Under the JIRCAS international research project entitled "Development of low-input technology for reducing postharvest losses of staples in Southeast Asia," the research team studied the growth-inhibitory effect against stored-product insect pests of various botanicals. The target insect pests were the rice weevil (Sitophilus oryzae), maize weevil (S. zeamais), red flour beetles (Tribolium castameum) and rice moth (Corcyra cephalonica), which are commonly found in stored rice worldwide. The project team found a strong growth-inhibitory effect of a spice plant, long pepper (Fig. 1, Piper retrofractum) against weevils and the beetle. Long pepper is a vine in the family Piperaceae; the fruit has a hot taste similar to black pepper. It is widely used in many types of food and medicines in tropical and subtropical regions. In the experiment, dried and powdered long pepper fruits were mixed into feed and its effect against insect pests was examined. Long pepper completely suppressed adult emergence from eggs of weevils at a dose of 0.5% (w/w), and the growth of 1–7-day-old red flour beetle larvae by approximately 90% at a dose of 5% (w/w).
In addition, long pepper showed a definite suppressive effect against red flour beetles in the model experiment using imitation warehouse-like boxes in which brown rice and starch powder mixed with long pepper powder were separately placed. Long pepper also inhibited the development of another important insect pest, the rice moth (Corcyra cephalonica), but the effect was not as strong as against Coleoptera. The growth-inhibitory component of long pepper was then isolated by activity-guided fractionation using weevils. The obtained pale yellow crystals of the active principle were identified as piperine by mass and nuclear magnetic resonance (NMR) spectrometry. Piperine is an alkaloid found in the black pepper seed that has a hot flavor. Purified piperine substantially suppressed the adult emergence of weevils by 50% (ED50) at a dose of approximately 50 ppm (Fig. 2). The risk of long pepper extract or piperine to human health is likely to be much lower than most other synthetic insecticides, since it has a long history of use as a spice. We therefore believe that long pepper has the potential to provide a safe alternative to synthetic insecticides. The project team is currently studying how to apply long pepper or piperine extracts on an individual scale in rice storage.
(G. Trakoontivakorn, R. Juntarawimoon, Y. Hanboonsong and K. Nakahara)
Fig. 1. Long pepper (Piper retrofractum), fresh fruit and leaves (left bottom); dried fruits (top right).
Fig. 2. Growth- inhibitory effect of piperine against weevils.
Eggs of weevils were reared in the feed pellet containing purified piperine at the indicated doses. Emerged adult weevils during 8 weeks were counted. The chemical structure of piperine is shown in the panel.
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