Impact of the weevils Neochetina eichhorniae and N-bruchi (Coleoptera: Curculionidae) on water hyacinth, Eichhornia crassipes (Pontederiaceae), in Benin, West Africa

Between 1991 and 1993, two weevils species, Neochetina eichhorniae (Warner) and N. bruchi Hustache (Col.: Curculionidae), were released at several localities on the Oueme and Zou River systems in Benin for biological control of water hyacinth, Eichhornia crassipes. In Benin, N. eichhorniae became the dominant and ubiquitous species, whereas N. bruchi remained confined to the release localities. The impact of these weevils was assessed at two localities (Tevedji and Lihu) between 1992 and 2000, by measuring water hyacinth cover, aspects of plant growth, and adult feeding damage. At both sites, water hyacinth cover was reduced from 100 % to 5 %. Leaf and petiole lengths were reduced by about 45 % at Lihu and 60 % at Tevedji, indicating declining vigour of surviving plants. The number of adult feeding scars peaked 2-3 years after release of the weevils.