Samir A. M. Abdelgaleil

Essential oils from 20 Egyptian plants were obtained by using hydrodistillation. The chemical composition of the isolated oils was identified by gas chromatograph/mass spectrometer. Fumigant and contact toxicities of the essential oils were evaluated against the adults of Tribolium castaneum. In fumigation assays, the oil of Origanum vulgare (LC₅₀ = 9.97 mg/L air) displayed the highest toxicity towards the adults of T. castaneum. In contact assays, the oils of Artemisia monosperma (LC₅₀ = 0.07 mg/cm²) and O. vulgare (LC₅₀ = 0.07 mg/cm²) were the most potent toxicants against the adults of T. castaneum. Biochemical studies showed that the tested oils caused pronounced inhibition of acetylcholinesterase (AChE) and adenosine triphosphatases (ATPases) isolated from the larvae of T. castaneum. The oil Cupressus macrocarpa (IC₅₀ = 12.3 mg/L) was the most potent inhibitor of AChE, while the oil of Calistemon viminals (IC₅₀ = 4.4 mg/L) was the most potent inhibitor of ATPases.

The extracts of three plants, namely Zygophyllum coccineum L., Majorana hortensis (Moench) and Mentha microphylla L. were tested for their insecticidal activity against the fourth instar larvae of the Egyptian cotton leafworm, Spodoptera littoralis and the adults of the bean aphid, Aphis fabae. All of the tested extracts were found to be toxic against both insects. However, the degree of toxicity was varied based on plant extract, plant species and insect species. Among the tested extracts, petroleum ether extract of M. hortensis (LD50 = 2.76 μg/larva) was the most potent against S. littoralis followed by chloroform extract of the same plant (LD50 = 3.36 μg/larva). The petroleum ether and ethanol extracts of Z. coccineum showed the less activity against S. littoralis. In the case of A. fabae, the petroleum ether and chloroform extracts of M. hortensis, the petroleum ether and ethanol extracts of M. microphylla showed pronounced toxic effect with LC50 values of 2.21, 2.44, 2.62 and 2.25 g/l, respectively. The essential oil of M. hortensis was isolated by hydrodistillation and analyzed by gas chromatography-mass spectrometry (GC-MS). Terpinen-4-ol (30.0%), γ-terpinene (11.3%), and trans-sabinene hydrate (10.8%) were the major components of the oil. This oil revealed remarkable insecticidal activityagainst S. littoralis and A. fabae. Repetitive column chromatography of the essential oil of M. hortensis led to isolate the two major constituents (terpinen-4-ol and γ-terpinene). The structure of these compounds were determined based on their spectral data of C NMR and MS. γ-Terpinene was more toxic than terpinen-4-ol towards S. littoralis and A. fabae. The isolated compounds were less toxic than the essential oil against both insects.

The acaricidal activity of 12 monoterpenes against the two-spotted spider mite, Tetranychus urticae Koch, was examined using fumigation and direct contact application methods. Cuminaldehyde and (−)-linalool showed the highest fumigant toxicity with LC₅₀ = 0.31 and 0.56 mg/l, respectively. The other monoterpenes exhibited a strong fumigant toxicity, the LC₅₀ values ranging from 1.28 to 8.09 mg/l, except camphene, which was the least effective (LC₅₀ = 61.45 mg/l). Based on contact activity, the results were rather different: menthol displayed the highest acaricidal activity (LC₅₀ = 128.53 mg/l) followed by thymol (172.0 mg/l), geraniol (219.69 mg/l) and (−)-limonene (255.44 mg/l); 1-8-cineole, cuminaldehyde and (−)-linalool showed moderate toxicity. At 125 mg/l, (−)-Limonene and (−)-carvone caused the highest egg mortality among the tested compounds (70.6 and 66.9% mortality, respectively). In addition, the effect of molecular descriptors was also analyzed using the quantitative structure activity relationship (QSAR) procedure. The QSAR model showed excellent agreement between the estimated and experimentally measured toxicity parameter (LC₅₀) for the tested monoterpenes and the fumigant activity increased significantly with the vapor pressure. Comparing the results of the fumigant and contact toxicity assays of monoterpenes against T. urticae with the results of acetylcholinesterase (AChE) inhibitory effect revealed that some of the tested compounds showed a strong acaricidal activity and a potent AChE inhibitory activity, such as cuminaldehyde, (−)-linalool, (−)-limonene and menthol. However, other compounds such as (−)-carvone revealed a strong fumigant activity but a weak AChE inhibitory activity.

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.

A comparative study was conducted to assess the contact and fumigant toxicities of eleven monoterpenes on two important stored products insects—, Sitophilus oryzae, the rice weevil, and Tribolium castaneum, the rust red flour beetle. The monoterpenes included: camphene, (+)-camphor, (−)-carvone, 1-8-cineole, cuminaldehyde, (l)-fenchone, geraniol, (−)-limonene, (−)-linalool, (−)-menthol, and myrcene. The inhibitory effect of these compounds on acetylcholinesterase (AChE) activity also was examined to explore their possible mode(s) of toxic action. Although most of the compounds were toxic to S. oryzae and T. castaneum, their toxicity varied with insect species and with the bioassay test. In contact toxicity assays, (−)-carvone, geraniol, and cuminaldehyde showed the highest toxicity against S. oryzae with LC₅₀ values of 28.17, 28.76, and 42.08 μg/cm², respectively. (−)-Carvone (LC₅₀ = 19.80 μg/cm²) was the most effective compound against T. castaneum, followed by cuminaldehyde (LC₅₀ = 32.59 μg/cm²). In contrast, camphene, (+)-camphor, 1-8-cineole, and myrcene had weak activity against both insects (i.e., LC₅₀ values above 500 μg/cm²). In fumigant toxicity assays, 1-8-cineole was the most effective against S. oryzae and T. castaneum (LC₅₀ = 14.19 and 17.16 mg/l, respectively). Structure-toxicity investigations revealed that (−)-carvone—, a ketone—, had the highest contact toxicity against the both insects. 1-8-Cineole—, an ether—, was the most potent fumigant against both insects. In vitro inhibition studies of AChE from adults of S. oryzae showed that cuminaldehyde most effectively inhibited enzyme activity at the two tested concentrations (0.01 and 0.05 M) followed by 1-8-cineole, (−)-limonene, and (l)-fenchone. 1-8-Cineole was the most potent inhibitor of AChE activity from T. castaneum larvae followed by (−)-carvone and (−)-limonene. The results of the present study indicate that (−)-carvone, 1,8-cineole, cuminaldehyde, (l)-fenchone, and (−)-limonene could be effective biocontrol agents against S. oryzae and T. castaneum.

Chemical investigation of the diethyl ether extract of the stem bark of Khaya ivorensis A Chev (Meliaceae) afforded ten limonoids of angolensates, ring D‐opened limonoids and mexicanolides. The structures of the limonoids isolated were determined by comparison of their ¹H and ¹³C NMR data with those reported in the literature. These compounds were evaluated for their antifungal activity against the plant pathogenic fungus Botrytis cinerea Pers. Methyl 6‐hydroxyangolensate and 3,7‐dideacetylkhivorin were also tested for their antifungal and antibacterial activities on several fungal and bacterial species. Methyl angolensate and 1,3,7‐trideacetylkhivorin displayed the highest antifungal activity against B cinerea, with respectively 62.8 and 64.0% mycelial growth inhibition at 1000 mg litre⁻¹, and 73.3 and 68.6% mycelial growth inhibition at 1500 mg litre⁻¹. 3,7‐Dideacetylkhivorin showed stronger antifungal and antibacterial activities than methyl 6‐hydroxyangolensate against all of the test fungi and bacteria except Penicillium expansum Link. This is the first report on the antifungal and antibacterial effects of these limonoids. Structure‐antifungal activity relationships of the limonoids isolated are discussed. Copyright © 2004 Society of Chemical Industry