This document discusses the role of chemical ecology, volatile compounds, and secondary plant substances in plant resistance to insects. It provides background on chemical ecology and how plant chemicals can affect insect behavior. Secondary plant metabolites like alkaloids, terpenoids, and phenolic compounds adversely impact insect growth, development, and metabolism. Specific examples are given of compounds that act as feeding deterrents or have toxic effects on insects. The document also discusses how plant volatile compounds can attract or repel insects.

1. 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

2. 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.

3. Feeding deterrents produced by plants
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
Hagen et al., 1984
Influence of plant volatiles on insects

4. 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
inhibitor
Colorado potato beetle
(Leptinotarsa
decemlineata)
Effect on
digestion

5. 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

6. 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.

7. 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

8. 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 cut-
worm, 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

9. 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,
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).

10. 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.
Figure 6. Plant volatile mediated in sect induced plant defense
Tritrophic interaction

11. Examples of Chem. Mediated Tritrophic Relationships
Plant – 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 defence
The 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

12. 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).

13. 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).
Figure 2. Release of plant volatiles, their dispersion and
perception by insects Visser, 1986

14. 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

15. 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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