Ethion resistance in the cattle tick (Boophilus microplus)in New Caledonia

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Abstract
Following reports of ethion resistant cattle ticks (Boophilus microplus (Canestrini)) in New Caledonia, a series of dosage mortality tests was undertaken in an attempt to establish the extent of this resistance. Thirty samples of B. microplus which was introduced from Australia in 1942, were taken from various locations on the main island, and the results compared with similar tests carried out using an Australian susceptible reference strain (Yeerongpilly) and an Australian resistant strain (Biarra). Larvae 6–15 days after hatching were exposed to different concentrations of ethion in olive oil for 24 h. The study showed that varying levels of ethion resistance had developed in New Caledonia, and that it was most strongly marked in tick populations from the east coast region. Some very low levels of resistance are probably of no practical importance, but the LC50 value of the most resistant population was eight times that of the Yeerongpiily reference strain. Ethion resistance has appeared on the island after seven years of application. Reasons for the observed regional variation in resistance level are discussed. It is suggested that the concentration of ethion cattle dipping baths should be increased before introducing new acaricides. Alternative control methods should be considered.