What is the use of biological organisms or products derived from them to control crop pests and diseases called?

Which of the following is used as a microbial biocontrol agent for plant disease control?

The utility of a microorganism like $Trichoderma$ $harzianum$ has been proven in which of the following?

What is the ecological principle behind the biological control method of managing pest insects?

Ladybird and dragonfly are very useful in getting rid of $........$ respectively.

Consider the following statements:
$I.$ Ladybirds and dragonflies are used to get rid of aphids and mosquitoes.
$II.$ The bacteria $Bacillus$ $thuringiensis$ $(Bt)$ are used to control butterfly caterpillars.
$III.$ $Trichoderma$ $sp.$,free-living fungi,are present in root ecosystems where they act against several plant pathogens.
$IV.$ $Rhizobium$ is a symbiotic bacterium that lives in the stem of legumes.
Which of the statements given above are correct?