Introduction to Microbial Biocontrol Agents:
Biocontrol, also known as biological control, is the natural process of removing and managing insects, pests, and other disease-causing agents by utilizing their natural, biological enemies.
Living microorganisms known as microbial biocontrol agents (MBCAs) are employed to suppress plant diseases and pests, providing a more environmentally friendly method of managing pests and diseases than conventional pesticides. Microbes including fungi, bacteria, yeast, and viruses have a lot of potential to be used as biocontrol agents in place of traditional chemical-based pesticides. Microbial bio-pesticides are used in integrated crop management (ICM) methods all over the world and are significant in crop protection.
Mechanisms of Action of Microbial Biocontrol Agents:
- Enhances resistance of plants against pathogens
- Enhances resistance of plants against pests
- Produces antimicrobial antibiotics against pathogens
- Helps plants cope up with subsequent infection by pathogens
- Acting as hyperparasites to control pathogen growth
- Helps better absorption of nutrients in soil
Fungal biocontrol agent against plant pathogen:
Trichoderma spp
Trichoderma spp mostly works against soil-borne plant pathogens (especially fungi) eg: Fusarium oxysporum, Alternaria alternata by producing antibiotics, parasitizing fungal hyphae and inducing systemic resistance in plants (enhancing defense capacity in plants without direct pathogen attack). Some Trichoderma strains produce compounds with herbicidal activity, meaning they can kill or inhibit the growth of weeds.
Beauveria bassiana
Beauveria bassiana works against fungal disease in honeybees. It mainly works against pests but some study show it works against fungi as well.
Several fungi are being investigated and used as biocontrol agents against fungal diseases in honeybees. Beauveria bassiana is a prominent example of entomopathogenic fungi (fungi that kill insects) that have shown promise in controlling pests like the Varroa destructor mite, which can weaken bee colonies. These fungi are applied to the bees or their environment, and when the mites come into contact with the fungal spores, they can become infected and die.
Penicillium citrinum
Penicillium citrinum helps plant defend abiotic stress like drought and extremely high temperature by enhancing plant’s tolerance to environmental stress. It produces plant growth hormones like auxin, gibberlin and cytokinin for plant growth. It also helps in nutrient absorption, availability of secondary metabolites and water retention.
Candida oleophila
It is a yeast biocontrol agent that works against post-harvest fungal pathogens like Penicillium expansum, Botrytis cinerea on fruits and vegetables. It does so by producing chitinase, glucanase degrading fungal cell wall. Candida oleophila can outcompete harmful fungi for resources like nutrients and space on the fruit’s surface.
Viral biocontrol agent:
Baculovirus
Baculovirus is used against pests (larva of moth, butterfly, flies, wasps). Larva digests baculovirus. It multiplies in mid gut of insect and spread in whole body. Death occurs. Viruses are released as insect decays and infects new larva
Bacterial biocontrol agent:
Bacillus thuringiensis (Bt)
Bacillus thuringiensis controls mosquito larva in aquatic environment. It is ingested by insect pests. Bt toxin (Cry protein) activates and punches hole in gut of insect. Pest dies due to starvation. moths and butterflies, beetles, and flies, mosquito
Pseudomonas
It is a common genus of phosphate solubilizing bacteria. It converts organic phosphates into hydrogen phosphate and dihydrogen phosphate which are soluble forms. It secretes organic acids and lowers pH to dissolve bound phosphates.
Rhizobium
Rhizobium helps in nitrogen fixation by symbiotic relationship with rood nodules of leguminous plants. These nodules houses Rhizobium, which convert atmospheric nitrogen into usable form of nitrogen that plants can use.
Azospirillum spp, Azotobacter spp, Frankia spp are free-living bacteria that aid in nitrogen fixation.
Bacillus subtilis
It is commonly used in agriculture as biopesticide. Bacillus subtilis is used as a bio-pesticide in agriculture because it can effectively control plant diseases by colonizing plant roots and competing with or suppressing disease-causing fungi. It also promotes plant growth and enhances stress tolerance, reducing the need for chemical pesticides and fertilizers. Bacillus subtilis, particularly strain GBO3, colonizes the developing root system of plants, competing with and suppressing harmful fungi like Rhizoctonia spp, Fusarium spp, and Aspergillus spp.
Bacillus pumilis
It is known for its ability to degrade pollutants like oil contaminant and industrial wastes. It produces Pumilacidins that kills Fusarium spp, Pseudomonas syringae etc. It has oxido reductase. It can solubilize phosphate but not as potent as Bacillus subtilis.
Nematode-Based Biocontrol Agents:
Steinernema feltiae
It is an entomopathogen that works against nematode pests in agriculture and insects. Steinernema feltiae nematodes are typically applied in their infective juvenile (IJ) stage, which is a non-feeding, highly mobile stage specialized for finding and infecting hosts. These nematodes actively search for their prey, using chemical and temperature cues to locate insect larvae, including those of fungus gnats, thrips, and other pests. Once a nematode encounters a suitable host, it enters through natural openings like the mouth, anus, or spiracles. Inside the host, the nematode releases symbiotic bacteria (usually Xenorhabdus spp) from its gut. These bacteria multiply rapidly within the host’s body, releasing toxins and causing septicemia (blood poisoning), which rapidly kills the host within 24-48 hours.