Efficacy of three insecticides against Anopheles dirus and Anopheles minimus, the major malaria vectors, in Kanchanaburi Province, Thailand

Abstract

We conducted this study to determine the insecticide susceptibility of two malaria vectors, Anopheles dims and Anopheles minimus from Kanchanaburi Province, Thailand. The mosquitoes were collected and reared under laboratory conditions. The test was carried out on unfed F-l female mosquitoes using a standard WHO testing protocol. The LD 50 , and LD 90 of deltamethrin in both species were tested for by exposing the mosquitoes to various doses of deltamethrin for 1 hour. The lethal time was also tested among mosquitoes by exposing them to deltamethrin (0.05%), permethrin (0.75%) and malathion (5%), for different exposure times, ranging from 0.5 to 15 minutes. Percent knockdown at 60 minutes and mortality at 24 hours were calculated. The resistance ratio (RR) was determined based on the LD 50 and LT 50 values. LD 50 of deltamethrin against An.dims and An.minimus were 0.00077% and 0.00066%, respectively. LT 50 values for deltamethrin (0.05%), permethrin (0.75%) and malathion (5%) against An.dims and An.minimus were 1.20, 3.16 and 10.07 minutes and 0.48, 1.92 and 5.94 minutes, respectively. The study revealed slightly increased tolerance by both mosquito species, compared with laboratory susceptible strains, based on LD 50 values. The two anopheline species had the same patterns of response to the three insecticides, based on LTjg values, although the LT 50 values were slightly higher in the An. dims population. Both An. dims and An. minimus were fully susceptible to all the insecticides tested, with 100% mortality at 24 hours post-exposure. Deltamethrin was the most effective insecticide, followed by permethrin and malathion.