Submitted to: Livestock Insect Worker's Conference Annual Meeting
Publication Type: Abstract Only
Publication Acceptance Date: June 21, 2009
Publication Date: June 23, 2009
Citation: Li, A.Y., Barre, N., Miller, R., Davey, R.B., George, J. 2009. Evaluation of Pyrethroid and Amitraz Mixtures for the Control of the Southern Cattle Tick Boophilus microplus. Livestock Insect Worker's Conference Annual Meeting. June 21-23, 2009. French Lick,IN. Technical Abstract: The control of the southern cattle tick, Boophilus microplus (Canestrini), in Mexico and other countries relies on chemical acaricides. B. microplus has developed resistance to all major classes of acaricides. Tick strains from Mexico and Brazil demonstrated 49.4- to over 672.2-fold resistance to permethrin, and up to 94.5-fold resistance to amitraz. Pesticide mixtures have been used successfully to control resistant insect pest species. We conducted a study to evaluate the usefulness of pyrethroid-amitraz mixture for the control of B. microplus. The FAO larval packet test was used to determine if one acaricide can synergize another at a sub-lethal concentration. A significant synergism was found between permethrin and amitraz when one acaricide was tested in the presence of another. Synergism ratios ranged from 1.5 to 54.9 when amitraz was tested as a synergist for permethrin. Similar synergism ratios were obtained when permethrin was tested as a synergist for amitraz. Permethrin caused virtually no mortality in one of the most pyrethroid-resistant tick strain, even at the highest concentration tested. Adding a low concentration of amitraz to permethrin led to a dramatic increase in larval mortality. We also conducted study 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 B. microplus on cattle at two dairy farms in New Caledonia. Results of laboratory bioassays 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 occurred only when the deltamethrin concentration 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 and adult ticks. In contrast, adding deltamethrin to an amitraz formulation did not increase control efficacy. Results from this study may lead to the adoption of an acaricide mixture strategy for the control of pyrethroid-resistant B. microplus in New Caledonia and elsewhere.