1. Chinhoyi University of Technology Invasive Species Research and Management Unit AQUATIC (WATER) INVASIVE SPECIES IN ZIMBABWE Water hyacinth (Eichhornia crassipes (Martius) Origin: South America in the Amazon Basin in north- eastern Brazil, Venezuela, Guyana and Surinam. Description: Floating aquatic weed that belongs to the family Pontedereceae which is closely related to lily ( Liliceae) family. Damage caused by the species: Grows rapidly and forms thick impregnable mats that block navigation and deny access to rivers and dams for fishing, tourism and recreational uses. Severe infestations are associated with high levels of pollution of water with industrial and sewage waste. Water hyacinth displaces other species from the ecosystem. Despite the introduction of biological weevils, Neochetina eichhorniae and N. bruchi in Zimbabwe, it remains the most problematic aquatic weed in Zimbabwe For more information contact Professor A. B. Mashingaidze. Iabmashingaidze@cut.ac.zw abmash@yahoo.com mobile 0774999054abmashingaidze@cut.ac.zwabmash@yahoo.com Azolla or Red Water Fern (Azolla filiculoides Lam) Origin: According to Lumpkin and Plucknett (1980), A. filiculoides is native to the Rocky Mountain states of the western USA and Canada, through Central America and to most of South America. It has been introduced to Europe, North and sub-Saharan Africa, China, Japan, New Zealand, Australia, the Caribbean and Hawaii. Description: A. filiculoides is a small aquatic heterosporous fern, rarely larger than 25 mm (O'Keeffe, 1986). The genus is unique in that it grows in association with a nitrogen-fixing heterocystous cyanobacterium (blue-green alga), Anabaena azollae Strasburger (Nostocales: Nostocaceae), which is located in cavities in the dorsal leaf-lobes (Ashton and Walmsley, 1984). Damage caused by the species: A. filiculoides spread to Zimbabwe in about 1980 and now infests water bodies throughout the country. It forms thick mats which eliminate submerged plants and algae, prevent photosynthesis and oxygen diffusion from the air and anaerobiosis, which eliminate fish and other animals from the water beneath them, reducing biodiversity (Gratwicke and Marshall, 2001). In South Africa it was effectively controlled through the release of a front feeding Stenopelmus rufinasus Gyllenhal (Coleoptera: Curculionidae), that was imported from Florida, USA (MaConnachie et al., 2003). Kariba weed or Giant Salvinia (Salvinia molesta Mitchell) Origin: It is native in south-eastern Brazil and has spread throughout the tropics and subtropics in the Americas, Africa and Asia. Description: It is a free-floating aquatic fern belonging to the family Salviniaceae. The fronds are 0.5–4 cm long and broad, with a bristly surface caused by the hair-like strands that join at the end to form eggbeater shapes. The bristles trap air, the “salvinia effect”, allowing the fern to float on the water surface. Damage caused by the species: The floating fern showed explosive growth after the creation of Lake Kariba in 1985. By 1962 it covered 22% of the Lake surface but later declined to a stable level of 10-15%. Coverage declined again in 1973 to 5% and by 1980 only 1% of the lake was coved by the fern. The decline was attributed to a biological control agent, the Neotropical grasshopper, Paulina acuminata De Geer and nutrient stress on the fern as other components of the ecosystem grew and competed for nutrients (Marshall and Junor, 1981). Effective biological control of the fern is now being accomplished by the introduction of weevil, Cyrtobagous salviniae, into water impoundments infested by the weed (Chikwenhere and Keswani, 1997). Water lettuce or Nile Cabbage (Pistia stratiotes L.) Origin: It has a cosmopolitan distribution throughout tropical and sub-tropical regions. In South and Central America, Africa and South-East Asia it is considered an endogenous species. Description: P. stratiotes is a perennial monocotyledonous aquatic plant that floats on the water surface, with roots hanging below floating leaves belonging to the family Araceae. Damage caused by the species: First observed in Mukuvisi river in 1937, and 50 years later was the major aquatic weed in northern Zimbabwe. By 1988, it had infested Manyame river, Seke, Darwandale, Chivero, Kariba, Chakoma, Chivake and Kaitano dams. Introduction of the biological control weevil Neohydronomas affins in 1988 reduced infestations of the weed by more than 90% in all the impoundments (Chikwenhere, 1994). No further outbreaks of the weed have so far reported attesting to the effectiveness of the biological control agent.