Phenolic monoterpene-rich essential oils from Apiaceae and Lamiaceae species: insecticidal activity and safety evaluation on non-target earthworms

The genera Oliveria (Apiaceae), Thymus and Satureja (Lamiaceae) encompass important medicinal and aro- matic plants of Iran, which contains noteworthy amounts of essential oils (EOs) that are worthy of exploitation on an industrial level, including pesticide formulation development. In this study, the efficacy of five EOs obtained from O. decumbens, T. daenensis, S. sahendica, S. khuzistanica and S. rechingeri, was evaluated against three insects of economic relevance, the mosquito Culex quiquefasciatus, the housefly Musca domestica, and the moth Spodoptera littoralis. Potential non-target effects of these EOs were assessed on earthworms, Eisenia fetida. The chemical composition of the five EOs was determined by gas chromatography (GC-FID) and gas chromatography-mass spectrometry (GC-MS). EOs from O. decum- bens, T. daenensis, S. khuzistanica and S. rechingeri were characterized by oxygenated monoterpenes (70.3, 91.6, 89.7 and 89.4%, respectively), whereas the EO from S. sahendica was dominated by monoterpene hydrocarbons (63.3%) followed by oxygenated monoterpenes (35.3%). In all cases, the biogenetically related thymol (0.3–36.7%), carvacrol (1.3–89.6%), p-cymene (2.1–13.7%) and γ-terpinene (1.5–41.3%) were found as the characteristic volatile compounds of the five EOs, with high quantitative variation according to the species considered. The five EOs exhibited high toxicity on the three target insects, with LC50/LD50 values in the range of 16.2–29.3 μg mL-1, 18.1–48.1 μg adult-1 and 7.4–23.1 μg larva-1 on C. quin- quefasciatus, M. domestica and S. littoralis, respectively. Toxicity of the five EOs on non-target soil invertebrates was mini- mal, leading to no or very low (5.0–7.5%) mortality on E. fetida when tested at the concentration of 200 mg kg-1, at variance with the positive control α-cypermetrin, which caused 100% mortality at 0.1 mg kg-1 of soil. Overall, our study discloses new opportunities to farmers growing these Iranian endemic plants, shedding light on the potential of these EOs to formu- late effective and eco-friendly insecticides.