The essential oil from leaves of Majorana hortensis Moench (Lamiaceae) was isolated by hydrodistillation with a yield of 1.6% (wt/wt). The insecticidal activity of the oil was evaluated against fourth instars of Spodoptera littoralis Boisduval (Lepidoptera: Noctuidae) and adults of Aphis fabae L. (Hemiptera: Aphididae). The oil showed a remarkable toxic effect against S. littoralis in a topical application assay (LD50 = 2.48 μg per larva) and in a residual film assay (LC50 = 3.14 g/l). The oil of M. hortensis also exhibited a pronounced toxic effect against A. fabae adults with LC50 values of 1.86 and 2.27 g/l in rapid dipping and residual film assays, respectively. Gas chromatography‐mass spectrometry analyses of M. hortensis essential oils revealed the presence of 31 compounds and the main components were terpinen‐4‐ol (30.0%), γ‐terpinene (11.3%), and trans‐sabinene hydrate (10.8%). Repeated column chromatography of M. hortensis oil on silica gel led to the isolation of two major constituents, which were characterized based on 1H‐nuclear magnetic resonance and mass spectrometric data, as terpinen‐4‐ol and γ‐terpinene. These two components were examined for their insecticidal and synergistic activities towards S. littoralis and A. fabae. Terpinen‐4‐ol and γ‐terpinene exhibited a significant insecticidal activity against both insects, but γ‐terpinene was more toxic than terpinen‐4‐ol. When tested in a binary mixture with the synthetic insecticides profenofos and methomyl, it was found that both compounds enhanced the insecticidal activity of these insecticides by two‐ to threefold. These results show that terpinen‐4‐ol and γ‐terpinene have a synergistic effect on the insecticidal activities of synthetic insecticides profenofos and methomyl.

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