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 mealybugs. 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 invasive plants and, of these, 41 species (31%) have been completely controlled.
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