Question

Explain how the following microbes act as biocontrol agents :

(a) Bacillus thuringiensis

(b) Nucleopolyhedrovirus

Hint:

Bt toxin makes pores in the larval gut; NPV is a species-specific insect virus.

Answer 1

Step 1: Understanding the Concept:
A biocontrol agent is a living organism used to control pests and diseases in place of chemical pesticides.
It is safer for the soil, water and non-target organisms.

Step 2: (a) Bacillus thuringiensis (Bt):
Bt is a bacterium that is sold as dried spores, which are sprinkled or sprayed on plants such as brassicas and fruit trees.
Caterpillars (butterfly and moth larvae) eat the leaves along with the spores.
In the alkaline gut of the larva the toxin protein is activated and dissolves.
It makes holes in the gut lining, the cells swell and burst, and the larva dies.
The toxin is made as an inactive protoxin, so it does not harm the bacterium itself.

Step 3: (b) Nucleopolyhedrovirus (NPV):
NPV belongs to the genus Baculovirus and attacks insects and other arthropods.
It is species specific and has a narrow range, so it infects only the target pest.
It has no negative effect on plants, mammals, birds, fish or even non-target insects.
It is therefore a good choice in integrated pest management, where useful insects must be protected.

Final Answer:
Bt kills caterpillars by its toxin, which makes holes in the larval gut. NPV is a species-specific virus that infects and kills the pest without harming other organisms.