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The essential oils of Thymus vulgaris L. and Origanum vulgare L. were investigated for their toxicity and physiological aspects on the lesser mulberry pyralid Glyphodes pyloalis Walker in controlled condition (24±1°C, 75±5% RH and 16L : 8D). The leaf disc method was used to study acute toxicity, the effect of LC10, LC30, and LC50 on the feeding efficiency of 4th instar larva, and biochemical indices. The essential oil doses of LC10, LC30 and LC50 were estimated to be 0.107%, 0.188% and 0.279% for T. vulgaris, and 0.67%, 1.173% and 1.728% for O. vulgare, respectively. The results showed that T. vulgaris was more toxic than O. vulgare. The essential oil sublethal dose LC30 affected the nutritional indices of 4th instar larvae of G. pyloalis. The essential oils reduced total protein, carbohydrate, and lipid. Some concentrations of essential oils changed the activity level of α-amylase, protease, lipase, general esterases, and glutathione S-transferase (GST) but others showed no effect on these enzymes. It was concluded that the used essential oil concentrations were both toxic to G. pyloalis and showed irreversible effects on key metabolic processes, therefore, the used essential oil concentrations may be considered as alternatives to the classic pest control agents.

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