Africa Invaded: Kariba weed

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Salvinia molesta is a free-floating water fern that is native to Brazil. It was first recorded in Africa in 1948, when it was found on the Zambezi River, but is now widely distributed throughout southern Africa. The species has also invaded other parts of the continent, as well as warm regions around the world, where it is commonly referred to as ‘giant salvinia’. It is usually introduced as an ornamental plant for ponds and aquaria.

Rapid expansion

Kariba weed was only recognised as a problem species in Africa in the early 1960s, when the Zambezi River was impounded by Lake Kariba. As the dam began filling up, organic-rich runoff and decomposition of flooded plant material enriched the water with nutrients, providing an effective fertiliser. By 1963 dense mats of the weed covered about 22% of the dam surface, threatening the operation of the hydroelectric power plant.

Similar scenarios are reported wherever Kariba weed invades. The plants grow rapidly, and in favourable conditions may double in number within a week. In the early stages of an infestation, the plants are small and have green leaves that lie almost flat on the water surface. Over time the leaves turn yellowish-green to brown and fold, causing them to interlink when pressed together in dense infestations.

Severe environmental and socio-economic impact

The resulting mats – sometimes up to a metre thick – tend to block waterways, obstructing boat traffic and disrupting fishing activity. They impede access to water by rural communities and their livestock, and clog intake pipes for water supply facilities, irrigation schemes and hydro-electric power plants. They also pose a health risk as they provide a safe and ideal habitat for mosquitoes and other vectors of disease.

Apart from these socio-economic impacts, the dense mats have a variety of negative effects on the environment. They out-compete indigenous species by crowding out floating weeds and reducing the light available to submerged plants and phytoplankton. By blanketing the surface of waterbodies, they prevent atmospheric oxygen from entering the water.

As the plants die and sink to the bottom, bacterial decomposition further depletes oxygen levels, creating conditions unsuitable for invertebrates and fish. The overall effect is a decline in water quality and a reduction in biodiversity. Kariba weed reproduces  vegetatively, and is able to regenerate from any fragment that includes a node. This facilitates its spread by water currents, by birds and mammals, and by boats and vehicles that enter infested waters.


Attempts have been made to control Kariba weed by physical removal, but the plant outgrows most efforts. Herbicides such as terbutryn, diquat and glyphosate have sometimes proved effective, but these put other species at risk as they are non-selective. They also need to be reapplied on an ongoing basis. Biological control using the host-specific weevil Cyrtobagous salviniae is the most sustainable option for controlling Kariba weed. The adult weevils, which are only about 2 millimetres long, feed on leaf buds and young terminal leaves, while the larvae tunnel in the rhizome and also feed externally. The resulting feeding damage causes the plants to become waterlogged, and they eventually sink.

The weevil was first introduced to Africa in 1983 in Eastern Caprivi, Namibia, where it proved very effective. It was subsequently released in a number of other African countries, in many cases reducing Kariba weed by more than 90% in less than a year. In cooler areas, control takes longer, but usually no more than three years. However, the weed’s tissue nitrogen content must be above 1% dry weight, or the weevils fail to establish.

More ornamentals turned invasives

Other invasive aquatic weeds that are particularly problematic in Africa are red water fern Azolla filiculoides, parrot’s feather Myriophyllum aquaticum and water lettuce Pistia stratiotes. All are native to South America and were introduced as ornamental plants to Africa, where they thrived in the absence of natural enemies. Fortunately, biological control agents have been identified for all of these species, and the prospect of bringing the weeds under control is very favourable, should the necessary control initiatives be put in place.

Lessons from Papua New Guinea

During the 1980s, a severe outbreak of salvinia on the island of New Guinea in Papua New Guinea seriously affected the livelihoods of the island community. The lives of the people of the region are linked very closely with the river, as a main source of food and water and as a principal means of travel in the more remote parts. By completely dominating the river system, the salvinia invasion caused some villages to be abandoned altogether when access became impossible, leaving communities without critically needed medical care and food aid assistance. Although largely undocumented, the negative impact on biodiversity in the region was most likely equally significant.

An innovative control programme was initiated during 1982–85 by UNEP and CSIRO Australia, where the biological control agent, Cyrtobagous salviniae was introduced into the more accessible parts of the river system. The challenge was to redistribute it to the rest of the affected area – and this is where radio proved to be a remarkably effective means of spreading the weevil.

The radio-transmitted message was simple: visit already infested lagoons and collect bags of material (salvinia and weevils), take these home and introduce them into the salvinia-invaded areas. Using canoes to ferry weevil-infested bags of salvinia, the local community actively supported the operation, thus ensuring a quicker establishment of the control agent. This method proved to be extremely successful and the weevil quickly infiltrated the entire infested system with resounding success. The resultant rapid control of salvinia on this island is still quoted as one of the most successful cases of biological control to date.

Reference: Matthews S. & Brandt K.   Africa Invaded: The growing danger of invasive alien species  Global Invasive Species Programme 2004

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