natural-products-in-plant-pest-management

12 N.K. Dubey et al.(a)OCO(b)CHCH 33 OOCH 3Fig. 1.8. (a) Juvabione (b) Precocenes.O(Varma and Dubey, 1999). Analogues of insect juvenile hormones such asjuvocimenes in Ocimum basilicum have also been reported (Balandrin et al.,1985). Precocenes isolated from essential oils of Matricaria recutita interferewith the normal function of insect glands that produce juvenile hormonesresulting in the suppression of insect growth while moulting (Fig. 1.8b).Many plant chemicals deter insects from feeding, thereby showing anantifeedant effect. Azadirachtin and limonoids such as limonin and nomilinfrom different plant species in Meliaceae and Rutaceae (e.g. from Citrusfruits) have long been used successfully for insect control, especially in India.Azadirachtin protects newly grown leaves of crop plants from feeding damage,thereby showing systemic antifeedant properties (Varma and Dubey,1999).1.6 Essential OilsSince the middle ages, essential oils have been widely used for bactericidal,virucidal, fungicidal, antiparasite, insecticidal, medicinal and cosmetic applications,especially nowadays in the pharmaceutical, sanitary, cosmetic, andagricultural and food industries. In nature, essential oils play an importantrole in the protection of the plants as antibacterials, antivirals, antifungals,insecticides and also against herbivores by reducing their appetite for suchplants. They also may attract some insects to favour the dispersion of pollensand seeds, or repel undesirable others. Some essential oils have been recognizedas an important natural source of pesticides. Aromatic plants producemany compounds that are insect repellents or act to alter insect feedingbehaviour, growth and development, ecdysis (moulting), and behaviour duringmating and oviposition. Recently researchers have demonstrated suchcompounds showing larvicidal and antifeedant activity (Adebayo et al., 1999;Larocque et al., 1999; Gbolade, 2001), capacity to delay development, adultemergence and fertility (Marimuthu et al., 1997), deterrent effects on oviposition(Naumann and Isman 1995; Oyedele et al., 2000), and arrestant and repellentaction (Landolt et al., 1999). Plants with strong smells, such as Frenchmarigold and coriander, act as repellents and can protect the crops nearby.Most insect repellents are volatile terpenoids such as terpenen-4-ol. Otherterpenoids can act as attractants. In some cases, the same terpenoid can repelcertain undesirable insects while attracting more beneficial insects. For instance,geraniol will repel houseflies while attracting honey bees (Duke, 1990).