Following introductions of natural enemies, pest densities may be reduced, in some cases by 90-99% or more. This has been achieved for a variety of pest insects, including caterpillars, sawflies, aphids, scales, whiteflies, and mealy bugs. Over the last 125 years, more than 1200 insect biological control projects have been attempted. Of these, 60% have resulted in a reduction of the pest's density. In 17% of cases, control was complete and no further effort was needed. Introductions of specialized herbivores have been made for over 133 species of inva sive plants and, of these, 41 species (31%) have been completely controlled.
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