Submitted to: Journal of Medical Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/5/2005
Publication Date: 11/1/2005
Citation: Rosario-Cruz, R., Guerrero, F., Miller, R., Rodrigues-Vivas, R.I., Dominguez-Garcia, D.I., Cornel, A.J., Hernandez-Ortiz, R., George, J.E. 2005. Roles played by esterase activity and by a sodium channel mutation involved in pyrethroid resistance in populations of Boophilus microplus (Acari: Ixodidae) collected from Yucatan, Mexico. Journal of Medical Entomology. 42(6):1020-1025.
Interpretive Summary: Boophilus microplus, the southern cattle tick, is responsible for severe losses to cattle producers in Mexico. Although eradicated from the United States, the tick is a threat to the United States cattle industry because of the potential to enter the country on cattle imported from Mexico. Outbreaks of this tick are becoming more frequent along the border of Texas and Mexico. Control of the tick is becoming problematic in Mexico because of the development of acaricide resistance, especially in the southern and coastal regions of the country. Outbreaks of resistant ticks will present a difficult control problem to United States ranchers. A survey of the state of Yucatan, Mexico was conducted to discover the extent of resistance to pyrethroid acaricides in this state. The survey found there was a significant correlation between resistance in tick larval bioassays and the presence of a sodium channel gene mutation. This mutation has been correlated with pesticide resistance in other arthropod species and likely plays an important role in resistance to the pyrethroid class of acaricides in Boophilus microplus. Esterases are a family of metabolic enzymes which can play a role in resistance. However, in this tick, there was no correlation found between general esterase activity and bioassay resistance.
Technical Abstract: Pyrethroid resistance in Boophilus microplus (Canestrini) (Acari: Ixodidae) was studied by correlating discriminating-dose (DD) bioassay results and esterase activity or the frequency of a sodium channel mutation known to be involved in pyrethroid resistance in nine field strains of B. microplus from Yucatan, Mexico. Two tick strains (P67 and B74) were identified as susceptible to cypermethrin, deltamethrin, and flumethrin by DD, one strain (P65) was susceptible to cypermethrin and resistant to deltamethrin and flumethrin, and six strains were resistant to cypermethrin, deltamethrin, and flumethrin (T11, M10, C54, R49, B71, and T66). By using polymerase chain reaction, only 6.0 and 6.2% of resistance allele (R) was found in the susceptible strains (P67 and B74, respectively). In the T66 strain, with 100% of larval survival to the DD of pyrethroids as measured by the larval packet test (LPT), 98.0% of the gene pool contained the R allele. Positive correlations between the larval survival and the percentage of the R allele were found (deltamethrin r2 = 0.8875, P < 0.01; cypermethrin r2 = 0.8563, P < 0.01; and flumethrin r2 = 0.8491, P < 0.01). There were no significant correlations between the level of larval survival and esterase-based hydrolytic activity. It was concluded that within the B. microplus populations studied, resistance to flumethrin, deltamethrin, or cypermethrin was because of the novel sodium channel mutation (Phe Ile amino acid substitution in the S6 transmembrane segment of domain III), and there was a correlation between tick mortality by pyrethroid exposure (larval survival) and the presence of R allele. It was not determined whether enhanced esterase-based hydrolytic activity was involved in pyrethroid resistance in the populations tested.