Biocontrol agents and biofertilizers are two pillars of ecologically sustainable agriculture tested regularly in NEET — not as heavy calculation questions, but as recall and direct-application items where precise organism–function pairing decides the mark.
Biological control (biocontrol) uses living organisms to suppress pest populations without synthetic pesticides. Key agents:
Biofertilizers are organisms that enrich soil nutrient quality:
The common NEET trap here is organism–role mismatch: confusing Rhizobium (symbiotic, legume-specific) with Azotobacter (free-living, non-specific), or misidentifying Trichoderma as a biofertilizer when it is a biocontrol agent.
(Reference: NCERT Class 12 Biology Chapter 8, page 152)
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
Which of the following is a free-living nitrogen-fixing bacterium used as a biofertilizer?
Answer: C. Azotobacter is a free-living (non-symbiotic) nitrogen-fixing soil bacterium used as a biofertilizer (NCERT Class 12 Biology Chapter 8, page 158). Unlike Rhizobium, it does not require a legume host.
Why A is wrong: A is wrong because Rhizobium is a symbiotic nitrogen fixer — it colonises root nodules of legumes only; it is not free-living.
Why B is wrong: B is wrong because Trichoderma is a biocontrol agent (fungal parasite of pathogenic fungi), not a nitrogen fixer or biofertilizer.
Why D is wrong: D is wrong because Bacillus thuringiensis is a biocontrol agent producing insecticidal Cry protein; it has no nitrogen-fixing role.
The Cry protein produced by Bacillus thuringiensis becomes active in:
Answer: A. Bt Cry protein (protoxin) is activated in the alkaline pH environment of the insect larval gut, where it is converted to active toxin that creates pores in the midgut epithelium (NCERT Class 12 Biology Chapter 8).
Why B is wrong: B is wrong because the mammalian gut is acidic (pH ~2 in stomach); Cry protein remains inactive at acidic pH, which is why Bt is safe for vertebrates.
Why C is wrong: C is wrong because plant sap has near-neutral pH and is not the site of Cry protein activation — the toxin acts inside the insect gut after ingestion.
Why D is wrong: D is wrong because the insect larval gut is alkaline (pH 9–10), not acidic; this alkaline environment is the specific trigger for protoxin activation.
Mycorrhiza (e.g., Glomus) primarily helps the plant in absorption of:
Answer: D. Mycorrhizal fungi form associations with plant roots and extend their hyphal network into the soil, significantly enhancing phosphorus uptake — phosphorus being relatively immobile in soil (NCERT Class 12 Biology Chapter 8, page 158).
Why A is wrong: A is wrong because nitrogen fixation is the role of Rhizobium, Azotobacter, and cyanobacteria — not mycorrhizae. Mycorrhizae specialise in phosphorus absorption.
Why B is wrong: B is wrong because potassium uptake is not the primary function of mycorrhizal associations; their documented role is phosphorus absorption from soil.
Why C is wrong: C is wrong because iron absorption is not the primary documented benefit of mycorrhiza; phosphorus is the nutrient whose uptake is most enhanced by fungal hyphae.
Which biocontrol agent is species-specific and has no negative impact on non-target organisms, plants, or mammals?
Answer: B. Baculoviruses (NPV) are species-specific — they attack only their target insect host. They have no negative effects on non-target organisms, making them ideal for ecologically sensitive IPM programs (NCERT Class 12 Biology Chapter 8).
Why A is wrong: A is wrong because ladybird beetles are generalist predators of aphids — they are not species-specific in the strict sense the question demands.
Why C is wrong: C is wrong because Trichoderma parasitises multiple species of pathogenic fungi; it is genus-broad, not species-specific in the way baculoviruses are.
Why D is wrong: D is wrong because Bt toxin, while safe for mammals, can affect multiple insect species whose larvae have alkaline guts — it is not as narrowly species-specific as baculoviruses.
Azolla is used as a biofertilizer in rice paddies because it harbours:
Answer: B. The aquatic fern Azolla forms a symbiotic association with the cyanobacterium Anabaena azollae in its leaf cavities; Anabaena fixes atmospheric N₂, enriching the paddy water with nitrogen when Azolla decomposes (NCERT Class 12 Biology Chapter 8).
Why A is wrong: A is wrong because Rhizobium forms symbiosis with legume roots, not with ferns like Azolla. Azolla's nitrogen-fixing partner is Anabaena.
Why C is wrong: C is wrong because Glomus is a mycorrhizal fungus that associates with plant roots for phosphorus absorption — it does not inhabit Azolla or fix nitrogen.
Why D is wrong: D is wrong because Azotobacter is a free-living soil bacterium, not an endosymbiont of Azolla. The fern's symbiont is the cyanobacterium Anabaena.
In integrated pest management, Trichoderma is effective against:
Answer: C. Trichoderma is a free-living fungus in the root ecosystem that acts as a biocontrol agent by parasitising and outcompeting several plant-pathogenic fungi (e.g., Fusarium, Pythium) in the soil (NCERT Class 12 Biology Chapter 8).
Why A is wrong: A is wrong because insect larval control is the domain of Bt or baculoviruses, not Trichoderma. Trichoderma is a fungal antagonist, not an insecticide.
Why B is wrong: B is wrong because Trichoderma acts against pathogenic fungi, not viruses. Viral biocontrol in agriculture uses baculoviruses (NPV) targeting insect vectors.
Why D is wrong: D is wrong because nematode biocontrol uses organisms like Paecilomyces or Pasteuria; Trichoderma's primary documented action is against soil-borne pathogenic fungi.
A farmer wants to improve phosphorus availability for a non-leguminous crop without chemical fertilisers. The most appropriate biofertiliser recommendation is:
Answer: A. Glomus (a mycorrhizal fungus) forms associations with roots of most plant species (not limited to legumes) and specifically enhances phosphorus uptake from soil — matching both constraints: non-leguminous crop + phosphorus improvement (NCERT Class 12 Biology Chapter 8, page 158).
Why B is wrong: B is wrong because Azospirillum is a nitrogen-fixing bacterium (associated with grasses); it enhances nitrogen, not phosphorus availability.
Why C is wrong: C is wrong because Rhizobium is a symbiotic nitrogen fixer specific to legumes; it cannot colonise non-leguminous crops and does not improve phosphorus availability.
Why D is wrong: D is wrong because Nostoc is a free-living cyanobacterium that fixes nitrogen; it does not specifically enhance phosphorus absorption.
Which of the following correctly pairs the organism with its role?
Answer: D. Anabaena is a cyanobacterium that can live free in soil/water AND form a symbiotic association with the aquatic fern Azolla, fixing atmospheric nitrogen in both states (NCERT Class 12 Biology Chapter 8).
Why A is wrong: A is wrong because Nostoc is a free-living cyanobacterium (not in legume root nodules). The symbiotic nitrogen fixer in legume nodules is Rhizobium.
Why B is wrong: B is wrong because Trichoderma is a biocontrol agent (parasitises pathogenic soil fungi), not a biofertiliser. Phosphorus-enhancing biofertiliser = mycorrhiza (Glomus).
Why C is wrong: C is wrong because NPV (baculovirus) is species-specific (narrow host range), which is the opposite of broad-spectrum. Its advantage is ecological specificity, not broad killing.
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Given
A NEET question asks: "Which of the following is used as a biocontrol agent against plant diseases caused by soil-borne fungi?"
Options: (A) Glomus (B) Rhizobium (C) Trichoderma (D) Anabaena
Required
Identify the organism whose documented biological function is control of pathogenic soil fungi.
Concept
Biocontrol = using living organisms to suppress pests/pathogens. Key distinction: Trichoderma is a fungal antagonist (kills other fungi); Glomus is a mycorrhizal symbiont (helps phosphorus uptake); Rhizobium and Anabaena are nitrogen fixers.
Classification rule
Biocontrol of soil fungi → Trichoderma (parasitises Fusarium, Pythium, etc.)
Biofertiliser (P) → Glomus
Biofertiliser (N, symbiotic with legumes) → Rhizobium
Biofertiliser (N, cyanobacterium) → Anabaena
Application
The question specifies "biocontrol agent" + "soil-borne fungi" — both conditions point exclusively to Trichoderma.
Evaluation
• Glomus is a mutualist (helps plants), not an antagonist of pathogenic fungi.• Rhizobium and Anabaena are nitrogen-fixers with no documented antifungal role.• Only Trichoderma parasitises soil-borne pathogenic fungi.
Final answer
(C) Trichoderma
Common trap
Selecting Glomus because it is a soil fungus associated with roots. But Glomus is a biofertiliser (enhances P uptake via symbiosis), NOT a biocontrol agent against pathogenic fungi. The organism–function mismatch (Glomus ≠ antagonist) is the designed distractor.
Similar NEET-style question
"A farmer observes Fusarium wilt in his tomato field. Which organism can be applied to the soil as a biological control measure?"
(A) Azotobacter (B) Trichoderma (C) Glomus (D) Nostoc
Answer: (B) — Trichoderma parasitises Fusarium in soil; others are biofertilisers (N or P), not biocontrol agents.
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Biocontrol: ladybird → aphids; dragonfly → mosquitoes. Bacillus thuringiensis (Bt): toxin against insects. Trichoderma vs fungal pathogens. Biofertilizers: Rhizobium (legumes), Azospirillum, Azotobacter (free-living N fixers), cyanobacteria (Anabaena, Nostoc), mycorrhiza (Glomus — P uptake).
-- NCERT Class 12 Biology, Chapter 8, p. 157More in Biology and Human Welfare: 4 exam traps and mistakes · 1 question pattern from its other lessons.
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