In countries where a particular weed species is native or indigenous, the plant is kept in check by its natural enemies or predators. If some of these species are introduced to other countries, their natural enemies, which have evolved with them in their native areas are not present. The plant can then out compete native vegetation and become a noxious weed. The biological control of weeds involves the introduction of host specific organisms to control weed populations. It usually involves the introduction of one or more of the natural enemies of the weed from its native area in an effort to bring the weed invasion under control. It is especially attractive in controlling aquatic weeds where herbicides cannot be used. The aim of biological control is reduction and not eradication of the weed population. Indeed for it to be successful, small populations of weeds must remain present to ensure survival of the natural enemy. Biological control agents can be arthropods, plant pathogens (fungi, bacteria, viruses, nematodes), fish and birds.
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Species/ Crop tolerance:
The biological agent is host specific and desirable species are not affected.
Additional information:
Biological control of weeds actually started by accident in 1795. Opuntia vulgaris, the prickly-pear cactus, native to South America, had become a major problem in India. One cactus-feeding insect, a mealybug called Dactylopus coccus, yielded a red dye and was exploited commercially. An imported mealybug called Dactylopus cyclonicus, which produced negligent amounts of the dye was found to rapidly kill prickly-pear cactus. It was species specific and no other Opuntia species were harmed. St John’s Wort, a plant containing hypericin, causes photo toxicity in light skinned animals. The meat and wool of affected animals is of poor quality. In 1952, this was the first plant in Canada targeted for biological control. Two defoliating beetles, Chrysolina quadrigemina and Chrysolina hypericum were released in an attempt to control the plant. Although slow at first, the beetle population had increased sufficiently in thirteen years to control the plant and chemical control for this plant was no longer a necessity. Biological control is inexpensive and once introduced the control agents are self multiplying and spread on their own. Non-target organisms are under little or no threat and little or no additional maintenance or effort is required once the biological control mechanism is established. However there are always some concerns that a host shift may result in an organism feeding on a desirable plant. Biological control is slow and weed populations may net be brought under control quickly enough to satisfy public opinion. Finding and screening organisms needs time and money and it can take years before a biological control mechanism is released. However, in an era where concerns about the impact of pesticides on the environment and non-target organisms, are growing, it could in some cases be a beneficial long-term option.
