Unprecedented incidence of
dengue has been recorded in Sri Lanka in recent times. Source reduction and use of
insecticides in space spraying/fogging and larviciding, are the primary means of controlling the vector mosquitoes Aedes aegypti and Ae. albopictus in the island nation. A study was carried out to understand
insecticide cross-resistance spectra and mechanisms of insecticide resistance of both these vectors from six administrative districts, i.e. Kandy, Kurunegala, Puttalam, Gampaha, Ratnapura and Jaffna, of Sri Lanka. Efficacy of the recommended dosages of frequently used
insecticides in space spraying and larviciding in
dengue vector control programmes was also tested.
Insecticide bioassay results revealed that, in general, both mosquito species were highly resistant to
DDT but susceptible to
propoxur and
malathion except Jaffna Ae. aegypti population. Moderate resistance to
malathion shown by Jaffna Ae. aegypti population correlated with
esterase and
malathion carboxylesterase activities of the population. High levels of
acetylcholinesterase (AChE) insensitivity in the absence of
malathion and
propoxur resistance may be due to non-synaptic forms of AChE
proteins. Moderate
pyrethroid resistance in the absence of high
monooxygenase levels indicated the possible involvement of 'kdr' type resistance mechanism in Sri Lankan
dengue vectors. Results of the space spraying experiments revealed that 100% mortality at
a 10 m distance and >50% mortality at a 50 m distance can be achieved with
malathion, pesguard and deltacide even in a ground with dense vegetation. Pesguard and deltacide spraying gave 100% mortality up to 50 m distance in open area and areas with little vegetation. Both species gave >50% mortalities for deltacide at a distance of 75 m in a dense vegetation area. Larval bioassays conducted in the laboratory showed that a 1 ppm
temephos solution can maintain a larval mortality rate of 100% for ten months, and the mortality rate declined to 0% in the eleventh month. In the field, where 1 ppm concentration is gradually decreased with water usage, 100% mortality was observed only for the first four months, <50% mortality for the next two months, and 0% mortality was observed eight months after the application of
temephos. Deltacide can be effectively used for space spraying programmes in Sri Lanka. Larval control can be successfully achieved through
temephos with public participation.