Plant antifeedants

In the first practical application of plant antifeedants for forest crop protection and wildlife management, snowshoe hare, L. americanus, fed less on coniferous tree seedlings after they had been sprayed with pinosylvin or pinosylvin methyl ether (Sullivan eta/., 1992). 

Influence of the plant antifeedant, pinosylvin, on suppression of feeding by snowshoe hzres. Journal of ChemicalEcobgy 18,1573-1561. 

A SN reaction-based domino route to clerodane diterpenoid tanabalin (2-488) [258] has been described by Watanabe s group (Scheme 2.111) [259]. This natural product is interesting as it exhibits potent insect antifeedant activity against the pink bollworm, Pectinophora gossypiella, a severe pest of the cotton plant The domino sequence towards the substituted trans-decalin 2-487 as the key scaffold is induced by an intermolecular alkylation of the (5-ke toes ter 2-484 with the iodoalkane 2-483 followed by an intramolecular Michael addition/aldol condensation (Robin-... 

Condensation of coumaric acid with malonic acid yields the basic chalcone and stilbane skeletons (see Fig. 3.6). Stilbenes are found in most vascular plants, where they exhibit fungicidal and to a lesser extent antibiotic properties. They function as both constitutive and inducible defense substances. Some stilbenes inhibit fungal spore germination and hyphal growth, whereas others are toxic to insects and parasitic nematodes (round-worms). They also possess antifeeding and nematicide properties in mammals. For example, resveratrol (a stilbene in red wine) suppresses tumor formation in mammals. 

A more detailed understanding of the biochemical pathways and enzymes involved in saponin biosynthesis will facilitate the development of plants with altered saponin content. In some cases, enhanced levels of saponins or the synthesis of novel saponins may be desirable (for example, for drug production 4 or improved disease resistance3,5,6), while for other plants, reduction in the content of undesirable saponins would be beneficial (for example, for legume saponins that are associated with antifeedant properties in animal feed7). This chapter is concerned with recent progress that has been made in the characterization of the enzymes and genes involved in the synthesis of these complex molecules and focuses on triterpenoid saponins. 

Herbivores have evolved many offensive adaptations that counteract the defensive adaptations of plants, including antifeeding compounds (Rhoades, 1985). 

Clay eating probably was extremely important in human evolution it enabled hominids who did not use fire to eat plants with toxic antifeedants. However, the glycoalkaloids of the potato are heat stable and insoluble in water. Domestication of tomatoes and potatoes probably went hand in hand with clay eating. Johns (1986) suggested that geophagy is the most basic human detoxification technique with behavior antecedents that are prehominid. ... 

The levels of antifeedants can be raised safely only in plants such as forest trees or fiber crops, not in food plants for humans or livestock. Such problems have arisen inadvertently. For instance, a new insect-resistant potato cultivarhad to be withdrawn from the market because it contained high levels of the carcinogens solanine and chaconine (Renwick etal, 1984). In another example, an insect-resistant celery had 10 times the usual concentration of the carcinogen 8-methoxypsoralen (and related psoralens), which caused dermatitis in produce handlers (Seligman etal, 1987). 

The trans-chrysanthemic acid 88 is an essential component of naturally occurring pyrethrin esters which are present in the flower of Chrysanthenum cinera-riaefolium and has a defense function in these plants [122]. Very effective as an antifeedant for herbivores, it presents a broad spectrum as an insect repellent. 

Diterpenoids have a wide range of biological activities. Their role in plant-insect interactions, both as antifeedants and growth inhibitors, is reviewed. Four ent-kauranes, kaur-16-en-19-oic acid, (-)-kauran-l > -ol, 15 t-hydroxy-(-)-kaur-16-en-19-oic acid, and 17-hydroxy-(-)-kaur-15-en-19-oic acid, have recently been isolated from the leaves of Solidago nemoralis.These compounds were found to have antifeedant activity against Trirhabda canadensis. 

Bicyclic diterpenes-clerodanes. An important group of Insect antifeedants are the clerodane diterpenoids, which have been isolated from several different plant families (13)(Figure 1). Particularly well studied are the antifeedant activities of caryoptln and clerodin, and their derivatives, from Clerodendron and Caryopteris, Verbenaceae, against the tobacco cutworm Spodoptera litura L. (25-27). 

Twenty two new limonoids have been isolated from a Meliaceae plant Metia toosendan. Their antifeeding activity was tested against the larvae of Spodoptera insects <99H(50)595>. Noelaquinone 7, a new hexacyclic triazine quinone, has been isolated from the Indonesian spongt Xestospongia sp. <98H(49)355>. 

Many triterpenes also have anti herbivore activity. In general, those which are highly oxygenated seem to be more active in this regard M). The role of cardiac glycosides, insects and their predators has been reviewed (91-94). A number of metabolically altered triterpenes from the Rutaceae, Meliaceae and Simaroubaceae are antifeedants. Extracts of neem tree seeds (Azadirachta indica. Meliaceae) were shown to be repellent to a number of insects when applied to various crop plants at low concentrations. The probable active compound is tetranortriter-pene, azadirachtin (9 ). This compound from the leaves and fruits... 

Furocoumarins such as isopimpinellin, bergapten, and kokusagin have antifeedant activity against Spodoptera litura (54). A number of similar compounds from umbelliferous plants have been demonstrated to be active antifeedants against Spodoptera litura. Periplaneta americana. Musca domestica. Blattela germani ca, and Stylopyga rhombifolia (98). 

Most acetylenic compounds in plants are derived from meta-bolically altered fatty acids. These often are active in plant-insect relationships. 8-cis-Dihydromatricaria acid is also found in the defensive secretion of the soldier beetle (Chaulioqnathus 1econtei) (125), and has subsequently been shown to have anti-feedant properties against Phidippus spp. (jumping spiders) (126). As previously mentioned matricaria ester has antifeedant properties to the pink bollworm, bollworm and tobacco budworm (115). 

Plant parts other than seeds have been less well studied and little attention has been placed on insecticidal modes of action of the defenses of the neotropical Meliaceae. Our study of the insect growth-reducing activity of 50 extracts of bark, leaf and wood of Central American Meliaceae showed the potent effect of these extracts against lepidopteran larvae and the potential for isolation of bioactive compounds from alarge number of these species. Extracts from the genera Trichilia and Cedrela (Xie etal., 1994 Ewete etal., 1996a Wheeler etal., 2001) show exceptional activity. Trichilia americana extracts have strong antifeedant activity to Spodoptera... 

These natural substances are of great interest due to both their unusual structures and the potent wide-ranging spectrum of biological activities that they possess antitumor activity [10] in vitro and/or in vivo, antiinflammatory activity [7], fungicidal activity [11], herbivorous mammalian antifeedant activity [12], insecticide activity against house-fly larvae and other insects [13] and, lastly, a potent plant growth regulatory activity, both as inhibitors and as stimulants [14-15]. 

Optically active cyanohydrins widely occur in an O-glycosylated form in plants and in many insects where they serve as antifeedants. The formation of... 

During the course of screening plants for insect antifeedants, the extract of the bulbs of Lycoris radiata Herb, was found to exhibit antifeeding activity when tested against the larvae of the yellow butterfly Eurema hecabe mandarina (61). 

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