Insecticide resistance and synergism of three field-collected strains of the German cockroach Blattella germanica (L.) (Dictyoptera: Blattellidae) from hospitals in Kermanshah, Iran.

The development of insecticide resistance in the German cockroach, Blattella germanica (L.) is a serious problem in controlling this medically important household pest. The insecticide resistance status in three hospital-collected strains of the German cockroach using four commonly used insecticides from different classes (permethrin, cypermethrin, bendiocarb and chlorpyrifos) was detected by topical bioassay method and preliminary information on possible involvement of monooxygenases in permethrin resistant strains employing synergist piperonyl butoxide (PBO) was obtained. For each insecticide, four to six concentrations resulting in >0% and <100% mortality were used. Three to six replicates of 10 cockroaches per concentration were conducted. For synergism studies, 100 μg PBO per gram body weight of cockroach as the maximum sublethal dose was administered to the first abdominal segment 1 h before insecticide treatment. The differences between LD50 (μg/g) values were considered statistically significant only when the 95% confidence intervals did not overlap. The resistance ratio and synergism ratio were calculated for each insecticide. All three hospital-collected strains of the German cockroach showed different levels of resistance to permethrin and cypermethrin based on resistance ratios compared with SUS strain. Permethrin and cypermethrin resistance ratios ranged from 11.61 to 17.64 and 11.45 to 26.45 at LD50 levels, respectively. Low to moderate levels of bendiocarb resistance and low level of chlorpyrifos resistance were also observed in the hospital-collected strains under study. The synergist piperonyl butoxide (PBO) significantly enhanced the toxicity of permethrin to all strains with different degrees of synergist ratio, 2.45-, 1.87-, 2.51- and 2.38-fold, suggesting monooxygenase involvement in permethrin resistance.