Which of the following is a biocontrol agent commonly used to manage…

The correct answer is Option (2) → Bacillus thuringiensis

Microbial biocontrol agents are living microbes used against pests and plant diseases. Each one acts on its own target.

Option (1) Trichoderma — Incorrect. Trichoderma is a free-living fungus of root ecosystems. It is a biocontrol agent of several plant pathogens, not of caterpillars.

Option (2) Bacillus thuringiensis — Correct. Bacillus thuringiensis (Bt) is the bacterium used to control butterfly caterpillars. Its dried spores are mixed with water and sprayed on plants such as brassicas and fruit trees; larvae that eat them are killed by the toxin released in their gut, while other insects are left unharmed.

Option (3) Pseudomonas — Incorrect. Pseudomonas is a soil bacterium, but NCERT does not list it among the biocontrol agents used against butterfly caterpillars. The microbe named for this job is Bacillus thuringiensis.

Option (4) Rhizobium — Incorrect. Rhizobium is a biofertiliser. It fixes nitrogen in the root nodules of leguminous plants and does not kill insects.

NCERT Extract:
Microbes as Biocontrol Agents – “… An example of microbial biocontrol agents that can be introduced in order to control butterfly caterpillars is the bacteria Bacillus thuringiensis (often written as Bt). These are available in sachets as dried spores which are mixed with water and sprayed onto vulnerable plants such as brassicas and fruit trees, where these are eaten by the insect larvae. In the gut of the larvae, the toxin is released and the larvae get killed. The bacterial disease will kill the caterpillars, but leave other insects unharmed.”