An Assignment

Submitted In Partial Fulfillment

Of

ENT-509

(PLANT RESISTANCE TO INSECTS)

On

ROLE OF CHEMICAL ECOLOGY, TRITROPIC RELATIONS, VOLATILES AND SECONDARY PLANT SUBSTANCES ON HOST

PLANT RESISTANCE TO INSECT

SUBMITTED TO:

Dr. P. K. Borad

Professor and head,Dept. of Entomology,BACA, AAU, Anand.

SUBMITTED BY:

MAYANK V.PATEL

2nd Sem. 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 an

integrative research field. It is driven by the recognition that organisms of diverse kinds make use of chemical signals to interact (Karban and Baldwin 1997). It promises an understanding of the molecular and genetic mechanisms of biological signal transduction in species interactions, which can help to ultimately understand the evolution of complex species interactions.

Plant chemicals which affect behaviour have also been classified into two main groups. There are those which the insect can utilize as nutrients as well as behavioural cues, and then there are those which have no apparent nutrient value but which serve only as sign stimuli, enabling the insect to select the appropriate food or 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 the orientation of the insect with respect to the plant (Dethier et al., 1960). It may influence larval or adult orientation or both. e.g. 3rd instar grass grubs are strongly attracted by the odour of fresh ryegrass root and that the odour of some legumes is even more attractive but the exact root volatiles involved was not identified (Sutherland, 1972).

CHEMICALS IN PLANT–INSECT INTERACTIONS

Chemical communication can be studied at various levels of integration reaching from the expression of genes involved in biosynthesis of signal molecules to ecological consequences of the resulting organismal interactions on the community level. When studying plant–insect interactions we observe an exchange of signals that reciprocally influence the interacting partners and consequently include a complex crosstalk across all the levels of integration. Moreover, plant–insect interactions are played out in an arena that is much bigger than the plant itself. It includes interferences on the cellular level that have been extensively studied in plant–pathogen interactions (e.g. Lam et al. 2001; Van Breusegem et al. 2001) as well as interactions at the whole-plant and the community level. The latter result from multitrophic and inter-guild interactions, which are frequently mediated by the plants’ chemical defences (Agrawal 2000; Dicke and Van Loon 2000; Karban and Agrawal 2002; Kessler and Baldwin 2002).

SECONDARY CHEMICALS

Different chemical constituents of plants that are the result of its primary and secondary metabolism makes the plant resist against various insects. It will adversely affect the growth, development and other vital metabolic 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 plant secondary metabolic products and other compounds like, alkaloids, terpenoids, flavanoids, glycosides, phenolic compounds, essential oils, isothiocyanates, coumarins, tannins and aromatic fatty acids are also having very important role in plant defense.

Hagen et al., 1984

Feeding deterrents produced by plants

Influence of plant volatiles on insects

Feeding deterrents include many different chemicals and some are amongst the normal constituents of plants. Larvae of the tropical army worm Spodopteralitura is a polyphagous pest but there are some plants which they do not eat, and if extracts from the leaves of these plants are painted on their host plants, it will deter the Spodoptera larvae. Three such unacceptable plants are Cocculustrilobus, (Presence of an alkaloid-Isoboldine) Clerodendrontrichotomum, (presence of diterpenes) and Parabenzointriloburn, (because of two sesquiterpenes) (Kato et al., 1973). One of the very few cases where resistance has been positively linked to an identified feeding deterrent is in sweet clover resistance to the blister beetle, Epicauta sp. Varieties of sweet clover (Melilotus) containing high concentrations of coumarin are resistant, whereas those with low concentrations are susceptible (Gorz et al., 1972). But where nature fails to impart resistance to a plant, man may succeed and the artificial creation of resistance holds great potential. For instance azadirachtin is a potent feeding deterrent derived from the neem tree Azadirachtaindica. If the chemical is applied to the soil in which young bean plants are growing, it is absorbed by the bean roots, translocated to the growing points and protects treated beans from attack by migratory locusts (Schistocercagregaria) (Gill and Lewis, 1971).

Table 1: Adverse effect of plant metabolic products on insects

Plant metabolic product

Source plant

Chemical group

Insect species Effect

Gossypol Cotton Isoprenoids Cotton pink boll worm (Pectinophora gossypiella)

Adverse effect on fecundity,

longevity

DIMBOA ( 2,4- dihydroxy 7-

methyl, 1,4- benzoxazin- 3one)

Maize Acetogenins European corn borer (Ostrinia nubilalis)

Feeding deterrent

Quercetin Maize Aromatic acids

Corn ear worm (Heliothis zea)

Reduced development Salicylic acid Rice Aromatic

acids Rice yellow stem borer

(Scirpophaga incertulas)

Pilocereine, lophocereine

Cactus Alkaloids Pomace fly(Drosophila spp.)

Feeding repellent

Trypsin inhibitors Potato Protease Colorado potato beetle Effect on

inhibitor (Leptinotarsa decemlineata)

digestion

L-canavanine Tobacco Non protein amino acids

Tobacco horn worm (Manduca sexta)

Reduce the volume of

haemolymph

Sinigrin Crucifers Glycosides Green peach aphid (Myzus persicae)

Feeding deterrent

(Juniper and Southwood, 1986)

Table 2. Hazardous chemicals produced by plants against insects

Chemical Source plant Insect species Effects Reference

Quercetin Gossypium spp.

Anthonomus grandis Feeding

stimulant

Hedin et al.,1974

Pectinophora gossypiella

Reduced development

Helothis zeae Reduced development

Helithis virescens Reduced development

Myristicin Quercus macrocarpa

Bombyx mori Growth inhibitor

Isogai et al., 1973

Morin Quercus macrocarpa

Heliothis virescens Feeding excitant

Hamamura, 1970

Sesamin, Kobusin

Magnolia kobus Bombyx mori Growth inhibitor

Kaminkado et al., 1975

Case studies

Plant biochemical that have adverse effects on insect feeding behavior may thereby reduce the probability for survival, particularly among species in which the larval forms are incapable of locating a more suitable host. Insect mortality may then result from starvation, or semi-starvation, combined with unfavorable environmental forces.

A distinction needs to be drawn between resistance to feeding and resistance that acts by interfering with the physiological processes underlying growth, metamorphosis, and reproduction. Such physiological effects may be caused by metabolic inhibitors in the plant tissues, or by the plant's failing to provide specific nutrients or nutrient balances required by the insect. Physiological inhibitors.-Research on the resistance of solanaceous 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 that influence feeding behavior and those that act as physiological inhibitors. A toxic action against the beetle larvae has been postulated in a number of cases.

Very young corn plants have long been known to be highly resistant to the establishment and survival of larvae of the European corn borer. Some genetic lines of corn become very susceptible to larval survival as they mature; 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 two types of plant biochemicals that inhibited the growth of young borer larvae: ether-soluble substances, which they termed, Resistance Factor A; and ether-insoluble factors, designated Resistance Factor B.

Subsequently, the ether-soluble fraction was shown to contain two resistance factors, necessitating introduction 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 a brief controversy over the question of whether or not RFA, as such, occurs in uninjured plant tissue. Its natural occurrence has, however, been unquestionably demonstrated.

The ether-insoluble RFB has never been isolated or characterized, but appears to be of relatively minor importance to borer-resistance in most of the corn varieties investigated (U, 12). Tissue concentrations of resistance factors were found to change as the corn plant matured, but varietal differences were found, not only in the amounts of RFA and RFC present but also in respect to the relative amounts present in different plant tissues at different stages of growth.

The leaves of a borer-susceptible corn variety were found to contain large amounts of RFA, but at a growth stage in which leaf-feeding by borers does not occur. Similarly, very young tassel buds of several corn inbreds were found to contain high RFA concentrations; but at the stage of growth where borer larvae invade the tassels, the tassels contained little or no RFA.

Borer resistance was dependent upon the presence of an effective concentration of resistance factors in the right tissues at the right stage of growth. Although the importance of coordinating chemical sampling and analysis with the biological pattern of the insect-plant combination would appear to be logical and obvious, it has been too frequently overlooked in studies of resistance.

Benzoxazolinone is the demethoxyl analogue of RFA, and was demonstrated to be an antifungal agent in rye 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, including bacteria, free living and pathogenic fungi, and a number of insects. The growth inhibitory effects of a series of benzoxazole analogues was studied by Beck & Smissman.

Inhibition of fungal growth and inhibition of corn borer growth appeared to be associated with different structural 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 phenolic hydroxyl. 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 or thiazole grouping, and phenolic compounds were of generally low inhibitory activity. Under laboratory conditions, no correlation could be found between growth inhibitory and feeding deterrent activities of benzoxazolinone analogues. European corn borer larvae tended to become conditioned to synthetic diets containing feeding deterrents, and their growth was then dependent upon the metabolic effects of the adjuvants.

Some analogues, such as benzothizole, inhibited growth but did not deter feeding; others such as phenylbenzothiazole, 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 under field 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 rutabaga varieties 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 been isolated from the roots of parsnips (5-allyl-l-methoxy-2,3-methyl enedioxybenzene). The latter substance was shown 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 of cabbage. One such substance was identified as indole-3-acetonitrile. Other insect growth inhibitors that have been isolated from both cabbage and alfalfa include salicylic acid and 2-ethyl-1- hexanol phthalate. Unidentified inhibitors 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 a nonglycosidic 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 and toxic to Bombyx mori larvae, but not to the southern armyworm, Prodenia eridana (Cramer). Tobacco, Nicotiana tabacum Linnaeus, is among the many hosts of the green peach aphid, Myzus persicae (Sulzer). The aphid feeds in 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 extremely difficult. In the absence of techniques for rearing aphids on synthetic diets, it has not been possible to determine the role of plant-borne toxins in plant resistance. The required techniques' are now being developed. Much of the 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 behavioral factors. The finding that aphid biotypes differ markedly in their performance on aphid-resistant plant varieties further complicates the study of plant resistance to aphids.

Nutritional deficiencies.-In order to be fully adequate, a host plant must provide the nutritional factors required by the insect. But the insect is dependent on the plant for much more than nutrients alone; chemo

stimulants, physical factors, and micro-environmental factors all play a role in determining the adequacy of a given 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 opposite view, that nutritional deficiencies cannot play a part in resistance, was advocated by Fraenkel. This view was based on the unproved assumptions that all phytophagous insects have the same nutritional requirements and that all plants are capable of meeting these requirements. It would now appear that the role of nutritional factors 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 best on mixed diets. The finding that plant species differed in suitability as food plants does not yield any information as to their relative nutritional values, because of non-nutritional factors that contribute to the total effect. The superiority of mixed diets compared to monotypic diets might be taken as evidence of differences in nutritional value, but caution must be exercised in offering such an interpretation because nothing is known of the effects of multiple choice diets on feeding behavior and ingestion rates.

Quantitative studies of the rate of food intake, the efficiency of digestion, and conversion to body tissues have disclosed differences in the suitability of different plants as hosts, but without accomplishing a clarification of the role of the insect's nutritional requirements in its host plant relationships. Smith compared the 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, but the amounts eaten were greater in the case of wheat and western wheat grass than in the case of oats. Growth was best on wheat. Smith concluded that oats was nutritionally satisfactory, but the insects did not eat enough of 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 found dandelion foliage (Taraxacum) to be as good as tomato (Lycopersicon esculentum Miller) in respect to larval growth and adult reproduction. Burdock (Arctium) was somewhat inferior; mullein (Verbascum) and Catalpa were poor host plants. Determination of the efficiency of conversion showed that tomato, dandelion, and burdock were utilized with greater efficiency than were mullein and Catalpa. These effects might have been caused by the presence of growth inhibitors, by nutritional deficiencies, or by differences in digestibility among the plant species.

The resistance of wheats (Triticum spp.) to the wheat stem sawfly, Cephus cinctus, is thought to be mainly ovipositional and secondarily physical, as discussed above. In addition to these factors, nutritional factors have been thought to be involved in the mortality of partly-grown larvae. Comparisons of moisture and nitrogen concentrations in the pith and stem walls of several wheat varieties disclosed both varietal and plant developmental differences, but no correlation with resistance could be demonstrated. Nor could resistance be correlated with varietal and growth stage differences in soluble carbohydrates. Larvae of the pale western cutworm, Agrotis orthogonia Morrison, were fed different wheats, and varietal differences in nitrogen content were reflectcted by correlated tissue nitrogen differences and growth rates in the larvae. Cutworm growth was not significantly influenced by varietal differences in carbohydrates, but the larvae were found to be quite sensitive to amino acid imbalances.

The resistance of wheat varieties to the Hessian fly, Phytophaga destructor (Say), could not be accounted for on the basis of hydrogen ion concentrations, protein contents, or mineral ion contents. The larvae

were found to secrete a hemicellulase that aided in the dissolution of cell structure; plant resistance to this species 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 the significance of these differences to plant resistance were uncertain.

Crison found that the sugar and lecithin contents of potato foliage fed to Colorado potato beetles exerted a. marked influence on the insect's fecundity. Old foliage was found to contain relatively high sugar but low lecithin concentrations as compared to young leaves. The beetles laid fewer eggs per day when fed old leaves 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 (Euproctis phaeorrhea Donovan and Malacosoma neustria Linnaeus) reared on young and senescent apple leaves led to similar conclusions.

The requirements of European corn borer larvae for sugars and protein were found to change during growth, 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 greatly diminished in the borer larvae fed on substrates containing relatively large amounts of sugars. The experimental evidence favored the idea that the resistance factor was detoxified as a glucuronide, and that the sugar content of the plant tissue influenced the plant's resistance. Nutritional factors have been implicated in the resistance of a number of plant species to aphids. Much of the evidence is circumstantial, and more experimental work is needed.

A number of workers have pointed out that the effects of leaf age and physiological state on the fecundity of aphids can best be explained on the basis of nutritive changes in the plant tissue. The work of Auclair 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 in amino acids, and aphids on resistant plants tend to grow more slowly than normal, secrete less honeydew, and produce fewer progeny. These effects have been interpreted as indicating that the resistant peas are less nutritious than are susceptible pea varieties.

Experiments in which pea aphids were fed on pea leaves that had been perfused with selected amino acids 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 than normal amounts of free amino acids improved the growth and reproduction of pea aphids feeding on the affected plant parts.

Insect induced indirect plant defense

Apart from self-defenses, plants rely on indirect defenses that facilitate control of herbivores mediated by parasitoids, predators, and pathogens that exploit the herbivores as hosts or prey. Induced defenses require plant sensing the nature of injury, such as wounding from herbivore attack as opposed to wounding from mechanical damage. Plants therefore use a variety of cues, including salivary enzymes of the attacking herbivore. In a study to test whether plants can distinguish mechanical damage from insect herbivory attack,

Figure 6. Plant volatile mediated in sect induced plant defense

Tritrophic interaction

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 mechanical damage in plants. While chemicals released in wounding responses are the same in both cases, the pathway in which they accumulate are separate. All herbivore attack always does not begin with feeding, but may involve insect egg laying on the plant. The adults of butterflies and moths do not feed on plants directly, but lay eggs on plants which are suitable food for their larva. In such cases, plants have been demonstrated to induce defenses upon contact from the ovipositing of insects.

Many insect herbivores change the quality of their host plants, affecting both inter and intra specific interactions. Higher- trophic level interactions, such as the performance of predators and parasitoids, may also be affected by host plant quality. Herbivore feeding and mechanical damage can induce certain responses in plants that will invite various predators and parasitoids of the herbivore (Gols et al., 2003).

Tritrophic Relationships

Tritrophic 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 emit green leaf volatiles.Insect wounded plants emit various blends of terpenoids.Parasitoids can differentiate between mechanically damaged and insect wounded plants.

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. persimilis

Cotesia 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 distress

signals 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 also

specifically initiated by exposure to herbivore salivary secretions. Although some volatile compounds are stored

in plant tissues and immediately released when damage occurs, others are induced by herbivore feeding and released 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 of volatiles 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 information that allows parasitoids to discriminate even closely-related herbivore species at long range (Moraes et al ., 2000)

Elicitors of plant volatiles

So far two elicitors of plant volatiles have been identified in the oral secretions of insect herbivores. In that beta-glucosidase, in cabbage butterfly, Pieris brassicae caterpillars elicits the release of volatiles from cabbage leaves (Mattiaci et al., 1995). The major active elicitor of the oral secretion of beet armyworm larvae is recently identified as (N-[17-hydroxylinolenoyl]-L-glutamine) and, named as volicitin. Both of its natural and synthesized 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 be involved in the transduction sequence that triggers synthesis of volatile compounds by plants. In the case of volicitin, which is an octadecatrienoate conjugated to an amino acid, this may suggest that the elicitor molecule interacts with the octadecanoid pathway in herbivore damaged plants (Alborn et al. 1997).

Figure 2. Release of plant volatiles, their dispersion and perception by insects Visser, 1986

Biosynthesis of induced plant volatiles

The isopropenoid precursor isopentenyl pyrophosphate serves as a substrate for monoterpenes and sesquiterpenes, the fatty acid lipoxygenase pathway generates green leaf volatiles and jasmone, and the shikimic acid/tryptophan pathway results in the nitrogen containing product indole (Mann, 1987). Green leaf volatiles 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 to herbivore damage and generally released not only from damaged tissue but also from undamaged leaves (Turlings et al., 1991).

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 variable across 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 insects

Plant chemicals are also involved in the egg-laying behaviour of many phytophagous insects but here their role is less predominant and they take their place beside other factors such as texture, surface configuration, and colour. Plant chemicals can also indirectly affect the sexual behaviour of insects. e.g. Danaus chrysippus. Males of this butterfly possess hair pencil pheromone glands which disseminate an aphrodisiac pheromone (dihydropyrrolizine), making females receptive to mating. This insect is attracted to the Heliotropium plants by olfactory cues and spends considerable periods in licking the leaf surface in order to obtain pheromone precursor (Schneider, 1975). So here the link between the plant chemicals and insect behaviour isindirect and very intimate indeed.

Oviposition

Polyphagous 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 the balance between visual, olfactory and tactile cues that act as attractants and deterrents for egg laying. (Papaj and Rausher, 1987). In insects with long-lived adults, adult feeding is often crucial for reproduction. Many insects largely rely on adult-derived resources for reproduction (Tammaru and Haukioja, 1996). Host plant quality also affects insect reproductive strategies such as egg size and quality, the allocation of resources to eggs, and the choice of egg laying site. Oviposition rate may be the parameter to be affected as a response to low host quality and egg maturation rates may be dependent on presence or quality of larval hosts (Leather and Burnand, 1987). Time to initiation of oviposition has often been reported to be dependent on the quality of the substrate (Gupta and Thorsteinson, 1960).

Insect defense against plant producing chemicals

In contrast to the plant defensive chemicals insect will also find some defensive mechanisms in order to detoxify or break the plant chemicals.

Detoxification

This can be achieved by various metabolic processes like, mixed function oxidation and different detoxifying enzymes. The detoxifying enzymes are mainly synthesized by microsomes (membranous particles in the cytoplasm) and also endoplasmic reticulum that traps the toxins and renders them non toxic. Some well known detoxifying enzymes are dehydroxy chlorinase and carboxyl esterase.

Avoidance and limited contact with resistant host plants

In order to avoid unwanted effects of plant chemicals, insect fly away from those host plants or make less period of contact with that plant, so as to protect them.

Less digestion of toxic chemicals and increasedexcretion

Less 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 toxic chemicals into the insect system.

Biotype development

A biotype is a population capable of damaging and survival of plants previouslyknown to be resistant against other populations of the same species which are all growing under similar conditions (Kogan, 1994). Biotypes are morphologically similar with normal insect types but they are physiologically differing from them. The continuous growing of insect-resistant varieties may lead to development of certain physiologically and behaviourally changed biotypes, which are capable of feeding and developing on same resistant varieties. These insects will then survive on the host plant and destroy them. Ultimately the development of insect biotypes happen. That has posed a serious threat to the success of plant defense. Biotypes are developed more on varieties having more biochemical defense than the varieties offering physical defense.

Conclusion Tritrophic relationships involve complex chemical interactions.Plants can differentiate between

mechanical damage, insect wounding and even between pest species and types of damage.Parasitoids and 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 THE END ! (except the pest ).

This reviewer is confident that the development of many highly resistant plant varieties will be accomplished in the future, and that the rate of progress realized will be closely correlated with the rate of accumulation of fundamental biological and biochemical knowledge concerning the complex interactions between insects and their host plants. In the past, studies of the mechanisms underlying plant resistance have always come after the fact of resistance. As greater understanding of insect and plant biology, chemistry, and ecology is attained, we will be able to approach the goal of developing agronomic plants that are deliberately and foresightedly designed to be insect-resistant.

Theory of interdependency and survival of the fittest are the common phenomenain nature. Each organism should depend on others in order to exploit and exchange the energy and matter in an efficient 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 on insects as well as their host plants. So depending upon the influence of environmental factors on plants and insects their survival and dominance will be decided. But nature wants to maintain an equilibrium condition in the environment. So when there is dominance of one factor, it will automatically be controlled by nature, by giving different stresses. Ultimately a balanced level of insect and their host plant will be maintained.

Allowing natural balance of ecosystem, by making minimal interventions in the habitat, is one of the most promising tools to reduce the dependence of pesticides in agriculture.

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