Microbes in Human Welfare - Notes | Class 12 | Part 3: Microbes as Biocontrol agents and Biofertilizers

Microbes as Biocontrol Agents and Biofertilizers

Microbes as Biocontrol Agents

Biocontrol is the use of biological methods for controlling plant diseases and pests. For example, ladybird beetles control aphids, and dragonflies control mosquitoes.
Chemical pesticides and insecticides kill both useful and harmful organisms and cause pollution. Biocontrol methods have no such problems.

Bacillus thuringiensis (Bt): Used to control butterfly caterpillars.
- Dried spores of Bt are mixed with water and sprayed onto vulnerable plants such as brassicas and fruit trees. These are eaten by caterpillars, releasing toxin in their gut, which kills the larvae.
- Scientists have introduced B. thuringiensis toxin genes into plants, such as Bt cotton.
Trichoderma sp. (fungus): Free-living fungi present in root ecosystems that control several plant pathogens.
Baculoviruses (especially genus Nucleopolyhedrovirus): Attack insects and other arthropods. Suitable for species-specific, narrow-spectrum insecticidal applications and desirable in Integrated Pest Management (IPM) programs to conserve beneficial insects.

Biofertilizers are organisms that enrich the nutrient quality of the soil, such as bacteria, fungi, and cyanobacteria.
Rhizobium (symbiotic bacteria in root nodules of leguminous plants) fix atmospheric N₂.
Free-living bacteria in the soil, such as Azospirillum and Azotobacter, enrich the nitrogen content of the soil.
Mycorrhiza: Symbiotic association of fungi (e.g., genus Glomus) with plants. The fungus gets food from the plant, and the fungal symbiont performs the following:
- Absorbs phosphorus from soil and passes it to the plant.
- Provides resistance to root-borne pathogens and tolerance to salinity and drought.
- Contributes to overall increase in plant growth and development.
Cyanobacteria (blue-green algae): Autotrophic microbes that fix atmospheric nitrogen, such as Anabaena, Nostoc, and Oscillatoria. In paddy fields, cyanobacteria serve as important biofertilizers, adding organic matter to the soil and increasing its fertility.

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