AGP - Sri Lanka - Water weeds

The problem of water weeds in Sri Lanka

By Lucky Amarasinghe and Ricardo Labrada


Sri Lanka is an agricultural country with a population of 19 million people. Majority of the farming community relies on irrigation water to raise successful crops as the country experiences periodic droughts during cropping season. In order to provide assured supply of water for agriculture activities, over ten thousand reservoirs which locally known as minor tanks, and a vast number irrigation networks have been developed over centuries in the country.

Regular maintenance of reservoirs, lakes, rivers and irrigation networks, thus, becomes a crucial factor for overall sustenance of irrigated agriculture and the livelihood of farming communities associated with wetland habitats. Among the several factors that constraints the productivity of inland water bodies in the country, the excessive growth of aquatic weeds, such as floating, submerged and rooting emerged species. Some of these aquatic species are exotic species that have been entered the country at different periods during the last century. Water hyacinth, one of the worst aquatic weeds in the world, has been intentionally introduced for ornamental purpose in 1904 and it is widely distributed in Sri Lanka despite its declaration as a prohibited weed under Water Hyacinth Act in 1909 and subsequently under Plant Protection Act in 1924. Water fern, Salvinia molesta, is also an exotic plant, brought into the country in 1939 for experimental purposes by the Colombo University around 1943. This floating weed rapidly invaded over large extents of aquatic habitats, engulfing over 10,000 ha of rice fields in the Western province by 1954. Incidentally, Salvinia was declared as a noxious weed under Plant Protection Act in 1952.  

A recent conducted survey showed that 60% of the water bodies are infested with Salvinia, 45% with water hyacinth and 17% with water lettuce (Pistia stratiotes). The results further disclosed that of the local aquatic species, Lotus and Typha javanica also have developed into epidemic proportions, occupying over 40 to 60 % of the water bodies in the district.

Submerged species Hydrilla verticillata is a prevailing weed in the Mahaweli irrigation networks, while alligator weed (Alternanthera philoxeroides) presents minimal populations, which can quickly reach unmanageable levels.

The main approach for the control of aquatic weeds in Sri Lanka has been classical biological control, which has been practised from early fifties when Salvinia was declared as a prohibited weed under Plant Protection Act. Initial attempts made on controlling aquatic weeds followed mechanical approach supplemented with chemical control. However, all these methods failed to provide long lasting effects due to rapid multiplication of the weeds and the relatively low efficacy of control methods. Subsequently, control attempts were diverted towards biological control when an effective biocontrol agent was introduced on to Salvinia in late seventies.  The insect, Cyterbagous salviniae, gave successful control of Salvinia in most of the aquatic habitats at low elevations. However, the insect failed to provide effective control at elevations above 1000 m from sea level due to low temperatures. Successful results observed in Salvinia made an impetus for further expansion of biological control activities and as a result the weevil Neochetina eichhorniae brought from Thailand was introduced in 1981 to control water hyacinth. Recent observations have revealed that although weevil shows a widespread distribution in the aquatic habitats across the country, it has failed to provide an effective degree of control of the weed.

An adequate sample of Neochetina bruchi, another bio-control agent for the control of water hyacinth, was also recently imported.

Most of the present work has been on setting up an unit for insect rearing at Kandhi, conducting training on rearing / release of bio control agents, preparing relevant materials on this subject, awareness programmes for rural communities on release of bio-control agents.

However, there is much work to be done, such as continuing weed survey, improving the quality of the rearing process of biocontrol agents of water hyacinth and Salvinia checking possible diseases caused by microsporidia in both species of Neochetina in rearing colonies; introducing the water lettuce weevil (Neohydronomus affinis) as soon as possible before this weed increases to unmanageable populations; examine the possibility of introducing alligator weed biocontrol agents, especially, flea beetles, and hydrilla leaf-mining flies (Hydrellia pakistanae and H. balciunasi) for management of H. verticillata.

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