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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. [ABSTRACT FROM AUTHOR]

Background: Rhipicephalus (Boophilus) microplus is a highly-invasive tick that transmits the cattle parasites (Babesia bovis and B. bigemina) that cause cattle fever. R. microplus and Babesia are endemic in Mexico and ticks persist in the United States inside a narrow tick eradication quarantine area (TEQA) along the Rio Grande. This containment area is threatened by unregulated movements of illegal cattle and wildlife like white-tailed deer (WTD; Odocoileus virginianus)., Methods: Using 11 microsatellite loci we genotyped 1,247 R. microplus from 63 Texas collections, including outbreak infestations from outside the TEQA. We used population genetic analyses to test hypotheses about ecological persistence, tick movement, and impacts of the eradication program in southern Texas. We tested acaricide resistance with larval packet tests (LPTs) on 47 collections., Results: LPTs revealed acaricide resistance in 15/47 collections (32%); 11 were outside the TEQA and three were resistant to multiple acaricides. Some collections highly resistant to permethrin were found on cattle and WTD. Analysis of genetic differentiation over time at seven properties revealed local gene pools with very low levels of differentiation (FST 0.00-0.05), indicating persistence over timespans of up to 29 months. However, in one neighborhood differentiation varied greatly over a 12-month period (FST 0.03-0.13), suggesting recurring immigration from distinct sources as another persistence mechanism. Ticks collected from cattle and WTD at the same location are not differentiated (FST = 0), implicating ticks from WTD as a source of ticks on cattle (and vice versa) and emphasizing the importance of WTD to tick control strategies. We identified four major genetic groups (K = 4) using Bayesian population assignment, suggesting multiple introductions to Texas., Conclusions: Two dispersal mechanisms give rise to new tick infestations: 1) frequent short-distance dispersal from the TEQA; and 2) rare long-distance, human-mediated dispersal from populations outside our study area, probably Mexico. The threat of cattle fever tick transport into Texas is increased by acaricide resistance and the ability of R. microplus to utilize WTD as an alternate host. Population genetic analyses may provide a powerful tool for tracking invasions in other parts of the world where these ticks are established.