|White, W. Hunter - ELANCO, GREENFIELD, IN|
Submitted to: Journal of Medical Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 13, 2006
Publication Date: March 1, 2007
Citation: Miller, R.J., Davey, R.B., White, W.H., George, J.E. 2007. A comparison of three bioassay techniques to determine amitraz susceptibility in Boophilus microplus (Acari: Ixodidae). Journal of Medical Entomology. 44(2):283-294. Interpretive Summary: Amitraz has been used to control cattle ticks around the world for over 40 years. However, little is known about the resistance to amitraz these ticks may have. Recently, three bioassay techniques have been developed for the study of amitraz resistance in cattle ticks. Since these techniques are new it is important to study and compare the results obtained from each technique so that researchers can determine which technique will best suit their needs. This study compared and evaluated the Miller, Soberanes, and White techniques used to measure amitraz susceptibility in cattle ticks. The strengths and weaknesses of each technique were discussed as well as suggestions for improvements. This paper also included a discussion on the analysis of bioassay data and provided examples from experimental data for the improvement of the precision of the analysis. Together the information provided in this manuscript will help researchers to choose the proper bioassay techniques and to better analyze their results. This will lead to a better understanding of the epidemiology of amitraz resistance in the world, information transfer to producers, and higher production levels for producers.
Technical Abstract: The Miller, Soberanes, and White techniques were compared for their ability to describe amitraz susceptibility in 3 different populations of Boophilus microplus. When a susceptible population was evaluated, all 3 techniques adequately described amitraz susceptiblitiy by producing a full range of mortality that correlated with increasing concentration of amitraz. However, when resistant populations were evaluated, only the Miller and the Soberanes techniques could adequately define the dose-response relationship. Lethal concentration estimates generated from probit analysis did not provide precise estimates of the observed data when all the data were included in the analysis across the entire dose-mortality range for every population and technique tested. Better estimates were obtained when excerpts of continuous data within the range of interest were used in the probit analysis. For the Soberanes technique, the slope about the LC50 estimate was steeper for the Brazilian resistant population, Santa Luiza, and for the Texan susceptible population, Muñoz, than for the heterozygous Mexican population, San Alfonso. The pattern was different when the same populations were tested with the Miller technique. The slope at the LC50 estimate did not differ for the San Alfonso and the Muñoz, but the Santa Luiza had a steeper estimated slope than the other populations. Resistance ratios were much greater when the Soberanes technique was used than when the Miller technique was used on the same populations. However, neither technique produced enough separation between susceptible and resistant populations to develop a traditional discriminating dose (DD) test that requires a concentration of 2 X LC99.998 estimate. A modified DD test would be possible for both techniques. The strengths and weaknesses of the three techniques are discussed including potential improvements to the White technique. The White technique has the greatest potential for use in detailed synergist studies to determine the mechanisms of amitraz resistance. 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 world-wide because they use commercially available formulated amitraz that is easy and inexpensive to obtain.