natural-products-in-plant-pest-management

136 R. PavelaIn many cases not a single semiochemical has an effect on its own butdifferent groups of chemicals in a precisely defined mixture act in an effectivelycombined manner. In general, it can be said that the whole group ofallelochemicals may find its application in plant protection. Nevertheless,metabolites falling in the allomone group represent the most studied groupwith the most potential at present.6.2 Allomones – A Prospective Group of Substances forAlternative Plant ProtectionAs explained above, this group of substances includes plant metabolites thathave some type of negative effect on insect behaviour providing benefit tothe plants. The chemicals can be divided as follows according to their modeof action:1. Repellents2. Anti-ovipositants3. AntifeedantsRepellents are substances that directly deter insects from settling on theplant. They include an entire group of simple aromatic hydrocarbons, whichmay be released into the environment, thus having a direct effect on insectchemoreceptors (Koul, 2005). Repellency often tends to be connected withanti-oviposition, as such substances deter females from settling on nutritiveplants and prevent oviposition at the same time. However, anti-ovipositionneed not always be connected with repellency. It may be connected with antifeedancyvery often – when upon settling on the plant, the female finds thatthe plant cannot provide food of good quality or acceptable for its descendants,and thus she flies off to seek a more suitable plant. The group of substanceswith an anti-oviposition effect includes a whole range of chemicalsfrom simple aromatic terpenes, phenols or alkaloids to molecules falling inthe group of polyphenols or limonoids (Koul, 2005).The last group of substances – antifeedants – deter phytophagous insectsagainst food consumption. This group of substances has been studied on alarge scale in recent times and the use of such substances in plant protectionagainst pests is connected with their significant potential. This group ofchemicals also range from simple aromatic terpenes, phenols or alkaloids tomolecules falling in the group of polyphenols or limonoids. At present, thereare high expectations for this group of substances due to their practical use inplant protection (Isman, 1994; Koul, 2005; Pavela, 2007b).The good prospects for the practical use of antifeedants are based onseveral factors. Most of the repellent substances fall into the group ofaromatic hydrocarbons, which are volatile in the environment and their efficiencytime is therefore reduced depending on period of application anddosage used. Numerous substances belonging to the polyphenols andhigher terpenes are included in the category of antifeedants, which mayhave a much longer persistence time, thereby extending the efficiency time