| Rice is a staple food for nearly half of the world population. Its consumption is estimated to be around 475.64 million metric tons from 2009 to 2017, and more than 90%of rice is consumed in Asia. China occupies the first rank among the rice producer and consumer (more than 65% of Chinese population depend on rice) in the world, with 29.4 million hectares of rice-growing area per year. China will need to expand the current production by 20% more in 2030 to meet the demand of its growing population. However,rice infestation by pests reduces farmers’ income by decreasing yield and value at harvest.The green rice pest leafhopper (GRL), Nephotettix cincticeps (Uhler) is a rice crop-devastating insect pest in China and many regions of Asia. N. cincticeps is a multivoltine pest that ingests the intracellular nutrients from the phloem and xylem, causing the decrease of the crop yield. The long-term utilization of synthetic pesticide has resulted in failure to pest management through pest resistance and risk to the non-target organisms including natural enemies. The essential oils are thought to be the perfect alternatives to the conventional insecticides in Integrated Pest Management (IPM).In this study, we evaluated the effectiveness of twelve ethanol extract essential oils and two chemical compounds (linalool and piperine) derived from these oils in repelling the female adults and 3rd-instar nymphs of N. cincticeps, in the laboratory, greenhouse and field conditions. In addition, we investigated the effects of the essential oils on the activities of Glutathione S Tranferase (GST), Carboxyl esterase (CarE), and Acetyl cholinesterase (AchE) of N. cincticeps in the laboratory. The effects of essential oils on orientation and predation of Licossa pseudannulata, the relevant predator of N. cincticeps,which plays a key role in regulating the density of N. cincticeps in paddy fields, were also assessed in the laboratory and field conditions.In the paired-choice assay under the laboratory, one of the plants in each pot was applied with 0.5 mL of essential oil solutions (5mg/mL and 10mg/mL) as the treatment,while the other plant in the pot (control) was applied with 0.5 mL of solvent (DMSO+distilled water and Tween 20(?)). 1ml of chemical compounds (1 mg/mL,0.1 mg/mL,and 0.01 mg/mL) was applied as the same manner of essential oil. In no choice assay, one plant in a pot was provided, this plant was applied with of essential oil solutions and the other plant in another pot received the solvent as control. The chemical compounds was applied as the same manner of essential oil. The same volume and concentrations of essential oils and chemical compounds in the paired-choice were used in the no choice assays. The Insects on control and treated plants were counted after 24 and 48h for essential oils assays and 12 and 24h for the chemical compounds. The effect of P. nigrum,L. cubeba, and their mixture (1:1) on the orientation and predation of L. pseudoannulata was also assessed in laboratory. For the test of orientation, the L. pseudoannulata was allowed to choose between treated and no treated zone in a transparent plastic tube.O.5mL of the essential oil solutions (1000, 100, 10, or 1ppm) were applied in one zone using filter paper while the other zone received only the solvents as control. For the test on the predation, 20 active leafhopper were pretreated with each oil treatments (100ppm,lOppm or lppm) before caging with one L. pseudoannulata for 24h. The essential oils from Piper nigrum, Litsea cubeba, Zanthoxylum bungeanum, Curcuma longa, Stemona japonica, and Hypericium hemsleyanum repelled the adults and nymphs of N. cinticeps.P. nigrum and C. longa oils were the most and the least potent, with maximum repellency indexes recorded 90±4.47% and 19. 3±2.23% respectively. Linalool,with maximum repellency index 58.26±1.72%,was the best repellent among the single-tested compounds under laboratory conditions. The mixture of P. nigrum and L. cubeba caused a decrease in predation of (P < 0.001) L. pseudoannulata at 10 and 100ppm. The P.nigrum oil caused a reduction of the predation, but it did not affect the orientation of L.pseudoannulata. L. cubeba oil did not have any influence on both orientation and predation of L. pseudoannulata.For the test of enzyme activity, 30-35 adult leafhoppers were feeding the plant treated with tested oils (volume = 0.5 mL; concentration =10 mg/mL) for 24h, before collecting for the bioassay following the instructions supplied with the kits. GST, CarE,and AchE activities of leafhoppers were significantly enhanced by exposure to the P.nigrum, L. cubeba,Z. bungeanum, C. longa oils. The GST activities in leafhoppers exposed to essential oils of L. cubeba, P. nigrum, C. longa, and Z. bungeanum were 1.67,1.78, 1.49 and 1.44 times higher than GST activity in leafhoppers exposed to the control plants. The CarE activities in P. nigrum, L. cubeba, C. longa, and Z. bungeanum were 2.78, 2.04, 1.43, and 1.42 times higher (respectively) than that of controls. The AchE activity P. nigrum, L. cubeba, Z. bungeanum, and C. longa were 1.51, 1.45, 1.26, and 1.12 times higher (respectively) than that of leafhoppers exposed to the controls.In the greenhouse (both paired-choice and no-choice), one pot of plant in the cage was sprayed with 10 ml of essential oil solution (10mg/mL) as the treatment pot, and the other pot was sprayed with 10 ml of solvent (control plant). 20 mL of 1 mg/mL chemical compounds (linalool and piperine and their mixture (1:1)) was sprayed in the same way of the essential oil. The insects on control and treated plant were counted after 24 and 48h for essential oils assays and 12 and 24h for the chemical compounds. The results of greenhouse test shows that essential oils from Piper nigrum, Litsea cubeba, Zanthoxylum bungeanum, Curcuma longa, Stemona japonica, and Hypericium hemsleyanum repelled the adults and nymphs of N. cinticeps. The highest repellency was obtained in the P.nigrum (66.64±4.03%) at the choice method. The highest repellency of chemical compounds was observed in the mixture (62.98±1.71%) of linalool and piperine. The repellency of the tested oils and chemical compounds in the paired-choice assays was significantly higher relative to the no-choice assays (P < 0.05) in all experimental assays.The oils from Stemona japonica was the best for oviposition deterrent with deterrence indexes 69.71±4.37% and 56.66±4.23% in laboratory and greenhouse condition, respectively. That from C. longa exhibited the minimum deterrence indices in both laboratory (17.34±2.13%) and greenhouse (6.81±2.141%) respectively.In the open field, treated and control plots designs were used to evaluate the orientation effect of P. nigrum, L. cubeba and their mixture (1:1) on the N. cincticeps and L. pseudoannulata for ten days. For each treatment, treated plots were sprayed with a 7L of essential oil solution (20mg/mL) while the control plots received the solution without the essential oils. The application of mixture P. nigrum and L. cubeba in the field decreased the density of leafhopper at 2, 4,6 and 8 days(P< 0.05),and triggered significant attraction of the predator at 2 and 4 days(P< 0.05).This study demonstrates the repellent action of these essential oils and chemical compounds against the rice pest in all the experimental conditions, and recommends their insertion in the integrated pest management. It provides new information regarding the potential use of plant derivate oils and chemical compounds to ecofriendly control the rice pest N. cincticeps and recommends their insertion in the integrated pest management.Moreover,these findings constitute a basic caution concerning the selection and application of essential oils to prevent the erosion of the natural enemies in the rice fields.Further researches are required to assess the effectiveness of more chemical compounds to control this pest in the open field. It is fundamental to study the longtime effect of these essential oils and chemical compounds on both the N. cincticeps and L. pseudoannulata.We believe that repellent application can be coupled with other methods as part of an integrated,pest-management strategy,such as the "push-pull" method to enhance the efficiency of the biological control of pest in rice fields. |
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