In crop science, biological control has two distinct meanings. The first involves the deliberate importing of biological control agents to solve weed or pest problems. Classic examples are the control of weed cactus in Australia by insect parasites imported from Mexico, and the control of rabbits with the myxomatosis virus. This form of control can be extremely effective, but it is usually limited to the control of an imported, foreign pest by parasites from that pest’s centre of origin.

The second meaning refers to the use of the normal biological control agents of an indigenous pest or pathogen. These control agents may be predators, hyper-parasites, antagonistic micro-organisms, or organisms that trigger defence reactions in the host. The cultivation of such control agents for release into greenhouses can be an effective technique.

Biological anarchy is the converse of biological control. It is a situation in which biological control has failed, either because the control agents are absent from the area in which a foreign host species is growing, or because the control agents of an indigenous host have been largely destroyed with pesticides. The pest or pathogen then runs riot and can be an infernal nuisance. The best way to restore the indigenous biological control agents is with horizontal resistance. And the best way to enhance the effects of horizontal resistance is to restore the biological controls. The two phenomena are mutually re-enforcing.

Integrated pest management (IPM) is a technique used mainly by entomologists to enhance biological controls. It involves careful monitoring of insect populations in order to reduce insecticide applications to the absolute minimum.