Microbes as Biocontrol Agents & as Biofertilizers | Biology Class 12 - NEET PDF Download

Microbes or microorganisms are tiny single-celled or simple multicellular organisms that cannot be seen with the naked eye. Some microbes are harmful and cause disease, while many are beneficial and play essential roles in agriculture and the environment. This chapter explains how microbes are used as biocontrol agents to manage pests and diseases, and as biofertilizers to improve soil fertility and plant growth.

What is Biocontrol?

Biocontrol (biological control) is the use of living organisms or their products to reduce the population of pests and pathogens and to limit damage caused by them. Biocontrol relies on natural ecological relationships such as predation, parasitism, antibiosis and competition. Compared to chemical pesticides, biocontrol is usually more specific, environmentally safer and less likely to cause long-term harm to soil health and human health.

Biocontrol Agents

Biocontrol agents are organisms used to reduce pests or disease incidence in crops. These include predatory insects (for example, ladybirds for aphids, dragonflies for mosquitoes), parasitic wasps, nematodes, and a wide range of microbes (bacteria, fungi, viruses and protozoa). Successful use of biocontrol agents requires knowledge of the pest's life cycle, ecology and the interaction between pest, host and control agent.

Microbial Biocontrol Agents

Microbes are important biocontrol agents and act in several ways:

• Parasitism / infection: Microbes infect and kill pests or pathogens (for example, entomopathogenic fungi infect insect pests).
• Antibiosis: Microbes produce antibiotics or toxins that inhibit or kill other microbes or pests.
• Competition: Beneficial microbes compete with pathogens for nutrients and space, reducing pathogen establishment.
• Predation or direct attack: Some microbial predators or hyperparasites attack pests or their eggs.
• Induced resistance: Certain microbes stimulate the plant's own defence systems, making plants less susceptible to pathogens.

Common microbial biocontrol agents and examples:

• Bacillus thuringiensis (Bt): A bacterial species whose spores contain protein toxins (cry toxins). When insect larvae ingest Bt spores on plant surfaces, the toxins act on the gut epithelium and kill the larvae. Bt formulations are sprayed on crops; genes from Bt have also been introduced into some crop varieties (for example, Bt cotton) to confer insect resistance.
• Entomopathogenic fungi: Beauveria bassiana, Metarhizium anisopliae and Paecilomyces infect and kill many insect pests by penetrating the cuticle and growing inside the insect body.
• Trichoderma spp.: Fungi used against fungal pathogens in soil and on roots. They act by mycoparasitism, competition and production of antifungal compounds, and are widely used to control root rots and wilts.
• Baculoviruses (Nucleopolyhedrovirus): Viruses that infect specific insect pests (often caterpillars). They are highly specific, often infecting only a narrow range of insects, and are used as biological insecticides.
• Pseudomonas fluorescens and Bacillus subtilis: Bacterial antagonists that suppress soil-borne pathogens by antibiosis and competition, and can stimulate plant growth.

Advantages and Limitations of Microbial Biocontrol

• Advantages: environmentally friendly; target-specific (less harm to non-target organisms); reduce chemical pesticide residues; compatible with integrated pest management (IPM).
• Limitations: some agents are specific to certain pests and cannot control a wide spectrum of pests; effectiveness can be affected by environmental conditions (temperature, humidity, UV light); development, mass production and formulation of microbial agents require expertise and cost; storage and shelf-life of live agents require care.

Biofertilizers

"Biofertilizers are substances that contain living microorganisms which, when applied to seeds, plant surfaces or soil, colonise the rhizosphere or the interior of the plant and promote growth by increasing the supply or availability of primary nutrients to the host plant."

What is a Biofertilizer?

Biofertilizers contain beneficial microbes that increase the nutrient availability for plants. These living organisms include nitrogen-fixing bacteria, phosphate-solubilising bacteria and fungi, cyanobacteria (blue-green algae), arbuscular mycorrhizal (AM) fungi, and others. They improve soil fertility naturally and reduce dependence on chemical fertilisers.

Nitrogen fixation is a key biofertilizer function: certain microbes convert atmospheric di-nitrogen (N2) into ammonia and other usable forms of nitrogen for plants. Other microbes solubilise phosphates, mineralise organic matter, or enhance uptake of nutrients and water by roots.

Types of Biofertilizers

1. Symbiotic Nitrogen-Fixing Bacteria

Rhizobium forms nodules on the roots of leguminous plants. Inside these nodules, Rhizobium fixes atmospheric nitrogen into forms usable by the plant. This is a mutualistic symbiosis: bacteria receive carbohydrates and shelter from the plant, while the plant receives fixed nitrogen.

Rhizobium

2. Associative or Loose Association Nitrogen-Fixers

Azospirillum associates with the rhizosphere of many non-leguminous plants. It does not form nodules but lives close to roots, utilises root exudates and contributes to nitrogen supply and growth stimulation. This relationship is often called associative mutualism.

3. Symbiotic Nitrogen-Fixing Cyanobacteria

Cyanobacteria (blue-green algae) form symbiotic associations with certain plants and lower plants. Examples include Anabaena in association with fern and Azolla (a water fern) in rice fields. These cyanobacteria fix atmospheric nitrogen and release nitrogenous compounds upon decomposition, thereby enriching the soil.

4. Free-living Nitrogen-Fixing Bacteria

Free-living soil bacteria such as Azotobacter and some anaerobic bacteria like Clostridium can fix nitrogen independently in the soil. They do not form associations with plant roots but contribute to the pool of available nitrogen.

5. Phosphate-Solubilising Microorganisms (PSMs)

Certain fungi and bacteria solubilise insoluble phosphorus compounds in the soil and make phosphorus available to plants. Common examples include Penicillium, Aspergillus, Bacillus and Pseudomonas.

6. Mycorrhizal Fungi

Arbuscular mycorrhizal (AM) fungi (for example, Glomus spp.) form mutualistic associations with plant roots. They increase the effective surface area for water and nutrient absorption (particularly phosphorus) and improve plant resistance to stress.

Among these, Rhizobium and Azospirillum are widely used in practical agriculture as biofertilizers.

Components and Common Preparations of Biofertilizers

Biofertilizer products are typically formulated as carrier-based preparations where a carrier material supports the survival of the beneficial microorganism until application. Common components and preparations include:

• Bio-compost: Decomposed organic waste (for example, from sugar industries) enriched with beneficial microbes; used to improve soil organic matter and microbial activity.
• Tricho-Card: A Trichoderma-based product that is non-pathogenic and used across many crops. It acts as an antagonist and hyperparasite against several fungal pathogens and insect egg stages.
• Azotobacter preparations: Carrier-based products containing Azotobacter strains that colonise the rhizosphere, protect roots from pathogens and fix atmospheric nitrogen.
• Phosphate-solubilising formulations: Contain microbes that hydrolyse or solubilise insoluble phosphorus compounds to release soluble phosphate for plant uptake.
• Vermicompost: Organic fertilizer produced by earthworms; rich in nutrients, hormones, vitamins and beneficial microbes that enhance soil fertility and plant growth.

Importance of Biofertilizers

Biofertilizers are important for sustainable agriculture for the following reasons:

• They improve soil structure and increase soil fertility and crop yield.
• They suppress soil-borne pathogens and reduce disease incidence.
• They are eco-friendly, renewable and cost-effective compared to chemical fertilisers.
• They reduce environmental pollution and chemical residues in food and soil.
• They can detoxify or reduce harmful substances in the soil.
• They remain effective under a range of conditions, including semi-arid regions, when managed properly.

Applications and Methods of Using Biofertilizers

Biofertilizers can be applied in different ways depending on crop and local practice:

• Seed treatment: Seeds are coated with a biofertilizer slurry (for example, nitrogen- or phosphate-fixing microbes) and sown soon after drying. This ensures early root colonisation by beneficial microbes.
• Seedling root dip (root dipping): Seedlings, particularly rice seedlings, are floated or dipped in a microbial suspension for several hours before transplanting to allow root colonisation.
• Soil treatment or soil application: Biofertilizer formulations (often mixed with compost) are applied to the soil at or before sowing so that microbes can colonise the rhizosphere.

Quality, Storage and Practical Considerations

To be effective, biofertilizer products must contain viable and appropriate strains of microbes in sufficient numbers. Practical points to ensure effectiveness:

• Use certified products from reliable sources and follow label instructions for application rates and methods.
• Carrier-based formulations (using materials such as peat or other organic carriers) help maintain viability; protect products from extreme heat, direct sunlight and moisture during storage.
• Apply biofertilizers under favourable soil moisture and temperature conditions for microbial activity.
• Combine biofertilizers with good crop management (organic matter addition, appropriate irrigation and soil pH) for best results.
• Biofertilizers work best as part of an integrated nutrient and pest management strategy rather than as a sole input.

Summary

Microbes play dual beneficial roles in agriculture: as biocontrol agents they reduce pests and diseases using natural biological mechanisms, and as biofertilizers they increase nutrient availability and improve soil health. Using microbial solutions reduces dependence on chemical pesticides and fertilisers, supports sustainable farming and helps maintain ecological balance. Proper selection, formulation and application are essential for achieving consistent benefits in the field.