Resistance to insecticides and effect of synergists on permethrin toxicityin Pediculus capitis (Anoplura : Pediculus) from Buenos Aires

Permethrin-resistant colonies of Pediculus capitis (De Geer) from Buenos Ai res were used to establish a resistance profile and to examine resistance m echanisms. All permethrin-resistant head lice (resistance ratio from 52.8 t o >88.7) were also resistant to d-phenothrin (resistance ratio from 40.86 t o >48.39) and deltamethrin (resistance ratio from 16.24, to 38.06). No cros s-resistance to carbaryl was found in any of the pyrethroid-resistant P. ca pitis tested. Otherwise, all resistant colonies showed low to high levels o f resistance to beta-cypermethrin. This pyrethroid had never been applied a s a pediculicide in Argentina; however, the high level of resistance found in these permethrin-resistant colonies (resistance ratio from 9.74 to 50.97 ) demonstrated that pyrethroid cross-resistance occurred to this novel inse cticide. Treatment with piperonyl butoxide (PBO) or triphenylphosphate (TPP ) significantly decreased the toxicity of permethrin in the four colonies t ested. The esterase inhibitor TPP produced lower enhancement of toxicity th an the multifunction oxidase inhibitor PBO in the colonies having the highe st resistance levels. Results presented here concerning the cross-resistanc e profile and synergism by enzyme inhibitors in permethrin-resistant head l ice demonstrated that enhanced metabolism was involved in the pyrethroid re sistance. However, the substantial degree of resistance that remained after synergism suggested the presence of another resistance mechanism. Cross-re sistance to pyrethroid and susceptibility to the carbamate carbaryl suggest ed a common action mechanism.