Microbes as Biocontrol Agents

Biocontrol refers to the use of biological methods for controlling plant diseases and pests. It is a natural method of eliminating and controlling insects, pests, and other disease-causing agents by using their natural biological enemies. 

Biocontrol agents are the agents employed for this purpose. These include microbes such as bacteria, fungi, viruses, and protozoans. 

• Chemical insecticides, pesticides, and weedicides are toxic and harmful to human beings and animals. 
• These chemicals pollute soil, groundwater, fruits, vegetables, and crop plants. 
• Biocontrol helps reduce dependence on toxic chemicals and pesticides. 
• In biological farming, pests are not necessarily eradicated; they are kept at manageable levels through natural checks and balances within the ecosystem. 
• A biologically balanced field supports both pests and their natural predators, helping maintain a sustainable system. 

1. Insects as Biocontrol Agents

• Ladybird is useful to get rid of aphids. 
• Dragonflies are useful for getting rid of mosquitoes. 

2. Bacteria as Biocontrol Agents

• Bacillus thuringiensis (Bt) is used against butterfly caterpillars and other insect pests. 
• It is available as dried spores mixed with water and sprayed on vulnerable plants such as Brassicas and fruit trees. 
• These spores are eaten by insect larvae. In the gut of the larvae, the toxin is released, and the larvae are killed. 
• Bt toxin genes have also been introduced into plants, making such plants resistant to insect pests. Bt-cotton is one example. 
• Agrobacterium radiobacter strain K-84 is used to control crown gall disease of stone fruits. 
• Pseudomonas is used to control fungal diseases of plants, such as damping off of cotton seedlings caused by Pythium species. 

3. Fungi as Biocontrol Agents

• Trichoderma species are free-living fungi found in the rhizosphere, the soil surrounding plant roots. 
• They are effective biocontrol agents against several soil-borne and plant pathogenic fungi. 
• They produce substances such as viridin, gliotoxin, and gliovirin, which inhibit pathogens that attack roots and rhizomes and cause root diseases. 
• Other fungal biocontrol agents mentioned include Aschersonia aleyrodis, Beauveria bassiana, Metarhizium anisopliae, and Verticillium lecanii, which are used to control a range of insect pests. 

4. Viruses as Biocontrol Agents

• Viruses are used to control outbreaks of insect pests such as gypsy moth, leaf roller, bud worm, bollworm, and earworm. 
• Baculoviruses, especially those belonging to the genus Nucleopolyhedrovirus (NPV), are important biological control agents against insects and other arthropods. 
• These viruses are species-specific and narrow-spectrum insecticidal agents. 
• They have no negative impact on plants, mammals, birds, fish, or non-target insects. 
• They are especially useful where beneficial insects need to be conserved as part of integrated pest management. 

5. Protozoans as Biocontrol Agents

• Nosema locustae is used against grasshoppers, caterpillars, corn borers, and crickets. 

Bacillus thuringiensis (Bt)

• Bt is a soil bacterium. 
• It produces crystals of a protein called Bt toxin in its endospore form. 
• It is used to control butterfly caterpillars and other insect pests on plants. 
• The toxin is released in the gut of insect larvae after ingestion, leading to their death. 
• It is also used to control mosquito larvae in malaria control. 

Trichoderma

• Trichoderma is a free-living fungus common in root ecosystems. 
• It acts as an effective biocontrol agent against several plant pathogens. 
• It inhibits soil-borne pathogens by producing antifungal substances. 

Baculoviruses

• Baculoviruses attack insects and other arthropods. 
• Nucleopolyhedrovirus and Granulovirus are mentioned as examples. 
• They are suitable for narrow-spectrum insecticidal applications. 
• They do not negatively affect beneficial organisms or ecologically sensitive areas. 

Bioherbicides are organisms used to destroy weeds without harming useful plants. 

Microbial Herbicides and Their Use

Pathogenic fungi as mycoherbicides:

• Phytophthora palmivora controls milkweed in orchards. 
• Alternaria crassa controls water hyacinth. 
• Fusarium species control most weeds. 
• Colletotrichum gloeosporioides is also mentioned as a mycoherbicide used against various weed species. 

Bacterial pathogens as herbicides:

• Pseudomonas species attack several weeds. 
• Xanthomonas species attack several weeds. 
• Agrobacterium species attack several weeds. 

Insects as herbicides:

• Tyria moth controls the weed Senecio jacobeae / Senecio jacobeacii. 
• Cactoblastis cactorum controls cacti weeds. 

• Smother crops - do not allow weeds to grow near them; examples include sweet clover, soybean, and alfalfa. 
• Mycoherbicides - fungal spores sprayed over weeds to kill them. 
• Transgenic plants - genetically engineered plants that develop resistance against herbicides or pests; Bt cotton is mentioned as an example. 
• Predator herbivore - insects used to control weeds, such as Cactoblastis cactorum and Tyria moth. 

• Meaning - Biocontrol agents are microbes that control pests by causing disease, competing with, or killing them, reducing dependence on chemical pesticides.
• Biopesticides - Safer, eco-friendly alternative to chemical pesticides. Key agents: Bacillus thuringiensis (Bt) → caterpillars, beetles; Beauveria bassiana → aphids, mites, white flies; Nosema locustae (protozoa) → grasshoppers, crickets; NPV & Granulovirus → gypsy moths, ants, wasps.
• Bacillus thuringiensis (Bt) - Dried spores mixed with water and sprayed on plants to kill butterfly caterpillars. Most widely used bacterial biocontrol agent.
• Trichoderma - A fungus that acts as a biocontrol agent specifically against soil-borne fungal plant pathogens.
• Microbial Herbicides - Phytophthora palmivora → milkweed; Alternaria crassa → water hyacinth; Fusarium spp. → most weeds; Pseudomonas, Xanthomonas, Agrobacterium → several weeds.
• Insects as Herbicides - Tyria moth controls Senecio jacobeae weed; Cactoblastis cactorum controls cacti weeds.