Detection of insecticides resistance status in Culex quinquefasciatus and Aedes aegypti to four major groups of insecticides

The resistance to various insecticides from 4 major groups (organochlorine, organophosphate, carbamate and pyrethroid) was investigated in a field strain of Culex quinquefasciatus from Baan Suan community, Nonthaburi province, Thailand by using a standard World Health Organization susceptibility test. The Baan Suan strain was completely resistant to DDT and highly resistant to deltamethrin, permethrin, fenitrothion and propoxur but this strain was still found to be highly susceptible to malathion. This strain displayed high resistance to cypermethrin since the result revealed that the resistance ratio of the 50% lethal concentration value (RR50) between the field and the laboratory strains (NIH strain) was 16. The study indicated that mosquitoes were resistant to almost all insecticide tested except malathion and this should be an alternative for Cx. quinquefasciatus control in this area. Moreover, Aedes aegypti, which is a main dengue vector in Baan Suan community was also tested with deltamethrin, permethrin and fenitrothion. The results showed that dengue mosquitoes are clearly resistant to permethrin and tolerant to deltamethrin, but was 100% susceptible to fenitrothion. The cause of insecticide resistance in Cx. quinquefasciatus may be due to the continuous use of insecticide for dengue vector control programme in Baan Suan community.

Insecticide susceptibility and selection for resistance in a population of Aedes aegypti from Ratchaburi province, Thailand

The insecticide susceptibility of Aedes aegypti larvae and adults from four areas of Ratchaburi Province, Thailand was investigated using World Health Organization standard procedures. The larvae of Ae. aegypti in all areas were found to be susceptible to temephos. The strain with the highest level of temephos resistance was subjected to temephos selection. The resistance ratios increased to 2.74-fold from their parental non selected strain after 5 generations of selection. For the adult of Ae. aegypti, the study was conducted to determine the effects of 0.75 % permethrin and 0.05 % deltamethrin impregnated paper selection on the strain having the highest with the highest LT50 level. The results showed the increase in the LT50 to 7.46 and 1.18-fold after the third selected generation with 0.75 % permethrin and 0.05 % deltamethrin respectively. Mortality rates after exposing adults to discriminating concentrations showed that field populations in Ratchaburi Province were resistant to permethrin and deltamethrin. Alternative non-chemical measures need to be used in new of the emergence of resistance in the mosquito population.

Biochemical detection of pyrethroid resistance mechanism in Aedes aegypti in Ratchaburi province, Thailand

The emergence of insecticide resistance in mosquito vectors was an important issue to be considered as one of factors influencing the success of vector control. The early detection of resistance could help the health personnel to plan and select appropriate alternative control measures or insecticide for effective control. Therefore biochemical assay of enzymes in mosquito was conducted to detect the emergence of insecticide resistance and to define the machanisms involved in pyrethroid resistance. Adults of Aedes aegypti from two localtities in Ratchaburi province were subjected to permethrin and deltamethrin selection in laboratory. After three generations of selection, permethrin-selected and deltamethrin-selected strains were established. Their LT 50 increased to 7.46 and 1.18 folds in the F3 strains that were selected with permethrin and deltamethrin respectively. The enzymes of these mosquitoes were assayed biochemically to study the mechanisms of resistance. The results revealed significant increase of esterase activity and monooxygenase levels in both strains when compared with labolatory susceptible strain. Glutathione-S-transferase activity was found to increase in permethrin-selected strain but not in deltamethrin-selected strain. This suggested that not only esterase and monooxygenase but also glutathione-S-transferase were associated with permethrin resistance in Ae. aegypti. The exposing of permethrin-selected and deltamethrin-selected mosquitoes to diagnostic concentration of permethrin (0.75%) and deltamethrin (0.05%) indicated no cross resistance for permethrin to deltamethrin while slight cross resistance from deltamethrin to permethrin was evident. It seemed that glutathione S-tranferase was not associated with cross resistance since its activity in deltamethrin-selected strain remained unchanged as compared with that of laboratory susceptible strain.