Effect of Lactone Glycoside on Feeding Deterrence and Nutritive Physiology of Tobacco Caterpillar Spodoptera litura Fabricius (Noctuidae: Lepidoptera)

Authors: Selvamuthukumaran Thirunavukkarasu, Arivudainambi Sundararajan

Abstract:

The plant active molecules with their known mode of action are important leads to the development of newer insecticides. Lactone glycoside was identified earlier as the active principle in Cleistanthus collinus (Roxb.) Benth. (Fam: Euphorbiaceae). It possessed feeding deterrent, insecticidal and insect growth regulatory actions at varying concentrations. Deducing its mode of action opens a possibility of its further development. A no-choice leaf disc bioassay was carried out with lactone glycoside at different doses for different instars and Deterrence Indices were worked out. Using regression analysis concentrations imparting 10, 30 and 50 per cent deterrence (DI10, DI30 & DI50) were worked out. At these doses, effect on nutritional indices like Relative Consumption and Growth Rates (RCR & RGR), Efficiencies of Conversion of Ingested and Digested food (ECI & ECD) and Approximate Digestibility (AD) were worked out. The Relative Consumption and Growth Rate of control and lactone glycoside larva were compared by regression analysis. Regression analysis of deterrence indices revealed that the concentrations needed for imparting 50 per cent deterrence was 60.66, 68.47 and 71.10 ppm for third, fourth and fifth instars respectively. Relative consumption rate (RCR) and relative growth rate (RGR) were reduced. This confirmed the antifeedant action of the fraction. Approximate digestibility (AD) was found greater in treatments indicating reduced faeces because of poor digestibility and retention of food in the gut. Efficiency of conversion of both ingested and digested (ECI and ECD) food was also found to be greatly reduced. This indicated presence of toxic action. This was proved by comparing growth efficiencies of control and lactone glycoside treated larvae. Lactone glycoside was found to possess both feeding deterrent and toxic modes of action. Studies on molecular targets based on this preliminary site of action lead to new insecticide development.

Keywords: Spodoptera litura Fabricius, Cleistanthus collinus (Roxb.) Benth, feeding deterrence, mode of action