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The bioactivity of 22 essential oils from aromatic and medicinal plants was tested uponAcanthoscelides obtectus Say (Coleoptera, Bruchidae), a pest of kidney bean (Phaseolus vulgaris L.). The insecticidal effect was evaluated by determination of 24- and 48-hr LC50 and LC50 (from 1.50 mg/ dm(3) to more than 1000 mg/dm(3)). Isoprenoids and phenylpropanoids were identified by gas chromatography. The most efficient essential oils were extracted from plants belonging to Labiatae.Origanum marjorana andThymus serpyllum essential oils were the most toxic.

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Resume.-Des observations ethnobotaniques ont montre qu'il existe dans le sud-ouest de Ia France une pratique traditionnelle de protection des graines de legumineuses par les plantes odorantes. L'efficacite de cette methode a ete evaluee en laboratoire. L'activite insecticide des plantes aromatiques s'exerce a deux niveaux : un eifel lethal sur les populations adultes et une inhibition de Ia reproduction. Les plantes les plus actives appartiennent a Ia famille des Labiees.Apres extraction des huiles essentielles, l'etude de l'activite biologique sur les insectes des differentes fractions botaniques montre que l'effet insecticide n'est pas le fait du seul extrait hydrodistille. L'activite protectrice des plantes aromatiques resulte de l'action de plusieurs composes allelochimiques, notamment terpeniques et polyphenoliques, que les plantes synthetisent au cours du metabolisme secondaire. L'utilisation de ces molecules dans le cadre de Ia lutte ecochimique pourrait contribuer a diversifier les methodes de Iuiie contre les insectes phytoravageurs.Summary.-Some ethnobotanical observations show that aromatic plants are used as traditional protectants of leguminosae seeds to avoid beetles attacks in southwestern France. The efficiency of this method was tested in laboratory. Aromatic plants present a double insecticidal effect : by direct toxicity on adults and also inhibition of the reproduction. The most efficient plants belong to Labiatae family. The insecticidal effect of water extracted and residual botanicals were tested. Several allelochemicals, especially monoterpenes and polyphenols, synthetized by the plants' secondary metabolism, are involved in this toxicity. The toxic ecochemical activity of these compounds could be considered as alternative or complementary method to classical pesticides for a better insect pest management.

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