Prolonged sublethal effects of essential oils from non-wood parts of nine conifers on key insect pests and vectors

Conifers are of high economic importance due to their exploitation for timber, gardening and landscaping, construction and paper, varnishes, adhesives and waterproof coatings, perfumes and pharmaceuticals, bioplastics and biodiesel. They are endowed in all parts with secretory channels producing a semi-solid resin and a liquid fraction composed of a mixture of monoterpenes and sesquiterpenes, which are obtainable as a steam distillable essential oil. In most of industrial processes involving conifers, a huge biomass, made up mostly of green twigs, is produced, causing a waste disposal concern. The reuse of this material to produce essential oils may be important in a circular economy scenario. On this basis, in the present study we selected nine conifer species grown in central Italy (Abruzzo), namely Cupressus arizonica, x Cupressocyparis leylandii, Cupressus lusitanica, Juniperus chinensis cv. ‘stricta’, Juniperus communis, Juniperus × pfitzeriana, Picea abies, Pinus halepensis, and Pinus sylvestris, and evaluated the insecticidal potential of their essential oils from the non-wood renewable parts (twigs). For comparative purposes, in the case of J. communis two essential oils obtained from both male and female individuals were considered. The essential oils were obtained by hydrodistillation (yield in the range 0.3–2.0 %), and analyzed for chemical composition by gas chromatography-flame ionization detection (GC-FID) and gas chromatography-mass spectrometry (GC–MS), and for the enantiomeric distribution of the main optically active isomers by chiral chromatography. The essential oils were assessed for their insecticidal activity against three important insect species, i.e. the larvae of a moth pest, Spodoptera littoralis, the larvae of the mosquito vector Culex quinquefasciatus, and the adults of the housefly, Musca domestica. Pyrethrum extract was the positive control. The essential oils with the highest potential as insecticide were those obtained from twigs of x C. leylandii, P. abies and P. sylvestris. These oils were made up mainly of monoterpenes, with δ-3-carene (20.5 %), α-pinene (16.7 %), limonene (15.4 %) and sabinene (10.4 %); camphene (14.5 %) and limonene (13.4 %), bornyl acetate (15.6 %) and borneol (11.2 %); α-pinene (25.7 %), limonene (15.6 %) and δ-cadinene (11.4 %), as the main constituents, respectively. α-Pinene occurred in these oils mostly with the (+) enantiomeric form; limonene was present mainly as (-)-enantiomer in C. leylandii whereas in P. abies and P. sylvestris the (+)-enantiomer prevailed. Furthermore, being exposed to LD₂₀₋₃₀/LC₃₀ of these three essential oils significantly reduced the longevity, development and vitality of the target insects. Overall, our findings showed that the x C. leylandii, P. abies and P. sylvestris essential oil bioactivity fairly overpassed that of the large majority of essential oils considered in earlier studies, allowing us to successfully candidate these botanical byproducts for further insecticide formulations.