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A comparison of three bioassay techniques to determine amitraz susceptibility in Boophilus microplus (Acari:
Robert J Miller1, Ronald B Davey, W Hunter White
1USDA-ARS, Cattle Fever Tick Research Laboratory, Moore Air Base, Building 6419, 22675 North Moorefield Road, Edinburgh, TX 78541, USA. robert.miller@ars.usda.gov
Abstract:
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.
Insights
The Miller and Soberanes bioassay techniques effectively measure amitraz resistance in ticks, unlike the White technique. These methods are crucial for understanding and managing acaricide resistance in Boophilus microplus populations globally.
Area of Science:
- Veterinary Entomology
- Acarology
- Pest Management
Background:
- Amitraz is a key acaricide used to control Boophilus microplus (ticks).
- Tick resistance to acaricides poses a significant threat to livestock health and agricultural economies.
- Accurate bioassay techniques are essential for monitoring and managing acaricide resistance.
Purpose of the Study:
- To compare the efficacy of three bioassay techniques (Miller, Soberanes, and White) in assessing amitraz susceptibility in different Boophilus microplus strains.
- To evaluate the suitability of these techniques for estimating lethal concentrations and resistance ratios.
- To determine the potential of each technique for developing discriminating dose tests and studying resistance mechanisms.
Main Methods:
- Three strains of Boophilus microplus (susceptible and two resistant) were tested using Miller, Soberanes, and White bioassay techniques.
- Dose-response relationships were analyzed using probit analysis, with comparisons made using full datasets and subsets.
- Resistance ratios and slopes of probit regressions were calculated to compare technique performance.
Main Results:
- Miller and Soberanes techniques accurately described amitraz susceptibility in susceptible strains.
- Only Miller and Soberanes adequately estimated dose-response relationships in resistant strains.
- Neither technique allowed for the development of a traditional discriminating dose (DD) test at LC99.9, but LC99 was feasible.
- Resistance ratios were higher with the Soberanes technique compared to the Miller technique.
- The White technique showed potential for synergist studies, while Miller and Soberanes are suitable for epidemiological studies.
Conclusions:
- Miller and Soberanes bioassay techniques are more reliable than the White technique for evaluating amitraz resistance in Boophilus microplus.
- These techniques provide valuable data for epidemiological studies of acaricide resistance.
- Further refinement of the White technique could enable detailed studies on resistance mechanisms.
