Topic: Biological Control Agents — Visual Overview
Bacillus thuringiensis (Bt) Mechanism:
Bt spore ingested by lepidopteran larva
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Alkaline midgut (pH 9-10) solubilizes crystal
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Gut proteases cleave protoxin → Active Cry toxin
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Cry toxin binds cadherin receptors on midgut brush border
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Pore formation in midgut membrane
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Osmotic imbalance → Cell lysis → Larval death
Reference Image — Bt Crystalline Inclusion:
Figure: Phase-contrast micrograph of Bacillus thuringiensis showing spores (oval, bright) and adjacent diamond-shaped crystalline protein inclusions (parasporal crystals containing Cry protoxin). Both structures are released upon sporulation.
Comparative Summary:
| Agent | Type | Target | Mechanism | Specificity |
|---|---|---|---|---|
| Bacillus thuringiensis (Bt) | Bacterium (Cry protein) | Lepidoptera larvae | Receptor-mediated gut cell lysis | High (order-specific) |
| Nucleopolyhedrovirus (NPV) | Baculovirus | Specific insect species | Viral replication → cell death | Very high (species-specific) |
| Trichoderma spp. | Fungus | Soil fungal pathogens | Mycoparasitism + antibiosis | Moderate (targets fungi) |
| Ladybird beetles | Insect (predator) | Aphids | Direct predation | Moderate (broad aphid range) |