Submitted to: Acarology International Congress Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: 4/26/2010
Publication Date: N/A
Technical Abstract: Resistance to acaricides in the cattle fever tick, Rhipicephalus microplus, remains a major problem for the successful control of this economically important ectoparasite of cattle in many countries, including Mexico and Brazil. Resistance to coumaphos and other acaricides is also a major concern to the USDA’s Cattle Fever Tick Eradication Program (CFTEP). During the past 10 years, research at the USDA-ARS Knipling-Bushland U.S. Livestock Insects Research Laboratory in Kerrville, Texas, has focused on characterization of mechanisms of resistance to three major classes of acaricides (pyrethroid, organophosphate, and formamidine) and development of new acaricide formulations to combat resistance. Using both the traditional and modified FAO larval packet tests, we measured the levels of resistance to various acaricides in tick samples collected from Mexico. Resistant tick colonies were established and maintained at the USDA-ARS Cattle Fever Tick Research Laboratory in Mission, Texas. The mechanisms of resistance were first studied using synergist bioassays to determine the involvement of metabolic enzymes, and then using molecular and biochemical techniques to identify and characterize resistant genes and the key metabolic enzymes involved in resistance. It has been determined that resistance to organophosphates is conferred by both insensitive AChE and the enhanced activity of cytP450s, while a sodium channel mutation and a detoxifying esterase (CzEst9) are responsible for pyrethroid resistance. Although the mechanism of resistance to amitraz is still not fully understood, there is evidence suggesting that the gene(s) which confer resistance to amitraz may be closely linked to the ones contributing to pyrethroid resistance. To develop synergized acaricide formulations, the effectiveness of pyrethroid and amitraz mixtures was evaluated for the control of resistant R. microplus on cattle. Significant synergism was observed when amitraz was used as a synergist in pyrethroid toxicity bioassays under laboratory conditions. Results of the on-animal efficacy trials of pyrethroid and amitraz alone and mixtures of the two at different concentrations revealed a similar pattern of synergism. Adding amitraz to a pyrethroid formulation led to dramatic increases of percent reduction of both immature ticks and engorging female ticks. However, adding pyrethroid 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.