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Bogged down with aquatic weeds?

Water Hyacinth
Water Hyacinth

"Beauty is in the eye of the beholder". This is certainly the case with Water Hyacinth, which has invaded waterways worldwide after being introduced as an attractive ornamental plant. But its eye-catching purple and violet flowers belie the damage it does to aquatic eco-systems and the livelihoods of people depend on them. Now infamous as the "world's worst water weed", the introduction of Water Hyacinth (Eichhornia crassipes) to Africa's wetlands alone has caused billions of dollars of damage. But it is not the only aquatic water weed to invade Africa's waters.

In an IUCN report "Alien Invasive Species in Africa's wetlands" published in February 2003, four types of aquatic weeds were listed in the top seven "worst invasive species". Besides Water Hyacinth, Azolla (Azolla filiculoides), Water Lettuce (Pistia stratiotes) and Water Fern (Salvinia molesta) affect a wide number of African countries, whilst Parrot's Feather (Myriophyllum aquaticum), currently a major problem in South Africa, has the potential to become more widespread. Other non-weed invasive species include the Lousiana crayfish and the common carp.

Economic cost

Of all invasive weed species, aquatic weeds are perhaps the most pernicious. With the warm tropical sunlight, they reproduce and spread to rapidly cover lakes and rivers. The thick weed mats make fishing impossible and disrupt water transport, irrigation systems and hydroelectric schemes. Water quality is also affected as the dense vegetation prevents sunlight from reaching other plants, which die and decompose. A loss of oxygen from the water increases its acidity and, as a result, aquatic biodiversity starts to decline. Thick vegetative cover also hides crocodiles and snakes, making it hazardous for local people to collect water. But more hazardous to humans is the risk of diseases (such as malaria and schistomiasis), as the weeds provide an ideal breeding ground for the vectors (e.g. mosquitoes and snails).

The management of Africa's wetlands, as well as lakes and rivers, increasingly has to reflect the impact of these weeds. Huge amounts of time and money are involved in attempts to restore aquatic ecosystems and to bring the weeds under control as, once present, they can rarely be eradicated. Africa's greatest success is perhaps best demonstrated through the collaborative efforts to bring Water Hyacinth under control in Lake Victoria, although unfortunately not before this and other invasive species, including Nile Perch, caused enormous ecological damage. The Lake is said to have experienced the greatest number of extinctions in modern times.

Battling for biological control

Lake with water weed, Ghana
Lake with water weed, Ghana

It is now well documented that a weevil or two won the war against Water Hyacinth and biological control for this and other aquatic weeds appears to be the best way forward. Mechanical control has been used in the past but, as Water Hyacinth and Salvinia can double their mass in only 12 days, these plants tend to grow faster than they can be cleared. Other weeds, such as Parrot's Feather, reproduce from fragmented rhizomes so mechanical clearance is more likely to stimulate rather than stop further weed spread. Chemical control is also an option and, despite fears of polluting waterways and killing other plants, it has been widely practised in various African countries. On the whole, however, use of herbicides against aquatic weeds has not been a success. Use of machines and chemicals are also expensive in both cost and labour and are viewed as suitable for short-term control only. Biological control, on the other hand, is relatively inexpensive; it requires only the initial investment to rear the insects. However, it is not a "quick-fix" solution as insect colonies take time to establish but, once they have been established, little further input is required.

Insects for biological control, many from South America where most of the invasive weeds have originated, have now been identified for all the major aquatic weeds affecting Africa's waterways. Neochetina spp. (N. eichorniae and N. bruchi) were key to the successful clearing of Water Hyacinth in Lake Victoria. They were first introduced into Sudan, and over the last two decades have been widely put to use wherever water hyacinth is a problem. In Benin, for instance, IITA estimates that the beetles have saved the economy US $260 over twenty years. But beetles are not the only weapons in the biological armoury; an Argentine moth (Niphograpta albiguttalis), whose larvae feed on the weed, a leaf-mining mite (Orthogalumna terebrantis) and a grasshopper (Cornops aquaticum), have all been tested and released by the Plant Protection Research Institute in South Africa. An added ally may be found in the use of mycoherbicides, developed from fungal pathogens. Recent selections made by the DANIDA funded "International Mycoherbicide Programme for Eichhornia crassipes Control in Africa" (IMPECCA) include Alternaria eichhorniae as it is indigenous to Africa, virulent and specific to Water Hyacinth. The use of the South American rust fungus (Uredo eichhorniae) is also being investigated.

Date published: March 2004

 

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