Multi-insecticide susceptibility evaluation of dengue vectors Stegomyia albopicta and St. aegypti in Assam, India

BACKGROUND: Dengue is rapidly expanding mosquito-borne viral infection globally facing operational challenges due to insecticide resistance in dengue vectors. We have studied the susceptibility status of potential dengue vectors St. albopicta and St. aegypti to the commonly used insecticides. METHODS: Stegomyia larval bioassays were carried out to determine LC(10), LC(50) and LC(99) values and resistance ratios (RR(50) and RR(99)) for temephos. Adult susceptibility bioassay to 4% DDT, 0.05% deltamethrin, 5% malathion was assessed following standard procedure to determine the corrected mortality. Knock-down times (KDT(50) and KDT(99)) were estimated and the knock-down resistance ratios (KRR(50) and KRR(99)) were calculated. RESULTS: St. albopicta wild population (WP) of Sotia was resistant to temephos as the LC(99) value was 0.12 mg/l and found to be 2.3 fold high than the reference population (RP). St. aegypti WP of Borgong, Kusumtola and Serajuli displayed a RR(99) of 2.5, 5.4 and 4.5 respectively suggesting high level of resistance to temephos. Results suggested that both St. albopicta and St. aegypti WP were fully resistant to DDT (mortality < 90%) in all the study locations. Both the species were completely susceptible to deltamethrin and malathion (corrected mortality > 98%), except for St. albopicta at Sotia which displayed low level of resistance to malathion (corrected mortality =95.4%). The estimated KDT values for both the species indicated high level of knock-down resistance to DDT and susceptibility to deltamethrin. CONCLUSION: WP of both the dengue vectors showed varied response to temephos, while resistant to DDT and completely susceptible to deltamethrin. Both the species were susceptible to malathion at majority of the testing sites. Current results strongly advocate that DDT is no longer effective against dengue vectors, while thorough monitoring of malathion susceptibility in geographical area at dengue risk is inexorable to ascertain whether or not the resistance to malathion is focal. Information generated herein may be useful in better planning and implementing in dengue control strategy using insecticides.