Biocontrol agents can work several ways

When the biocontrol agent grows faster, or uses its food source better, it crowds the pathogen out and takes over. The pathogens don't stand a chance!

Reproducing faster crowds out the pathogens!

When a biocontrol agent makes some product that slows down or kills the pathogen that gets close to it, it is called antibiosis

The BCA on the right battles the pathogen on the left

When a biocontrol agent causes a plant to make a product that discourages or kills the pathogen, it is called induced resistance. The plant actually fights back.

A plant with "induced resistance" fights back

When a biocontrol agent feeds directly on or in a pathogen, it is called parasitism. In this way the pathogen is destroyed.

The BCA is coiling around the pathogen

Some biocontrol agents use only one of these actions and others can use several. The most successful biocontrol agents use several of these methods.