The water hyacinth Eichhornia crassipes is considered the world’s worst invasive aquatic weed. Indigenous to the Amazon Basin of South America, it was introduced to many parts of the world as an ornamental plant, and today occurs in more than 50 countries on five continents.In Africa, it was first recorded in the 1890s from the River Nile in Egypt, but has since become widespread throughout the continent. The plant thrives in still and slow-moving water-bodies that have become nutrient-enriched through eutrophication, and dense mats of water hyacinth now blanket many of Africa’s dams, lakes, rivers and canals.
Understanding our worst invader
Water hyacinth is a perennial aquatic weed that is usually free-floating, although its long, feathery hanging roots may anchor it in shallow water. Individual plants are typically 100-200 millimetres high, but can reach a height of 1 metre when growing in dense mats. The showy flowers are lilac-blue with yellow markings, and each produces about 300 seeds. The seeds sink after being released from the seed capsule and can remain viable for up to 20 years, contributing to the plant’s success as an invader.
Once a seed germinates, on moist sediments or in warm shallow water, the plant grows rapidly and can flower within 10 to 15 weeks. Individual plants or small clumps of water hyacinth may disperse downstream and can easily spread to new areas during floods. The population increases mainly through vegetative re p roduction, the plants budding to form daughter plants that break off and become entangled in dense mats.
Water hyacinth infestations are associated with a variety of socio-economic and environmental impacts. Dense mats that block waterways inhibit boat traffic, and hence disrupt trade, fishing and recreational activities. They also clog irrigation canals and pumps, and threaten hydro-electric power schemes. By impeding water flow and trapping particles in suspension they increase siltation of rivers and dams. They adversely affect the quality of drinking water, and pose a health risk by creating conditions suitable for mosquitoes and bilharzia-carrying snails.
The thick mats reduce light penetration into the water, which causes a decline in phytoplankton concentrations that support the zooplankton-fish food chain, resulting in ecosystem changes. Rotting material depletes oxygen levels in the water, further impacting aquatic biodiversity. Furthermore, vast quantities of water hyacinth can damage road and rail bridges when swept downriver during floods.
As a readily available resource, water hyacinth has been used in paper, rope, basket and biogas production, as fodder for livestock, as mulch and compost for crop cultivation, and as a biological filter in water treatment schemes. Although some of these uses are successful as cottage industries, they are not commercially viable on a large scale.
This is because water hyacinth is more than 90% water, so it is not cost-effective to remove and transport.
Manual removal of water hyacinth, although very labour-intensive, can be useful in controlling small infesta-tions. For example, community groups using rakes and pitchforks removed over 200 tonnes of the weed from landing beaches in the Mara district of Tanzania in one year.
However, in many parts of Africa such work carries the risk of exposure to attack by snakes and crocodiles and to waterborne diseases such as bilharzia. Even mechanical harvesters are impractical in infes-tations larger than a hectare, due to the rapid rate of increase of the weed. These machines are also very expensive to purchase and operate, and the harvested material must be removed for utilisation or proper disposal to prevent plants and seeds returning to the water.
Nevertheless, mechanical harvesters have been successful in some areas. On the Ugandan side of Lake Victoria, for instance, they have been used to provide ships’ access to Port Bell and prevent weeds from entering the intake pipes of the hydroelectric power scheme at Owen Falls Dam. Floating booms have also helped protect the hydroelectric schemes at Owen Falls Dam and Zambia’s Kafue Gorge Dam, and have been widely used in other areas to contain the weed.
On the Vaal River in South Africa, cables spanned across the river have been used to accumulate isolated plants moving downstream, allowing them to be more easily treated with herbicides. Herbicides such as glyphosate, diquat and 2,4-D amine, sprayed from aircraft, boat-mounted units or knapsack sprayers, provide a relatively cheap control option, and rapid results can be obtained. However, although relatively safe if applied by skilled operators, these herbicides are non-selective, and require ongoing follow-up spraying to control reinfestation.
Biological control – a sustainable option
Biological control is the only control option that is sustainable in the long term. The first successful biological control programme in Africa was in Sudan, after the weevils Neochetina eichhorniae and N. bruchi were released on the White Nile in the 1970s. These biocontrol agents have proved so effective that they have since been released in about 20 countries across the continent.
The adult weevils feed on the leaves of water hyacinth, while the larvae eat their way down the petioles and into the crown, the growth point of the plant. This feeding damage stunts growth, impedes reproduction of the plants, and at high intensities causes them to rot, die and sink. Wind and wave action help to break up water hyacinth mats already weakened by the weevils.
Other biocontrol agents have also been introduced to supplement the effectiveness of the two weevils. In South Africa, these include a petiole-boring moth Niphograpta albiguttalis, a sap-sucking mirid Eccritotarsus catarinensis, a leaf-mining mite Orthogalumna terebrantis and a fungal pathogen Cercospora rodmaini, while the potential of a number of other natural enemies of water hyacinth are currently being considered.
The existing biocontrol agents have been successful in controlling water hyacinth in some areas of South Africa, but not in others. This is partly because the worst areas of infestation are in high-altitude regions subject to cold winters. The mite O. terebrantis cannot establish in such a climate, and population increase of the weevil N . eichhorniae is suppressed. The moth N. albiguttalis can withstand cold winters, but since it prefers feeding on young or actively growing plants that are not always found in mature infestations, its distribution is patchy, seasonal and temporary.
The weevil N. eichhorniae also seems poorly adapted to the Western Cape’s Mediterranean climate and nutrient-enriched conditions. Although N. bruchi is more effective in such eutrophic conditions, and is also cold-tolerant, populations of both species have been negatively impacted by seasonal flooding and mechanical control operations to remove water hyacinth.
Indeed, the failure of biological control in many instances can be attributed to inappropriate integration with chemical and mechanical control. It is therefore imperative that integrated management plans are imple-mented on a site-specific basis. Furthermore, since eutrophication and reduction in water flow create a stable and nutrient-rich environment in which water hyacinth flourishes, these plans should include nutrient and hydro-logical control where possible.
Lake Victoria - a success story in biological control
One of the most notorious cases of water hyacinth infestation and subsequent control occurred at Africa’s Lake Victoria, the second largest freshwater lake in the world. During the 1980s, water hyacinth invaded the headwaters of Rwanda’s Kagera River, which empties into Lake Victoria at the Tanzania-Uganda border. The weed was first noticed in the lake in 1989, but it thrived in the eutrophic conditions and spread rapidly.
By the mid-1990s it covered about 12 000 hectares of the lake surface. Dense mats of water hyacinth collected against the shorelines of Kenya, Tanzania and Uganda, disrupting fishing and trade by blocking ports and landing areas. In Uganda, the weed interfered with operations at the Owen Falls Dam hydroelectric scheme, resulting in power cuts that affected the country ’s industrial output. The water hyacinth mats also impeded water circulation, creating an ideal breeding ground in the stagnant water for malarial mosquitoes and the snail hosts of bilharzia. The cumulative affect of these impacts was a downturn in the region’s economic productivity.
Biological control was considered the only viable method of dealing with the weed, so the two weevils Neochetina eichhorniae and N. bruchi were released there in 1995. A number of weevil-rearing stations were set up around the lake and along the Kagera River. With the help of local fishing communities, several million weevils were released. Some manual removal and mechanical control efforts continued, but by late 1999 much of the weed
had died off . Many bays were already clear of weed, and the remaining mats were heavily damaged by weevils. Fishing activities had recommenced, and the lake’s transport system had been revived.
Today water hyacinth has been effectively brought under control, and now covers only 2 000 hectares of lake surface.
Reference: Matthews S. & Brandt K. Africa Invaded: The growing danger of invasive alien species Global Invasive Species Programme 2004