Ecological constraints and chemically mediated avoidance behavior in Sitophilus zeamais infesting stored rice

Sitophilus zeamais (Coleoptera: Curculionidae) is a globally distributed pest of stored grains; however, the ecological and biological constraints governing its infestation process remain incompletely understood. In this study, we investigated host-stage utilization and chemically mediated behavioral responses of S. zeamais in relation to stored rice. First, we examined survival and reproductive success on paddy rice and brown rice to clarify the infestation stage at which this species can complete its life cycle. Survival and progeny emergence were evaluated on multiple rice varieties. After 20 days, more than 86.7% of adults survived on all brown rice varieties tested (Black, White, Tsukushiakamochi, and Beniroman), and successful progeny emergence was observed. In contrast, survival on paddy rice was extremely low (0–16.7%), and no progeny emergence occurred. These results demonstrate a clear host-stage limitation, indicating that S. zeamais is unable to establish a viable population before threshing and that infestation is restricted to post-threshing rice. Second, taking advantage of the innate tendency of S. zeamais to move upward, we examined behavioral avoidance responses to leaves of Juniperus chinensis var. kaizuka. The leaves exhibited significant repellent activity against S. zeamais, although the effect declined with repeated exposure. Gas chromatography–mass spectrometry analysis revealed that the leaves were rich in volatile compounds, with bornyl acetate (17.98%), D-limonene (8.64%), linalool (5.70%), ethyl laurate (5.12%), and β-myrcene (4.62%) as major constituents. Monoterpenes and monoterpenoids accounted for a substantial proportion of the extract, suggesting their primary role in mediating repellency. This study advances our understanding of S. zeamais ecology, shedding light on how this species navigates the boundary between natural plant habitats and human-managed storage environments.