Resistance to OP acaricides developed in Mexican B. microplus populations in recent years, and poses a major threat to the continued success of the cattle fever tick eradication program (CFTEP) in the United States. Acaricide-resistant strains of B. microplus tick have been collected from Mexico, and are maintained at the USDA-ARS Cattle Fever Tick Research Laboratory in Mission, TX. Bioassays with the FAO Larval Pack Test revealed 3- to 10-fold resistance to coumaphos in the tick strains. Synergist studies were conducted to test the role of metabolic mechanisms in OP resistance in those ticks. Bioassays with TPP (triphenyl phosphate) showed enhanced toxicity of coumaphos in both coumaphos-resistant and -susceptible strains. Synergism of TPP was not particularly enhanced in coumaphos-resistant tick strains. Therefore, resistance to coumaphos detected in B. microplus larvae was likely not conferred by esterases. However, bioassays with PBO (piperonyl butoxide) revealed a major difference between resistant and susceptible strains. In the susceptible strains, coumaphos toxicity was decreased by PBO, an inhibitor of the cytochrome P450 monooxygenases (cytP450). This may be caused by inhibition of cytP450 by PBO, and cytP450 is required for the oxidation of coumaphos to produce more toxic metabolites in ticks. In resistant ticks, however, adding PBO synergized the toxicity of coumaphos. The results suggest that cytP450 may be involved in coumaphos resistance, possibly by enhanced cytP450 activity that is involved in detoxification process. Two metabolic processes involving cytP450 might coexist in resistant ticks, one that activates coumaphos and one that detoxifies coumaphos and/or its metabolites before the molecules inhibit their target, acetylcholinesterases. The detoxification pathway is probably much more enhanced in the resistant strains. This is the first evidence that suggests the involvement of cytP450 in coumaphos resistance in the B. microplus tick, and further biochemical and molecular studies are needed to confirm the finding.
Species 1: Acari Ixodidae Boophilus microplus (southern cattle tick, cattle tick)
Keywords: organophosphate, acaricide
The ESA 2001 Annual Meeting - 2001: An Entomological Odyssey of ESA