The largest group of insecticides produced by plants is .......

Which insect is used to control the spread of prickly pear cactus $(Opuntia)$?

Which fish controls diseases like malaria and filariasis by feeding on mosquito larvae?

Assertion $A$: The use of biocontrol agents prepared by microorganisms has become necessary as an alternative to chemical pesticides.
Reason $R$: The use of biological control agents helps maintain the balance of the ecosystem.
Which option is correct for Assertion $A$ and Reason $R$?

The ladybird beetle and dragonflies are useful to get rid of . . . . . . and . . . . . . respectively.

Gambusia is a/an: