Insecticide susceptibility of mosquitoes invading tsunami-affected areas of Thailand

In order to control the mosquitoes invading tsunami-affected areas of Thailand, the insecticide susceptibility status of field larvae and mosquitoes (Anopheles sundaicus and Culex sitiens) was tested under laboratory conditions. Larval bioassay tests were conducted using the WHO standard method. Three larvicides: temephos, malathion, and plant extract (ethanolic extract of the Southeast Asian long pepper. Piper retrofractum Vahl), were used in the experiments. The results revealed that Cx. sitiens was more susceptible to temephos than malathion and the plant extract, with LC50 ranges of 0.0008-0.0014 mg/l, 0.0046-0.0078 mg/l, and 5.3180-10.1030 mg/l, respectively. Cx. quinquefasciatus showed greater tolerance to every tested larvicide than Cr sitiens. Adult bioassay tests using a WHO test kit and diagnostic doses of 5% malathion, 0.75% permethrin, 0.05% deltamethrin, and 4% DDT were also conducted. The results revealed that Cx. sitiens and An. sundaicus were susceptible to all tested insecticides. The LT50 of 5% malathion ranged between 25.7-26.0 minutes for Cx. sitiens, and 44.7 minutes for An. sundaicus. In addition, Cx. quinquefasciatus showed susceptibility to malathion, with LT10 of 19.7 minutes. However, it showed resistance to both pyrethroid insecticides, with LT50 of 33.1 minutes for 0.75% permethrin, and 19.6 minutes. for 0.05% deltamethrin; it showed low percentage mortality at 24 hour post-exposure, of 48 and 32%, respectively. In conclusion, every tested larvicide could be used for controlling Cx. sitiens larvae, even in brackish water, pyrethroid insecticides for adult Cx. sitiens and An. sundaicus, and malathion for all three species.