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The brown planthopper (Nilaparvata lugens, BPH), white-backed planthopper (Sogatella furcifera, WBPH) and small brown planthopper (Laodelphax striatellus, SBPH) are three serious pests in the rice cultivation of Asia. Using plant extracts or their derived compounds to control rice planthoppers could be sustainable, as they are reliable sources of bio-pesticide. In this study, we evaluated the repellency of eighteen plant ethanolic extracts and their derived compounds against these planthoppers. Our results indicated that ethanolic extracts from Litsea cubeba, Zanthoxylum bungeanum, Piper nigrum, Curcuma longa, and Cinnamomum camphora possessed repellency effects on female adults and nymphs of BPH. The petroleum ether fractions of L. cubeba, Z. bungeanum, and C. longa ethanolic extracts, the ethyl acetate fractions of P. nigrum ethanolic extracts were analyzed with GC–MS. Based on earlier reports and our GC–MS analysis, nine compounds derived from the above ethanolic extracts were selected to screen their repellency to BPH in H-tube olfactometer. Two effective compounds, 1-(2-hydroxy-4, 6-dimethoxyphenyl)-ethanone (xanthoxyline) and caryophyllene oxide, were chosen to investigate their repellency on three species of rice planthoppers. The two compounds showed significant repellency to BPH, WBPH and SBPH under laboratory and greenhouse conditions at 10–1000 ppm. Field experiments showed that the releasers which contained 1000 ppm xanthoxyline or caryophyllene oxide suppressed the planthoppers population significantly. Additionally, xanthoxyline and caryophyllene oxide showed no adverse effect on the predator, Cyrtorhinus lividipennis, in the laboratory and field conditions. These results may contribute to the IPM program for the development of green planthoppers repellents applied in the rice field. [ABSTRACT FROM AUTHOR]

Seven allelochemicals, namely R-(+)-limonene (A), vanillin (B), xanthoxyline (C), vanillic acid (D), linoleic acid (E), methyl linoleate (F), and (±)-odorine (G), were investigated for their herbicidal activities on Chinese amaranth (Amaranthus tricolor L.). At 400 μM, xanthoxyline (C) showed the greatest inhibitory activity on seed germination and seedling growth of the tested plant. Both vanillic acid (D) and (±)-odorine (G) inhibited shoot growth, however, apart from xanthoxyline (C), only vanillic acid (D) could inhibit root growth. Interestingly, R-(+)-limonene (A) lightly promoted root length. Other substances had no allelopathic effect on seed germination and seedling growth of the tested plant. To better understand and optimize the inhibitory effects of these natural herbicides, 21 samples of binary mixtures of these seven compounds were tested at 400 μM using 0.25% (v/v) Tween® 80 as a control treatment. The results showed that binary mixtures of R-(+)-limonene:xanthoxyline (A:C), vanillin:xanthoxyline (B:C), and xanthoxyline:linoleic acid (C:E) exhibited strong allelopathic activities on germination and seedling growth of the tested plant, and the level of inhibition was close to the effect of xanthoxyline (C) at 400 μM and was better than the effect of xanthoxyline (C) at 200 μM. The inhibition was hypothesized to be from a synergistic interaction of each pair of alleochemicals. Mole ratios of each pair of allelochemicals ((A:C), (B:C), and (C:E)) were then evaluated, and the best ratios of the binary mixtures A:C, B:C and C:E were found to be 2:8, 2:8, and 4:6 respectively. These binary mixtures significantly inhibited germination and shoot and root growth of Chinese amaranth at low concentrations. The results reported here highlight a synergistic behavior of some allelochemicals which could be applied in the development of potential herbicides. [ABSTRACT FROM AUTHOR]