Role of Microbes as Biocontrol Agents

Biocontrol, or biological control, refers to the use of biological methods — rather than synthetic chemicals — to control plant diseases and pests. The underlying idea is simple: many insects, pests and other disease-causing agents already have natural biological enemies in the wild, and biocontrol works by deliberately introducing or encouraging those natural enemies (biocontrol agents) against a target pest. This matters because chemical insecticides and pesticides, while effective, are extremely harmful both to human health and to the wider environment; using biocontrol measures instead greatly reduces how dependent agriculture needs to be on those toxic chemicals.

Microbial biocontrol agents work in one of three ways: they may directly cause disease in the target pest, they may compete with it for resources or space, or they may kill it outright. Four broad groups of organisms serve as microbial biocontrol agents — bacteria, fungi, viruses and protozoans — and the chapter gives worked examples from two of these groups. Bacillus thuringiensis (Bt) is used against caterpillars of the butterfly/moth group: dried Bt spores are mixed with water and sprayed onto vulnerable crops such as Brassicas and fruit trees, the insect larvae then eat the spores while feeding, and once inside the larval gut the bacterium releases a toxin (the Cry protein) that kills the larva. Trichoderma species are free-living fungi found naturally in the rhizosphere (the root ecosystem) and are effective against several soil-borne fungal plant pathogens; they work by producing inhibitory substances such as viridin, gliotoxin and gliovirin, which suppress the other soil-borne fungi that would otherwise attack roots and rhizomes and cause rot disease. In terms of documented host relationships: the bacteria Bacillus thuringiensis, B. papilliae and B. lentimorbus target caterpillars, cabbage worms and adult beetles; the fungi Beauveria bassiana, Entomophthora pallidaroseum and Zoophthora radicans target aphids, mealy bugs, mites and whiteflies; the protozoan Nosema locustae targets grasshoppers, caterpillars and crickets; and the viruses Nucleopolyhedrovirus (NPV) and Granulovirus (GV) target caterpillars, gypsy moths, ants, wasps and beetles.

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