Structural changes in sodium channels due to mutations may decrease the interaction between pyrethroids and its target site, and thus reduce the sensitivity of arthropods to these acaricides (Dong, 2007). Three mutations in the sodium channel have been associated with resistance to pyrethroids in R. microplus

populations ( He et al., 1999, Chen et al., 2009, Morgan et al., 2009, Jonsson et al., 2010 and Guerrero et al., 2012). He et al. (1999) identified a point mutation in the S6 segment of domain III of the para-type sodium channel of Mexican strains of R. microplus resistant to permethrin. This mutation involves the substitution of a thymine by an adenine (T2134A), resulting in the replacement Compound C purchase of a phenylalanine by an isoleucine at susceptible and resistant individuals, respectively. The mutation described by Morgan et al. (2009) is located at domain II S4-5 linker of the para-sodium channel gene and it is a substitution of a cytosine in the susceptible strain to an adenine in the resistant strain (C190A).

This substitution led to a leucine to isoleucine replacement that was correlated to pyrethroid resistance ( Morgan et al., 2009). Jonsson et al. (2010) reported another substitution in tick populations from Australia: G214T in the domain II S4-S5 linker, which is a glycine to valine change that is associated with resistance to the pyrethroid Talazoparib flumethrin only. Both detection of the levels of acaricide resistance and understanding the mechanism of resistance in R. microplus are important to the development of an effective

tick control program. A rational use of pesticides will help to delay the development of resistance and reduce pesticide contamination of the environment as well as chemical residues in meat and milk. Linifanib (ABT-869) This study aimed at evaluating (i) the susceptibility of Brazilian field populations of R. microplus to the synthetic pyrethroid cypermethrin and the organophosphate chlorpyriphos and (ii) the role of target site insensitivity mediated by T2134A and C190A substitutions. In April 2010, 100 engorged females of R. microplus were collected from 10 cattle ranches in the ‘Triângulo Mineiro’ and ‘Alto Paranaíba’ regions within the state of Minas Gerais in Southeastern Brazil. The state has the highest milk production in the country and is a leading producer of beef cattle ( Pesquisa, 2009). After collection, ticks were stored in plastic containers and sent by post to the Laboratory of Parasitic Diseases, School of Veterinary Medicine, Federal University of Minas Gerais, Belo Horizonte. The bioassay, larval packet test (LPT) (Stone and Haydock, 1962), recommended by FAO (2004), was conducted to detect resistance to cypermethrin and chlorpyriphos.