Determination of host-associated variability in the shape of the mandible of white rice stem borer Scirpophaga innotata (Lepidoptera: Pyralidae)

Host-plant modifications are known to promote high taxonomic diversity andecomorphological disparity among its insect-herbivores. Studies on mouthpart morphology specificallythe mandible are central to understanding these adaptive modifications as they are used as majorfeeding apparatus. In this study, mandible shape of a monophagous white rice stem borer, Scirpophagainnotata Walker (Lepidoptera: Pyralidae) which were obtained from different host-plant rice cultivarswere assessed using image analysis and Elliptic Fourier techniques. Contours of the mandibles wereextracted via chain-coding and principal component analyses were performed to determine patterns ofshape differences. Observed variation ranges from the arrangement and length of its teeth, from thebasal and external margins and the contour of the side of attachment from the body. The greatestvariations accounted by the first two principal components (PC’s) are on the arrangement, length andnumber of its teeth which is attributable to continuous wear of the individual mandible in effect of thebiochemical properties of the plant. Intra-population variation is associated with the influence of riceplant morphological characteristics associated with resistance or susceptibility to white stem borer.