Possible mechanisms of organophosphorus and carbamate insecticide resistance in German cockroaches (Dictyoptera: Blattelidae) from different geographical areas.

The resistance status of 14 strains of Blattella germanica (L.) from four countries was determined for chlorpyrifos and propoxur compared with a standard reference susceptible strain. Thirteen strains were resistant to chlorpyrifos; 12 strains were resistant to propoxur. Resistance ratios for chlorpyrifos ranged from 8- to 462-fold at LC90; for propoxur, resistance ratios ranged from 4- to 46-fold. One cockroach strain from Denmark had negative cross-resistance to chlorpyrifos, and one strain from the United States had negative cross-resistance to propoxur. Slopes of probit regressions indicated that all resistant strains were heterogeneous for resistance to both chlorpyrifos and propoxur. Synergist studies with piperonyl butoxide indicated that multifunction mono-oxidases are probably involved in resistance to chlorpyrifos in six strains and in resistance to propoxur in seven strains. Esterase activity was elevated in 10 strains; of these strains, two had only slightly elevated esterase activity as measured with the substrates 1- and 2-naphthyl acetate. The remainder had higher levels of elevated esterase activity to both substrates. Strains with elevated esterase activity were resistant to a broad spectrum of organophosphates, pyrethroids, or both. Increased levels of glutathione S-transferase activity were found in four strains. Another two strains had a low frequency (1%) of individuals with high glutathione S-transferase activity. Elevated glutathione S-transferase activity was not correlated with the observed levels of organophosphate or carbamate resistance. One strain from Dubai had an altered acetylcholinesterase-based mechanism that conferred broad-spectrum resistance to a range of organophosphates and carbamates.