Efficacy of multiple dip treatments (one to three) at 0.3% active ingredient (AI) coumpahos applied at 7- or 10-d intervals was determined against organophosphate (OP)-resistant Boophilus microplus (Canestrini). None of the treatments totally prevented repletion of all females. In the 7 d after one treatment, the percentage of reduction of female ticks was 24.8%, whereas in the 10 d after one treatment, the reduction of female numbers was 47.1%. Application of two or three dips at either interval provided substantially higher reductions in female numbers than either single treatment (range 80.2–88.8%). Percentage reduction of the index of fecundity of females (designated as percentage of control) in the 7 d after one dip provided 46.8% control, whereas control in the 10 d after one treatment was 65.6%. Again, application of two or three dips at either interval provided substantially higher control (range 92.9 to >99.9%). Although control levels after two or three dips were similar, only application of three dips at either interval provided a high enough level of control (>99.5%) to ensure that the U.S. Boophilus eradication program would not be at risk of inadvertently dispersing viable ticks. Thus, although no treatment regime prevented repletion of all treated females, application of three dips at either interval virtually eliminated production of reproductively active females, thereby eliminating the possibility that ticks would become established. It should be noted that ticks possessing different OP resistance mechanisms than ticks in this study could have higher reproductive capabilities than were observed against these treatment regimes.
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