Key Words  
  Multi-resistant, pyrethroids, Blattella, organophosphate resistance, pyrethroid resistance, glutathione-S-transferase
  Multi-resistant strains of Blattella germanica are now common, and strategies for their continued control using conventional insecticides need to be devised rationally if long-term suppression of these nuisance insects is to be achieved. Newer insecticides such as growth regulators may have a role in such strategies, but these cannot be used wholly as substitutes for conventional insecticides, due to their high cost and mode of action, which may result in a residual nymphal population being present over a long period. This may, wrongly, be perceived as control failure by householders. In order to devise control strategies for multi-resistant B. germanica the resistance status of thirty strains from four countries was determined for a range of pyrethroids. Strains were collected on the basis of recently reported pyrethroid resistance. Greater than 2-fold resistance to a range of pyrethroids occurred in fifteen of these strains and, of these fifteen, thirteen strains were resistant to chlorpyrifos and twelve to propoxur. All the field strains tested were heterogeneous with regard to resistance. The mechanisms of resistance were determined in all fifteen multi-resistant strains. The elevated esterase and oxidase-based resistance mechanisms were the most prevalent, with eleven and ten strains respectively having evidence of these mechanisms. Pyrethroid resistance could be synergised by piperonyl butoxide in strains with only the oxidase mechanism. In some strains the elevated esterase mechanisms detected were primarily correlated with organophosphate resistance, while in others they conferred resistance to both organophosphates and pyrethroids. Increased levels of glutathione- S-transferase activity were found in four strains and an altered acetylcholinesterase-type, organophosphate and carbamate resistance mechanism was seen in one strain. There was no evidence of resistance to commercially available growth regulators in any of the strains tested. On the basis of the types of pyrethroid resistance mechanisms detected in field populations of B. germanica a two- tiered approach to control of these populations was devised, the optimum strategy being determined by the characteristics of the population to be controlled. Where oxidase-based resistance alone occurs, a synergist/pyrethroid combination can be used, where multiple resistance mechanisms occur a pyrethroid/growth regulator combination is used. The growth regulator is used at a low dosage, which effectively sterilises both male and female cockroaches. The fast-acting pyrethroid controls the majority of the population, resulting in rapid perceived control, while the highly pyrethroid resistant remainder of the population which survive are sterilised by the growth regulator, giving longer term control.