CBSE Class 12-science Answered

Biological control of pests and pathogens is preferred over the conventional use of chemical pesticides because:

(i) The chemicals cause pollution of water bodies as well as groundwater. They also enter the plant body through the soil.

(ii) The chemicals are extremely harmful to human beings and other animals.

Microbes as biocontrol agents:

(a) Bacillus thuringiensis: Bt toxin genes when introduced into a plant, the plant becomes resistant to attack by insect pests. The protein toxins are available in the form of dried spores, which are mixed with water and sprayed onto the vulnerable plants. When these toxins are ingested by the insect larvae, they are released in the insect gut where they become active and kill the larvae. Specific Bt toxin genes obtained from B.thuringiensis are incorporated in several crop plants which make them resistant to insect pests.

(b) Nucleopolyhedrovirus: These viruses are an excellent source for species-specific, narrow spectrum insecticidal applications. This is especially desirable when beneficial insects are being conserved to aid in an overall integrated pest management (IPM) programme.