Origanum syriacum subsp. syriacum: from an ingredient of Lebanese 'manoushe' to a source of effective and eco-friendly botanical insecticides

Origanum syriacum subsp. syriacum, also known as ‘Za’tar’ is an aromatic shrub native to Lebanon and cultivated in other Middle East countries. The plant leaves enjoy a high reputation as a traditional remedy against cardiovascular, respiratory and infectious diseases. In addition, they are a famous component of the Lebanese pizza (“manoushe”). Starting from its safety for humans, here O. syriacum subsp. syriacum was selected to assess the insecticidal efficacy of its leaf essential oil (EO) and its major constituent carvacrol against two key agricultural pests, namely the noctuid Spodoptera littoralis and the aphid Myzus persicae, as well as on the fly pest Musca domestica. Furthermore, the Za’tar EO potential impact on beneficial organisms such as the aphid predator Harmonia axyridis and the earthworm Eisenia fetida, which is used in the vermicomposting process, was assessed. GC–MS analysis highlighted the phenolic monoterpene carvacrol as the predominant component (83%) of Za’tar EO. Toxicity of O. syriacum subsp. syriacum EO was noteworthy, showing LC50/LD50 of 103.3 μg larva−1, 2.1 mg L−1 and 58.7 μg adult−1 on S. littoralis, M. persicae and M. domestica, respectively, which were partly consistent with those of its major component carvacrol (38.3 μg larva-1, 1.6 mL L−1 and 59.3 μg adult-1, respectively). When tested up to 3.8 mL L−1 and 200 mg kg−1 on H. axyridis and E. fetida, this EO was not toxic, at variance with α-cypermethrin, which caused 100% mortality at 1 mL L−1 and 25 mg kg−1, respectively. Taken together, these results promote carvacrol-rich Za’tar EO as a promising reservoir of green insecticides to be used for managing insect pests and vectors of economic relevance.