An AssignmentSubmitted In Partial FulfillmentOfENT-509(PLANT RESISTANCE TO INSECTS)OnROLE OF CHEMICAL ECOLOGY, TRITROPIC RELATIONS,VOLATILES AND SECONDARY PLANT SUBSTANCES ON HOSTPLANT RESISTANCE TO INSECTSUBMITTED TO:Dr. P. K. BoradProfessor and head,Dept. of Entomology,BACA, AAU, Anand.SUBMITTED BY:MAYANK V.PATEL2ndSem. M.Sc. (agri.)REG No: 04-1904-2012
CHEMICAL ECOLOGY AND ITS USE IN HPR:Chemical ecology is a discipline that emerged during the past half century and is by definition anintegrative research field. It is driven by the recognition that organisms of diverse kinds make use of chemicalsignals to interact (Karban and Baldwin 1997). It promises an understanding of the molecular and geneticmechanisms of biological signal transduction in species interactions, which can help to ultimately understandthe evolution of complex species interactions.Plant chemicals which affect behaviour have also been classified into two main groups. There are thosewhich the insect can utilize as nutrients as well as behavioural cues, and then there are those which have noapparent nutrient value but which serve only as sign stimuli, enabling the insect to select the appropriate foodor host-plant. These are known as secondary plant chemicals (Kennedy and Booth, 1951).Plant odours can attract or repel insects. In either case the volatile plant constituent affects theorientation of the insect with respect to the plant (Dethier et al., 1960). It may influence larval or adultorientation or both. e.g. 3rdinstar grass grubs are strongly attracted by the odour of fresh ryegrass root and thatthe odour of some legumes is even more attractive but the exact root volatiles involved was not identified(Sutherland, 1972).CHEMICALS IN PLANT–INSECT INTERACTIONSChemical communication can be studied at various levels of integration reaching from the expression ofgenes involved in biosynthesis of signal molecules to ecological consequences of the resulting organismalinteractions on the community level. When studying plant–insect interactions we observe an exchange of signalsthat reciprocally influence the interacting partners and consequently include a complex crosstalk across all thelevels of integration. Moreover, plant–insect interactions are played out in an arena that is much bigger than theplant itself. It includes interferences on the cellular level that have been extensively studied in plant–pathogeninteractions (e.g. Lam et al. 2001; Van Breusegem et al. 2001) as well as interactions at the whole-plant and thecommunity level. The latter result from multitrophic and inter-guild interactions, which are frequently mediatedby the plants’ chemical defences (Agrawal 2000; Dicke and Van Loon 2000; Karban and Agrawal 2002; Kesslerand Baldwin 2002).SECONDARY CHEMICALSDifferent chemical constituents of plants that are the result of its primary and secondary metabolismmakes the plant resist against various insects. It will adversely affect the growth, development and other vitalmetabolic process of insect species. Primary metabolic products like carbohydrates, sugars, proteins, enzymes,lipids and certain organic acids play very important role in this process. Apart from these different plantsecondary metabolic products and other compounds like, alkaloids, terpenoids, flavanoids, glycosides, phenoliccompounds, essential oils, isothiocyanates, coumarins, tannins and aromatic fatty acids are also having veryimportant role in plant defense.
Feeding deterrents produced by plantsFeeding deterrents include many different chemicals and some are amongst the normal constituents ofplants. Larvae of the tropical army worm Spodopteralitura is a polyphagous pest but there are some plantswhich they do not eat, and if extracts from the leaves of these plants are painted on their host plants, it willdeter the Spodoptera larvae. Three such unacceptable plants are Cocculustrilobus, (Presence of an alkaloid-Isoboldine) Clerodendrontrichotomum, (presence of diterpenes) and Parabenzointriloburn, (because of twosesquiterpenes) (Kato et al., 1973). One of the very few cases where resistance has been positively linked to anHagen et al., 1984Influence of plant volatiles on insects
identified feeding deterrent is in sweet clover resistance to the blister beetle, Epicauta sp. Varieties of sweetclover (Melilotus) containing high concentrations of coumarin are resistant, whereas those with lowconcentrations are susceptible (Gorz et al., 1972). But where nature fails to impart resistance to a plant, manmay succeed and the artificial creation of resistance holds great potential. For instance azadirachtin is a potentfeeding deterrent derived from the neem tree Azadirachtaindica. If the chemical is applied to the soil in whichyoung bean plants are growing, it is absorbed by the bean roots, translocated to the growing points and protectstreated beans from attack by migratory locusts (Schistocercagregaria) (Gill and Lewis, 1971).Table 1: Adverse effect of plant metabolic products on insectsPlant metabolicproductSourceplantChemicalgroupInsect species EffectGossypol Cotton Isoprenoids Cotton pink boll worm(Pectinophoragossypiella)Adverse effecton fecundity,longevityDIMBOA( 2,4- dihydroxy 7-methyl, 1,4-benzoxazin- 3one)Maize Acetogenins European corn borer(Ostrinia nubilalis)FeedingdeterrentQuercetin Maize AromaticacidsCorn ear worm(Heliothis zea)ReduceddevelopmentSalicylic acid Rice AromaticacidsRice yellow stem borer(Scirpophaga incertulas)Pilocereine,lophocereineCactus Alkaloids Pomace fly(Drosophila spp.)FeedingrepellentTrypsin inhibitors Potato ProteaseinhibitorColorado potato beetle(Leptinotarsadecemlineata)Effect ondigestion
L-canavanine Tobacco Non proteinamino acidsTobacco horn worm(Manduca sexta)Reduce thevolume ofhaemolymphSinigrin Crucifers Glycosides Green peach aphid(Myzus persicae)Feedingdeterrent(Juniper and Southwood, 1986)Table 2. Hazardous chemicals produced by plants against insectsChemical Source plant Insect species Effects ReferenceQuercetin Gossypium spp.Anthonomus grandisFeedingstimulantHedin et al.,1974PectinophoragossypiellaReduceddevelopmentHelothis zeae ReduceddevelopmentHelithis virescens ReduceddevelopmentMyristicin QuercusmacrocarpaBombyx mori GrowthinhibitorIsogai et al., 1973Morin QuercusmacrocarpaHeliothis virescens FeedingexcitantHamamura, 1970Sesamin,KobusinMagnolia kobus Bombyx mori GrowthinhibitorKaminkado et al.,1975Case studiesPlant biochemical that have adverse effects on insect feeding behavior may thereby reduce theprobability for survival, particularly among species in which the larval forms are incapable of locating a moresuitable host. Insect mortality may then result from starvation, or semi-starvation, combined with unfavorableenvironmental forces.A distinction needs to be drawn between resistance to feeding and resistance that acts by interferingwith the physiological processes underlying growth, metamorphosis, and reproduction. Such physiological
effects may be caused by metabolic inhibitors in the plant tissues, or by the plants failing to provide specificnutrients or nutrient balances required by the insect. Physiological inhibitors.-Research on the resistance ofsolanaceous plants to the Colorado potato beetle has disclosed that a number of alkaloids and alkaloid-glycosides are involved. However, no proper experimental distinction has been made between those thatinfluence feeding behavior and those that act as physiological inhibitors. A toxic action against the beetle larvaehas been postulated in a number of cases.Very young corn plants have long been known to be highly resistant to the establishment and survival oflarvae of the European corn borer. Some genetic lines of corn become very susceptible to larval survival as theymature; others retain much of their juvenile resistance. Beck & Stauffer demonstrated the presence of borer-toxic substances in the tissues of both very young corn plants and borer-resistant varieties. They found twotypes of plant biochemicals that inhibited the growth of young borer larvae: ether-soluble substances, whichthey termed, Resistance Factor A; and ether-insoluble factors, designated Resistance Factor B.Subsequently, the ether-soluble fraction was shown to contain two resistance factors, necessitatingintroduction of the term, Resistance Factor C. Resistance Factor A (RFA) was identified as 6-methoxybenzoxazolinone Resistance Factor C (RFC) has been shown to be 2,4-dihydroxy-7-methoxy-1,4-benzoxazine-3-one. The latter has been demonstrated to be a biochemical precursor of RFA, and there was abrief controversy over the question of whether or not RFA, as such, occurs in uninjured plant tissue. Its naturaloccurrence has, however, been unquestionably demonstrated.The ether-insoluble RFB has never been isolated or characterized, but appears to be of relatively minorimportance to borer-resistance in most of the corn varieties investigated (U, 12). Tissue concentrations ofresistance factors were found to change as the corn plant matured, but varietal differences were found, not onlyin the amounts of RFA and RFC present but also in respect to the relative amounts present in different planttissues at different stages of growth.The leaves of a borer-susceptible corn variety were found to contain large amounts of RFA, but at agrowth stage in which leaf-feeding by borers does not occur. Similarly, very young tassel buds of several corninbreds were found to contain high RFA concentrations; but at the stage of growth where borer larvae invadethe tassels, the tassels contained little or no RFA.Borer resistance was dependent upon the presence of an effective concentration of resistance factors inthe right tissues at the right stage of growth. Although the importance of coordinating chemical sampling andanalysis with the biological pattern of the insect-plant combination would appear to be logical and obvious, ithas been too frequently overlooked in studies of resistance.Benzoxazolinone is the demethoxyl analogue of RFA, and was demonstrated to be an antifungal agent inrye leaves. Resistance Factor A has also been reported as occurring in the roots of Coix grass, leaves of wheat,and the roots of corn. Benzoxazolinone and RFA act as growth inhibitors against a variety of organisms, includingbacteria, free- living and pathogenic fungi, and a number of insects. The growth inhibitory effects of a series ofbenzoxazole analogues was studied by Beck & Smissman.Inhibition of fungal growth and inhibition of corn borer growth appeared to be associated with differentstructural features of the molecule. Fungal growth inhibition was dependent on the presence of a lipid-solubilizing group on the benzoid nucleus and the presence of a nitro or amino group adjacent to the phenolichydroxyl. Antifungal activity did not depend on the oxazole ring.
Inhibition of larval growth, on the other hand, was closely associated with the presence of an oxazoie orthiazole grouping, and phenolic compounds were of generally low inhibitory activity. Under laboratoryconditions, no correlation could be found between growth inhibitory and feeding deterrent activities ofbenzoxazolinone analogues. European corn borer larvae tended to become conditioned to synthetic dietscontaining feeding deterrents, and their growth was then dependent upon the metabolic effects of theadjuvants.Some analogues, such as benzothizole, inhibited growth but did not deter feeding; others such asphenylbenzothiazole, had a strong deterrent effect on feeding, but once the larvae were conditioned to the diet,growth was normal. It was concluded that the two effects of RFA both contributed to plant resistance underfield conditions.Plant resistance to soil forms has long been observed. In addition to resistance to oviposition,biochemical resistance to larval survival has been detected. Swailes reported that resistance of rutabagavarieties to the cabbage maggot, Hylemya brassicae, was in part due to the presence of larval growth inhibitors.An inhibitor of insect growth and survival was isolated from the roots of turnips and identified as 2-phenylethylisothiocyanate, but its role in plant resistance has not been determined. A toxic factor has also beenisolated from the roots of parsnips (5-allyl-l-methoxy-2,3-methyl- enedioxybenzene). The latter substance wasshown to be toxic to several species of insects, but its importance to plant resistance is not known.A number of antifungal and insect-toxic substances have been isolated from the leaves and roots ofcabbage. One such substance was identified as indole-3-acetonitrile. Other insect growth inhibitors that havebeen isolated from both cabbage and alfalfa include salicylic acid and 2-ethyl-1- hexanol phthalate. Unidentifiedinhibitors of insect growth have been detected in a number of plant species and varieties, including barley,Solanum spp., oats, alfalfa, wheat, cabbage, kale, and beets. Fraenkel et al. described the presence of anonglycosidic factor in Petunia foliage that was toxic to larvae of Prataparee sexta.These workers also reported the occurrence of a substance in Nicandra that was both repel- lent andtoxic to Bombyx mori larvae, but not to the southern armyworm, Prodenia eridana (Cramer). Tobacco, Nicotianatabacum Linnaeus, is among the many hosts of the green peach aphid, Myzus persicae (Sulzer). The aphid feedsin the phloem, and not in the nicotine-transporting xylem, and thereby avoids the powerful toxin.NicotianagosseiDominica, is resistant to the green peach aphid, because of the production of a toxin.The toxic substance is exuded from leaf hairs, and produces nicotinelike symptoms in contacted aphids.Apterous aphids are relatively sessile, making a distinction between nonpreference and antibiosis extremelydifficult. In the absence of techniques for rearing aphids on synthetic diets, it has not been possible to determinethe role of plant-borne toxins in plant resistance. The required techniques are now being developed. Much ofthe existing literature on resistance to aphids contains interpretations as to the basis of resistance by antibiosis,but must be considered speculative in regard to the relative importance of toxins, nutritional, and behavioralfactors. The finding that aphid biotypes differ markedly in their performance on aphid-resistant plant varietiesfurther complicates the study of plant resistance to aphids.Nutritional deficiencies.-In order to be fully adequate, a host plant must provide the nutritional factorsrequired by the insect. But the insect is dependent on the plant for much more than nutrients alone; chemostimulants, physical factors, and micro-environmental factors all play a role in determining the adequacy of agiven plant as host for a given insect. A resistant plant, therefore, is not necessarily nutritionally inadequate.Painter suggested that some instances of resistance might be attributed to the complete absence of specific
nutrients required by an insect; no evidence could be presented in support of this view, however. The oppositeview, that nutritional deficiencies cannot play a part in resistance, was advocated by Fraenkel. This view wasbased on the unproved assumptions that all phytophagous insects have the same nutritional requirements andthat all plants are capable of meeting these requirements. It would now appear that the role of nutritionalfactors in plant resistance is far too complex to fit either of these views.Phytophagous insects of relatively polyphagous food habits have been found to grow faster, live longer,and reproduce better on some plant species than on others. The insects frequently were found to perform beston mixed diets. The finding that plant species differed in suitability as food plants does not yield any informationas to their relative nutritional values, because of non-nutritional factors that contribute to the total effect. Thesuperiority of mixed diets compared to monotypic diets might be taken as evidence of differences in nutritionalvalue, but caution must be exercised in offering such an interpretation because nothing is known of the effectsof multiple choice diets on feeding behavior and ingestion rates.Quantitative studies of the rate of food intake, the efficiency of digestion, and conversion to bodytissues have disclosed differences in the suitability of different plants as hosts, but without accomplishing aclarification of the role of the insects nutritional requirements in its host plant relationships. Smith comparedthe utilization of wheat, western wheat grass, and oats by the migratory grasshopper, Melanoplus bilituratus(Walker). The efficiency of conversion of the plant tissue into insect tissue was about the same in each case, butthe amounts eaten were greater in the case of wheat and western wheat grass than in the case of oats. Growthwas best on wheat. Smith concluded that oats was nutritionally satisfactory, but the insects did not eat enoughof it. Western wheat grass was fed on quite readily, but was nutritionally inferior to wheat.Working with maxillectomized larvae of the tobacco hornworm, Protolarce sexta, Waldbauer founddandelion foliage (Taraxacum) to be as good as tomato (Lycopersicon esculentum Miller) in respect to larvalgrowth and adult reproduction. Burdock (Arctium) was somewhat inferior; mullein (Verbascum) and Catalpawere poor host plants. Determination of the efficiency of conversion showed that tomato, dandelion, andburdock were utilized with greater efficiency than were mullein and Catalpa. These effects might have beencaused by the presence of growth inhibitors, by nutritional deficiencies, or by differences in digestibility amongthe plant species.The resistance of wheats (Triticum spp.) to the wheat stem sawfly, Cephus cinctus, is thought to bemainly ovipositional and secondarily physical, as discussed above. In addition to these factors, nutritional factorshave been thought to be involved in the mortality of partly-grown larvae. Comparisons of moisture and nitrogenconcentrations in the pith and stem walls of several wheat varieties disclosed both varietal and plantdevelopmental differences, but no correlation with resistance could be demonstrated. Nor could resistance becorrelated with varietal and growth stage differences in soluble carbohydrates. Larvae of the pale western cut-worm, Agrotis orthogonia Morrison, were fed different wheats, and varietal differences in nitrogen contentwere reflectcted by correlated tissue nitrogen differences and growth rates in the larvae. Cutworm growth wasnot significantly influenced by varietal differences in carbohydrates, but the larvae were found to be quitesensitive to amino acid imbalances.The resistance of wheat varieties to the Hessian fly, Phytophaga destructor (Say), could not beaccounted for on the basis of hydrogen ion concentrations, protein contents, or mineral ion contents. The larvaewere found to secrete a hemicellulase that aided in the dissolution of cell structure; plant resistance to thisspecies was thought to involve a high hemi- cellulose content and a relatively low free water content.Chromatographic comparisons of extracts of wheats that were susceptible and resistant to Hessian fly larvae
disclosed that the resistant varieties lacked the sugar cellulose and the polyhydric alcohol sorbitol (lOS), but thesignificance of these differences to plant resistance were uncertain.Crison found that the sugar and lecithin contents of potato foliage fed to Colorado potato beetlesexerted a. marked influence on the insects fecundity. Old foliage was found to contain relatively high sugar butlow lecithin concentrations as compared to young leaves. The beetles laid fewer eggs per day when fed oldleaves than when fed young leaves. Supplementing the leaves with glucose tended to reduce egg production;whereas lecithin supplements increased both egg production and adult longevity. Grison concluded that carbon:nitrogen ratios were of less importance to reproduction than were sugar:lecithin ratios.Comparison of larval growth rates and adult fecundity of two lepidopterous species (Euproctisphaeorrhea Donovan and Malacosoma neustria Linnaeus) reared on young and senescent apple leaves led tosimilar conclusions.The requirements of European corn borer larvae for sugars and protein were found to change duringgrowth, but no evidence was obtained that plant resistance could be accounted for on such a nutritional basis.How- ever, the growth inhibiting effect of the resistance factor 6-methoxybenzoxazo- linone was greatlydiminished in the borer larvae fed on substrates containing relatively large amounts of sugars. The experimentalevidence favored the idea that the resistance factor was detoxified as a glucuronide, and that the sugar contentof the plant tissue influenced the plants resistance. Nutritional factors have been implicated in the resistance ofa number of plant species to aphids. Much of the evidence is circumstantial, and more experimental work isneeded.A number of workers have pointed out that the effects of leaf age and physiological state on thefecundity of aphids can best be explained on the basis of nutritive changes in the plant tissue. The work ofAuclair and his associates on the importance of amino acids in the resistance of peas to the pea aphid,Acyrthosiphon pisum (Harris), is well known. They have found that resistant pea varieties tend to be deficient inamino acids, and aphids on resistant plants tend to grow more slowly than normal, secrete less honeydew, andproduce fewer progeny. These effects have been interpreted as indicating that the resistant peas are lessnutritious than are susceptible pea varieties.Experiments in which pea aphids were fed on pea leaves that had been perfused with selected aminoacids yielded results tending to support the interpretation that resistance is at least partially nutritional.Similarly, Maxwell & Harwood observed that herbicides causing the plant tissues to accumulate greater thannormal amounts of free amino acids improved the growth and reproduction of pea aphids feeding on theaffected plant parts.Insect induced indirect plant defenseApart from self-defenses, plants rely on indirect defenses that facilitate control of herbivores mediatedby parasitoids, predators, and pathogens that exploit the herbivores as hosts or prey. Induced defenses requireplant sensing the nature of injury, such as wounding from herbivore attack as opposed to wounding frommechanical damage. Plants therefore use a variety of cues, including salivary enzymes of the attackingherbivore. In a study to test whether plants can distinguish mechanical damage from insect herbivory attack,showed that the accumulation of induced defense transcription products occurred more rapidly in potato(Solanum tuberosum L.) leaves chewed by caterpillars than in mechanically damaged leaves (Korth and Dixon,1997).
Distinct signal transduction pathway is activated in response either to insect damage or mechanicaldamage in plants. While chemicals released in wounding responses are the same in both cases, the pathway inwhich they accumulate are separate. All herbivore attack always does not begin with feeding, but may involveinsect egg laying on the plant. The adults of butterflies and moths do not feed on plants directly, but lay eggs onplants which are suitable food for their larva. In such cases, plants have been demonstrated to induce defensesupon contact from the ovipositing of insects.Many insect herbivores change the quality of their host plants, affecting both inter and intra specificinteractions. Higher- trophic level interactions, such as the performance of predators and parasitoids, may alsobe affected by host plant quality. Herbivore feeding and mechanical damage can induce certain responses inplants that will invite various predators and parasitoids of the herbivore (Gols et al., 2003).Tritrophic RelationshipsTritrophic relationships are three way interactions. Plant chemical cues elicit natural enemies to ‘defend’herbivore infested plants. Plants know when they are under attack.Mechanically damaged plants only emitgreen leaf volatiles.Insect wounded plants emit various blends of terpenoids.Parasitoids can differentiatebetween mechanically damaged and insect wounded plants.Figure 6. Plant volatile mediated in sect induced plant defenseTritrophic interaction
Examples of Chem. Mediated Tritrophic RelationshipsPlant – Pest – Parasitoid Corn ( Zea mays )– Beet Armyworn ( S. exigua ) – C. marginiventris Tobacco (N. attenuata)– Tobacco budworm (H. virescens) – C. nigriceps Field elm(Ulmus minor) – Elm leaf beetle (X. luteola ) – O. gallerucae Vicia fabia ( broad bean ) – T. urticae – P. persimilisCotesia marginiventris Native to Cuba and West Indies. Found throughout the US and South America General parasitoid of Noctuid moths Corn infested by beet armyworms Spodoptera exigua ( Noctuidae, Lepidoptera ) send out distresssignals which attract C. marginiventris. Volicitin from beet armyworm saliva initiates corn to synthesize and emit semiochemicals.Oomyzus gallerucae Egg parasitoid of elm leaf beetle Xanthogaleruca luteola (Chrysomelidae, Coleoptera ). Ovipositor wounding, not feeding, initiates plant chemical release. Field elms emit a different chemical blend when fed on by elm leaf beetle. O. gallerucae can differentiate between oviposition and feeding. Have succesfully been employed in biological control.Phytoseiulus persimilis P. persimilis is used as biological control agent of two spotted spider mites Tetranychus urticae. Bean plants infested with TSSM emit terpenoids and methyl saliclylate. Important for biological control.Volatile compounds in host plant defenceThe release of volatile signals by plants occurs not only in response to tissue damage but is alsospecifically initiated by exposure to herbivore salivary secretions. Although some volatile compounds are storedin plant tissues and immediately released when damage occurs, others are induced by herbivore feeding andreleased not only from damaged tissue but also from undamaged leaves.Thus, the damage localized to only a few leaves also results in a systemic response and the release ofvolatiles from the entire plant. New evidence suggests that, in addition to being highly detectable and reliable
indicators of herbivore presence, herbivore-induced plant volatiles may convey herbivore-specific informationthat allows parasitoids to discriminate even closely-related herbivore species at long range (Moraes et al ., 2000)Elicitors of plant volatilesSo far two elicitors of plant volatiles have been identified in the oral secretions of insect herbivores. Inthat beta-glucosidase, in cabbage butterfly, Pieris brassicae caterpillars elicits the release of volatiles fromcabbage leaves (Mattiaci et al., 1995). The major active elicitor of the oral secretion of beet armyworm larvae isrecently identified as (N-[17-hydroxylinolenoyl]-L-glutamine) and, named as volicitin. Both of its natural andsynthesized forms, induces corn seedlings to release the same blend of volatiles induced by herbivore feeding.This blend has been exploited as a host location cue by the parasitic wasps that attack this herbivore.Jasmonic acid which is produced from linolenic acid by the octadecanoid signalling pathway, may beinvolved in the transduction sequence that triggers synthesis of volatile compounds by plants. In the case ofvolicitin, which is an octadecatrienoate conjugated to an amino acid, this may suggest that the elicitor moleculeinteracts with the octadecanoid pathway in herbivore damaged plants (Alborn et al. 1997).
Biosynthesis of induced plant volatilesThe isopropenoid precursor isopentenyl pyrophosphate serves as a substrate for monoterpenes andsesquiterpenes, the fatty acid lipoxygenase pathway generates green leaf volatiles and jasmone, and theshikimic acid/tryptophan pathway results in the nitrogen containing product indole (Mann, 1987). Green leafvolatiles are produced when leaves are damaged, by insects. They are typically mixtures of C 6 alcohols,aldehydes, and esters produced by oxidation of membrane-derived fatty acids.In contrast, many monoterpenes, homoterpenes and sesquiterpenes are produced in response toherbivore damage and generally released not only from damaged tissue but also from undamaged leaves(Turlings et al., 1991).Figure 2. Release of plant volatiles, their dispersion andperception by insects Visser, 1986
In the case of cotton, several monoterpenes and sesquiterpenes, along with the lypoxygenase products,are released immediately in response to damage. So the release of plant volatile compounds is highly variableacross plant species and varieties and is also sensitive to the species of the herbivore (Dicke et al., 1990).Influence of plant chemicals on sexual and reproductive behaviour of insectsPlant chemicals are also involved in the egg-laying behaviour of many phytophagous insects but heretheir role is less predominant and they take their place beside other factors such as texture, surfaceconfiguration, and colour. Plant chemicals can also indirectly affect the sexual behaviour of insects. e.g. Danauschrysippus. Males of this butterfly possess hair pencil pheromone glands which disseminate an aphrodisiacpheromone (dihydropyrrolizine), making females receptive to mating. This insect is attracted to theHeliotropium plants by olfactory cues and spends considerable periods in licking the leaf surface in order toobtain pheromone precursor (Schneider, 1975). So here the link between the plant chemicals and insectbehaviour isindirect and very intimate indeed.OvipositionPolyphagous insects have been shown to preferentially select certain host plant species for oviposition(Renwick and Chew, 1994). The proximate basis for relative preferences for different host species are from thebalance between visual, olfactory and tactile cues that act as attractants and deterrents for egg laying. (Papajand Rausher, 1987). In insects with long-lived adults, adult feeding is often crucial for reproduction. Manyinsects largely rely on adult-derived resources for reproduction (Tammaru and Haukioja, 1996). Host plantquality also affects insect reproductive strategies such as egg size and quality, the allocation of resources toeggs, and the choice of egg laying site. Oviposition rate may be the parameter to be affected as a response tolow host quality and egg maturation rates may be dependent on presence or quality of larval hosts (Leather andBurnand, 1987). Time to initiation of oviposition has often been reported to be dependent on the quality of thesubstrate (Gupta and Thorsteinson, 1960).Insect defense against plant producing chemicalsIn contrast to the plant defensive chemicals insect will also find some defensive mechanisms in order todetoxify or break the plant chemicals.DetoxificationThis can be achieved by various metabolic processes like, mixed function oxidation and different detoxifyingenzymes. The detoxifying enzymes are mainly synthesized by microsomes (membranous particles in thecytoplasm) and also endoplasmic reticulum that traps the toxins and renders them non toxic. Some well knowndetoxifying enzymes are dehydroxy chlorinase and carboxyl esterase.Avoidance and limited contact with resistant host plantsIn order to avoid unwanted effects of plant chemicals, insect fly away from those host plants or make less periodof contact with that plant, so as to protect them.Less digestion of toxic chemicals and increasedexcretionLess digestion or direct excretion without digesting the toxic plant chemicals will safely dispose them outside.Resistant strains of insects are found to be having more of fat body which will help them to insulate the toxic
chemicals and expel it out. Resistant insects will have thicker cuticle which helps in lesser penetration of toxicchemicals into the insect system.Biotype developmentA biotype is a population capable of damaging and survival of plants previouslyknown to be resistant againstother populations of the same species which are all growing under similar conditions (Kogan, 1994). Biotypes aremorphologically similar with normal insect types but they are physiologically differing from them. Thecontinuous growing of insect-resistant varieties may lead to development of certain physiologically andbehaviourally changed biotypes, which are capable of feeding and developing on same resistant varieties. Theseinsects will then survive on the host plant and destroy them. Ultimately the development of insect biotypeshappen. That has posed a serious threat to the success of plant defense. Biotypes are developed more onvarieties having more biochemical defense than the varieties offering physical defense.Conclusion Tritrophic relationships involve complex chemical interactions.Plants can differentiate betweenmechanical damage, insect wounding and even between pest species and types of damage.Parasitoidsand predators can recognize varying semiochemicals from different plants in different states of distress.Chem. Mediated tritrophic relationships can be implemented in biological control.Everybody wins in THEEND ! (except the pest ). This reviewer is confident that the development of many highly resistant plant varieties will beaccomplished in the future, and that the rate of progress realized will be closely correlated with the rateof accumulation of fundamental biological and biochemical knowledge concerning the complexinteractions between insects and their host plants. In the past, studies of the mechanisms underlyingplant resistance have always come after the fact of resistance. As greater understanding of insect andplant biology, chemistry, and ecology is attained, we will be able to approach the goal of developingagronomic plants that are deliberately and foresightedly designed to be insect-resistant. Theory of interdependency and survival of the fittest are the common phenomenain nature. Eachorganism should depend on others in order to exploit and exchange the energy and matter in anefficient way. Naturally insects are influenced by many factors like size, shape, age and biotic potential.Similarly host plant quality is decided by many physical and chemical factors. In contrast to these,different biotic and abiotic factors of environment will cause both positive and negative influence oninsects as well as their host plants. So depending upon the influence of environmental factors on plantsand insects their survival and dominance will be decided. But nature wants to maintain an equilibriumcondition in the environment. So when there is dominance of one factor, it will automatically becontrolled by nature, by giving different stresses. Ultimately a balanced level of insect and their hostplant will be maintained. Allowing natural balance of ecosystem, by making minimal interventions in the habitat, is one of themost promising tools to reduce the dependence of pesticides in agriculture.
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