Insecticide resistance profiles and possible underlying mechanisms in German cockroaches, Blattella germanica (Linnaeus) (Dictyoptera: Blattellidae) from Peninsular Malaysia

Insecticide resistance and possible underlying mechanisms were studied in 52 strains of the German cockroach, Blattella germanica (L.) field-collected from Peninsular Malaysia. These strains were assayed using a lethal time assay at lox susceptible LC95 concentrations of propoxur (l.10,ug/cm2), chlorpyrifos (18.40,ugl cm2), permethrin (1.70 ,ug/cm2) and deltamethrin (0.55 ,ug/cm2) using a surface-contact exposure method with reference to the susceptible ICI strain. Results indicated that propoxur and permethrin resistance were highly prevalent (73.0% and 80.2% of LT50 RR > lox, respectively) while low prevalence of deltamethrin (29.6%) and chlorpyrifos (4.0%) resistance (RR >5) were detected. Resistance level was partially suppressed in most of the strains when piperonyl butoxide (lOO,ug/insect) and S,S,S,-tributylphos- phorotrithioates (30 ,ug/insect) were used, indicating possible involvement of elevated monooxygenase and esterase. In addition, altered acetylcholinesterase was also suspected in eight strains. The percentage mortality of field populations were found to be distributed at two extremes when treated topically with deltamethrin at LC99 (0.056 ,ug/insect), indicating higher numbers of homogenous susceptible and resistant strains, respectively in those populations tested. Results obtained using topical application (percentage mortality) and surface-contact exposure method (resistance ratio) were not well correlated (r2=0.169, P=0.023).