Thymol enhances the acaricidal effects of cypermethrin through the disruption of oocyte development in Rhipicephalus microplus (Acari: Ixodidae).

The cattle tick Rhipicephalus microplus is one of the most important ectoparasites in tropical and subtropical regions. Synthetic pyrethroids are widely used to control this tick, although the selection of resistant populations is a worldwide concern. Thymol, a natural monoterpene, alone or combined with other compounds, has shown efficacy against tick species. In R. microplus engorged females, thymol increases cypermethrin concentration in various tissues by enhancing its penetration at the cuticular level. Data on internal organ histopathological changes from the thymol-cypermethrin combination remain limited in different R. microplus populations. This study evaluated the combined effects of cypermethrin and thymol on ovarian morphology of engorged females from two populations with low or high susceptibility to cypermethrin. Combinations of cypermethrin at different concentrations with a fixed thymol concentration (1.3 mg/mL) were tested. Eight treatment groups (n = 20 engorged females) were tested, including cypermethrin alone, thymol alone, and combinations. Ovarian changes induced by the combination were more pronounced than those caused by either compound alone, suggesting a synergistic effect. The main alterations in oocytes II-V included large peripheral vacuoles, reduced yolk granules, altered oocyte shape, chorion irregularities, and fragmented yolk granules, resulting in incomplete vitellogenesis. These findings provide histological evidence that thymol enhances cypermethrin efficacy by impairing ovarian development, supporting its potential as an adjuvant in tick control strategies.