BiologyCBSE Class 12 · Microbes in Human Welfare

Microbes as Biocontrol Agents

Updated 15 October 2026 · Fact-checked

Biocontrol is the use of biological methods to control plant diseases and pests instead of toxic chemicals. Ladybirds eat aphids and dragonflies eat mosquitoes; Bacillus thuringiensis kills butterfly caterpillars; the fungus Trichoderma controls plant pathogens; and baculoviruses (Nucleopolyhedrovirus) kill specific insects, which makes them ideal for integrated pest management.

Match each biocontrol agent to its target: predators eat pests, while Bt, Trichoderma and baculoviruses attack caterpillars, pathogens and specific insects.

Illustration drawn for QuestionBuddy.

Understand Microbes as Biocontrol Agents

Why biocontrol. Modern farming relies heavily on chemical insecticides, pesticides and weedicides. These are toxic and extremely harmful to humans and animals, and they pollute soil, groundwater, fruits, vegetables and crops. Biocontrol means using biological methods to control plant diseases and pests, which greatly reduces dependence on toxic chemicals.

The organic farmer's view. An organic farmer takes a holistic approach, trying to understand the web of interactions among the many organisms in a field. Eradicating 'pests' completely is undesirable, because the beneficial predatory and parasitic insects that depend on them as food or hosts would then not survive. So the farmer studies the life forms in the field, predators as well as pests, their life cycles, feeding patterns and preferred habitats, to design the right biocontrol.

Biocontrol agents to know.

Agent	Type	Controls	How
Ladybird	beetle (predator)	aphids	eats them
Dragonfly	insect (predator)	mosquitoes	eats them
Bacillus thuringiensis (Bt)	bacterium	butterfly caterpillars	dried spores, mixed with water, sprayed on vulnerable plants (brassicas, fruit trees); larvae eat them; toxin released in the larval gut kills them; other insects are unharmed
Trichoderma	free-living fungus, common in root ecosystems	several plant pathogens	acts against disease-causing organisms around roots
Baculoviruses (genus Nucleopolyhedrovirus)	viruses	insects and other arthropods	species-specific, narrow-spectrum insecticides

Why baculoviruses are special. They have no negative effects on plants, mammals, birds, fish or even non-target insects. This makes them the agent of choice in an integrated pest management (IPM) programme that aims to conserve beneficial insects, and for treating ecologically sensitive areas.

From spray to gene. With genetic engineering, the *B. thuringiensis* toxin genes have been introduced into plants, making them resistant to insect pests: Bt cotton is an example cultivated in some states of India (details in Biotechnology and its Applications).

Biocontrol vs chemical pesticides.

Specific to target pests vs broad-spectrum (kills useful insects too).
Leaves little or no toxic residue vs residues in soil, water and food.
Conserves natural predators vs eliminates them, often causing pest resurgence.
Self-sustaining in many cases (living agents multiply) vs repeated applications needed.

Flow chart (Bt spray): Bt spores sprayed on leaves → caterpillar eats leaves → in the larva's gut the toxin is released → gut damaged → larva dies → other insects that do not eat the leaves are unaffected.

Draw and label (flow diagram, no organism drawing needed): an IPM plan. Checklist: a central box 'crop field' with arrows from four boxes: 'ladybirds (aphids)', 'Bt spray (caterpillars)', 'Trichoderma in soil (root pathogens)', 'NPV (specific insect pests)'; a fifth box 'minimal chemical use only when needed'. Labels: predator, bacterium, fungus, virus, IPM.

Microbes as Biocontrol Agents: key facts

Predators: $ladybird \to aphids; dragonfly \to mosquitoes$; Insect predators used as biocontrol.
Bacterial agent: $B a c i l l u s t h u r in g i e n s i s \to butterfly caterpillars (toxin in larval gut)$; Sold as dried spores in sachets.
Fungal agent: $T r i c h o d er ma \to plant pathogens (root ecosystem)$; Free-living fungus.
Viral agent: $Baculovirus (N u c l eo p o l y h e d r o v i r u s) \to specific insects$; Narrow-spectrum; safe for plants, mammals, birds, fish, non-target insects.
IPM principle: $control pests below damaging level + conserve beneficial organisms$; Biocontrol agents are its core tools.

How to answer Microbes as Biocontrol Agents questions

Biocontrol questions ask you to match agents to pests, explain how an agent works, or argue why biocontrol is better than chemicals.

1Name the agent and its type (predator insect, bacterium, fungus, virus).
2Name the target (aphids, mosquitoes, caterpillars, plant pathogens, specific insects).
3Explain the mechanism: predation, toxin in the larval gut, action against pathogens, species-specific infection.
4Give the advantage: species-specific, no harm to non-target organisms, no toxic residue, conserves predators.
5For 'why not eradicate pests', explain that predators and parasites depend on them, so complete eradication harms the ecosystem.
6Link Bt sprays to Bt crops when asked about the genetic engineering application.

Microbes as Biocontrol Agents in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Choosing biocontrol in IPM

Suppose a vegetable farmer near a bird sanctuary finds aphids on his beans, caterpillars on his cabbages, and root rot caused by a soil fungus in his chillies. He has been spraying a broad-spectrum chemical pesticide, but pests keep returning and ladybirds have disappeared.

Suggest a biocontrol plan for each problem and explain why the chemical spray made things worse.

Try it first, then show the solutionSolution

Aphids on beans: release or protect ladybirds, which eat aphids.
Caterpillars on cabbages: spray Bacillus thuringiensis spores mixed with water; caterpillars eating the leaves are killed by the toxin released in their gut, while other insects are spared.
Root rot in chillies: apply Trichoderma, a free-living fungus of the root ecosystem that controls plant pathogens.
Near the sanctuary, a baculovirus (NPV) specific to any remaining insect pest can be used, since it harms no birds, fish, mammals or non-target insects.
The broad-spectrum chemical killed the ladybirds and other natural enemies, so pests rebounded with nothing to check them, and it polluted soil and water.

Answer: Ladybirds for aphids, Bt spray for caterpillars, Trichoderma for root pathogens, NPV where safety matters; chemicals killed natural predators, causing pest resurgence.

Memory hook: Use the pest's enemies, not poison for everyone.

Tricks to remember Microbes as Biocontrol Agents

Short memory aids. Say each one aloud once, then use it on a question the same day.

Ladies love Aphids; Dragons dine on Mosquitoes
Ladybird → aphids; dragonfly → mosquitoes.
Use it when: Predator-prey pairs.
Bt = Butterfly terminator
Bacillus thuringiensis kills butterfly caterpillars.
Use it when: Bt target.
NPV = Narrow, Precise, Virus
Nucleopolyhedrovirus is narrow-spectrum and precise.
Use it when: Baculovirus advantages.

Microbes as Biocontrol Agents: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Match the agents · 1 mark · MCQBoard level

Match: (a) Ladybird (b) Dragonfly (c) Bacillus thuringiensis (d) Trichoderma with (i) plant pathogens (ii) mosquitoes (iii) aphids (iv) butterfly caterpillars.

Try it first, then show the solutionSolution

Ladybirds eat aphids; dragonflies eat mosquitoes.
Bt kills butterfly caterpillars; Trichoderma controls plant pathogens.

Answer: (a)-(iii), (b)-(ii), (c)-(iv), (d)-(i).

Example 2: How Bt works as a spray · 3 marks · Short answerBoard level

Explain how Bacillus thuringiensis is used to control caterpillars and why other insects are not harmed.

Try it first, then show the solutionSolution

Bt is available as dried spores in sachets; these are mixed with water and sprayed on vulnerable plants such as brassicas and fruit trees (1 mark).
Insect larvae eating the leaves take in the spores; the toxin is released in the gut of the larvae and kills them (1 mark).
The bacterial disease kills the caterpillars but leaves other insects unharmed (1 mark).

Answer: Spore spray → eaten by larvae → toxin in gut → larvae die; specific to caterpillars.

Example 3: Baculoviruses in IPM · 2 marks · Very short answerHarder board level

Why are baculoviruses considered excellent candidates for integrated pest management, especially in ecologically sensitive areas?

Try it first, then show the solutionSolution

Baculoviruses (mostly genus Nucleopolyhedrovirus) are species-specific, narrow-spectrum insecticidal agents (1 mark).
They have no negative effects on plants, mammals, birds, fish or non-target insects, so beneficial insects are conserved and sensitive ecosystems are not harmed (1 mark).

Answer: Species-specific; safe for non-target organisms; conserve beneficial insects.

Example 4: Why not eradicate pests? · 2 marks · Very short answerHarder board level

An organic farmer holds that eradicating pests completely is undesirable. Explain.

Try it first, then show the solutionSolution

Beneficial predatory and parasitic insects depend on pests as food or hosts (1 mark).
Eradicating pests would remove these beneficial organisms too, disturbing the web of interactions in the field; it is better to keep pests at low levels through natural enemies (1 mark).

Answer: Natural enemies need pests to survive; eradication breaks the field's food web.

Example 5: Chemicals vs biocontrol · 3 marks · Short answerHarder board level

Give three disadvantages of chemical pesticides and explain how biocontrol overcomes each.

Try it first, then show the solutionSolution

Toxic to humans and animals → biocontrol agents such as baculoviruses are specific and harmless to non-targets (1 mark).
Pollute soil, groundwater and food with residues → living agents leave little or no toxic residue (1 mark).
Kill natural predators, so pests resurge → biocontrol conserves predators like ladybirds and dragonflies, keeping pests in check (1 mark).

Answer: Toxicity, pollution, predator loss; biocontrol is specific, residue-free and conserves predators.

Example 6: Designing a study before biocontrol · 3 marks · Short answerHOTS / competency

Suppose you are advising a village that wants to switch from pesticides to biocontrol for its paddy fields. What must they study first about the pests and predators in the field, and why? Give one risk if they skip this step.

Try it first, then show the solutionSolution

They must identify the life forms in the field, both pests and predators, and learn their life cycles, feeding patterns and preferred habitats (1 mark).
This tells them which agent to use, when to release or spray it (e.g. when larvae are feeding) and how to protect natural enemies already present (1 mark).
Risk: an agent may be applied at the wrong stage or against the wrong pest, failing to control it, or a beneficial species may be harmed, causing crop loss and a return to chemicals (1 mark).

Answer: Study pests and predators, their life cycles, feeding and habitats, to choose and time the right agent; skipping it risks failure or harm to beneficial species.

Where students lose marks in Microbes as Biocontrol Agents

The slips examiners see most often, and what to write instead.

Mistake: Calling Trichoderma a bacterium
Microbes are mixed up.
Wrong working
The bacterium Trichoderma controls pests.
Right working
Trichoderma is a free-living fungus that controls plant pathogens.
Do this: Learn the type column of the agent table.
Usually costs: 1 mark.
Mistake: Saying Bt kills all insects
Its specificity is missed.
Wrong working
Bt spray kills all insects on the crop.
Right working
Bt kills caterpillars that eat it; other insects are unharmed.
Do this: Stress specificity.
Usually costs: 1 mark.
Mistake: Writing 'baculoviruses attack plants'
Pathogen of pests vs pathogen of crops confused.
Wrong working
Baculoviruses are plant pathogens used in IPM.
Right working
Baculoviruses attack insects and other arthropods; they do not harm plants.
Do this: Remember: no negative impact on plants.
Usually costs: 1 mark.
Mistake: Omitting the advantage
Students only name agents.
Wrong working
Ladybirds and Bt are biocontrol agents.
Right working
They control specific pests without toxic chemicals, conserving useful organisms.
Do this: Always add why biocontrol is better.
Usually costs: ½-1 mark.

Exam tips

Give each agent with its type and target pest in one line.
Write the genus Nucleopolyhedrovirus when baculoviruses are asked.
In 3-mark answers, end with the advantage over chemicals.
Mention the organic farmer's holistic view in 'why biocontrol' questions.
Keep Bt spray (this topic) and Bt cotton (biotechnology chapter) linked but distinct.

Practice questions on Microbes as Biocontrol Agents

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The biocontrol agent used against butterfly caterpillars is:
(a) Trichoderma (b) Bacillus thuringiensis (c) Nucleopolyhedrovirus against fungi (d) Rhizobium
Hint
Bt.
Answer
(b) Bacillus thuringiensis.
Q2 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Baculoviruses are preferred in ecologically sensitive areas.
Reason (R): Baculoviruses are broad-spectrum insecticides that kill all insects.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Narrow or broad?
Answer
(c). They are narrow-spectrum and species-specific, which is why they are preferred.
Q3 · 1 mark · Very short answerBoard level
Name a fungus used as a biocontrol agent, where it lives, and what it controls.
Hint
Root ecosystem.
Answer
Trichoderma; a free-living fungus common in root ecosystems; it controls several plant pathogens.
Q4 · 2 marks · Very short answerHOTS / competency
Suppose a farmer sprays Bt spores on a mustard crop at a time when there are no caterpillars, only adult butterflies. Will the spray be effective? Explain using the mode of action of Bt.
Hint
Who eats the leaves?
Answer
Not much. Bt must be eaten by the larvae (caterpillars) feeding on leaves; the toxin acts in the larval gut. Adult butterflies feed on nectar, not leaves, so they are not affected. The spray should be timed for when caterpillars are feeding.
Q5 · 4 marks · Case-basedHarder board level
Read and answer.
Suppose a mango orchard owner keeps dragonfly-friendly ponds nearby, encourages ladybirds, sprays a bacterial product only on young shoots attacked by caterpillars, and uses a virus product that affects only one moth species.
(a) Which pests do the dragonflies and ladybirds control? (1)
(b) Name the bacterial product and explain where it acts in the caterpillar. (1)
(c) Name the virus group and explain why this combined approach is called integrated pest management. (2)
Hint
IPM uses several methods while conserving beneficial organisms.
Answer
(a) Dragonflies control mosquitoes; ladybirds control aphids. (b) Bacillus thuringiensis; its toxin is released in the caterpillar's gut and kills it. (c) Baculoviruses (Nucleopolyhedrovirus). It is IPM because several biological methods are combined to keep pests below damaging levels while conserving beneficial insects and avoiding broad chemical use.
Q6 · 2 marks · Very short answerHarder board level
How did Bt as a spray lead to Bt crops? Name one Bt crop grown in India.
Hint
Toxin genes.
Answer
Genetic engineering allowed scientists to introduce the B. thuringiensis toxin genes into plants, so the plants themselves make the toxin and resist insect pests without spraying. Example: Bt cotton.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Which predator controls aphids?Show answer
Ladybird.
2.Which predator controls mosquitoes?Show answer
Dragonfly.
3.In what form is Bt sold to farmers?Show answer
Dried spores in sachets, mixed with water and sprayed.
4.Where does Bt toxin act?Show answer
In the gut of the insect larva.
5.Which genus do most biocontrol baculoviruses belong to?Show answer
Nucleopolyhedrovirus.
6.What does IPM stand for?Show answer
Integrated pest management.

Microbes as Biocontrol Agents: frequently asked questions

What is biocontrol?

The use of biological methods, such as predators, bacteria, fungi and viruses, to control plant diseases and pests instead of toxic chemicals.

How does Bacillus thuringiensis kill caterpillars?

Its spores, sprayed on plants, are eaten by caterpillars; a toxin released in their gut kills them, while other insects are not harmed.

What is Trichoderma used for?

Trichoderma is a free-living fungus of root ecosystems that acts as a biocontrol agent against several plant pathogens.

Why are baculoviruses good biocontrol agents?

They are species-specific and narrow-spectrum, with no harmful effect on plants, mammals, birds, fish or non-target insects.

Why should pests not be completely eradicated?

Beneficial predatory and parasitic insects depend on pests as food or hosts; eradicating pests would eliminate them too.