Association of the kdr and superkdr sodium channel mutations with resistance to pyrethroids in Louisiana populations of the horn fly, Haematobia irritans irritans (L.)

Pyrethroid resistance in three horn fly populations in Louisiana was monitored by weekly fly counts, filter paper bioassays, and diagnostic PCR assays for the presence of pyrethroid resistance-associated mutations in the sodium channel gene coding region. The PCR assay for the knockdown resistance (kdr) and superkdr sodium channel mutations was used to determine the frequency of the target site insensitivity mechanism in the populations of horn flies, which possessed varying degrees of insecticide resistance. The bioassays and frequency of homozygous-resistant (RR) kdr genotypes were relative predictors of the fly control subsequently observed. Flies exposed to filter paper impregnated with a discriminating concentration of one of four different insecticides were collected when 50% mortality was estimated. Genotypes for the dead flies and the survivors were determined by the PCR assay. The results of the PCR assays indicated that the genotype at the kdr locus of the flies exposed to the two pyrethroids had an effect upon whether the flies were considered to be alive or dead at the time of collection. The kdr genotype of flies exposed to either diazinon or doramectin was unrelated to whether the flies were considered to be alive or dead, except for a single comparison of flies exposed to diazinon. When possible interactions of the kdr and superkdr mutations were compared, we found that there were no associations with the response to diazinon and doramectin. For one location, there were no survivors of the 75 flies with the SS-SS (superkdr-kdr) homozygous susceptible wild type genotype exposed to pyrethroids, while there were survivors in all of the other five genotypes. The SS-RR genotype flies were more susceptible to the pyrethroids than the SR-RR flies, but that was not the case for exposure to diazinon or doramectin. In the St. Joseph population, there were an adequate number of flies to demonstrate that the SS-SR genotype was more susceptible to pyrethroids than the SS-RR and that flies with the SR-SR genotype were more susceptible to pyrethroids than the flies with the SR-RR genotype.