The beetle used as a biocontrol agent for aphids and mosquitoes is:

The correct answer is Option (3) → Ladybird

Option (1) Trichoderma — Incorrect. Trichoderma is a free-living fungus found in the root ecosystem. It is used as a biocontrol agent against plant pathogens. It is a fungus, not a beetle.

Option (2) Dragonflies — Incorrect. Dragonflies are used to get rid of mosquitoes, but they are not beetles. They belong to a different group of insects.

Option (3) Ladybird — Correct. The ladybird is the familiar red and black beetle. In the book it is the predator of aphids, while dragonflies take care of mosquitoes.

Option (4) Silver fish — Incorrect. The silverfish is a small wingless insect that feeds on paper, starch and cloth in houses. It is a household pest, not a biocontrol agent, and not a beetle.

NCERT Extract:
Microbes as Biocontrol Agents – “… The very familiar beetle with red and black markings - the Ladybird, and Dragonflies are useful to get rid of aphids and mosquitoes, respectively. 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).”