Which one of the following is an example of carrying out biological control of pests/diseases using microbes

Options

A. Nucleopolyhedrovirus against white rust in Brassica
B. Bt-cotton to increase cotton yield
C. Lady bird beetle against aphids in mus- tard
D. Trichoderma sp. against certain plant pathogens

Explanation

To solve the question regarding biological control of pests/diseases using microbes, let's analyze the options provided step by step.

### Step-by-Step Solution:

1. **Understanding Biological Control**: Biological control refers to the use of living organisms, such as microbes, to control pest populations or diseases. This can include bacteria, fungi, or viruses that specifically target harmful organisms without causing harm to the crops or the environment.

2. **Analyzing the Options**:
- **Option A: Nucleopolyhedrovirus against a white rust**: This virus is indeed used in pest control, but it primarily targets specific insects rather than being a general biological control agent for diseases. Therefore, it does not fit the definition of biological control using microbes in the context of the question.

- **Option B: BT cotton to increase the cotton yield**: BT cotton is genetically engineered to produce a toxin from the bacterium *Bacillus thuringiensis* (Bt). This toxin is effective against certain insect pests, making it a clear example of biological control using a microbe. Thus, this option is a strong candidate.

- **Option C: Ladybirds against aphids**: Ladybirds are predators of aphids, but they are not microbes. They do help in controlling pest populations but do not represent microbial biological control.

- **Option D: Trichoderma sp. against certain plant pathogens**: While *Trichoderma* is a fungus that can control plant pathogens, it is not the best example of biological control in the context of the question, which focuses on pests rather than pathogens.

3. **Conclusion**: After analyzing all the options, the correct answer is **Option B: BT cotton to increase the cotton yield**. This option effectively illustrates the use of a microbe (Bacillus thuringiensis) in biological pest control.

### Final Answer:
**B: BT cotton to increase the cotton yield**

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