Monoterpene-Rich Nanoemulsion from Thymus vulgaris as a Promising Acaricidal Strategy Against Tetranychus mexicanus: Effects on Survival and Fecundity

Mounting acaricide resistance in Tetranychus mexicanus (McGregor) (Acari: Tetranychidae), among the most damaging phytophagous mites in tropical and subtropical crops, has intensified the search for botanical alternatives. An oil-in-water nanoemulsion of Thymus vulgaris essential oil (TVEO-NE) was developed and evaluated for lethal and sublethal effects on adult females of T. mexicanus. TVEO, composed mainly of thymol (45%) and p-cymene (37%), was formulated by low-energy emulsification yielding stable dispersions (~200 nm; PDI < 0.25; zeta potential of −22.2 mV). At 30.0 mg a.i./mL, TVEO-NE caused 68.3% corrected mortality at 72 h and suppressed fecundity by ~44–52%; vehicle controls exerted only moderate effects, identifying the essential oil as the primary bioactive driver. Morphological examination revealed collapse of female idiosomata and disruption of excretory pellet architecture, corroborating the bioassay data. Molecular docking against a cathepsin L homology model revealed that thymol and p-cymene interact exclusively via hydrophobic contacts and display substantially lower ChemPLP fitness scores than the reference cysteine protease inhibitor E64, indicating weak predicted binding affinity and arguing against enzyme inhibition as the primary mechanism. Taken together, bioassay, morphological, and docking are consistent with supporting membrane partitioning as a plausible primary mode of action, positioning TVEO-based nanoemulsions as promising botanical tools for T. mexicanus management.