Thymol-Based Natural Deep Eutectic Solvents: New Trends in the Development of Bioinsecticides

The growing concern about the harmful effects of insecticides on human health and the environment has led to an expansion of the search for sustainable substitutes. In this context, the topical bioactivity of different hydrophobic natural deep eutectic solvents (NADES) prepared from plant secondary metabolites was evaluated for the first time as bioinsecticides against Tribolium castaneum and Sitophilus oryzae. The studied NADES included thymol/eucalyptol (T/E), thymol/benzyl alcohol (T/B), and thymol/menthol (T/M), evaluated in a range from pure NADES to a 1:3 molar dilution with acetone. Additionally, the toxicities of the individual compounds and their acetonic noneutectic mixtures were assessed. In a preliminary assay, T/M showed the most acute toxicity. In contact assays, T/M achieved 85% mortality for T. castaneum and 100% for S. oryzae. In repellent tests, T/M had activity against S. oryzae for 16 days but showed no repellency against T. castaneum. In order to confirm the existence of eutectic mixtures in the dilutions employed in biological tests and to understand the spatial interactions between their components, all the NADES were analyzed by NMR spectroscopy. The obtained results allow us to conclude that the evaluated NADES, especially T/M, can be considered as new alternatives for the development of bioinsecticides.