Tick resistance to pyrethroids in New Caledonia.

Boophilus microplus is the common cattle tick of great economic importance in New Caledonia. Since 1986, deltamethrin has been used for dipping. In 1992, an increase of tick infestations was seen on some ranches. Field and laboratory studies were conducted to determine if resistant ticks were present. Ticks resistant to deltamethrin were detected on three ranches, with resistance factors from 8.3 to 97.7. All deltamethrin-resistant isolates were also resistant to fenvalerate, but only one was also resistant to flumethrin. The combination of deltamethrin and ethion seemed to be active on isolates. This is the second description of isolates of Boophilus microplus resistant to pyrethroids, the first being in Australia and points to future problems in the management of tick control. Some organophosphates like chlorpyriphos or fenthion were studied as replacements for deltamethrin, in spite of the fact that most ticks are resistant to ethion. The authors propose a rotation strategy based on the alternation of several compounds together with a minimal frequency of dipping.