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Quantitative post-release evaluation of biological control of Pistia stratiotes Linnaeus (Araceae) by the weevil Neohydronomus affinis Hustache (Coleoptera: Curculionidae) in Senegal

The South American floating aquatic plant Pistia stratiotes (Araceae) was introduced into Senegal in the early 1990s and by 1994 had covered some 2 034 ha on Lake Guiers. The biological control agent Neohydronomus affinis (Coleoptera: Curculionidae) was released in this area between September 1994 and April 1995. A quantitative post-release evaluation showed that the weed cover decreased by about 25% every two months, completely eliminating the water lettuce mats within eight months. The weevils dispersed some 150 km in 18 months and controlled water lettuce in several water bodies in the Djoudj National Park. A resurgence of the weed in the Senegal River Delta in 2005 prompted further releases of the weevil, once again resulting in effective control. The biological control programme against water lettuce in Senegal has been highly successful and the weed is no longer considered problematic.