# Biological control

Biological control is the utilization of antagonistic alive organisms which are natural enemies against different pests (like Nematodes, insects, fungi, bacteria, vertebrate animals). Biological control usually involves the freedom and propagation of alive organisms into habitats of the population (Rey, et al., 2000).
Some insect species threaten human welfare by transmitting disease, reducing agricultural productivity or damaging forests. If insect population density exceeds an arbitrary density level (the economic injury level) above which the insect interferes with health, comfort, convenience or profit, then the insect is considered a pest. In those circumstances, management efforts must be undertaken (Rechcigl & Rechcigl, 2000).
Humans have attempted to eradicate, control or manage these pests using several chemical, biological, cultural, and mechanical control methods. Biological control is the deliberate exploitation of a natural enemy for pest control (DeBach & Rosen, 1991). This can be accomplished either through (1) importation of exotic enemies (i.e. classical biological control), or (2) conservation or (3) augmentation of enemies that are already in the environment. DeBach (1964) considered conservation as an environmental modification to protect and enhance natural enemies. Augmentation involves mass-rearing natural enemies in the laboratory and releasing them into the environment.
The definition of alive organisms includes predators, parasitoids, nematodes, fungi, bacteria, protozoa and viruses (Eilenberg, 2006). Natural enemies are classified into three major groups: predators, parasitoids or pathogens (Sunderland et al., 1997). Most predators feed on a wide variety of prey, consume many of them throughout their life and rapidly devour all or most of their prey. Predators include representatives from most of the major orders in the class Insecta.
A parasitoid is an organism that spends a significant portion of its life attached to or within a single host organism, and it sterilizes or kills, and sometimes consumes, the host (Godfray, 1994). A parasitoid ultimately always kills its hosts, whereas a true zoological parasite (e.g., tapeworm) does not. Almost all of the major parasitoid species occur in the insect orders Hymenoptera (wasps) and Diptera (flies). Generally, parasitoids have a narrow host range, feed on only one host throughout their life span, feed on their host only during their immature stage, and are free-living as adults (Sabelis, 1992).
The pathogens used for biological control of insects include bacteria, fungi, viruses, protozoans, and nematodes. Naturally occurring pathogens commonly attack insects, causing illness and sometimes death. Often the sub-lethal effects of pathogens can alter insect behavior to prevent insect reproduction (Rechcigl and Rechcigl, 2000).
The purpose of biological control is not to eliminate the pest, it is to keep the pest population under a threshold where they cannot damage the host population. These biological control agents do not eliminate the target organisms, but they reduce their population density (Larsen, 2000).
Biological control has two fundamental ecological principles:
* An organism may be used to control another organism.
* The control organism has a reduced range of hosts.
Host range refers to a target species group on which control organisms may feed and develop. The use of biological control requires a control organism which has the capability to survive, to reproduce, to disperse and to develop into the environment where it is used, without damaging the rest of the organisms. Biological control is a safe and effective practice; an effective technology that benefits the environment and is capable of self-sustaining during long periods without cost interventions (Mc Evoy, 1996).
Biological control may be applied to viruses, bacteria, nematodes, insects, acari, scrub and pest vertebrates which invade several environments and exert a negative influence on the natural source, agriculture, stored products and urban environments (Charudattan et al., 2006).