Rhipicephalus (Boophilus) microplus (Acari: Ixodidae) resistance to fipronil in Uruguay evaluated by in vitro bioassays.

Rhipicephalus (Boophilus) microplus obtained from four local populations in Uruguay (2007-2008) were subjected to various bioassay techniques to determine the presence of fipronil resistance within the country. Resistance ratios (RRs) obtained by larval immersion test varied between 3.3 and 3635 for tick populations subjected to treatment with fipronil for the last 3-7 years. The highest RR was observed in the population which received fewer treatments. Using discriminating concentration (8ppm) for larval immersion test, all field strains were correctly diagnosed as fipronil-resistant. This study presents the first diagnoses of cattle tick resistance to fipronil in Uruguay's field populations. It also highlights the importance of the possible conflict between programs to control agricultural pests and cattle ticks. The findings provide valuable information for selection and adoption of new control alternatives to manage drug resistance exhibited by cattle ticks.