Describing Shapes of Mandible in Selected Populations of the Rice Striped Stem Borers (Chilo suppressalis, Walker, 1863) Associated with Different Rice Types

The striped stemborer (SSB), Chilo suppressalis Walker, 1863 is a widely distributed pest of the rice agroecosystem. It bores and feeds inside the stalks of the rice plant during larval stage using its strong mandibles. Host-plant resistance has played a very important role in the management of this pest however; many cultivated types have succumbed to infestation even those with hard stems. Since the mandibles play a very important role in the entry of the larvae inside the stem, it is hypothesized that larvae with morphologically strong mandibles are those specifically associated with the genotypes of the plant. This study was therefore conducted to determine if mandibular variations are associated with rice types. We used an outline method of shape analysis, the elliptic Fourier analysis of geometric morphometrics as a tool in describing mandibular variations in SSB. Independent aspects of mandibular shape variation were identified where more than 91% were related to symmetrical variations and 87% belongs to asymmetrical variations. Observable variations between larvae associated with rice types include the form of the external and basal margins of the mandible, the length and arrangement of teeth, degree of basal angle and the shape of the side of attachment of the mandible to its body. While host-associated differentiation (HAD) may explain the variations observed in the larvae, more populations need to be investigated especially those feeding on other host plants.