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<p>The rapid escalation of acaricide resistance in <i>Rhipicephalus annulatus</i> poses a significant threat to livestock health and productivity, necessitating the development of effective resistance-breaking tools. This study characterized the high-level resistance status of <i>R. annulatus</i> populations in Mazandaran Province, Iran, and evaluated the synergistic potential of carvacrol in combination with permethrin. Field-collected ticks and a susceptible reference strain were evaluated in triplicate using adult immersion tests (AIT), ovicidal assays, larval immersion tests (LIT), and larval packet tests (LPT). Results revealed widespread permethrin resistance; resistance ratios (RR50) reached 111.32 in the LPT, with RR90 values in the LIT reaching 253.75. Biochemical assays indicated a robust metabolic basis for this resistance, with significantly elevated activities of alpha-esterase, beta-esterase, and glutathione-S-transferase (GST) in the resistant larvae (<i>p</i> &lt; 0.01–0.0001). Importantly, carvacrol demonstrated potent synergism when combined with permethrin. The 1:4 (permethrin: carvacrol) ratio yielded the highest synergistic rates (SR), reaching 38.00 in adults and 47.46 in larvae, with isobologram analysis confirming strong synergistic interactions (FIC &lt; 1) across all tested combinations. These findings provide the first report of carvacrol-permethrin synergism against <i>R. annulatus</i>. Integrating carvacrol into pest management strategies may provide a promising, sustainable approach to overcoming metabolic resistance and restoring the efficacy of conventional pyrethroids.</p>

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Synergistic activity of carvacrol in combination with permethrin against permethrin resistant Rhipicephalus annulatus

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2026年3月28日

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  • Mohammadreza Youssefi,
  • Milad Kashani Rad,
  • Mohadeseh Abouhosseini Tabari

摘要

The rapid escalation of acaricide resistance in Rhipicephalus annulatus poses a significant threat to livestock health and productivity, necessitating the development of effective resistance-breaking tools. This study characterized the high-level resistance status of R. annulatus populations in Mazandaran Province, Iran, and evaluated the synergistic potential of carvacrol in combination with permethrin. Field-collected ticks and a susceptible reference strain were evaluated in triplicate using adult immersion tests (AIT), ovicidal assays, larval immersion tests (LIT), and larval packet tests (LPT). Results revealed widespread permethrin resistance; resistance ratios (RR50) reached 111.32 in the LPT, with RR90 values in the LIT reaching 253.75. Biochemical assays indicated a robust metabolic basis for this resistance, with significantly elevated activities of alpha-esterase, beta-esterase, and glutathione-S-transferase (GST) in the resistant larvae (p < 0.01–0.0001). Importantly, carvacrol demonstrated potent synergism when combined with permethrin. The 1:4 (permethrin: carvacrol) ratio yielded the highest synergistic rates (SR), reaching 38.00 in adults and 47.46 in larvae, with isobologram analysis confirming strong synergistic interactions (FIC < 1) across all tested combinations. These findings provide the first report of carvacrol-permethrin synergism against R. annulatus. Integrating carvacrol into pest management strategies may provide a promising, sustainable approach to overcoming metabolic resistance and restoring the efficacy of conventional pyrethroids.