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Naivasha Virtual Fieldcourse

Weevils

Water hyacinth, a floating higher plant, was inadvertently introduced from South America in 1988 and rapidly covered a large portion of the lake surface due to the high availability of nutrients. Fishermen, as well as tourist operators, find its presence a nuisance and demand its eradication.

Because it comes from far, no organism living around the lake used to it. In 1996 and again in 2000, the weevil Neochetina originating from South America was introduced to the lake to consume Water hyacinth. Despite some success, the weevil is not able to match Water hyacinth’s rapid growth. Weevils tend to grow best on plants that are drier and slightly higher above the water level, right in the middle of the thickest hyacinth “mats”. Some 67 weevils were captured in early 2012 on a single day. Breeding weevils for enhanced biological control is a promising option for controlling the spread of Water hyacinth in the lake.

Control of floating plants, such as Salvinia and Eichhornia, was attempted through the introduction of the weevils Cyrtobagous salviniae in the early 1990s and of Neochetina bruchii, N. eichhorniae, and Neohydronomus affinis in 1996–2000 (IUCN, 2003). Nearly 20 years after introduction, adult weevils are commonly found within Water hyacinth mats, however it has been realised that their action is insufficient to keep Water hyacinth proliferation at bay.

 

Mottled Water hyacinth weevil (Neochetina eichhorniae, Coleoptera, Curculionidaea, Erirhinidae) caught in Lake Naivasha in 2012