Veterinary Parasitology 191 (2013) 323–331 Contents lists available at SciVerse ScienceDirect Veterinary Parasitology jo u rn al hom epa ge : www.elsevier.com/locate/vetpar Use of the Larval Tarsal Test to determine acaricide resistance in Rhipicephalus (Boophilus) microplus Brazilian field populations L. Lovis a,b,∗ , M.C. Mendes c , J.-L. Perret b , J.R. Martins d , J. Bouvier b , B. Betschart a , H. Sager b a University of Neuchâtel, Institute of Biology, Laboratory of Parasitology, 2000 Neuchâtel, Switzerland b Novartis Animal Health Research Center, 1566 St-Aubin (FR), Switzerland c Centro de Pesquisa e Desenvolvimento de Sanidade Animal, Instituto Biológico, São Paulo, Brazil d Laboratory of Parasitology, Instituto de Pesquisas Veterinárias Desidério Finamor, Fundac ¸ ão Estadual de Pesquisa Agropecuária, Eldorado do Sul, Rio Grande do Sul, Brazil a r t i c l e i n f o Article history: Received 8 June 2012 Received in revised form 7 September 2012 Accepted 10 September 2012 Keywords: Larval Tarsal Test Rhipicephalus (Boophilus) microplus Brazil Acaricide resistance Tick a b s t r a c t Acaricide resistance of the cattle tick Rhipicephalus (Boophilus) microplus is widespread in most of the countries where this parasite is present. Bioassays are used to diagnose the level and pattern of resistance in tick populations. In the present study, we describe a detailed protocol of the Larval Tarsal Test (LTT) using simplified equipment and data on the resistance of 17 tick field populations originating from 5 Brazilian states. Nine aca- ricidal compounds from 5 major classes were tested: organophosphates (OP), synthetic pyrethroids (SP), macrocyclic lactones (ML), phenylpyrazols (PYZ) and amidines. For com- parison, four of the tick populations were also tested with the Larval Packet Test (LPT) with one compound per class. The most common resistances were to SP, amitraz and OP, with frequencies of 94%, 88% and 82%, respectively. Resistance to PYZ was also found to be widespread (65%), suggesting a rapid development of fipronil resistance in Brazil. One case of ML resistance and 2 cases of suspected ML resistance were identified with the LTT. The LTT led to higher resistance ratios to all compounds than the LPT, reflecting its high sensi- tivity to detect resistance. Finally, the LTT allowed testing a larger number of compounds and doses with reduced labour in comparison to the LPT and turned out to be a reliable bioassay to detect resistance in field populations. © 2012 Elsevier B.V. All rights reserved. 1. Introduction The one-host cattle tick Rhipicephalus (Boophilus) microplus is a pest of major economic importance in tropical and subtropical countries. Treatments nearly exclusively rely on acaricides and multi-drug resistance has become widespread (Alonso-diaz et al., 2006; Jonsson and Hope, 2007; Martins et al., 2008; Castro-Janer et al., 2011). In Brazil, R. (B.) microplus is the most important ∗ Corresponding author at: Novartis Animal Health Research Center, Chemin de la Petite Glâne, 1566 Saint-Aubin (FR), Switzerland. Tel.: +41 26 679 14 15; fax: +41 26 679 14 10. E-mail address: leonore.lovis@unine.ch (L. Lovis). ectoparasite of cattle and its economic impact on the Brazil- ian cattle industry was estimated at 2 billion US dollars per year (Grisi et al., 2002). This amount includes losses due to the increased mortality caused by tick-borne parasites, losses due to decreased milk production and decreased weight gain, damage to the leather, and treatment costs to control infestations. In Brazil resistance successively emerged to arsenic in 1950 (Freire, 1953), to organophos- phates (OP) in 1974 (Amaral et al., 1974) and to synthetic pyrethroids (SP) in 1988 (Leite, 1988; Laranja et al., 1989). At the end of the 1990s, Farias (1999) pointed out that the widespread resistance to SP was a big issue in Brazil, especially considering that 90% of the acaricides available on the market at that time belonged to SP. In parallel, amitraz became an important alternative to control OP 0304-4017/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.vetpar.2012.09.011