Submitted to: Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 8/25/2003
Publication Date: 10/1/2003
Citation: LI, A.Y., DAVEY, R.B., FRAGOSO, H.S., GEORGE, J.E. CHARACTERIZATION OF AMITRAZ RESISTANCE IN BOOPHILUS MICROPLUS (ACARI: IXODIDAE) AND ITS IMPLICATIONS IN RESISTANCE MANAGEMENT. PROCEEDINGS OF V INTERNATIONAL SEMINAR OF ANIMAL PARASITOLOGY, 1-3 OCTOBER 2003, MERIDA, YUCATAN, MEXICO. 2003. p. 163-172. Interpretive Summary: Development of resistance to several major classes of acaricides, including amitraz, in Mexican populations of the cattle fever tick Boophilus microplus poses a major threat to the USDA's Cattle Fever Tick Eradication Program. It is very important to understand the mechanisms of resistance to amitraz and to have a sensitive bioassay technique for detecting amitraz resistance in order to develop resistance management strategies. We conducted a study to investigate the role of metabolic enzymes in amitraz resistance in several tick strains and to compare two bioassay techniques in their sensitivity to detect amitraz resistance. The results from this study suggested that metabolic detoxification may play only a minor role in amitraz resistance in B. microplus. It is likely that resistance to amitraz in the cattle tick may be the result of a mutation at the target site, the octopamine receptors. Larval immersion test gave a higher resistance ratio in the resistant strain, therefore, was more sensitive in the detection of amitraz resistance. Low levels of amitraz resistance in Mexican populations of B. microplus may also indicate low frequency of resistance genes, in contrast to the homogeneous Santa Luiza strain from Brazil. Results from this study advanced our knowledge on amitraz resistance in B. microplus, and will be helpful in our efforts to control this important pest.
Technical Abstract: A modified FAO larval packet test (LPT) was used to measure and characterize resistance to amitraz, a formamidine acaricide, in several strains of Boophilus microplus (Acari: Ixodidae). Results of bioassays revealed low levels of resistance to amitraz in two tick strains from Mexico, and a high level of resistance to amitraz in a Brazilian tick strain selectively challenged with amitraz during colonization in the laboratory. To determine the role of metabolic detoxification enzymes in amitraz resistance, the effects of three synergists (TPP, PBO, and DEM) on toxicity of amitraz were tested in both susceptible and resistant tick strains. Results of synergist bioassays demonstrated that metabolic detoxification mechanisms may play only a minor role in the amitraz resistance observed in the tick strains thus far studied. Results of bioassays of hybrid larvae from crosses between the resistant and susceptible strains indicate resistance is inherited as an incomplete recessive trait. A study was conducted to compare two bioassay techniques, the modified FAO-LPT and the modified Larval Immersion Test (LIT), for their sensitivity in detecting amitraz resistance. The results suggested that the LIT technique was more sensitive in detecting amitraz resistance, where resistance levels obtained from bioassays using LIT were generally higher in comparison to the modified FAO-LPT. The results from this study may provide significant implications on the management of amitraz resistance in B. microplus.