Plant predators are themselves subject to attack by predators, parasites, and microbes, all of which can indirectly help protect a plant and therefore are also considered biopesticides. An oak tree may have about 100 species of insect herbivores feeding on it. In turn, there can be up to 1,000 species of predators, parasites, and microbes feeding on the herbivores. The microbes, parasites, and predators attacking the herbivore populations are considered "biopesticides," as are any protective chemicals produced by the tree.
Such living biopesticides play a vital role in agriculture and nature, helping to control insect pests, plant pathogens, and weeds. Numerous organisms, including viruses, fungi, protozoa, bacteria, and nematodes, as well as insects, such as parasitic wasps, can attack pest insects and weeds. In some cases, biologists search around the world to find natural organisms to help control an insect, a plant pathogen, or weed populations.
The use of natural organisms as biopesticides is sometimes hampered by the presence of chemical pesticides, which can threaten populations of a pest insect's natural enemies. Pest outbreaks that result from chemical pesticides destroying a pest's natural enemies are estimated to cost the United States more than $500 million per year.
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