Tracking the increase of acaricide resistance in an invasive population of cattle fever ticks (Acari: Ixodidae) and implementation of real-time PCR assays to rapidly genotype resistance mutations.
The invasive cattle fever tick, Rhipicephalus (Boophilus) microplus (Canestrini) (Acari: Ixodidae), occurs in the United States only along the Texas border as incursions from Mexico. Intensive acaricide treatment of cattle herds in Mexico to reduce tick populations has resulted in the development of resistance to an array of acaricides. Resistance to permethrin has increased both in incidence and level in Texas over the last decade, even though permethrin is not used to control fever ticks on the U.S. side of the border. From 3 to 4% of submittals in 2008 to over 50% in 2016 and 2017 showed resistance based on standardized pesticide bioassays. Moreover, by 2017, the highly resistant ticks had spread to include all five of the southmost Texas counties and were found on both cattle and sylvatic hosts. Improved diagnostic tools developed by our group include a panel of DNA-based real-time PCR assays to detect mutations within the voltage-sensitive sodium channel (Vssc) that are involved in permethrin resistance. Further, a positive correlation between the presence of multiple Vssc mutations and phenotypic resistance was validated. These assays can be completed within days of receiving field collected ticks providing timely, valuable information to program managers. Microsatellite marker analysis of these pyrethroid resistant tick populations provided evidence that separate incursions of resistant ticks had entered Texas independently.