Evaluation of cypermethrin resistance in Boophilus microplus strains from cattle farms in the state of Yucatán, Mexico

Abstract

The purpose of this study was to determine the level of resistance of Boophilus microplus field strains to cypermethrin in cattle farms in the state of Yucatán, Mexico. B. microplus ticks from 31 farms were studied. The larval package technique was used and the Probit analysis was applied in order to obtain both lethal concentrations 50 and 99 % (LC50, LC99), and the slopes. In order to evaluate the level of resistance to cypermethrin in each strain, resistance factors (RF50 and RF99) were determined. Strains were considered as cypermethrin-susceptible, -tolerant or -resistant when RF values <3, 3-5, or >5 were obtained, respectively. RF differences were found in the strains analyzed. One strain (FC32) was identified as highly resistant to cypermethrin, with an RF50 of 2297.8. When the RF50 was used to determine resistance, 32.3 % of the strains were found to be resistant, while 67.7 % were susceptible. Nevertheless, when the RF99 was used, 74.2 % of the strains were resistant and 25.8 % were susceptible (P<0.05). When both RFs were used, 32.3 % of the strains were categorized as resistant and 25.8 % were susceptible. Therefore, 41.9 % of the strains were considered as either resistant or susceptible. Using both RFs, all susceptible strains showed slopes >2.5. It was concluded that the field B. microplus strains from Yucatán used in this study show variable levels of resistance to cypermethrin. In order to classify a strain as resistant or susceptible, both RFs (RF50 and RF99) and the slope need to be considered.