Efficacy of coumaphos applied as a dip for control of an organophosphorus-resistant strain of Boophilus microplus (Acari : Ixodidae) on cattle

The efficacy of coumaphos, an organophosphorus acaricide, was tested on a s train of Boophilus microplus (Canestrini) that was 9.5 times more resistant to the chemical than a susceptible strain. Cattle infested with larval, ny mphal, and adult ticks were dipped into a vat charged at concentrations of 0.031, 0.083, 0.160, and 0.279% (AI). Assessment of efficacy was based on s urviving tick numbers, female weight, egg mass weight, egg hatchability, an d index of reproduction. At 0.031% (AI), the level of control was lower tha n the other coumaphos treatments, and the weight, fecundity, and fertility of surviving females was similar to untreated females, indicating only mini mal adverse effects on the ticks. At 0.160 and 0.279% (AI), no differences in control, female weight, egg mass weight, or percentage egg hatch were ob served, but effects were greater than that of untreated ticks or ticks trea ted at 0.031% (AI). At 0.083% (AI), most biological parameters were interme diate, but there was no difference in control compared with the 2 higher co ncentrations. Thus, even with the increase in concentration of more than 3 times between treatments of 0.083 and 0.279% (AI), there was no substantial increase in control. The residual efficacy of all 4 treatments showed that none provided complete protection against larval reinfestation for even 1 wk after treatment. Therefore, the presence of organophosphorus-resistant t icks at U.S. ports-of-entry could jeopardize the success of the Boophilus e radication program.