Rice stem borers are considered the most serious insect pest of rice in Asia. The
white stem borer Scirpophaga innotata (Walker, 1863) is the most prevalent among
the different species that exist in the Philippines. This pest uses its strong mandibles
to penetrate and feed on its plant host. Rice varieties of various level of resistance
were deployed in different geographical locations in the Philippines but infestations
were still observed and have contributed to economic losses to the farmers. Although
differences in the resistance level among rice varieties makes an important factor in
the control of the pest, no rice variety has been reported to be completely resistant
against rice stem borer attacks. Several reports have indicated however that the level
of damage caused by rice stem borer attacks differs. Thus it was the major objective
of this study to determine if populations of white stem borer showed intraspecific
variation between different geographical locations and between rice varieties. Since
the mandible of the larvae is used in penetrating the plant, this morphological part of
the insect larvae was investigated. Outline-based geometric morphometric analysis
was used to test the hypothesis that the mandible shape of white stem borer differ with
respect to different rice varieties and geographical location. Results of the principal
component analysis showed variation in the shape of the mandible among populations
of white stem borer taken from different rice varieties but not in the same variety
collected from different geographical locations. Results of this study further confirm
the general argument that host plant resistance play an important role in the evolution
of pests.