First report of organophosphate-resistant Boophilus microplus (Acari: Ixodidae) within the United States.

Boophilus microplus (Canestrini), collected from Starr County, Texas, were determined to be resistant to the organophosphorus acaricides coumaphos and diazinon. Initial bioassay results from wild-collected ticks produced a probit regression slope (SE) of 3.96 (0.22), which was different from that obtained from a susceptible reference population 6.97 (0.38). Resistance ratios (RRs) (95% CI) indicated that the population was resistant to coumaphos 3.6 (3.4-3.8), 5.0 (4.5-5.5), and 6.5 (5.4-7.7) at the LC50,90,99, respectively. A second collection of wild ticks made 12 d after all cattle in the infested pasture were treated with coumaphos produced a slope (SE) that was not significantly different from a susceptible laboratory reference population. A second bioassay found these ticks to be resistant to diazinon,RR (95%CI) = 7.1 (6.5-7.7),11.7 (10.3-13.3),17.7 (14.5-21.5) at the LC50,90,99, respectively. The slope (SE) generated from the diazinon bioassay with the resistant ticks was different than that of a reference strain, 2.98 (0.12) and 6.09 (0.35), respectively. The high-dose strategy used by the Cattle Fever Tick Eradication Program was able to eradicate coumpahos-resistant B. microplus after just two treatments of coumaphos, 12 d apart.