Factors associated with the efficiency of acaricides on different populations of Rhipicephalus microplus
Tick infestation causes major problems in cattle. Tick parasitism accounts for significant economic losses in many beef and dairy herds in the vast majority of the states in the Brazilian territory including the State of Santa Catarina in the southern region of the country. Tick resistance to several active principles occurs due to a number of factors including the indiscriminate, injudicious and inadequate use of tick insecticides. Considering the great importance of fighting off tick infestations in cattle, we evaluated the efficiency of 8 different topical active principles against the ixodid tick R. microplus and identified the main factors that contribute to the development of ticks that are resistant to acaricides in the farms and bovine herds studied. For such purpose, R. microplus telegionae were collected in 39 farms locates in different municipalities of the West of Santa Catarina, south Brazil. At the time of sample collection, information about the management, the history of the acar used in the herd, the number of annual applications of these pesticides, the frequency of acaricide rotation, and the frequency of technical monitoring for strategic tick control were retrieved. We collected this data in order determine contributing factors to the development resistant R. microplus populations. For the sensitivity profile of these ticks to a number of different acaricides tested, 10 engorged females for each principle and a control were used. The results of our study show that most associations between pyrethroids and organophosphates had an efficiency between 96.6% and 100% in the control of R. microplus except for the combination of Alfacipermetrine 15%, Ethion 16%, and Chlorpyrifos 8.5% that had 93.4% of efficiency. Among the chemicals not associated with any other type of compound, Amitraz 12.5% was effective in 98.3% of the cases. In contrast, Cypermethrin 15% had an efficiency in only 25.8% of the farms/herds analyzed. Based on the results of the present study, we may infer that the methods of control and management used by producers may be related to the low indexes of resistance to topical acaricides in the different populations of ticks in the study area.
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