natural-products-in-plant-pest-management

Allelochemicals in Pest Management 139Studies of the chemosensoric insect system are only at the initial point ofresearch, and knowledge of the mode of action of the substances is superficialand should be studied in detail. Such a lack of knowledge is particularlycaused by the fact that suitable technologies for chemosensoric investigationshave been available only relatively recently, making it possible to performreliable measurements of response to substance at insect chemoreceptors.Although research of chemoreceptors is important for a general understandingof efficiency of individual antifeedant substances, from the practicalpoint of view, experiments based on simple biological tests are thosemostly used in antifeedancy studies.Bioassays against insects have been used for decades as a means ofelucidating the activity of many chemical components or extracts. The majorgoals achieved by employing bioassay techniques are to determine the rolesof naturally occurring chemicals, identify the mechanism of resistance incrop plants and to find various insect control agents. The basic design tostudy deterrents is to present to an insect a substrate with the candidatechemical and to measure the response of the insect. Therefore, substratechoice and presentation are important factors for a successful bioassay. Bothnatural and artificial substrates are used, depending upon the goal of theexperiment. On one hand one may emphasize that artificial substrates offeruniformity, but at the same time studies have shown that thresholds for thesame deterrent may vary as much as 1000 times between natural and artificialsubstrates (Schoonhoven, 1982), perhaps due to differences in porosity oruptake rates by the insect. For sucking insects, the principal artificial substrateused has been a chemically defined liquid presented between naturalor artificial membranes (Koul, 2005).However, whatever the substrate may be, it is important that no texturaldifferences should occur between the control and test substrates. Colour differencesmay also influence insects during testing. Care is needed to ensurethe least hindrance with the presentation to the insect chemoreceptors, whichshould be in the usual way. Natural substrates could be whole plant, leaves,leaf discs, or specialized substrates such as twigs, blocks of wood, board, andpaper towel discs. Artificial substrates usually include agar-based artificialdiets, simple liquid-based artificial diets, styropors, or discs of foamedpolystyrene, or polyurethane, and glass fibre discs (Koul, 2005).Leaf discs are commonly used in preference or consumption bioassayswith chewing insects. These assays are important in estimating the biologicalpotential of the antifeedant effect of plant extracts in screening studies, andthey correspond as much as possible to the conditions of the practical application.However, it must be emphasized that these assays are short term. Forthe purpose of practical use, further biological assays must be performed inextracts or substances showing the best biological activity, which will be of along-term nature and will provide evidence on the practical applicability ofthe substances or extracts in plant protection. Nevertheless, such assays areirreplaceable as screening assays.Individual types of biological assays used for evaluating antifeedantefficiency are discussed by Koul (2005). However, in general, such assays can