title: Salinity-tolerant larvae of mosquito vectors in the tropical coast of Jaffna, Sri Lanka and the effect of salinity on the toxicity of Bacillus thuringiensis to Aedes aegypti larvae.
abstract: BACKGROUND
  Dengue, chikungunya, malaria, filariasis and Japanese encephalitis are common mosquito-borne diseases endemic to Sri Lanka. Aedes aegypti and Aedes albopictus, the major vectors of dengue, were recently shown to undergo pre-imaginal development in brackish water bodies in the island. A limited survey of selected coastal localities of the Jaffna district in northern Sri Lanka was carried out to identify mosquito species undergoing pre-imaginal development in brackish and saline waters. The effect of salinity on the toxicity of Bacillus thuringiensis israelensis larvicide to Ae. aegypti larvae at salinity levels naturally tolerated by Ae. aegypti was examined.
  
  
  METHODS
  Larvae collected at the selected sites along the Jaffna coast were identified and salinity of habitat water determined in the laboratory. The LC₅₀ and LC₉₀ of B. thuringiensis toxin, the active ingredient of a commercial formulation of the larvicide BACTIVEC®, were determined with Ae. aegypti larvae. Bioassays were also carried out at salinities varying from 0 to 18 ppt to determine the toxicity of Bacillus thuringiensis to fresh and brackish water-derived larvae of Ae. aegypti.
  
  
  RESULTS
  Larvae of four Anopheles, two Aedes, one Culex and one Lutzia species were collected from brackish and saline sites with salinity in the range 2 to 68 ppt. The LC₅₀ and LC₉₀ of B. thuringiensis toxin for the second instar larvae of Ae. aegypti in fresh water were 0.006 ppm and 0.013 ppm respectively, with corresponding values for brackish water populations of 0.008 and 0.012 ppm respectively. One hundred percent survival of second instar fresh water and brackish water-derived Ae. aegypti larvae was recorded at salinity up to 10 and 12 ppt and 100% mortality at 16 and 18 ppt, yielding an LC₉₀ for salinity of 13.9 ppt and 15.4 ppt at 24 h post-treatment respectively for the two populations. Statistical analysis showed significantly reduced toxicity of B. thuringiensis to fresh and brackish water-derived Ae. aegypti larvae at high salinities.
  
  
  CONCLUSIONS
  A variety of mosquito vectors of human diseases undergo pre-imaginal development in brackish or saline waters in coastal areas of the Jaffna district in northern Sri Lanka. Salinity has a small but significant negative impact on the toxicity of B. thuringiensis toxin to Ae. aegypti larvae at salinity levels where Ae. aegypti larvae are found in the environment. This has implications for the use of B. thuringiensis toxin as a larvicide in brackish waters.
authors.affiliation: Department of Zoology, Faculty of Science, University of Jaffna, Jaffna 40000, Sri Lanka.
authors.forename: Pavilupillai J
authors.initials: PJ
authors.lastname: Jude
authors.forename: Tharmatha
authors.initials: T
authors.lastname: Tharmasegaram
authors.forename: Gobika
authors.initials: G
authors.lastname: Sivasubramaniyam
authors.forename: Meena
authors.initials: M
authors.lastname: Senthilnanthanan
authors.forename: Selvam
authors.initials: S
authors.lastname: Kannathasan
authors.forename: Selvarajah
authors.initials: S
authors.lastname: Raveendran
authors.forename: Ranjan
authors.initials: R
authors.lastname: Ramasamy
authors.forename: Sinnathamby N
authors.initials: SN
authors.lastname: Surendran
mesh_terms: D000330:Aedes / Q000187:drug effects* / Q000502:physiology*; D000818:Animals; D001427:Bacterial Toxins / Q000633:toxicity*; D017753:Ecosystem; D005368:Filariasis / Q000453:epidemiology / Q000635:transmission; D007303:Insect Vectors; D007306:Insecticides / Q000737:chemistry; D007814:Larva / Q000187:drug effects / Q000382:microbiology; D008288:Malaria / Q000453:epidemiology / Q000635:transmission; D054712:Salinity; D055049:Salt Tolerance / Q000502:physiology*; D012965:Sodium Chloride / Q000494:pharmacology*; D013188:Sri Lanka / Q000453:epidemiology; D014777:Virus Diseases / Q000453:epidemiology / Q000635:transmission
chemical_list: C049673:Bacillus thuringiensis ovicidal toxin; D001427:Bacterial Toxins; D007306:Insecticides; D012965:Sodium Chloride