A SURVEY ON CYRTOBAGOUS SALVINIAE: A PREDATORY INSECT OF SALVINIA MOLESTA IN TANK CASCADE SYSTEMS OF MEDAWACHCHIYA DIVISIONAL SECRETARIAT OF SRI LANKA

Abstract

bstract: Tank cascade systems are threatened by the presence of aquatic weeds, which interfere with the water storage, irrigation water delivery systems and mainte- nance of canals and drains. Salvinia molesta is one of the major noxious invasive weeds in the tank cascade systems of Sri Lanka, causing severe environmental prob- lems to their ecosystems. Control of this weed with biocontrol agents has been show- ing promising results in many parts of the world. Cyrtobagous salviniae, a biocontrol agent of this weed, was introduced to Sri Lanka in 1986 by the Department of Agri- culture. The lack of information on the ecology of these weeds and their distribution is one of the key problems in implementing a sound biological control program in the north-central province. Therefore, the objectives of study were to identify the abun- dance and distribution of S. molesta and the population density of C. salviniae in se- lected tanks in the Medawachichiya divisional secretariat. The quadrate sampling method was used to find predator and weed density in 28 selected tanks located in the Medawahchiya Divisional secretary areas of Sri Lanka. The index of dispersion and intraspecific mean crowding of predators were also calculated. Results found that the surfaces of the Medawachchiya and Kadawathrambewa tanks were covered more than 80% with the S. molesta while less than 40% of the water surfaces were covered in 16 other tanks. The abundance of C. salviniae was significantly different (P< 0.05) among the tanks, and higher predator densities were recorded in Kadawathrambawa (10±3.20) and Mahadiulwewa (9.06±3.45) and the lowest densities were recorded in Siyabagaswewa (1.3±1.25) and Periyakulama (0.9±0.88) tanks. Among the tanks, Dutuwewa, Siyabalagaswewa, and Periyakulama showed low intraspecific crowding, indicating a lower population. Therefore, the re-introduction of predators is urgently necessar