Submitted to: Veterinary Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/21/2008
Publication Date: 8/1/2008
Citation: Barre, N., Li, A.Y., Miller, R. J., Gaia, H., Delathiere, J., Davey, R.B., George, J.E. 2008. In vitro and in vivo evaluation of deltamethrin and amitraz mixtures for the control of Rhipicephalus (Boophilus) microplus (Acari: Ixodidae) in New Caledonia. Veterinary Parasitology. 155:110-119. Interpretive Summary: Acaricide resistance is a major problem that hinders the control of the southern cattle tick, Rhipicephalus (Boophilus) microplus (Canestrini), in many parts of the world. Resistance to major classes of acaricide, including organophosphates, pyrethroids and amitraz, in Mexican tick populations remains a major threat to the USDA’s Cattle Fever Tick Eradication Program. In order to develop an effective resistance management strategy, a study was conducted jointly by scientists of the USDA, ARS, Knipling-Bushland Livestock Insects Research Laboratory in Texas and of New Caledonia to evaluate the usefulness of acaricide mixtures in controlling resistant ticks. Both laboratory larval toxicity bioassays and on-animal efficacy trials were conducted in New Caledonia to evaluate the effectiveness of various mixtures of deltamethrin and amitraz against resistant ticks. The results from this study indicate that the addition of amitraz to deltamethrin solution significantly increases the toxicity of deltamethrin to tick larvae in laboratory bioassays. Compared to deltamethrin alone, the amitraz-synergized deltamethrin mixtures also led to significantly increased control efficacy against resistant ticks on animals. The results from this study may facilitate the adoption of an acaricide mixture strategy for the control and management of resistant R. microplus ticks in New Caledonia and elsewhere.
Technical Abstract: Deltamethrin and amitraz have been used to control R. microplus in New Caledonia for the past decade, and tick populations have developed resistance to both acaricides. A study was conducted to evaluate the effectiveness of deltamethrin and amitraz mixtures, through in vitro laboratory bioassays and in vivo on-animal efficacy trials, for the control of resistant R. microplus on cattle at two diary farms in New Caledonia. Results of laboratory bioassays using modified Larval Packet Tests (LPT) revealed up to 16.59-fold resistance to deltamethrin, and up to 5.86-fold resistance to amitraz. Significant synergism was observed when amitraz was used as a synergist in deltamethrin bioassays. Amitraz significantly increased deltamethrin toxicity to tick larvae, while deltamethrin was much less effective on amitraz toxicity. Synergism of amitraz by deltamethrin only occurred when the deltamethrin concentration used was relatively high. Results of on animal efficacy trials of deltamethrin and amitraz alone and mixtures of both at different concentrations revealed a similar pattern of synergism. Adding amitraz to a deltamethrin formulation led to dramatic increases of percent reduction of both immature ticks and engorging female ticks. On contrast, adding deltamethrin to an amitraz formulation did not increase control efficacy. Results from this study may lead to the adoption of new amitraz-synergized deltamethrin formulations for the control of pyrethroid-resistant R. microplus.