Molecular Characterization and Physiological Aspects of Lepidiota mansueta Burmeister (Coleoptera: Scarabaeidae

The present investigations were carried out in the Department of Entomology and Department of Soil Science, DBT-AAU Centre, Assam Agricultural University (AAU), National Bureau of Agricultural Insect Resources, Bangalore and Tezpur University, during the years 2011-2015 to generate a comprehensive information on the molecular phylogeny as well as some physiological aspects of Lepidiota mansueta, a major white grub species endemic to Majuli river island of Assam. Molecular analysis of genetic diversity revealed that the highest per cent polymorphism and Polymorphism Information Content (PIC) was found for BEM 22 marker which was 80 per cent and 0.799 respectively, while lowest percent polymorphism and PIC was recorded for BEM 11 i.e. 8 per cent and 0.083, respectively. Estimation of genetic similarities among the 18 species of scarab beetles suggested that all species were dissimilar to one another. The similarity value ranged from 0.130 to 0.714. The lowest similarity value was found in between Adoretus renardi and Apogonia blanchardi (0.118) followed by A. blanchardi and Holotrichia sp. (0.130) and the highest similarity value was found between Maladera insanabilis and Onitis philemon (0.769) followed by O. philemon and Adoretus bicolor (0.714). The species L. mansueta had a close similarity to Maladera insanabilis (0.643) while the same was observed to be the lowest with Apogonia blanchardi (0.250). Two major clusters were derived from the dendrogram in which, L. mansueta could be comprehended to be genetically close to M. insanabilis, Catharsius molossus, O. philemon, A. renardi, Anomala pellucida and A. bicolor. However, within this group, L. mansueta stands as an outgroup. Laboratory experiments also revealed distinct sexual dimorphism in the antennal segments between the male and female beetles of L. mansueta. The length and breadth (mean+SD) of pedicel (0.574+0.165; 0.326+0.057), flagellum (1.452+0.272; 0.472+0.113), proximal lamellae (1.699+0.378; 0.767+0.103), middle lamella (1.724+0.174; 0.729+0.092) and distal lamella (1.686+0.137; 0.652+0.097) was significantly higher in males than the females which was recorded to be 0.322+0.014 & 0.214+0.011, 0.797+0.058 & 0.293+0.046, 1.503+0.229 & 0.594+0.069, 1.572+0.190 & 0.577+0.080 and 1.460+0.214 & 0.532+0.027 for pedicel, flagellum, proximal lamellae, middle lamella and distal lamella, respectively. Sensilla located in the antennae of both sexes of L. mansueta beetles also exhibited dimorphism. Scanning Electron Microscopic studies revealed 3 types of sensilla in males and 7 types in females. Wind tunnel bioassays showed clear affinity of adult males and females to prothoracic region (PTR) extracts of males while only males were found to be attracted to the abdominal extracts of females. GC-EAG readings also exhibited clear response of both male and female antennae to the PTR extracts of males while significant response was observed only in male antenna to abdominal extracts of females. GC-MS/FID analysis revealed 4 different compounds in the PTR extracts of males viz., cis-9Hexadecenal, cis-9-Hexadecenoic acid, Octadec-9-enoic acid and 1-Hexacosene. Likewise, female abdominal extracts also registered 4 compounds viz., cis-9Hexadecenoic acid, 18-Nonadecenoic acid, Octadec-9-enoic acid and 9,19-Cyclolanost24-en-3-ol, acetate during the course of study. Microbial investigation of the gut content of third instar grubs of L. mansueta revealed 20 different bacterial cultures. Out of these, 5 bacterial cultures designated as B1, B6, B11, B15 and B19 had the population load with highest colony forming unit/ml. Bacterial flora was considerably varied in size, margins, elevation, gram staining and shape and the variations were based on utilisation of carbohydrates as well as their response to different enzymes. Out of the 5 bacterial cultures, B1, B6 and B15 exhibited cellulose degrading activities in laboratory conditions.