Infestation of rice weevils in rice grain in relation to drying procedures after harvest and the form of the grain at different stages in the milling process.

Abstract

Drying conditions after harvest and the form of the rice grain at different stages in the milling process affect the degree of infestation by pest insects. The reproduction and development of two rice weevils, Sitophilus zeamais and S. oryzae, were compared from life tables obtained at 30°C and 70% R.H, on rice grain at different stages in its processing (rough, hulled, or polished rice) and under different drying conditions for rough rice (nat-urally dried by hanging on racks or mechanically dried with heated air) .No progeny were found on rough rice dried naturally. Weevils were able, however, to deposit eggs into a split in the husk of rough rice that was produced by mechanical drying. The average length of the developmental period was shortest on hulled rice, polished rice probably being inferior to hulled rice in its content of nutrients. The longevity and oviposition period of adult weevils were longest on polished rice and shortest on rough rice. The reproductive potential changed with adult aging. The peak of natality (bx) curve came earlier on hulled rice than on polished rice. The reproductive potential during the adult life span was highest on hulled rice and smallest on rough rice.Our results are evidence that rough rice is the most desirable form in which to store rice grain to prevent its infestation by rice weevils.