Chemical profiles and insecticidal efficacy of the essential oils from four Thymus taxa growing in central-southern Italy

Thymus longicaulis subsp. longicaulis, Th. picentinus, Th. spinulosus, and Th. zygiformis var. magellensis are chamaephytes occurring in rocky places of central and southern Italy and are poorly explored for the chemical composition and biological activity of their essential oils. In the present work, we analysed the essential oil composition of twelve samples coming from the aforementioned species by gas chromatography-mass spectrometry (GC-MS). Furthermore, the taxa producing the highest amounts of essential oil, namely Th. longicaulis subsp. longicaulis and Th. spinulosus, were investigated for the first time for insecticidal efficacy against three target insect species, i.e., the larvae of Culex quinquefasciatus and Spodoptera littoralis as well as the adults of Musca domestica. Chemical and principal component (PCA) analyses allowed us to discriminate five main groups of samples, thymol-, carvacrol-, thymol/carvacrol-, geranyl acetate and (E)-nerolidol-types with the ones showing the highest yields (Th. longicaulis subsp. longicaulis and Th. spinulosus) being characterized by phenolic monoterpenes. The Th. spinulosus and Th. longicaulis subsp. longicaulis essential oils were assayed for toxicity against the three target insect species showing significant toxic effects, with LC50/LD50 values in the ranges 39.6–87.1 μg.larva−1, 21.7–62.4 μl.L-1 and 35.9–147.3 μg.adult−1, for S. littoralis, C. quinquefasciatus and M. domestica, respectively. Notably, the most active samples were those with the highest amounts of thymol that were also obtained from Th. spinulosus plants with the highest yield of essential oils. Overall, our results indicate very prospects for using the essential oil from Th. spinulosus in the development of environmentally safe botanical insecticides designed for protection against insects.