Key Factor Responsible for Fluctuations in Rice Yellow Stemborer Populations in Deepwater Rice Ecosystems

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Abstract
A series of life tables (22 generations) for the rice yellow stem borer, Scirpophaga incertulas (Walker), a major pest of rice in Bangladesh, was constructed. In a deepwater rice ecosystem, the S. incertulas population dropped to a low level in the second or third generation (May-June), then reached the annual peak level in the fifth generation during September-October. The population dropped again to the lowest level in the sixth generation (November-February) when the larvae remained in diapause for about 3 months. In general, the population suffered very high mortality (99%). The estimated egg mortality was 49%; about 96% of the hatched larvae failed to penetrate into the rice stem; 39% larvae and 27% pupae died inside the stem. The population suffered very high mortality (96 %) at the early larval stage and the resultant survivorship curve was type III. Key factor analysis revealed that the disappearance of newly hatched larvae before penetration into the rice stem was the key mortality factor responsible for population fluctuations between generations. Disappearance of young larvae and egg mortality were density-dependent, but of a weak type.