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 LC(50) values of 28.17, 28.76, and 42.08 microg/cm(2), respectively. (-)-Carvone (LC(50) = 19.80 microg/cm(2)) was the most effective compound against T. castaneum, followed by cuminaldehyde (LC(50) = 32.59 microg/cm(2)). In contrast, camphene, (+)-camphor, 1-8-cineole, and myrcene had weak activity against both insects (i.e., LC(50) values above 500 microg/cm(2)). In fumigant toxicity assays, 1-8-cineole was the most effective against S. oryzae and T. castaneum (LC(50) = 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.

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Essential oils of eight plants growing in Egypt were isolated by hydrodistillation. The chemical composition of the isolated oils was examined by gas chromatography/mass spectrometry (GC-MS). In the contact toxicity assay, the oils of Mentha microphylla and Artemisia judaica were the most potent against Sitophilus oryzae. When tested against Tribolium castaneum, the oils of M. microphylla, Eucalyptus camaldulensis and A. judaica showed the highest activity among the test oils with M. microphylla being the most potent. In the fumigant assay, the oil of M. microphylla displayed the strongest insecticidal activity against S. oryzae (LC 50 ϭ0.21 ml/L), while the oils of Lantana camara (LC 50 ϭ29.47 ml/L) and E. camaldulensis (LC 50 ϭ30.81 ml/L) revealed good activity. In the case of T. castaneum, the oil of M. microphylla (LC 50 ϭ4.51 ml/L) was the most effective, followed by Citrus reticulata (LC 50 ϭ19.47 ml/L) and Schinus terebenthifolius (LC 50 ϭ20.50 ml/L). The most interesting finding in this study is the potent contact and fumigant toxicity of M. microphylla oil against S. oryzae and T. castaneum. These results suggested that M. microphylla oil could be used as a potential control agent for S. oryzae and T. castaneum.

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/cm(2)) and O. vulgare (LC50 = 0.07 mg/cm(2)) 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.

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