The basis of various biological control methods is on the concept of predation. Predation is a biological interaction between the predator and the prey, whereby the predator feeds on the prey. Hence, the predators regulate the population of preys in a habitat, thereby helping in the management of pest insects.
its also based on different trophic level or food chain,secondary consumer (predator biological agent)consume primary consumer ( pest which take food from plant)
It is not just predation, biological control relies on parasitism also. Baculoviruses are used to control arthropod pests. Similarly fungi may be used to parasitise pests.