Susceptibility level of hospital–collected strains of German cockroach Blattella germanica (L.) to carbamate and organophosphorous insecticides using surface contact method in Tehran (2007-2008)

Background: The German cockroach, Blattella germanica (L), is a world wide hygienic pest that is very difficult to control. Extensive usage of insecticides may develop resistance to a variety of insecticides, causing failure to control. This study was designed to determine resistance of hospital-collected strains of the German cockroach to carbamate and organophosphorous insecticides.

Methods: The insecticide resistance status in nine hospital- collected strains of the German cockroach, Blattella germanica (L.) against two carbamate insecticides (bendiocarb, carbaryl) and two organophosphorous insecticides (malathion, chlorpyriphos) were detected using glass jar test method. For detection of resistance in adult males the susceptibility level of each field strain was compared with standard susceptible strain based on the mortality rate obtained from exposing them to a single discriminating dose.

Results: The results of mortality tests of four insecticides on adult males of susceptible strain demonstrated the discriminating doses for chlorpyriphos (6.63mg/m2), bendiocarb (66.34mg/m2) , malathion (179.14 mg/m2) and carbaryl (390.11mg/m2) respectively. The lowest and highest discriminating doses belonged to chlorpyriphos and carbaryl respectively. Among nine adult males strains two strains were resistant to bendiocarb and two strains were resistant to carbaryl. Some strains also showed a wide range of tolerance to malathion and chlorpyriphos.

Conclusion: This study proposed that carbamate insecticides (bendiocarb & carbaryl) did not have efficacy for control of German cockroach due to development of resistance in the strains. Our results showed that the resistance to organophosphorous insecticides is developing, consequently, the usage of these insecticides is not recommended. Investigation on chlorpyriphos resistance mechanisms is suggested for the possibility of rotational usage of this compound with pyrethroid insecticides could be evaluated.