Weevils in the salad
The water weed commonly referred to as water lettuce, Pistia stratiotes, is causing grave concern, particularly in the Senegal River valley. For several Years, some parts of Lake Guiers to the north of Dakar have been covered with a thick carpet of green. It looks like a prairie. The Diama dam, built in 1985, prevents the flow of salt water into the river and this has provided favorable conditions for water lettuce to grow unchecked. Since then it has invaded irrigation channels, blocked motor pumps and starved fish of oxygen. Ousseynou Diop, the head of a small team of scientists in Senegal, took up the challenge to find a means to control Pistia stratiotes. The water weevil, Neohydronomus Affinis is a natural enemy of Pistia and has been tested in trials in Zimbabwe. The weevil lays its eggs at the base of Pistia leaves. When the larvae hatch they attack the plant, making holes about a millimetre in diameter. The leaves become yellow and within 115 days the plants succumb, shrivel and die. Three months after the first release of the insects, the water lettuce had disappeared. As with any other biological control, it was essential to check that the natural enemy, the water weevil, would not itself become a pest. Trials using over 40 different host plants revealed that the weevil likes only Pistia ill its salad. Ousseynou Diop Projet de lutte biologique Service de lo protection des vegetaux BP 20084 Thiarye Dakar SENEGAL
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Technical Centre for Agricultural and Rural Cooperation
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