How Many Insect Species are Necessary for the Biological Control of Insects?

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Abstract
INSECTS INTRODUCED to new environments without their natural enemies frequently attain high population densities. Outbreaks of insects that are free of natural enemies and their suppression by introduced biological control agents are evidence that predators and parasitoids can limit or regulate host density (Huffaker 1970). However, most biological control projects are not successful. The introduction of predators and parasitoids has only reduced host densities sufficiently to replace chemical control in approximately 16% of more than 600 projects (Hall et al. 1980, Ehler & Andres 1983). This low success rate includes failures of the predators and parasitoids to become established and failures of established agents to reduce host density. These failures occur for several reasons but, it is possible that only a few species of predators and parasitoids have the potential to suppress host population density to any great extent. An alternative interpretation is that a complex of natural enemies is necessary to suppress host population densities and until this total complex is reestablished, successful biological control will not be achieved.

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