Susceptibilities of two colonies of the taxon An. dirus (one from Perlis and from Thailand) and one colony of An. balabacensis from Sabah to DDT, dieldrin, malathion, fenitrothion and propoxur were determined. DDT and fenitrothion tolerance was found in An. dirus species B and An. balabacensis. No resistant strain was isolated as the two colonies were not either homozygous or nearly so for resistance. Field testing of the susceptibility of the adults of An. balabacensis to DDT was carried out between 1957 to 1976. The results indicated a progressive rise in the LC50 levels greater than 1% in almost all instance. The variation in the number of sprays applied in some districts have resulted in varying sensitivities. Association between the changes in levels of DDT susceptibility and exophilic habit of An. balabacensis has been observed but needs further confirmation. The significance of these findings and the difficulties in distinguishing tolerant from truly resistant individuals are discussed in relation to accurate species identification.
Given the lack of molecular evidence in altered target-site insecticide resistance mechanism in Aedes albopictus (Skuse) worldwide, the present study aims to detect the presence of A302S mutation in the gene encoding the gamma aminobutyric acid receptor resistant to dieldrin (Rdl) in Ae. albopictus for the first time from its native range of South East Asia, namely Malaysia. World Health Organization (WHO) adult susceptibility bioassay indicated a relatively low level of dieldrin resistance (two-fold) in Ae. albopictus from Petaling Jaya, Selangor. However, PCR-RFLP and direct sequencing methods revealed the presence of the A302S mutation with the predomination of heterozygous genotype (40 out of 82 individuals), followed by the resistant genotype with 11 individuals. This study represents the first field evolved instance of A302S mutation in Malaysian insect species.
Extensive usage and heavy reliance on insecticides have led to the development of insecticide resistance in the German cockroach, Blattella germanica (L.). Six field-collected strains of B. germanica from Singapore were used to investigate resistance to fipronil and dieldrin. The three strains (Boat Quay, Cavenagh Road, and Ghimmoh Road) with greatest resistance to fipronil were subjected to selection with fipronil bait up to the F5 generation. Synergism assay and molecular detection of a target site mutation were used to elucidate the mechanism of fipronil resistance in these strains. With the exception of the Cavenagh Road strain, all parental strains were susceptible to dieldrin. This strain exhibited resistance to dieldrin and fipronil with resistance ratios of 4.1 and 3.0, respectively. Piperonyl butoxide and S,S,S-tributylphosphorotrithioate were antagonistic toward fipronil toxicity in all strains. Bait selection significantly increased fipronil and dieldrin resistance in the three chosen strains, either in topical bioassay or bait evaluations. There was a significant positive relationship [y = (6,852.69 +/- 1,988.37) x - (708.93 +/- 1,226.28), where x = fipronil toxicity and y = dieldrin toxicity] between dieldrin and fipronil resistance levels, indicating significant cross-resistance between the insecticides. High frequencies of individuals possessing the Rdl gene mutation were found in the F5 generation of the three strains selected with fipronil bait. The synergism assays indicated that monooxygenase and esterase were not involved in fipronil resistance in the strains studied herein. The A302S Rdl mutation was the major mechanism contributing to fipronil and dieldrin resistance in these strains.