Efficacy of amitraz applied as a dip against an amitraz-resistant strain of Rhipicephalus (Boophilus) microplus (Acari: Ixodidae) infested on cattle.

Selection pressure with the acaricide, amitraz was applied to a strain of Rhipicephalus (Boophilus) microplus (Canestrini) in 13 out of 18 generations, resulting in a 28.7-fold increase in resistance, indicating a shift in the phenotypic composition of the ticks, from a heterogenous mixture of both susceptible and resistant ticks when selection began to a homogenous composition of predominantly resistant ticks when selection ended. Resistant larvae of the last selected generation were infested on cattle three separate times at 20, 13, and 6 d (7 d intervals) before being dipped in a total immersion vat charged at 0.02, 0.044, and 0.096% active ingredient (AI) amitraz. There was a dose related, though not always significant, response to amitraz whereby each increase in concentration produced increased adverse effects. The number of ticks and index of fecundity (IF) of females recovered from cattle treated at 0.02 and 0.044% AI were not different from that of untreated ticks. However, female weight and egg mass weight of females recovered from cattle treated at 0.044% were lower than untreated females or females treated at 0.02% AI. All measured parameters obtained from ticks recovered from cattle treated at 0.096% AI were significantly lower than the untreated control group, but tick numbers and IF were not different from the other treated groups. At 0.02% AI the level of control was significantly lower (34.6%) than treatment at 0.096% AI (81.2%), while the level of control at 0.044% AI (57.6%) was intermediate between the other treated groups. Therefore, none of the amitraz treatments provided the required 99% control necessary for use in the U.S. Cattle Fever Tick Eradication Program, and the estimated line derived from regression analysis showed that a concentration of ca. 0.2% AI amitraz would be needed to achieve 99% control of the resistant ticks.