A comparison of three bioassay techniques to determine amitraz susceptibility in Boophilus microplus (Acari: Ixodidae).

The ability of the Miller, Soberanes, and White bioassay techniques to describe amitraz susceptibility in three different strains of Boophilus microplus (Canestrini) (Acari: Ixodidae) was compared. For a susceptible strain, all techniques adequately described amitraz susceptibility by producing a full range of mortality that corresponded with increasing concentration of amitraz. However, when resistant strains were evaluated, only the Miller and the Soberanes techniques adequately estimated the dose-response relationship. Lethal concentrations were not precisely estimated when all the data were included in the analyses for every strain and technique tested. Better estimates were obtained when subsets of data around the range of interest were subjected to probit analysis. For the Soberanes technique, the slope of the probit regression was steeper for the Brazilian resistant and Texan susceptible strains compared with the heterozygous Mexican strain. The pattern was different when the same strains were tested with the Miller technique. The slopes of the regressions for the Mexican and the Texan strains did not differ significantly, but the Brazilian strain had a steeper slope than the other strains. Resistance ratios were much greater when the Soberanes technique was used than when the Miller technique was used on the same strains. However, neither technique produced enough separation between susceptible and resistant strains to develop a traditional discriminating dose (DD) test that required a concentration of 2x LC99.9 estimate. A DD test at the LC99 would be possible for both techniques. We discuss the strengths and weaknesses of the three techniques, including potential improvements to the White technique. The White technique has the greatest potential to determine the mechanisms of amitraz resistance in detailed synergist studies. Currently, only the Miller method can fulfill this task. The Miller and Soberanes techniques are well suited for the study of the epidemiology of resistance worldwide, because they use commercially available, formulated amitraz that is easy and inexpensive to obtain.