Submitted to: Journal of Medical Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/18/2003
Publication Date: 11/15/2003
Citation: LI, A.Y., GUERRERO, F., GARCIA, C.A., GEORGE, J.E. SURVEY OF RESISTANCE TO PERMETHRIN AND DIAZINON AND THE USE OF A MULTIPLEX PCR ASSAY TO DETECH RESISTANCE ALLELES IN THE HORN FLY, HAEMATOBIO IRRITANS IRRITANS (L.). JOURNAL OF MEDICAL ENTOMOLOGY. 2003. v. 40. p. 942-949. Interpretive Summary: Resistance to insecticides is a major problem for the control of the horn fly, an important ectoparasite of cattle, in the United States and Mexico. Effective use of insecticides relies on our knowledge of current status of resistance, as well as our ability to detect and monitor resistance in horn flies from any particular region. A study was conducted in 2001 to test responses of horn flies from several ranches in Mexico and Texas to permethrin and diazinon, representing pyrethroid and organophosphate (OP) classes of insecticides. Filter paper bioassays were used to measure their susceptibility to those insecticides. A multiplex PCR assay was used to detect the presence in individual flies of sodium channel gene mutations associated with pyrethroid resistance, and an esterase gene mutation associated with OP resistance. Horn flies from ranches in Mexico had moderate to high resistance to permethrin and high susceptibility to diazinon. Horn flies from ranches in Texas had low level resistance to permethrin, and no resistance to diazinon. High proportions of horn flies from Mexico possessed the mutated sodium channel allele (kdr), while only small portion of horn flies from Texas ranches carried the kdr allele. A linear relationship between kdr allelic frequency and the log transformation of LC50 estimates was established. The data provide a foundation for use of PCR-based molecular diagnostic tool in area-wide survey and monitoring of horn fly resistance to pyrethroids.
Technical Abstract: A field survey was conducted in 2001 to evaluate resistance to pyrethroid and organophosphate (OP) insecticides of horn flies, Haematobia irritans irritans (L.), from seven ranches in the state of Tamaulipas, Mexico and from three locations in central Texas. Filter papers impregnated with either technical permethrin or diazinon were used to measure the levels of resistance to pyrethroids and OPs. A multiplex PCR assay was used on individual horn flies from the field collections to detect presence of the kdr and super- kdr alleles associated with pyrethroid resistance, and a mutated aE7 esterase allele associated with OP resistance. Relative to a susceptible laboratory (Kerrville) strain, horn flies from Mexico exhibited 5.1- to 28.3-fold resistance to permethrin at LC50, and 23.8- to 136-fold resistance at LC90. Horn flies from Texas ranches exhibited only 2 to 5 folds of resistance. All field populations of the horn fly were highly susceptible to diazinon, and no mutated aE7 esterase alleles were detected. The super-kdr allele was found only in a single fly from a ranch in Mexico. Results of PCR assays showed the kdr allele was present at various frequencies in all field populations of horn flies. A gender-related bias in distribution of kdr genotypes was found in horn flies from Mexico, but not in horn flies from Texas. Horn flies from Mexico had significantly higher proportions of homozygous resistant (RR) individuals in females than in males, and the overall kdr allelic frequencies were 23.2 to 37.8% higher in females than in males. Regression analysis revealed a significant correlation between kdr allelic frequencies and the levels of resistance to permethrin among the horn fly populations studied. The results validate the role of PCR-based molecular assay as a diagnostic tool in monitoring resistance to pyrethroids, and also provide useful information on population genetics of horn fly resistance to pyrethroids and OPs.