Chapter 11
Insecticidal and Antifeedant Activities of Plant Compounds Potential Leads for Novel Pesticides
Downloaded by MONASH UNIV on October 26, 2012 | http://pubs.acs.org Publication Date: December 20, 1993 | doi: 10.1021/bk-1994-0551.ch011
Pierre Escoubas, Labunmi Lajide, and Junya Mizutani Research and Development Corporation of Japan (JRDC), Eniwa RBP, Eniwa-Shi, Megumino Kita 3-1-1, Hokkaido 061—13, Japan
We have examined a variety of plants, chosen for their traditional medicinal or insect control uses, for insecticidal and insect antifeedant properties. Our test insects are crop pests such as the tobacco cutworm Spodoptera litura, medically important insects (e.g the yellow fever mosquito Aedes aegyptii) or wood-destroying organisms such as the termite Reticulitermes speratus. Using innovative methods, as well as classical bioassays, we have studied feeding deterrency, larval growth inhibition and acute toxicity of a number of Nigerian plant extracts. The results of our survey of bioactivies, as well as phytochemical studies are presented. Plants such as Xylopia aethiopica (Annonaceae), Aframomum melegueta (Zingiberaceae), Aristolochia albida (Aristolochiaceae), Zanthoxylum xanthoxyloides (Rutaceae), Dichapetalum barteri (Dichapetalaceae) and Detarium microcarpum (Leguminosae), have proven to be interesting sources of bioactive compounds. In their often quoted Science paper, Balandrin et al. (1) emphasize the importance of natural products as sources of useful therapeutic and commercial products. They also state the fact that plant resources in that respect, are still largely underexploited. In the past decade there has been renewed interest in the potential of natural products as sources of drugs or pesticides, due to some shortcomings of the "synthesis-only" approach. Natural products are therefore considered to provide a vast potential for the discovery of novel bioactive structures, new modes of actions or leads for the synthesis of interesting compounds (2). Certainly, in the case of the insecticides, past experience has shown this approach to be successful, with synthetic pyrethroids as the best example. Other commercially useful botanical pesticides include nicotine, pyrethrum, rotenone and several other alkaloids. Similarly, other natural compounds modifying feeding behavior or inhibiting the growth of insect larvae, are considered viable alternatives to acute toxins, for insect control (3). One of the most successful examples so far is Neem, extracted from the seeds of the tree Azadirachta indica.
0097-6156/94/0551-0162$06.00/0 © 1994 American Chemical Society In Natural and Engineered Pest Management Agents; Hedin, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
11. ESCOUBAS ET AL.
Insecticidal and Antifeedant Activities
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Downloaded by MONASH UNIV on October 26, 2012 | http://pubs.acs.org Publication Date: December 20, 1993 | doi: 10.1021/bk-1994-0551.ch011
Based on these assumptions, we have started investigating plants of various origins, for their insecticidal, antifeedant, or growth-inhibitory properties against several insect pests. Among random and non-random type search strategies, we have opted for a selective approach, relying either on ecological observation or on traditional use of the plants (4). Medicinal plants, usually containing a variety of bioactive substances, were thus considered a good choice as sources of interesting material. They are still widely used in Africa and are therefore readily available (5). Plants in temperate areas have also evolved chemical defenses against herbivores, and many temperate plant species have yet to be investigated in that respect. In this context, we present here the results of our activity screening of a series of plants from Nigeria, as well as plants collected in Hokkaido, Japan, against several insect pests. MATERIAL AND METHODS Sample collection: Nigerian plants were collected in Billiri, Northern Nigeria, and Ogbomoso Local Government area (Southwestern Nigeria). Japanese plants were collected in Nopporo and Tomakomai forest parks, Hokkaido, Japan. Preparation of the samples: Samples were dried and various extractions were performed, using non-polar (hexane) and polar solvents (methanol), as well as different temperatures (room T ., and boiling solvent). When the amount of material was limiting, 5% aqueous methanol was used. When possible, different parts of the plants (roots, bark, stems, leaves) were extracted separately. Solvents were removed under vacuum, the samples weighted, and an initial dilution to lOmg extract/ml was prepared in either methanol or acetone, for the bioassays. 0
Isolation of the active constituents was done using a bioassay-guided procedure. Depending on the extract, various combinations of chromatographic methods and solvent systems were used. Basic procedures included thin layer chromatography (analytical and preparative), as well as standard column chromatography. Structures were elucidated by a combination of NMR and MS spectrometric techniques. Details of the analyses will be reported elsewhere. BIOASSAYS Four different bioassays were performed with each plant extract: 1- Mosquito Larval toxicity: Ten second-instar larvae of Aedes aegyptii were placed in wells containing an aqueous solution of plant extract (100 μg/ml) in two replicates, and their mortality recorded after 24H (27°C, 16:8 L/D). Controls received solvent only (100 μΐ, acetone or methanol). 2
2- Leaf-disk Antifeedant Bioassay: Ten 1.0 cm sweet potato leaf disks were placed in marked wells in an agar-coated petri dish. Five disks were alternatively treated with 10 μΐ of plant extract (100 μg/cm2) or solvent (acetone or methanol). Five third instar Spodoptera litura (Lepidoptera Noctuidae) larvae per dish and three dishes were used per treatment. The treated dishes were placed in an incubator at 27°C and 75-80% RH for 16-18 hours in darkness. The leaf surface consumed was measured with a video camera interfaced to a personal computer as described earlier (6). The feeding index was calculated as 1= %T / (%T + %C) (%T = % of treated disks consumed, %C = % of control disks consumed). An arbitrary level of I
