Concepts: Biocontrol agents and biofertilisers

4.1 The trouble with chemicals

Biocontrol means using biological methods to control plant diseases and pests. Modern farming has leaned on insecticides and pesticides, and these chemicals are toxic to human beings and animals. They pollute soil and ground water, and traces remain on fruits, vegetables and crop plants. Weedicides used to clear weeds add to the soil load.

4.2 The biological approach

An organic farmer works with natural predation instead of introduced chemicals. The guiding belief is that variety supports health: a landscape with more kinds of life is more likely to last. So the aim is not to wipe out the insects we call pests but to keep them at manageable levels through checks and balances inside a living ecosystem. Conventional farming often kills useful and harmful life forms together. The biological approach asks the farmer to learn the field's life forms, their life cycles, their feeding patterns and the habitats they prefer.

4.3 Who does the work

The familiar red and black beetle called the ladybird, and dragonflies, help control aphids and mosquitoes respectively. Among microbial agents, the bacterium Bacillus thuringiensis, written Bt, controls butterfly caterpillars. Dried spores of Bt are sold in sachets. Farmers mix the spores with water and spray them over susceptible crops such as brassicas and fruit trees. Larvae feeding on the sprayed leaves take in the material, and the toxin is released in the larval gut, killing them. The bacterial disease kills the caterpillars but leaves other insects unharmed. Genetic engineering has moved the Bt toxin genes into plants, which makes those plants resistant to insect attack. Bt cotton is one such crop, grown in some states of India. The fungus Trichoderma is being developed as a biological control for plant disease. Species of Trichoderma live freely in the soil and are found in large numbers around root systems. They are effective biocontrol agents against several plant pathogens. One species, Trichoderma polysporum, is also the industrial source of cyclosporin A, so the genus earns its keep twice. Baculoviruses are pathogens of insects and other arthropods. Most baculoviruses used as biological control agents belong to the genus Nucleopolyhedrovirus. They attack a narrow range of species, which makes them excellent for species-specific insecticidal use. Tests have shown no harm to plants, mammals, birds, fish or even insects that are not the target. That matters where beneficial insects are protected under an integrated pest management plan, or where a fragile area is being treated.

4.4 Biofertilisers

Chemical fertilisers have raised farm output and added to pollution as well. Organisms that improve the nutrient content of soil are called biofertilisers. Bacteria, fungi and cyanobacteria are the main kinds. | Biofertiliser | Organism | How it helps | |---|---|---| | Symbiotic nitrogen fixer | Rhizobium in root nodules of legumes | Fixes atmospheric nitrogen into organic forms used by the plant | | Free-living nitrogen fixers | Azospirillum, Azotobacter | Fix atmospheric nitrogen while living free in the soil | | Mycorrhiza | Glomus and related fungi | The fungal partner takes up phosphorus and hands it to the plant | | Cyanobacteria | Anabaena, Nostoc, Oscillatoria | Fix atmospheric nitrogen, add organic matter, raise paddy fertility |

Such plants also resist root pathogens better, cope with salinity and drought, and grow more strongly overall. Cyanobacteria are autotrophic microbes found in aquatic and terrestrial environments; many fix atmospheric nitrogen. In paddy fields they serve as a valuable biofertiliser, and these blue green algae enrich the soil with organic matter too. Many biofertilisers are sold commercially now, and farmers use them to replenish nutrients and cut their dependence on chemical fertilisers.

4.5 Distinguish the terms

Biocontrol means controlling pests with living organisms or their products. Biofertiliser means enriching soil nutrients with living organisms. Both reduce dependence on chemicals but they act differently. A common error is to write that biofertilisers kill pests; they do not. Another is to forget that mycorrhiza supplies phosphorus, not nitrogen. In a five-mark answer, give one named example for each named group. Chemical fertilisers add nutrients in a soluble form that rain can wash into water bodies, which is one reason the syllabus pushes biofertilisers. A biofertiliser keeps the nutrient supply tied to a living partner, and that partner often brings a second benefit such as pathogen resistance or drought tolerance. A biofertiliser keeps the nutrient supply tied to a living partner, and the partner often brings a second benefit such as pathogen resistance or drought tolerance.

4.6 Biocontrol or biofertiliser?

Both practices reduce chemical use, but they answer different problems. Biocontrol agents act on pests and pathogens: Bt kills insect larvae, Trichoderma holds back root pathogens, and baculoviruses target specific insects. Biofertilisers act on nutrition: Rhizobium and Azotobacter add nitrogen, Glomus moves phosphorus into the plant, and cyanobacteria add both nitrogen and organic matter to paddy fields. A field may need both, because a pest problem is not a nutrient problem. In an exam answer, keep the two lists separate and give one example from each list before explaining the mechanism.

4.7 What to memorise

Four lists cover most of this section. First, the biocontrol agents: ladybird and aphids, dragonfly and mosquitoes, Bt and caterpillars, Trichoderma and root pathogens, Nucleopolyhedrovirus and insect pests. Second, the biofertiliser groups: symbiotic bacteria, free-living bacteria, mycorrhizal fungi, cyanobacteria. Third, the benefit in one line for each. Fourth, the two definitions, biocontrol and biofertiliser. Then practise writing each benefit in a sentence of your own instead of copying the textbook wording, because the board answer needs meaning rather than a phrase.