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Title: Tests to determine LC50 and discriminating concentrations for fipronil against Rhipicephalus (Boophilus) microplus (Acari: Ixodidae) and their standardization

item RIFRAN, L
item GIL, A
item NARI, A
item Miller, Robert

Submitted to: Veterinary Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/6/2010
Publication Date: 5/26/2010
Citation: Castro-Janer, E., Rifran, L., Piaggio, J., Gil, A., Nari, A., Miller, R., Schumaker, T. 2010. Tests to determine LC50 and discriminating concentrations for fipronil against Rhipicephalus (Boophilus) microplus (Acari: Ixodidae) and their standardization. Veterinary Parasitology. 162: 120-128.

Interpretive Summary: The southern cattle tick, Rhipicephalus (Boophilus) microplus, is one of the most damaging pests that attack bovines in tropical and subtropical regions of the world. These ticks cause severe economic losses by feeding on host blood and through the transmission of disease-causing agents. Fipronil is an important new pesticide that could mitigate much of the losses due to this tick but, until now, no tests have been developed or validated for the detection of fipronil resistance in the cattle tick. This study was successful in developing an in vitro bioassay technique for the diagnosis of fipronil resistance in cattle ticks and will be important to producers who must decide which is the most efficacious chemical to use against this pest in order to increase production. This work is also important to scientists in academics and government who track the development of resistance in order to make informed recommendations to producers. Lastly, this research benefits industry as it allows for the development and deployment of better and more efficacious products for use by producers.

Technical Abstract: Laboratory test were carried out on larvae and adults of the cattle tick, Rhipicephalus (Boophilus) microplus, to determine fipronil toxicity. Adult immersion test (AIT), larval immersion test (LIT) and larval packet test (LPT) were standardized using susceptible strain (Mozo). Curves dose-response was compared with a fipronil resistant strain. Four variables were analyzed from AIT results: mortality, weight of eggs on day 7 and on day 14, index of fertility, and percent reduction of oviposition. For larval test, curve dose mortality was analyzed. In spite of the high LC50 variability, all variables determined for AIT were appropriate to discriminate both strains. AIT and LIT had more sensitivity than LPT, with larger resistance factors. DCs (2xLC99.9) for mortality by AIT, LIT and LPT were 5 p.p.m., 8 p.p.m. and 2366 p.p.m., respectively, for Mozo strain. Protocols for AIT, LIT and LPT have been presented in this paper.