Throughout the tropical ...

Throughout the tropical regions of the world, the invasive aquatic plant Water Hyacinth is blocking vast areas of lakes, rivers and canals. Water Hyacinth can double in volume every 12 to 18 days. It blocks harbours, destroys economic fisheries, paralyses hydro-electric power stations and encourages the spread of diseases such as malaria, cholera and bilharzia.The three main current control mechanisms,

1. Biological,
2. Chemical and
3. Manual, have only limited effect and can produce undesirable environmental consequences.

Biological control

Biological control is the use of host specific natural enemies to reduce the population density of a pest. Several insects and fungi have been identified as control agents for water hyacinth. These include a variety of weevils, moth and fungi. Biological control of water hyacinth is said to be environmentally benign as the control agents tend to be self-regulating. Control programmes are usually inexpensive due to the fact that the control agents are known and only a small numbers of staff are required to run such programmes. One major drawback is that it can take a long time to initiate such projects because it can take several years for the insect population to reach a population density sufficient to tackle the pest problem.

Chemical control

The application of herbicides for controlling water hyacinth has been carried out for many years. The common herbicides are 2,4-d, Diquat and Glysophate. It has been found that there is a good success rate when dealing with small infestations but less success with larger areas. Application can be from the ground or from the air and requires skilled operators. As mentioned earlier the main concern when using herbicides is the environmental and health related effects, especially where people collect water for drinking and washing.

Physical control

Mechanical removal of water hyacinth is seen as the best short-term solution to the proliferation of the plant. It is however costly, using either land-based ‘clamshell’ bucket cranes, draglines or booms or, alternatively, water based machinery such as mowers, dredges, barges or specially designed aquatic weed harvesters. Such methods are suitable for only relatively small areas. Many of these techniques require the support of a fleet of water and land-based vehicles for transporting the large quantities of water hyacinth which is removed. Manual removal of water hyacinth is suitable only for extremely small areas. It is difficult, labour intensive work and in some areas there are serious health risks associated with the work (crocodiles, hippopotamus and bilharzia in Lake Victoria for example).

Reduction of nutrient run-off into the water

Besides these three mainstream forms of control Harley, Julien and Wright suggest another method, namely the reduction of nutrient inputs to the water. Although strictly speaking this is a preventative method, it can be argued that a reduction in nutrients in the water body will result in a reduction in the proliferation of water hyacinth. In recent decades there has been a significant increase in the level of nutrients dumped into waterways from industrial and domestic sources as well as from land where fertilisers are used or where clearance has caused an increase in run-off. It seems that the systematic use of successive vetiver hedges along the river and lake beds, beside the positive impact on the bank protection, shows positive results in retaining nutrients, pollutants and sediments from the run-off to flow in the water.