| Summary: | The effect of essential oil of Origanum vulgare L. a medicinal plant was studied on toxicity, physiology and biochemical characteristics of diamondback moth Plutella xylustella L. in controlled condition. The LC10, LC30 and LC50 values were estimated 0.234, 0.710 and 1.528 percent (v/v) respectively for 3rd instar larvae. Repellency of essential oil under LC10 and LC30 concentrations was 22.8 ± 6.64 and 49.8 ± 6.95 percent respectively. Effect of plant essential oil on feeding efficiency was also evaluated under LC10 and LC30 concentrations. Approximately Digestibility (AD), Efficacy of Conversion of Ingested Food (ECI), Efficacy of Conversion of Digested Food (ECD), Relative Consumption Rate (RCR) and Relative Growth Rate (RGR) of the treated larvae showed a significant difference compared with the control. The effectiveness of plant essential oil on digestive enzymes under LC50 concentration for the survivors after 24 and 48 h after treatment was analyzed. Total protein and triglycerides were decreased significantly compared with the control. Activity of alkaline phosphatase and protease in treated larvae decreased compared with the controls. The lipase was increased 24 h after treatment compared with the control. Significant differences in detoxifying enzymes such as glutathione S-transferase and esterase were noted compared to controls. It is concluded that the essential oil used in the present experiment shows toxicity in higher doses. It is also concluded that the essential oil used did show considerable changes in repellency, feeding efficiency, reduced larval weight and changes in biochemical properties which may nominate it for further investigation in key insect pests.
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