Laboratory Evaluation of the Efficacy of Wild Thyme Oil (Thymus syriacus Boiss) and Its Nano-Formulations Using Silica Nanoparticles and Polyethylene Glycol Against the Two-Spotted Spider Mite (Tetranychus urticae Koch) (Acari: Tetranychidae)

Abstract

The two-spotted spider mite, Tetranychus urticae Koch, is considered one of the most dangerous agricultural pests worldwide due to its wide distribution, broad host range, and rapid development of resistance to chemical pesticides. These characteristics have made its control using conventional methods increasingly difficult, prompting the search for safer and more effective alternatives. As a result, research has shifted toward evaluating the efficacy of biopesticides derived from aromatic plants. In this study, the efficacy of wild thyme (Thymus syriacus Boiss.) essential oil and its nano formulations prepared using silica nanoparticles and polyethylene glycol (PEG) was evaluated for the control of T. urticae at different developmental stages (adult, protonymph, and egg). The experiments were conducted using the leaf disc dipping method, in which treated discs were immersed for 5 seconds in various concentrations of the essential oil (0.5%, 1%, 2%, and 4%) and its nano-formulations (100, 250, 500, 1000, 2000, and 3000 ppm).  The results showed that mortality rates varied depending on the type of treatment, applied concentration, target stage, and the post-treatment period. The 4% concentration of thyme oil caused 100% mortality in both adult and protonymph stages from the fifth to the seventh day post-treatment. The 2% concentration also achieved high efficacy, with a maximum adult mortality rate of 87.14% on the seventh day, and 100% mortality of protonymphs starting from the fifth day. At a concentration of 2000 ppm, the thyme oil–nano-silica formulation induced female mortality rates of 34.67% on the first day, increasing to 97.14% by day seven. For the protonymph stage, mortality was 37.32% on day one and reached 100% from day five onward. In comparison, the PEG-based formulation at the same concentration exhibited a slower effect, with protonymph mortality rising from 28% on day one to 94.67% by day seven, while female mortality increased from 20% to 89.98% by the end of the trial. At a concentration of 3000 ppm, the nano-silica formulation exhibited a more rapid increase in efficacy during the initial days compared to the PEG-based formulation, which showed a more gradual effect. On day one, thyme oil–nano-silica induced mortality rates of 40% and 38.67% in protonymphs and females, respectively, whereas the PEG-based formulation caused equal mortality rates of 38.67% in both stages. As the experiment progressed, mortality continued to rise in both treatments, reaching 100% for both stages by day five. High-concentration treatments caused a noticeable delay in egg hatching. The essential thyme oil at 4% and 2% concentrations inhibited hatching by 76.87% and 73.98%, respectively. Meanwhile, the nano-silica and PEG-loaded formulations at 3000 ppm delayed hatching until the fifth day of the experiment, with inhibition rates of 67.38% and 62.45%, respectively. These results highlight the high efficacy of thyme oil and its nano-formulations, supporting their potential integration into integrated pest management (IPM).