Insect predators preying on other insects kill and consume their prey. Parasites and pathogens live in close association with their host, feeding on their host and reducing host fitness but not necessarily killing the host. Parasitoids may be free-living as adults but immatures develop on their host and usually kill it.

Parasites, predators and pathogens may reduce populations of potential biological control agents in the country of origin and in the country to which the agents are introduced. In the country of origin the effect may be to reduce the agent to insignificant densities. When introduced to the new country free of those parasites, predators and pathogens, agent populations may increase to much higher densities than in the country of origin.

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