Biology and diversity of Copepods in Sri Lanka and their potential use as predator of Aedes larvae

Abstract

BACKGROUND: Dengue is a major viral disease in tropical and subtropical countries. Although chemical control of the vector is efficient it causes much damage to the natural environment including destroy of other useful insects. There are many natural predators that prey on mosquito larvae such as copepods, dragonfly, damselfly and larvivorous fish. Among them the use of fish has been practiced extensively to control mosquito larvae. Another potential predator of mosquito larvae is a small aquatic organism called copepod, which is extensively used for biological control in South East Asian countries. We are conducting a preliminary survey on the copepods in Sri Lanka and their ability to prey on larvae of dengue vector Aedes spp. METHODS: Water collected from reservoirs in Kandy (species A), Batticaloa (species B) and Kurunegala (species C) were sieved with a net with 50μm mesh size. The trapped copepods were collected into sampling bottles by washing the net thoroughly. The copepods were observed under a stereo microscope to determine their size. The predation studies were conducted with two ratios: 25 first instar Aedes larvae and 5adult copepods(5:1) and 50 first instar Aedes larvae and 5 adult copepods (10:1). The 5:1 ratio was replicated four times while applying the 10:1 ratio thrice. RESULTS: The body length of the Batticaloa (species B) copepods were larger than the Kandy and Kurunegala copepods. The total mortality of larvae (TM) from the study of 5:1 was species A TM=45, species B, TM=70 and species C, TM=49. The average mortality (AM) was, speciesA, AM=11.25±5.6, species B, AM=17.5±3.1 and species C, AM=12.25±2.5.Mosquito larva predation rate per day by a copepod was, species A 2.25±1.12, species B 3.45±0.62 and species C 2.45±0.5.In the second study with a ratio of 10:1, the total mortality was, species A, TM= 86, species B, TM= 123 and species C, TM= 108. Average mortality was, species A, AM= 28.67±14.17, species B, AM=41±6.24 and species C, AM=39.33±14.53. Mosquito larval predation rate per day by a copepod was, species A, 2.87±1.42, species B, 4.1±0.62 and species C, 3.93±1.45. CONCLUSION: Species B from the Batticaloa, which were with larger in body size showed a better predation ability of Aedes when compared with species A (Kandy) and C (Kurunegala).