The fruiting and seed dynamics of the alien invasive aquatic species Pistia stratiotes L. (water lettuce; Araceae) was investigated in the seasonally flooded Selinda Canal and Zibadianja Lake of the Kwando-Linyanti River system and in the perennial Chobe River, Botswana, in 1999-2003. The mean number of 30.3 seeds fruit -1 in the Selinda Canal is the highest ever recorded. An artificial earthen barrier or dyke was constructed on the Selinda Canal to allow manipulation of water levels downstream. By drying and subsequently reflooding a region of the stream, seed germination was stimulated, followed by the manual removal of water lettuce seedlings before their seeds reached maturity, which resulted in a decline in seed germination in surface sediment samples from 63.5% in 2002 to 31.7% in 2003. Manipulation of flooding, followed by the physical removal of P. stratiotes at regular intervals prior to anthesis and seed maturity, is considered the most viable strategy for arresting further additions to the seed bank, and could lead to its eradication in seasonally flooded areas where the biocontrol weevil Neohydronomus affinis has become locally extinct since its introduction in 1987.

Aquatic ecosystems in Botswana have been under threat by the aquatic alien invasive plant species viz., salvinia Salvinia molesta Mitchell, water lettuce Pistia stratiotes L., and water hyacinth Eichhornia crassipes (Mart.) Solms-Laub. While salvinia has been termed the major threat to the Botswana wetlands, water lettuce and water hyacinth are considered to be of minor importance. This review presents the species biology, distribution, historical spread, negative impacts, control achieved right from their discovery in the country by referring to their control and management in the world.Having infested the Kwando-Linyanti-Chobe Rivers in the 1970s, salvinia was initially tried by the use of herbicides, paraquat and glyphosate, between 1972 and 1976. With the discovery of the host specific biological control weevil Cyrtobagous salviniae Calder and Sands in 1981, the weevil was introduced by Namibians on Kwando and Chobe Rivers in 1983 and by Botswana in 1986 in the Okavango Delta. While the control was slowly establishing in Kwando-Linyanti-Chobe Rivers, it became apparent that lakes and perennial swamps within and outside Moremi Game Reserve of the Okavango Delta were infested with salvinia from 1992 onwards.With continuous and sustained liberation of the weevil in the Kwando-Linyanti-Chobe Rivers and in the Okavango Delta between 1999 and 2000, salvinia control was achieved by 2003, and since then the weevil constantly keeps the weed at low levels. The success is mainly due to sustainable monitoring through the application of physical and biological control methods. However, salvinia is still threatening the Okavango Delta due to factors such as tourism activities, boat navigation fishing and transporttion by wild animals.The first occurrence of water lettuce was recorded on Kwando and Chobe Rivers in 1986. Its biocontrol weevil Neohydronomous affinis Hustache was released in the year 1987. The weevil became extinct in Selinda Canal and Zibadianja Lake on Kwando River due to dry and wet events for over 10 years and the weed had been under control biologically on Chobe River. Having surface covered the Selinda and a part of the Zibadianja in high flood and rainfall in 1999/2000 season, research was undertaken to contain water lettuce, which led to its eradication by 2005. Regular physical removal of the water lettuce prior to fruit maturity is an effective method of control or eradicating the weed in seasonal water bodies.The Limpopo Basin (shared by Botswana, South Africa, Zimbabwe and Mozambique) has become vulnerable to water hyacinth infestation. Water hyacinth infested the trans-boundary Limpopo River in 2010 sourced from Hartbeesport Dam on Crocodile River in South Africa. Botswana and South Africa have been consulting each other to implement integrated control of the weed jointly in the Limpopo River. Water hyacinth could be a continuous threat to the dams and the rivers in the Limpopo basin if its control is not taken seriously.These three species are found growing in Botswana in a range of pH...

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