Waterhyacinth (Eichhornia crassipes [Mart.] Solm) (Figure 2) is both a plant used in ornamental fish ponds and the world's most damaging aquatic weed. Its beautiful lavender flowers have led people to take it far from its native range in the Amazon basin of South America. Wherever waterhyancinth has been introduced into subtropical or tropical climates, it has escaped into the wild. There it forms gigantic mats that clog rivers and cover over bays and ponds, causing great ecological damage and harming economic activities such as fishing, transportation, and hydropower. Among the many places invaded by waterhyacinth is Lake Victoria in East Africa. It was first recorded there in 1980 and by the mid-1990s some 12 000 ha of weed mats were clogging bays and inlets around the lake. Economic losses were caused to fisheries (the mats impede the launching of boats and the use of nets) and to water and hydroelectric power works. Ecologically, the weed threatened one of evolution's

Figure 2 Waterhyacinth (Eichhornia crassipes [Mart.] solms) has invaded water bodies throughout the tropics and subtropics, causing great ecological and economic damage. Photo Courtesy of Chris Evans; Forestryimages.org.

greatest products - a radiation of some 200-400 species of endemic cichlid fishes. These fish, often separated by mating habits based on female color preference, were threatened by hybridization among species induced by low light under weed mats, where color-based visual-recognition mating systems could not be sustained.

Control efforts recommended to the governments of the affected countries (Uganda, Kenya, and Tanzania) included herbiciding the mats, use of harvester boats to cut the mats, and release of specialized herbivorous insects. Two weevils, Neochetina eichhorniae Warner and N. bruchi Hustache, known to be specialists on waterhya-cinth from earlier work in Florida, were chosen for release. In 1995, Uganda was first to release biological control insects against the weed, followed by the other two countries in 1997. On the Ugandan shore, weed mats began to show damage from herbivory by late 1998. By 1999 some 75% of the mats had died and sunk into the lake.

This project illustrates the power of classical biological control to provide control of invasive species damaging large natural systems without the recurrent costs of mechanical control or the pollution of pesticides.

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