neighboring areas, and control the pest at no cost beyond the initial cost of research - there is no commercial product to buy. The following textbook is recommended for people wanting more information about biological control (Van Driesche, R.G., Bellows, T.S. Jr. 1996. Biological Control. New York; Chapman & Hall. 539 pp.).