Fipronil resistance in a multi-resistant field strain of the German cockroach, Blattella germanica L. (Dictyoptera: Blattellidae)

Fipronil, a phenylpyrazole insecticide, has been in use for German cockroach control since 1999. Previously, dieldrin resistant field strains of the German cockroach were shown to have 7- to 17-fold resistance to fipronil. In the present study, we have identified a field strain (GNV-R) that has ~36-fold resistance to topically applied fipronil. To determine the mechanism(s) of fipronil resistance in this strain synergist bioassays were conducted. Piperonyl butoxide (PBO) and S,S,S,-tributyl-phosphorotrithioate (DEF) significantly synergized fipronil toxicity in the GNV-R strain, indicating cytochrome P450 monooxygenase and esterase mediated detoxification of fipronil in this strain . Additionally, electrophysiological recordings were done to test the sensitivity of the central nervous system to fipronil. Spontaneous electrical activity recorded after application of fipronil was significantly lower in the GNV-R strain as compared to the susceptible strain. In agreement with the results of the electrophysiological recordings, frequency of the resistance associated A302S mutation in the resistance to dieldrin (Rdl) gene was high (85%) in the GNV-R strain. The A302S mutation is known to confer high level of dieldrin resistance and low level of fipronil resistance in the German cockroach. Taken together, results of the synergism bioassays, electrophysiological recordings and A302S mutation frequency suggest that high-level of fipronil resistance in the GNV-R strain is caused by a combined effect of enhanced metabolism and target-site modification.