Microbes as Biocontrol Agents — Revision Notes

⚡ 30-Second Revision

Biocontrol: — Using living organisms to control pests.
Advantages: — Eco-friendly, specific, less resistance, no residues.
***Bacillus thuringiensis* (Bt): Bacterium, produces Cry proteins (toxins). Targets lepidopteran larvae (caterpillars), dipteran (mosquitoes), coleopteran (beetles). Mechanism: Toxins activated in alkaline insect gut**, bind to midgut cells, cause pores, death.
***Trichoderma* species: Free-living fungi. Targets soil-borne fungal plant pathogens** (e.g., damping-off, root rot by *Pythium*, *Rhizoctonia*). Mechanism: Mycoparasitism, antibiosis, competition, ISR.
Baculoviruses (Nucleopolyhedrovirus): — Viruses. Targets lepidopteran larvae. Key feature: High host specificity, safe for non-target organisms.

2-Minute Revision

Microbes as biocontrol agents offer a sustainable alternative to chemical pesticides, harnessing natural biological interactions to manage pests. The core principle is to use beneficial microorganisms to suppress harmful insects, plant pathogens, and weeds. This approach is environmentally friendly, highly specific, and reduces the risk of pest resistance, leaving no toxic residues.

Key examples include:

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***Bacillus thuringiensis* (Bt):** A bacterium that produces insecticidal protein crystals (Cry proteins). When ingested by susceptible insect larvae (like caterpillars), these protoxins are activated in the alkaline gut, creating pores in the midgut lining, leading to death. Different Bt strains target different insect orders.
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***Trichoderma* species:** These are free-living fungi found in soil. They act as biofungicides against soil-borne plant pathogens (e.g., *Pythium*, *Rhizoctonia*) through mycoparasitism (attacking other fungi), antibiosis (producing inhibitory compounds), and competition for resources.
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Baculoviruses: — Specifically, Nucleopolyhedrovirus, are highly host-specific viruses that infect and kill insect pests, primarily lepidopteran larvae. Their narrow host range makes them safe for non-target organisms, making them ideal for Integrated Pest Management (IPM). Remember the specific microbe, its target, and its basic mechanism for NEET.

5-Minute Revision

Microbes as biocontrol agents represent a crucial shift towards sustainable agriculture, moving away from the detrimental effects of chemical pesticides. This method utilizes living microorganisms to control pest populations, including insects, plant pathogens, and weeds. The primary advantages are their eco-friendliness, high specificity to target pests, reduced likelihood of pest resistance, and the absence of harmful chemical residues on food or in the environment.

Let's delve into the key examples:

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***Bacillus thuringiensis* (Bt):** This soil bacterium is a powerful bioinsecticide. During sporulation, it produces protein crystals containing insecticidal toxins called Cry proteins. When a susceptible insect larva (e.g., a caterpillar) ingests these crystals, the alkaline pH in its gut solubilizes the crystals, releasing protoxins. These protoxins are then activated by gut proteases into active toxins. The active toxins bind to specific receptors on the midgut epithelial cells, forming pores that disrupt the gut integrity, leading to paralysis of the digestive system, septicaemia, and ultimately, the death of the insect. Different Bt strains are specific to different insect orders: Bt *kurstaki* for lepidopterans, Bt *israelensis* for dipterans (mosquitoes), and Bt *tenebrionis* for coleopterans (beetles). Its gene can also be engineered into crops (Bt cotton).

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***Trichoderma* species:** These are common, free-living fungi found in soil and root systems. They are highly effective biofungicides against a broad range of soil-borne fungal plant pathogens, such as *Pythium*, *Rhizoctonia*, *Fusarium*, and *Sclerotium*, which cause diseases like damping-off and root rot. *Trichoderma* employs multiple mechanisms: mycoparasitism (directly attacking other fungi by coiling around their hyphae and secreting lytic enzymes like chitinases), antibiosis (producing inhibitory secondary metabolites), and competition for nutrients and space in the rhizosphere. They can also induce systemic resistance (ISR) in plants.

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Baculoviruses: — These are insect-specific viruses, with Nucleopolyhedrovirus (NPV) being a prime example used in biocontrol. Baculoviruses exhibit extremely high host specificity, meaning they infect and kill only a very narrow range of insect hosts, primarily lepidopteran larvae. This specificity is a major advantage, as they do not harm non-target organisms like beneficial insects (e.g., honeybees, ladybugs), birds, mammals, fish, or plants. When ingested, the viral particles replicate within the insect, leading to its death and liquefaction, releasing more virions to infect other pests. Their safety profile makes them ideal for Integrated Pest Management (IPM) programs, especially in ecologically sensitive areas.

For NEET, focus on the specific names, their target pests/diseases, and the core mechanism of action for each. Understand the comparative advantages of biocontrol over chemical methods.

Prelims Revision Notes

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Biocontrol Definition: — Using living organisms (microbes, insects) to control pests (insects, weeds, plant pathogens).
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Advantages of Microbial Biocontrol:

* Eco-friendly: No chemical pollution of soil, water, air. * Highly Specific: Targets only harmful pests, sparing beneficial organisms (e.g., pollinators, natural predators). * Reduced Resistance: Pests less likely to develop resistance due to complex modes of action. * No Residues: No toxic chemical residues on food or in the environment. * Sustainable: Promotes long-term ecological balance.

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*Bacillus thuringiensis* (Bt):

* Type: Bacterium (Gram-positive, spore-forming). * Mechanism: Produces Cry proteins (insecticidal crystal proteins/toxins) during sporulation. * Ingested by insect larvae $\rightarrow$ activated in alkaline gut $\rightarrow$ binds to midgut cells $\rightarrow$ forms pores $\rightarrow$ disrupts gut $\rightarrow$ death.

* Target Pests: Specific insect larvae (Lepidopteran - caterpillars; Dipteran - mosquitoes; Coleopteran - beetles). * Application: Sprays, or genetically modified (GM) crops (e.g., Bt cotton, Bt corn).

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*Trichoderma* species:

* Type: Free-living fungi, common in soil and root ecosystems. * Mechanism: Biofungicidal action through: * Mycoparasitism: Directly attacks other fungi (coils around hyphae, secretes lytic enzymes like chitinases).

* Antibiosis: Produces inhibitory compounds/antibiotics. * Competition: Competes for nutrients and space in the rhizosphere. * Induced Systemic Resistance (ISR): Enhances plant's natural defenses.

* Target Pests: Soil-borne fungal plant pathogens (e.g., *Pythium*, *Rhizoctonia*, *Fusarium* causing damping-off, root rot). * Application: Seed treatment, soil amendment.

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Baculoviruses (Nucleopolyhedrovirus - NPV):

* Type: Viruses, primarily infect insects and arthropods. * Mechanism: Ingested viral particles replicate within insect cells $\rightarrow$ systemic infection $\rightarrow$ death. * Key Feature: High host specificity (narrow spectrum).

Does not harm non-target organisms (beneficial insects, birds, mammals, fish, plants). * Target Pests: Primarily lepidopteran larvae (caterpillars). * Importance: Excellent for Integrated Pest Management (IPM) due to safety and specificity.

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Comparison with Chemical Pesticides: — Microbial agents are slower but safer, more specific, and lead to less resistance.

Vyyuha Quick Recall

Be Tiny, Target Lepidopterans, Thrive in Soil, Virus Specific!

Be Tiny: Refers to Bt (*Bacillus thuringiensis*), a bacterium.
Target Lepidopterans: Bt's primary target is Lepidopteran larvae (caterpillars).
Thrive in Soil: Refers to Trichoderma, a fungus found in soil.
Soil: Trichoderma targets Soil-borne fungal diseases.
Virus Specific: Refers to Viruses (Baculoviruses), which are highly Species-specific.