Background: Development of insecticide resistance is a serious problem in the control of German cockroach population in home and hospital. Knowledge about the resistance mechanisms would help us to control the pest. This study was conducted to determine the cyfluthrin resistance in German cockroaches using the piperonyl butoxide (PBO).

Materials and Methods: In this study, cyfluthrin (4-6 concentration ranges) was used on susceptible strains- for ten years without any contact with the lab-grown poison - with the mortality rate ranged from 0% to 100%. After determining the concentrations of the cyfluthrin in the susceptible strains, the level of susceptibility and resistance mechanism in the wild strains were determined using PBO in vivo. Data were analyzed using Probit program.

Results: Lethal dose (LD50) and resistance ratio (RR) for susceptible and three wild strains were 133.43-503.6 and 1.48-3.77, respectively. LD50 of cyfluthrin+PBO and synergist ratio for susceptible and three wild strains were 51.3-490.53 and 1.03-2.43, respectively.

Conclusion: All three collected strains showed different levels of resistance compared to the susceptible strains. In almost all strains, PBO considerably increases the cyfluthrin toxicity. The findings signify the involvement of monooxygenase in cyfluthrin resistance.