This is not an example of performing biological control of diseases/pests using microbes

The correct option is "Bt-cotton to increase yield".

To understand why this is the correct choice, let us break down the concepts of biological control and analyze each option step-by-step:

1. Understanding Biological Control using Microbes:
Biological control (or biocontrol) refers to a method of controlling pests, weeds, and plant diseases using other living organisms. When we specifically refer to biocontrol "using microbes," it means applying or introducing living microorganisms (such as fungi, bacteria, or viruses) directly into the environment to suppress or eliminate pest populations.

2. Analyzing the Options:
Let us evaluate how each option relates to this definition:

• Trichoderma sp. against some plant pathogens: Trichoderma is a genus of free-living fungi (microbes) that are highly effective biocontrol agents against various soil-borne plant pathogens. This is a direct example of using a microbe for biological control.


• Nucleopolyhedrovirus against insects and other arthropods: Nucleopolyhedroviruses are pathogens (viruses, which are microbes) that target specific insects and arthropods. They are used as narrow-spectrum bioinsecticides, representing another clear example of biocontrol using microbes.


• Ladybird beetle against aphids: Although the ladybird beetle is a biocontrol agent used to eradicate aphids, it is an insect (a macro-organism) and not a microbe.


• Bt-cotton to increase yield: Bt-cotton is a transgenic (genetically modified) crop. It contains genes derived from the bacterium Bacillus thuringiensis that code for insecticidal toxin proteins (Cry proteins), making the plant resistant to pests like bollworms. The primary objective here is to increase crop yield through genetic engineering (biotechnology) rather than the ecological application of living microbial agents to control diseases. Thus, it is classified under pest-resistant crop development via biotechnology rather than active microbial biological control.

Conclusion:
Therefore, among the choices provided, Bt-cotton to increase yield is not classified as an example of performing biological control of diseases/pests using microbes, making it the correct answer.