Biological control is simply using natural enemies to control the pest and diseases in agriculture. The natural enemies used for biological control classified under 3 groups. 

They are predators (predatory insects, directly prey on their hosts) , parasitoids (wasps lay their eggs on or inside of the host) and micro organisims  (bacteria, viruses, fungus,nematods). The most common biological control method using in agriculture is to use predators to suppress the pest population under the economic damage threshold levels. 

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