Morphometric Analysis of Intraspecific Variation of the Elytral Shape Between the Wild- and Mass-reared Sweet Potato Weevil Cylas formicarius

摘要

In an eradication program using the sterile insect technique (SIT), a large number of individuals are mass-reared, sterilized and then released into the field. Since the ratio of sterile to normal males in the field is used to monitor the progress of the program, an extensive mass-trapping survey is conducted in the whole target area. Although a definite marker for the released individuals is indispensable in this monitoring procedure, such a definite marker has not been developed for the sweet potato weevil Cylas formicarius. Consequently, the monitoring of the eradication program in this weevil has been hampered by the presence of less accuracy due to using an uncertain marker. Therefore, one needs to develop another marking system. Since the mass-rearing of the beneficial insect is conducted under highly artificial conditions compared to the field, such a condition prior to the emergence in the rearing facility, might lead to differences in morphological characters in the mass-reared individuals. In this study, the shape of the elytral contour between wild and mass-reared weevils was compared to assess the potential of an intraspecific differentiation pattern in C. formicarius. This comparison was conducted in spring and autumn for two successive years (four comparisons in total). The morphometric analysis using elliptic Fourier descriptors (EFDs) demonstrated that the elytral shape of the mass-reared weevil differed significantly to that of wild one in each comparison and the proportion of individuals that were correctly classified into original lineages was high (> 80%). Therefore, the EFDs are considered useful for identifying the source of the trapped weevils in an eradication program using the mass-reared individuals for the sweet potato weevil. Although the fine-scale intraspecific morphological variation is generally difficult to recognize, the present study suggests that the EFDs have potential to highlight these differences between closely related lineages within the same species.