Prevalence and Magnitude of Insecticide Resistance in the German Cockroach (dictyoptera, Blattellidae)

Numerous studies have shown that German cockroaches, Blattella germanica (L.), are often resistant to insecticides in settings where insecticides have been used extensively in the past. We have now examined resistance in German cockroaches collected in homes in which insecticide usage has been minimal, and resident-initiated at best. Resistance to propoxur, chlorpyrifos, pyrethrins, permethrin, and cypermethrin was evaluated with a discriminating dose technique. Logit analysis was used to identify topical dosages of the insecticides expected to kill 99% of adult males in an insecticide-susceptible strain. These dosages, or slightly higher ones, were then used to screen for resistance in males of field-collected strains. All strains exhibited resistance to every insecticide, but the degree of resistance varied. Propoxur was the only insecticide to kill, with great frequency, more than 10% of insects, whereas the other insecticides usually killed less than 5%. The magnitude of resistance was assessed by treating insects with 25-fold more insecticide than the discriminating dosage. This amount of chlorpyrifos and propoxur killed almost all insects, but large numbers of males from most colonies survived being treated with high dosages of pyrethrins, permethrin, and cypermethrin. We conclude that insecticide resistance is a ubiquitous phenomenon in the German cockroach.