Toxicity of Pyrethroid and Organophosphorous Insecticides against Two Field Collected Strains of the German Cockroach Blattella germanica (Blattaria: Blattellidae) from Hospitals in Hamadan, Iran

Background: The German cockroach, Blattella germanica is a major hygienic pest and mechanical vector for pathogenicagents in hospitals and residential areas. The development of insecticide resistance is a serious problem incontrolling of this pest. Toxicity of four commonly used insecticides (permethrin, cypermethrin, malathion andchlorpyrifos) against two hospital- collected strains of the German cockroach was investigated.Methods: Topical bioassay methods were carried out for detecting insecticide susceptibility of adult male cockroaches.For each insecticide, four to six concentrations resulting in >0% and <100% mortality were used. Three tosix replicates of 10 cockroaches per concentration were conducted. The differences between LD50 (μg/g) values wereconsidered statistically significant only when the 95% confidence intervals did not overlap.Results: Two hospital- collected strains of the German cockroach showed low to moderate levels of resistance tochlorpyrifos, permethrin, malathion and cypermethrin based on resistance ratios compared with susceptible strain.Conclusion: The low level chlorpyrifos resistance suggesting this insecticide may still provide adequate control ofthese strains. While the obsereved moderate levels of resistance to cypermethrin could imply developing resistanceto this compound.