Bt cotton is genetically modified variant which is designed to fight against pink bollworm. pink bollworm is the pest which affects the cotton crop which increases the yield of cotton crop and through reduce usage of pesticides and insecticide and reduce the losses pest

Biological control of pests and diseases refers to the use of living organisms to control or suppress populations of pests or pathogens. Among the given options, option A, Bt-cotton to increase cotton yield, is an example of carrying out biological control of pests using microbes.

The Bt in Bt-cotton stands for Bacillus thuringiensis, which is a naturally occurring soil bacterium. This bacterium produces a protein called Bt toxin, which is toxic to certain insects, particularly caterpillars. Bt-cotton is genetically modified to express the Bt toxin, making the cotton plants resistant to pest attacks.

Here is a detailed explanation of how Bt-cotton works as a biological control agent:

1. Bt toxin production: Bt-cotton plants are genetically engineered to produce the Bt toxin in their tissues, particularly in the leaves. The toxin is produced as an inactive precursor, which is activated when ingested by susceptible insects.

2. Insect feeding: When insect pests, such as cotton bollworm or pink bollworm, feed on the leaves or other plant parts of Bt-cotton, they ingest the Bt toxin.

3. Toxicity to insects: Once inside the insect's digestive system, the Bt toxin is activated by the alkaline pH and specific enzymes. The activated toxin binds to specific receptors in the gut lining, creating pores that disrupt the integrity of the cells. This leads to cell lysis and eventually kills the insect.

4. Selective toxicity: Bt toxin is highly specific to certain insect orders, particularly Lepidoptera (butterflies and moths). It does not harm beneficial insects, such as bees, ladybirds, or spiders, which are important for natural pest control.

5. Reduced pesticide use: By incorporating Bt toxin into cotton plants, farmers can reduce their reliance on synthetic insecticides. This reduces the environmental impact and potential risks associated with chemical pesticides.

6. Increased cotton yield: Since Bt-cotton plants are resistant to pest attacks, they suffer less damage from insect feeding. This leads to increased cotton yield and improved economic returns for farmers.

In conclusion, Bt-cotton is an example of biological control of pests using microbes, specifically Bacillus thuringiensis. The genetically modified cotton plants produce the Bt toxin, which is toxic to certain insect pests, providing a sustainable and environmentally friendly approach to pest management in cotton cultivation.