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Title: Critical evaluation of the modified-adult immersion test with discriminating dose bioassy for Boophilus microplus using American and Australian isolates

item Miller, Robert

Submitted to: Veterinary Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/1/2006
Publication Date: 5/31/2007
Citation: Jonsson, N.N., Miller, R.J., Robertson, J.L. 2007. Critical evaluation of the modified-adult immersion test with discriminating dose bioassy for Boophilus microplus using American and Australian isolates. Veterinary Parasitology. 146:307-315.

Interpretive Summary: Quantifacation of resistance in Boophilus microplus by placing adult females into various concentrations of pesticide has been used for years and many conclusions regarding resistance and susceptibility to these compounds have been made based on the results of these tests. Two adult immersion bio assay techniques were compared in this study using various pesticides with resistant and susceptible strains of ticks. We found that the techniques could discriminate between resistant and susceptible ticks, but it could not be done consistently with all tick strains and chemicals used. Therefore, we concluded that these techniques, while useful in a laboratory situation, are not likely to be useful in field survey studies where environmental conditions can not be controlled. This is important information for producers and scientists since many scientists and pharmaceutical companies use these techniques in the field to determine resistance and make critical management decisions based on the results of these tests. Our study should reduce the amount of "bad information" generated from these bioassay techniques.

Technical Abstract: Similar adult immersion tests (AITs) for acaricide susceptibility of Boophilus microplus were done in Texas, USA (Muñoz strain) and in Queensland, Australia (N strain and Ultimo strain). Engorged adult female ticks were immersed in one of a series of dilutions of commercial acaricide in water and then incubated at room temperature for 7 days. Data on oviposition were collected 7 days after exposure to acaricide and subjected to probit analysis. For most data, we observed poor fit to the probit model. Substantial differences in both LC50 and LC99 for the susceptible strains occurred between the respective laboratories and confidence intervals for all acaricides and all strains were unacceptably wide. For amitraz, the discriminating concentration (double the LC99.9 or LC99) recommended by FAO was 0.25%, but our estimates ranged from 0.46 to 9000%. For cypermethrin, the recommended DD was 0.0050%, with our estimates ranging from 0.00022 to 0.74%. For coumaphos the recommended DD was 0.50% but our estimates ranged from 0.66 to 130%. Finally, for moxidectin, the recommended DD was 0.10, while our estimates ranged from <0.0001 to 5.9%. The method does not provide a means to discriminate between amitraz susceptible and Bresistant, nor between cypermethrin susceptible and resistant B. microplus.