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Plants that are beneficial in their homeland, once they are introduced - accidentally, or intentionally - into other parts of the world, may become weed problems. This may happen when they find environmental conditions that are favourable for their growth and - most importantly - their populations are not limited by the natural enemies that are present in their original habitats. These natural enemies associated with the plants contribute to prevent the uncontrolled growth of their populations.

Exotic aquatic weeds, in the absence of their natural enemies, cause major problems in Africa. In this continent they rapidly invaded freshwater bodies and became widespread especially in tropical and warm-temperate regions. In particular the populations of some species, such as Water Hyacinth (Eichhornia crassipes (Martius) Solms-Laubach), Water Lettuce (Pistia stratiotes L.) and Water Fern (Salvinia molesta D.S. Mitchell) have increased dramatically during recent decades, becoming an agricultural, environmental and public health problem. They threaten freshwater bodies both in coastal areas and inland, where they (a) interfere with human activities, seriously affecting the national economy and social structure; (b) are deleterious for human health, since they offer a suitable habitat for vectors of many diseases; and (c) are a serious menace for the environment and biological diversity, including the displacement of native flora and fauna.

The control of these weeds in Africa is difficult and all sound options should be considered. When necessary, a combination of control measures may be used, depending on the nature of the local problem. Among the ways to control exotic weeds, environmentally friendly and sustainable methods should be adopted.

A sound option is the so-called classical biological control, which is the use of beneficial organisms - the natural enemies that effectively limit the populations of the host plant in its area of origin - to reduce the growth and population density of the weed in the area of introduction. The organisms used are host-specific and do not harm other desirable plants. Biological control is the preferred way to control aquatic weeds, since it is self-sustaining and its implementation relatively cheap and environmentally friendly. Also, the adoption of biological control instead of chemical control in water bodies, avoids pollution and any other undesirable impact on water used for drinking, or agriculture. Effective natural enemies of aquatic weeds have already been identified, studied and used successfully in parts of Africa, America, Asia and Australia.

The Food and Agriculture Organization of the United Nations (FAO) is strongly committed to contributing to the control of aquatic weeds and to help to solve the serious problems caused by these plants. During the past ten years FAO has conducted numerous efforts in the control of aquatic weeds at various levels, from the institutional level to the field, with the identification and implementation of effective strategies and through concerted and coordinated international actions. These efforts have also included the provision of advice and technical assistance, with the participation of relevant institutions. The aim is to conduct sound pest management, achieving sustainable, long-term control.

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