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Title: Discovery of the Rdl mutation in association with a cyclodiene resistant population of horn flies, Haematobia Irritans (Diptera: Muscidae)

Author
item DOMINGUES, LUISA - Louisiana State University Agcenter
item Guerrero, Felicito
item ALLISON, MONTGOMERY - Louisiana State University Agcenter
item FOIL, LANE - Louisiana State University Agcenter

Submitted to: Veterinary Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/19/2013
Publication Date: 11/15/2013
Citation: Domingues, L.N., Guerrero, F., Allison, M.W., Foil, L.D. 2013. Discovery of the Rdl mutation in association with a cyclodiene resistant population of horn flies, Haematobia irritans (Diptera: Muscidae). Veterinary Parasitology. 198(1-2):172-179.

Interpretive Summary: The horn fly, Haematobia irritans irritans, is an obligate blood-feeding parasite of cattle that causes significant economic impact in many countries. This pest is a continuing problem since it has became resistant to most of the chemicals available for its control. We investigated the resistance of a horn fly population from Louisiana to a new ear tag product containing endosulfan, a cyclodiene insecticide. Bioassays were performed in 2010 and 2011 in order to determine the resistance level of the population and a diagnostic PCR assay was developed to detect the presence of a gene mutation that causes target site resistance to endosulphan. Endosulfan tags provided 8 weeks of control in 2010 and only 1 week in 2011. Thus, only one summer of exposure to the endosulfan tags was enough to significantly increase the resistance ratio of the population such that economic control was lost. Before endosulphan treatment began, most flies surveyed by the diagnostic PCR assay did not contain the resistance-causing gene mutation. When present, the resistance-causing mutation was mainly present in the heterozygous condition with one copy of the susceptible gene and one copy of the mutated gene. After the first season of exposure to the endosulfan tags, the percentage of the R allele increased significantly. However, after 1 year without any treatment, the percentage of the R allele significantly dropped. The results indicate that target site resistance was at least in part responsible for the resistance and that there might be a fitness cost associated with the target site gene mutation.

Technical Abstract: The horn fly, Haematobia irritans irritans, is an obligate blood-feeding parasite of cattle that causes significant economic impact in many countries. This pest is a continuing problem since it has become resistant to most of the drugs available for its control. In this study, we investigated the resistance of a horn fly population from Louisiana/USA to endosulfan, a cyclodiene insecticide. Bioassays were performed in 2010 and 2011 in order to determine the resistance ratio of the population to endosulfan and a PCR assay was developed to detect the Rdl mutation, the replacement of an alanine with a serine at the GABA receptor locus that was previously described in other insect species and is associated with resistance to cyclodienes. Endosulfan tags provided 8 weeks of effective control in 2010 and only 1 week in 2011. Only one summer of exposure to the endosulfan tags was enough to significantly increase the resistance ratio of the population. Most flies surveyed by the PCR assay were homozygous susceptible for the Rdl mutation. The resistant (R) allele was mainly present in heterozygous flies and there was no difference in the percentage of the R allele between females and males. After the first exposure to the endosulfan tags, the percentage of the R allele increased significantly. However, after 1 year without any treatment, the percentage of the R allele significantly dropped. The results indicate that target site resistance was at least in part responsible for the resistance and that there might be a fitness cost associated with the Rdl mutation.