There has been no comprehensive study on biochemical characterization of insecticide resistance mechanisms in field populations of Malaysian Culex quinquefasciatus. To fill this void in the literature, a nationwide investigation was performed to quantify the enzyme activities, thereby attempting to characterize the potential resistance mechanisms in Cx. quinquefasciatus in residential areas in Malaysia.
The ability to identify the occurrence of different resistance genotypes in field populations of mosquito is considered important for the purpose of optimising chemical control operations. The recent development of rapid microassays of enzymes responsible for resistance has provided a means for rapidly assessing the genetic background of target mosquito populations. This concept is the topic of investigation in this study. Non-specific esterase activity, which is responsible for the resistance to organophosphates in Malaysian Culex quinquefasciatus Say adults, was determined in 3 field populations from Kuala Lumpur City using rapid enzyme assay. The optical density results were used to estimate the genotypic frequencies of the populations. Subsequently, time-dependent changes in the various frequencies were determined. Such techniques allowed rapid assessment of resistance genotypes for decision-making and its possible use in insect control merits further investigation.
Larvae and adults of Culex quinquefasciatus were used for the test undertaken for malathion resistant strain (F61 - F65) and permethrin resistant strain (F54 - F58). The results showed that the LC50 for both malathion (F61 - F65) and permethrin (F54 - F58) resistant Cx. quinquefasciatus increased steadily throughout the subsequent five generations, indicating a marked development of resistance. The adult female malathion resistant strain have developed a high resistance level to malathion diagnostic dosage with a resistance ratio of 9.3 to 17.9 folds of resistance compared with the susceptible Cx. quinquefasciatus. Permethrin resistance ratio remained as 1.0 folds of resistance at every generation. It was obvious that malathion resistance developed at a higher rate in adult females compared to permethrin. Enzyme-based metabolic mechanisms of insecticide resistance were investigated based on the biochemical assay principle. From the results obtained obviously shows that there is a significant difference (p < 0.05) in esterase level in both malathion and permethrin selected strains. Female malathion selected strain has the higher level of esterase activity compared to the female permethrin selected strain at (0.8 to 1.04) alpha-Na micromol/min/mg protein versus (0.15 to 0.24) alpha-Na micromol/min/mg protein respectively. This indicated increased level of non-specific esterase is playing an important role in resistance mechanism in female malathion selected strain. Permethrin selected strain exhibited non-specific esterase activity at a very low level throughout the different life stages compared to malathion selected strain. This study suggests that life stages play a predominant role in conferring malathion and permethrin resistance in Cx. quinquefasciatus.
Given that there is limited available information on the insensitive acetylcholinesterase in insect species in Malaysia, the present study aims to detect the presence of G119S mutation in the acetylcholinesterase gene of Culex quinquefasciatus from 14 residential areas across 13 states and a federal territory in Malaysia.