Dynamics of cypermethrin resistance in the field in the horn fly, Haematobia irritans

The toxicity of cypermethrin to the horn fly Haematobia irritans (L.) (Diptera: Muscidae) was determined for samples collected from untreated herds at a farm in central Argentina from October 1997 to May 2001. Field tests of the efficacy of cypermethrin against horn flies were first carried out at this farm in 1993, when the fly was shown to be susceptible to pyrethroids. Subsequently the horn fly populations on this farm were shown to have become resistant and, since 1997, the use of cypermethrin has been restricted to experimental purposes. In this study, fly samples collected in 1999, 2000 and 2001 were subjected to a polymerase chain reaction (PCR) to detect the presence of a specific nucleotide substitution in the sodium channel gene sequence, which has been associated with target site insensitivity to pyrethroids. This analysis showed that the level of cypermethrin resistance had diminished between 1997 and 2001. However, this was not sufficient to restore the efficacy of this pyrethroid to the level found prior to the onset of resistance. Heterozygous and homozygous resistant flies were detected in all samples of flies subjected to PCR diagnosis of alleles conferring target site resistance.