Antifeedant Phytochemicals in Insect Management (so Close yet so Far)

Recently, the role of plant-based pest control agents has been considered to have significant potential to act as an alternative to synthetic chemicals for integrated pest management as they pose little threat to the environment or to human health. Bioactivity of plant-based compounds is well documented in the literature and is a subject of increasing importance. An antifeedant approach for insect control has been extensively studied, at least at laboratory level, though only a handful of plant-based compounds are currently used in agriculture. The known active plant-based antifeedants belong to groups like chromenes, polyacetylenes, saponins, quassinoids, cucurbitacins, cyclopropanoid acids, phenolics, alkaloids, and various types of terpenes and their derivatives, which have been comprehensively discussed in numerous reviews and books. Therefore, this chapter will not discuss various classes of compounds but will emphasize some of the latest isolated molecules that have significant potential for inducing feeding deterrence in insects. It is also obvious from various studies that each insect species may process these allomones in a thoroughly idiosyncratic way, so that the same compound may have very different fates and consequences in different species of insects, thus pointing to different mechanisms involved in antifeedant action. It can also be visualized that insect feeding deterrents may be perceived either by stimulation of specialized deterrent receptors or by distortion of the normal function of neurons, which perceive phagostimulating compounds. Some plant antifeedants influence the feeding activity through a combination of these two principal modes of action. From a commercial point of view only a few highly active antifeedants have been researched, which makes it impossible to systemize or to predict any molecular motifs in feeding inhibition. Impairment in deterrence resulting from habituation has been suggested that could pose different implications for pest management than does decreased deterrence resulting from increased tolerance to toxic substances. Genetically modified plants, which could produce the active antifeedant substances in amounts high enough to protect the plants from further herbivorous damage, could be a possibility in the future.