Resistance to acaricides in Mexican strains of Boophilus microplus poses a major to the continued success of the USDA-VS's Cattle Fever Tick Eradication Program (CFTEP). Alternative acaricides are being evaluated for use in the dipping vats at the ports of entry for treating cattle. Amitraz is a potent acaricide and has been used to control pyrethroid- and organophosphate-resistant ticks in Mexico. The purpose of this study is to determine levels of resistance to amitraz in various Mexican strains of B. microplus and to investigate the role of metabolic detoxification in amitraz resistance. Adult female ticks were collected from various locations in Mexico. Several strains were established from those founding females at the USDA-ARS Cattle Fever Tick Research Laboratory in Mission, TX. A modified FAO Larval Packet Test was used to test the susceptibility of B. microplus larvae to amitraz. Bioassay results revealed low level (2-6x) of resistance to amitraz in those strains. Laboratory selection with amitraz of an amitraz-resistant strain from Brazil resulted in an increase of resistance ratio from 15 to 93 in only 8 generations. Preliminary results of synergist bioassays with TPP, PBO and DEM did not show consistent differences in synergism ratio between resistant and susceptible strains. Therefore, metabolic detoxification mechanisms may not play a major role in amitraz resistance. The presence of low level resistance to amitraz in Mexican strains of B. microplus and the potential for developing high resistance under selection demonstrated in this study underscore the risk of using amitraz in the dipping vats for the CFTEP.
Species 1: Acari Ixodidae Boophilus microplus (southern cattle tick, cattle tick)
Keywords: acaricides, chemical control
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