Establishment, spread and impact of Neochetina spp. weevils (Coleoptera: Curculionidae) on Water Hyacinth in Lake Victoria, Kenya
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The Kenya Agricultural Research Institute imported 12,300 curculionid weevils (Neochetina spp.) from diverse sources, for biological control of water hyacinth in Lake Victoria, as part of the World Bank-funded Lake Victoria Environmental Management Project in East Africa. In addition to the rearing and quarantine facility at Muguga, a second rearing facility was established in 1996 at Kibos, near Lake Victoria. The Kibos rearing facility and two community rearing facilities at the lakeshores, have produced approximately 100,000 adult weevils and 42,000 weevil eggs over a three-year period. Since January 1997, some 73,500 Neochetina weevils have been released at 29 sites and an additional 10,000 redistributed at several sites. Visual observations and regular sampling monitored the establishment and spread and also evaluated the impact of Neochetina weevils on water hyacinth. Within two years, weevils were established at 55% of release sites and were being recovered 50 km from release sites. Post-release sampling data from four release sites in Berkeley, Kisumu and Kendu bays, indicated a reduction in leaf length, laminar area and fresh weight of water hyacinth, and a significant increase in number of weevil feeding scars and adult weevils per square metre. Three years after the initial weevil releases, the combined mean number of weevils per plant for Kisumu, Nyakach, Kendu and Homa bays, was estimated to be six, well above the critical threshold of five weevils per plant. N. bruchi was the dominant species accounting for 73.3% of the total weevil population. Thus, under Lake Victoria conditions, the critical threshold was attained within 2–3 years of the initial releases.
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