High toxicity and specificity of the saponin 3-GlcA-28-AraRhaxyl-medicagenate, from Medicago truncatula seeds, for Sitophilus oryzae

It was observed that Medicago truncatula seed flour displayed a strong toxic activity towards the adults of the rice weevil Sitophilus oryzae (Coleoptera), a major pest of stored cereals. The molecule responsible for toxicity was purified, by solvent extraction and HPLC, and identified as a saponin, namely 3-GlcA-28-AraRhaxyl-medicagenate. Saponins are detergents, and the CMC of this molecule was found to be 0.65？mg per mL. Neither the worm Caenorhabditis elegans nor the bacteria E. coli were found to be sensitive to this saponin, but growth of the yeast Saccharomyces cerevisiae was inhibited at concentrations higher than 100？μg per mL. The purified molecule is toxic for the adults of the rice weevils at concentrations down to 100？μg per g of food, but this does not apply to the others insects tested, including the coleopteran Tribolium castaneum and the Sf9 insect cultured cells.This specificity for the weevil led us to investigate this saponin potential for pest control and to propose the hypothesis that this saponin has a specific mode of action, rather than acting via its non-specific detergent properties.Chemical pesticides in general, and insecticides in particular, are increasingly used around the world but are also increasingly stigmatized because of their persistence and their toxicity to non-target organisms (impacting amphibians, aquatic wildlife, beneficial insects, such as bees and ladybirds, and even causing mortality among farmers, particularly in developing countries [1-3]). Crop protection against two very important pests, namely cereal weevils and aphids, is currently carried out almost exclusively by chemical treatments. Some alternative methods exist in the fight against these insects, but they are either much less effective or prohibitively expensive compared with chemical control.Chemical treatments used to protect stored products are the source of the majority of chemical residues in cereals, subsequently found in processed products. High dose