Weevils to clear Africa's waterways
Clogged up water-ways in West Africa are gradually being cleared by weevils brought in from South America. Water hyacinth (Eichornia crassipes) and water lettuce (Pistia stratiotes), which have been gradually colonizing rivers, canals and dams for nearly 100 years, are now in retreat. Over 30 years ago researchers began their search for a biological control agent for water hyacinth. In South America, the home of water hyacinth weevils have been found which have proved to be very effective. Researchers at the International Institute of Tropical Agriculture (IITA), in Nigeria and Benin, first released the weevils on a weed-clogged river in Benin in 1992. Until the weevils were well established initial progress seemed slow, as the weevils only complete three generations in a year. At the first release site in Benin fishermen were frustrated at this slow progress and continued to remove the weed manually, which nearly eliminated the weevil and jeopardized the biological control. In other areas, where fishermen have been more patient, indigenous fish are beginning to return. Now, when researchers release the weevils in new areas, they spend time informing local people so they are made aware that initial progress is going to be slow. Since release in Benin, the weevils have spread across to Nigeria, and recent releases have also been made in Ghana. As in any biological control programme, complete elimination of water hyacinth is not the aim, since small pockets of the weed are necessary to maintain populations of the weevils. Furthermore, removal of water hyacinth has not been the end of the problem. Where it has been cleared, water lettuce has then come in to fill the gap. A different species of weevil, also from in South America, has been released to control the water lettuce. These weevils work more quickly, as they reproduce faster. The policy now is to release both species of weevils at the same time so that the water lettuce problem is tackled as soon as the hyacinth retreats. Biological Control Centre for Africa Interantional Institute of Tropical Agriculture B P 08-0932 Cotonou BENIN
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Technical Centre for Agricultural and Rural Cooperation
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