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Biological control is the deliberate use of living natural enemies (parasitoids, predators, pathogens, and herbivores) to manage populations of pests or invasive plant or animal species. It has a long history of success and, when practiced properly, is one of the most cost-effective and environmentally benign methods of pest management.

The most widely successful form of biological control against invasive plants is "classical biological control". This is most suitable for plants which have become invasive weeds after being introduced into new environments where their natural enemies do not occur. Classical biological control consists of carefully selecting plant-feeding insects, mites, or pathogens from the areas where the plant originated, and introducing them into the areas where it has become invasive.

The species used as biological control agents are extensively studied and tested before release to ensure that they are specific to their target weeds and present minimal risk of damaging non-target plants or causing other undesirable side-effects. All countries that practice classical biological control require thorough review and approval by regulatory agencies before a new agent can be released.

Biological control has been successful in reducing the impacts of invasive plant species in many countries around the world. Examples are leafy spurge, tansy ragwort, and St. John’s wort in the USA and Canada, floating fern in New Guinea, and prickly pear in Australia.

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