Submitted to: Journal of Medical Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/25/2002
Publication Date: N/A
Interpretive Summary: Bioassay methods involving the systematic exposure of ticks to a series of progressively lower dilutions of a pesticide are used to determine the relationship between the dose of a pesticide and the mortality of treated ticks. Bioassays are especially important for tests of a sample of a tick population to determine if resistance to a pesticide has evolved and the susceptibility of the ticks to the pesticide has decreased to the point that effective control cannot be obtained with a given product or chemical group of products. The Food and Agricultural Organization Larval Packet Test (LPT) is the internationally accepted standard method for evaluating the dose-mortality responses of ticks to the pesticides used to control them. This technique is useful for bioassays against many of the kinds of pesticides used against ticks, but suitable results can not be obtained with this method with the tickicide amitraz. Because of the importance of amitraz for the control of tick populations resistant to other pesticides, there is a need for a bioassay method useful for diagnosis of resistance to amitraz. A modification of the LPT has been developed by using commercially formulated amitraz to treat special nylon fabric, substituted for the paper normally used, on which ticks are exposed to different doses of amitraz. This procedures provides repeatable results and is being used to diagnose resistance to amitraz in several species of ticks.
Technical Abstract: Modifications of exposure time, substrate, and formulation were made to the Food and Agriculture Organization Larval Packet Test (LPT) to determine a combination suitable for measuring the susceptibility of Boophilus microplus (Canestrini) to amitraz. Exposure time influenced the slope of the dose-response when paper was used as a substrate for amitraz. However, time did not influence the dose-response slope when nylon fabric was used as an amitraz substrate. Formulated amitraz produced results with less deviation from the log-probit model than technical amitraz. The combination of formulated amitraz and nylon fabric as a substrate for amitraz produced results that best fit the log-probit model. The modified FAO procedure (formulated amitraz/nylon substrate combination) was used to assay a Brazilian strain of B. microplus and a Panamanian strain of Rhipicephalus sanguineus (Latreille). Resistance ratios (95% CI) of 26.3 (25.7-26.9) and 7.3 (5.5-9.9) were calculated for the B. microplus and R. sanguineus strains, respectively. A discriminating dose of 0.03% amitraz was determined for B. microplus. This technique will help to locate amitraz resistant tick populations, provide data for improved control practices, and aid in the discovery of resistance mechanisms through synergist studies and verification of molecular techniques.