The gut bacterial community of the red palm weevil Rhynchophorus ferrugineus (Olivier, Coleoptera, Curculionidae) larvae

Abstract

The red palm weevil (RPW), Rhynchophorus ferrugineus (Olivier, Coleoptera, Curculionidae) is the major pest of palm trees in the Mediterranean area. Damages are caused mainly by the endophytophagous larvae that consume the palm tender soft tissues making galleries and producing a wet “frass” with a characteristic fermented odour. The culturable bacterial community associated to the frass produced by RPW larvae inside Phoenix canariensis trees is mainly composed of facultative anaerobe gamma-proteobacteria that have their closest phylogenetic relatives in the gut microbiome of other endophytophagous and xilophagous coleopteran as bark beetles and other plant biomass-degrading insects (leaf-cutter ants). In RPW-bored galleries, these enterobacteria together with lactic acid bacteria, could be responsible for the synthesis of organic volatiles such as ethyl esters that are attractants for adults that deposit their eggs on the palm. The hypothesis that the frass-associated bacteria might be inoculated by the RPW larvae in order to obtain benefits for the insect (i.e. attraction of adults, plant polymers degradation, protection against fungal invasion, etc.) is under investigation. To this aim we started to characterize the bacterial community of the RPW larval gut by culture-based and culture-independent methods. Bacterial and DNA isolation was carried out from RPW larval guts after sterilization and dissection. About 107 CFU/gut were detected on Nutrient Agar plates and the phylogenetic analysis of 48 isolates is in progress. Cellulosolytic and hemicellulosolytic proteobacteria and actinobacteria were isolated from gut enrichment cultures on carboxymethyl cellulose and sterile filter paper but no cellulolytic activity was detected in the frass-associated bacterial community. Total gut DNA was used in 16S rRNA gene analysis by denaturing gradient gel electrophoresis and for the construction of a ribosomal gene library that is under screening.