Biocontrol Biofertilisers

10.10 Microbes as Biocontrol Agents

Modern agriculture's heavy reliance on chemical pesticides and herbicides has caused environmental contamination, biodiversity loss, and human health problems. Biocontrol refers to the use of biological methods — pest-eating organisms or pest-killing microbes — for controlling plant diseases and pests, replacing or reducing chemical pesticides.

Biocontrol uses (i) predators (ladybird beetles, dragonflies), (ii) parasites (parasitic wasps), and (iii) microbial agents — the focus of this section.

Bacillus thuringiensis (Bt) — the Insect-Killer Bacterium

Bacillus thuringiensis (Bt) is a bacterium that produces crystalline protein toxins (Cry proteins) during sporulation. These toxins, when ingested by certain insect larvae (butterflies, beetles, flies), dissolve in the alkaline midgut and kill the larva by perforating its gut.

How Bt is used:

• Spore preparations are sprayed on Brassica, fruit trees, etc. The larvae of butterflies and moths that eat these leaves are killed.
• Bt cotton, Bt brinjal — the Cry gene is transferred into the plant genome. The plant produces its own insecticide in its tissues. No spraying required.

Trichoderma — the Fungal Biocontrol Agent

Trichoderma species are free-living fungi commonly found in root ecosystems. They are effective biocontrol agents of several plant pathogens — particularly soil-borne fungi like Fusarium, Pythium, Rhizoctonia.

Mechanism: Trichoderma coils around and digests the pathogenic fungus (mycoparasitism); it also secretes antifungal molecules; and it stimulates the plant's own defence systems.

Baculoviruses — Viral Pesticides

Baculoviruses are pathogens that attack insects and other arthropods. The majority of baculoviruses used as biocontrol belong to the genus Nucleopolyhedrovirus (NPV). They are excellent candidates for IPM (Integrated Pest Management) because:

• Species-specific — kill only the target insect; do not harm other organisms.
• Have no negative impact on plants, mammals, birds, fish or non-target insects (bees, predators).
• Useful especially when only one pest needs to be eliminated and beneficial fauna preserved.

10.11 Microbes as Biofertilisers

Biofertilisers are organisms that enrich the nutrient quality of soil. The main sources of biofertilisers are bacteria, fungi and cyanobacteria. Their use is critical for the long-term productivity of soils — chemical fertilisers degrade soil health over time, while biofertilisers improve it.

Nitrogen-Fixing Bacteria

Rhizobium are bacteria that have symbiotic association with the roots of leguminous plants (gram, pea, peanut, soybean, clover). They form root nodules where atmospheric nitrogen (N₂) is fixed into ammonia (NH₃) — a form usable by plants.

Free-living nitrogen-fixers (not requiring a host plant):

• Azospirillum, Azotobacter — soil bacteria that fix atmospheric N₂ and provide it to crops.

Cyanobacteria (Blue-Green Algae)

Many cyanobacteria — Anabaena, Nostoc, and Oscillatoria — are autotrophic AND fix atmospheric nitrogen. They are abundant in paddy fields. Anabaena often lives in symbiosis with the water fern Azolla — together they enrich paddy fields with nitrogen.

Mycorrhiza — Fungal Roots

Mycorrhiza = "fungus root" — symbiotic association between fungi and plant roots. The fungus colonises the root surface and surroundings; in return for sugars from the plant, the fungal hyphae extend far into the soil and absorb water, phosphorus and other nutrients.

Major group: Fungal genus Glomus forms vesicular arbuscular mycorrhiza (VAM) — the most common mycorrhiza in agricultural crops.

Benefits to plant:

• Absorbs phosphorus from soil and passes to plant (P is often the limiting nutrient).
• Resistance to root-borne pathogens.
• Tolerance to salinity, drought.
• Overall increased plant growth and development.

Summary Table — Biofertiliser Types

10.12 Worked Examples