Mechanical, chemical, and biological control methods are commonly used to control WH (Julien et al, 2001), but no one method is suitable for all situations (Gopal, 1987).

Mechanical control includes harvesting by hand or machine (Villamagna & Murphy, 2010). The use of machinery to remove WH from water bodies is the most effective non-polluting control method (Mara, 1976), especially in critical areas such as hydroelectric dams and ports. The main advantage to the use of mechanical harvesting is the simultaneous removal of nutrients and pollutants from the water body, and may therefore act as a means of slowing or even reversing eutrophication (Wittenberg & Cock, 2001). Mechanical harvesting of WH has also resulted in rapid increases in dissolved oxygen, and improved suitability of the habitat to support fish. However, it requires recurring efforts involving machine and labor inputs (Mara, 1976). Mechanical removal with harvesters is also slow and therefore not suitable for large mats. Studies have shown that the costs of mechanical harvesting are on average US$ 600 to 1,200 per hectare, about six times more expensive than chemical treatment using glyphosate (Wittenberg & Cock, 2001).

Chemical herbicides are the principal means of control when an immediate solution to a WH problem is needed (Charudattan, 1986). Glyphosate and 2, 4- D [(2, 4-dichloro phenoxy) acetic acid] have been the most widely used herbicides and are considered effective and safe herbicides to control WH (Chen et al., 1989; Charudattan, 1986). They are relatively cheap, with costs per hectare for aerial application of US$ 25-200 (Wittenberg & Cock, 2001). Treated plants die and decompose in a few days to a few weeks. Despite such effectiveness of herbicides, the major disadvantages are that they are non-selective and could cause major environmental problems if incorrectly applied (Wittenberg & Cock, 2001). Chemical control needs to be carried out repeatedly as re-infestation of WH occurs from seeds or clonal multiplication of surviving plants (Chen et al., 1989; Charudattan, 1986).

When chemical control is economically unfeasible or harmful to the environment, biological control is recognized as a cost effective, permanent and environmentally friendly control method. Using natural enemies from their original ecosystem is a prime target for biological control. Since 1971, two South American weevils, Neochetina eichhorniae and Neochetina bruchi, have been widely introduced in Australia, Asia and Africa (Wittenberg & Cock, 2001). In some areas, they have provided considerable control, but this is not consistent in all areas. The principal drawback with biological control of WH is the time required to achieve control. In tropical environments, this is usually 2 to 4 years and is influenced by the extent of the infestation, climate, water nutrient status, and other control options (Wittenberg & Cock, 2001).