Use of micro-organisms to control insect pests, pathogens or weeds constitutes biological control, and the biological agents employed are called biocontrol agents. Micro-organisms used for biocontrol are bacteria, viruses, fungi and protozoa; some of them are being used at commercial scales. Biocontrol agents have been used to control mostly insect pests, and occasionally for weed and disease control.
Bioinsecticides
Micro-organisms used for insect control are often called Bioinsecticides, while the term biopesticides is used for all biocontrol agents.
Viruses, bacteria, fungi, protozoa and mites are employed to control a variety of insects attacking both plants and animals. Although a large number of micro-organisms attack insects, only a limited number of them have been found commercial application. The technology for production and application of biopesticides has been developed in India, and a Bacillus thuringinesis based insecticide is being commercially produced.
Bacillus thuringinesis is a spore forming bacterium, which produces a crystal protein as parasporic inclusion in the vegetative cell containing the spore. The crystal protein is a protoxin which is processed into a toxin by the proteases present in insect mid-gut. The toxin fragment binds to highly ultimately causes insect death. Several membranes of gut epithelium cells; this binding ultimately causes insect death. Several different types of crystal proteins are produced by different strains of bacterium, each protein having a specific and ordinarily narrow insect host range. This highlights a very important feature of biopesticides: they are specific to a well-defined range of target species. This is in contrast to the chemical pesticides which usually affect a very wide range of target species. This property must be well appreciated by the user of a biopesticides.
B. thuringinesis cells are produced in fermenters, as are other bacteria and fungi. The commercial preparations usually contain a mixture of spores, crystal proteins and inert carriers. The formulation may be in the form of a water dispersible powder, wettable powder, emulsifiable concentrate, flowable concentrate, granules or dust. It may be mixed with chemical insecticides or fungicides, if required. The crystal protein activity disappears usually within 24-40 hr after application, but the spores may persit for long periods and cause harm to nontarget useful insects, e.g. silkworm. This problem is sought to be overcome by developing mutants which produce the crystal protein but do not form spores. Such mutants have been isolated by Indian scientists, and their usefulness as biopesticides is being evaluated.
Bioherbicides
Fungal pathogens are attractive biocontrol agents for weed control in view of their host specificity and ease in production and inoculation in the field where, once established, they will spread on their own. Atleast two fungal pathogens have achieved commercial status in U.S.A.
Disease control
Biocontrol of plant diseases employs both bacteria and fungi, and at least three commercial examples are available. In India, the technology for large scale production of Trichoderma and its use for control of soil-borne fungal pathogens like Macrophomina phaseolona etc. and for seed treatment has been developed; efforts are being made for its commercialization.
Advantages and limitations
The interest is biopesticides is based in the advantages associated with chemical pesticides, some of which are as follows: (i) extensive pollution of the environment, (ii) serious health hazard due to the presence of their residues in food, fiber and fodder, and (iii) increasing cases of insects developing resistance, e.g. Helicoverpa (Heliothis) has become resistant to most of the insecticides.