This document provides an outline and overview of plant-insect interactions. It discusses different types of interactions like herbivory, mutualism, and parasitism. It describes how insects feed on plants through chewing, mining, boring, and sap sucking. It also discusses how plants defend against insects through production of secondary compounds, physical defenses, and induced responses. The document outlines how plants and insects have co-evolved over time through an evolutionary arms race of offense and defense traits.

1. PLANTS-INSECTS
INTERACTIONS

2. OUTLINE
 Introduction
 Coevolution
 How insect responses change over time
 Host/non-host odour recognition
 Host defence
 How plant responses change over time
 Insect effectors
 How plants recognize insects
 Interactions between insects and other organisms associated with plants
 Conclusion

3. General types of interactions
 competition -
 parasitism / predation
 mutualism
 commensalism
 Amensalism allelopathy
Lifestyles of herbivores
 Monophagous, single food type Specialist
 Oligophagous, few food types Specialist-tetranicha evansi
 Polyphagous, many food types Generalist

4. INTRODUCTION
Types of interactions
 Herbivory (phytophagy) leaf chewing , sap sacking, seed predation, gall inducing, leaf
mining
 Insect-plant mutualism –pollination and plant insect food for defence relationships
Herbivory
 Chewers Most diverse of the leaf chewing insects are the Coleoptera and
Lepidoptera . Other important groups Orthopteran, Hymenoptera
 Insects eat leaves, roots, shoots, stems, and flowers or fruits
 Chewing insects possess mandibulate mouthparts
 Mandibles serve to cut and grind food
 Mandibles are highly sclerotized to reduce wear
 High silica content and cellulose can act as resistance to herbivory

5. Mining and boring
 Insects live in between 2 epidermal layers of a leaf. Damage appears as tunnels , blotches or
blisters.
 Independently evolved in 4 orders : Diptera, Lepidoptera, Coleoptera, and Hymenoptera
 Different species may excavate different layers of leaf parenchyma or reside in particular leaf
 Fruit boring
 Stem boring
 Wood borers
 Stalk boring
 Plant boring

6. Sap sucking
 Drains plants resources by tapping into xylem and phloem
 Can retard growth and cause overall lower biomass
 Often vectors
 Hemipterans exemplify this strategy( haustellate mouth
parts
 Serves to pierce tissues and suck liquid food.
 Labium modified into a sheath enclosing stylet maxillae
 Stylets pierce cuticle and can change orientation
 Food channels empties into cibarial cavity

7. Chewers
 No relative size restrictions
 Heavy mechanical damage
 Faced with indigestible compounds and toxins
Suckers
 Restricted to a relatively small size
 Avoid mechanical damage (but still damaging)
 Avoid indigestible compounds and most toxins
 Xylem less suitable
Gall makers
 Galls consists of pathologically developed cells, tissues or organs of plants that have arisen
by hypertrophy and/or hyperplasia as a result of stimulation from foreign organisms.
 Orders that makes galls;
Hemiptera
Diptera
Hymenoptera

8. Introduction cont.,,,
 Insects are programmed to recognize and rapidly respond to patterns of host
cues.
 Particularly specialist insect species have to find specific plant species on which
they can feed and reproduce (host plants) among plant species that do not support
feeding and/or reproduction of the insects (non-host plants).
 Thus, in an environment with changing availability and quality of host plants,
phytophagous insects are under selection pressure to find quality hosts
 To maximize their fitness they need to locate suitable plants and avoid unsuitable
hosts
 Thus, they have evolved a finely tuned sensory system for detection of host cues
and a nervous system capable of integrating inputs from sensory neurons with a
high level of spatio-temporal resolution
 Time and space also influence plant responses to insects; The time dimension
is of major significance because whether or not odours arrive
simultaneously at the antenna can change the type of behavioural response
elicited in the insect

9. Introduction cont.,,,
 The huge number species of flowering plants on our planet (approximately 275 000) is
thought to be the result of adaptive radiation driven by the coevolution between plants
and their beneficial animal pollinators (Yuan et al., 2013). The
 fossil record shows that pollination originated 250 million years ago (Labandeira, 2013).
Some plants have evolved with their pollinators and produce olfactory messages which
make them unique for their specific pollinators (Grajales-Conesa et al., 2011).
 For example, certain orchid flowers mimic aphid alarm pheromones to attract hoverflies
for pollination (Stoekl et al., 2011).
 Furthermore, insect herbivores can drive real-time ecological and evolutionary change in
plant populations.
 Recent studies provide evidence for rapid evolution of plant traits that confer resistance
to herbivores when herbivores are present but for the evolution of traits that confer
increased competitive ability when herbivores are absent (Agrawal et al., 2012; Hare,
2012; Züst et al., 2012).
 While phytophagous insects have been adapting to exploit their hosts, the plants have
simultaneously been evolving defensive systems to counteract herbivore attack
(Anderson and Mitchell-Olds, 2011; Johnson, 2011).

10. Coevolution
 The huge number species of flowering plants on our planet (approximately 275 000) is
thought to be the result of adaptive radiation driven by the coevolution between plants and
their beneficial animal pollinators (Yuan et al., 2013).
 The fossil record shows that pollination originated 250 million years ago (Labandeira, 2013).
 Some plants have evolved with their pollinators and produce olfactory messages which make
them unique for their specific pollinators
 For example, certain orchid flowers mimic aphid alarm pheromones to attract hoverflies for
pollination
 Furthermore, insect herbivores can drive real-time ecological and evolutionary change in
plant populations.
 Recent studies provide evidence for rapid evolution of plant traits that confer resistance to
herbivores when herbivores are present but for the evolution of traits that confer increased
competitive ability when herbivores are absent (Agrawal et al., 2012)
 While phytophagous insects have been adapting to exploit their hosts, the plants have
simultaneously been evolving defensive systems to counteract herbivore attack (Anderson
and Mitchell-Olds, 2011)

11. Coevolution
 Many plant taxa manufacture novel secondary compounds that are mildly noxious
 Some insect taxa feed on plants with the compounds and reduce plant fitness
 Mutation/recombination introduce more compounds
 Insect feeding is reduced and toxicity in plants is selected
 The plant taxon goes through adaptive radiation
 Insects evolve tolerance or attraction to the novel compound and tend to specialize
on plants with that taxon. The insect taxon goes through adaptive radiation
 The cycle is repeated , resulting more phytochemicals and more feeding
specialization

12. Host/non-host odour recognition
The way in which insects use plant volatiles to recognize their host
plants usually involves blends of commonly occurring volatiles in
specific combinations or ratios

13. Host/non-host odour recognition
 Blend combinations play a crucial role as evidenced by a study with host odours
of the black bean aphid, Aphis fabae, in which odours presented individually in
an olfactometer were repellent but when put together as a blend became
attractive (Webster et al., 2010).
 .A combination of olfactory and visual cues can further enhance attraction
 This came from the finding of olfactory receptor neurones (ORNs) tuned to
specific non-host compounds, 3-butenyl isothiocyanate and 4-pentenyl
isothiocyanate, in the black bean aphid (Nottingham et al., 1991).
 When these isothiocynates were tested in an olfactometer bioassay, they were
found to be repellent.
 Ratios can also be important; for example, Cha et al. (2011) found that doubling
the concentration of any one of the components of a synthetic host volatile blend
of grape odours (comprising (E)- and (Z)-linalool oxides, nonanal, decanal, (E)-
caryophyllene, and germacrene-D), while keeping the concentration of the other
compounds constant, significantly reduced female attraction in a wind-tunnel

14. How insect responses change over time
 Insects have a nervous system and the capacity to learn which has consequences for
their responses to plant volatiles
 . Learning behaviour, such as when an odour is associated with a reward, can affect
the strength or even the type of response to plant stimuli.
 For example, hawkmoths (Manduca sexta) are innately attracted to blends of
particular night-blooming flowers, but, when there are not enough of these
hawkmoth-adapted flowers in the habitat, moths learn to associate the odours of
bat-pollinated Agave palmeri flowers which have a completely different smell
(Riffell et al., 2013).
 Thus, processing of stimuli through two olfactory channels, one involving an
innate bias and the other a learned association, allows the moths to exist within a
changing environment.
 The challenge of host recognition: herbivorous insects need to discriminate between
host and non-host and to select good quality hosts.
 Hosts already attacked by other insects may have defences induced and be lower
quality.

15. How insect responses change over time cont…
 Other biotic and abiotic stresses that change plant quality can also change the profile of
volatiles emitted thus providing further information to foraging insects.
 This was proved in a laboratory study where Spodoptera littoralis moths were trained to
extend their proboscis (a feeding response) in response to (Z,E)-9,11tetradecadienyl
acetate, which is a sex pheromone that usually elicits sexual behaviours
 Studies have shown that some odours are learnt better than others in particular insect–
plant interactions; for example, honey bees learn linalool and 2-phenylethanol better than
host odour blend
 Natural enemies can also learn. It appears that generalist egg and larval parasitoids
respond innately to herbivore-induced plant volatiles (HIPVs) whereas specialists rely
more on associative learning

16. How insect responses change over time
 The physiological condition of an insect has long been known to influence
insect–plant interactions.
 When the insect is satiated it will be less motivated to respond to food odours;
for example, the response of D. melanogaster to vinegar is modulated by hunger
 Similarly, when a female insect has already laid eggs she will be less attracted to
oviposition cues.
 Female insects are influenced by mating which can induce profound
physiological changes.
 After mating, S. littoralis switches its behavioural response to olfactory cues
from food associated ones to oviposition-associated ones (Saveer et al., 2012).
 Unmated females are strongly attracted to lilac flowers but, after mating,
attraction to floral odour is abolished and they fly instead to the green-leaf odour
of the larval host plant cotton (Gossypium hirsutum).

17. Plant defence
 Plants have had to defend themselves against insect attack.
 Being rooted to the ground they are unable to flee from attacking herbivores.
 They have evolved a wide range of sophisticated defence systems to protect their tissues
 These include toxic or anti-feedant secondary metabolites that represent a major barrier
to herbivory and physical defences such as lignin
 These provide direct defence via toxic, anti-nutritive or repellent effects on herbivores.
 Plant defences are orchestrated both in time and space by highly complex regulatory
networks that themselves are further modulated by interactions with other signalling
pathways

18. Plant defence
 Defences can be constitutive or induced.
Constitutive- defenses that are always present regardless of the presence of
herbivory eg. Cuticle, wax, spines etc
Induced- defenses that are only produced when there is feeding by an
herbivore eg. a HR( hypersensitive reaction), secondary metabolites.
 Primed plants respond more quickly and strongly when they are attacked again
 Metabolites and energy can, thus, be more efficiently allocated to defensive
activities when there is a mechanism for recognizing the herbivore challenge and
triggering precise timing of the adaptive modulation of the plant’s metabolism

19. Plant/host defence
Plant defense
non=-Nitrogenous
phenolics,
Terpenoids,
Photooxidants,
insect hormone or
pheromone analogs,
Pyrethroids,
aflatoxins
Nitrogenous
Elemental
Se, Mn, Cu,
Ni, Zn, Cd, Cr,
Pb, Co, Al,
non protein
amino acids,
cyanogenic
glucosides,
Glucosinolates,
alkaloids

20. 1. Nonnitrogenous Defences
Phenolics/flavonoids,
 Are distributed widely among terrestrial plants and are likely among the oldest plant
secondary (i.e., non metabolic) compounds.
 Provide support for vascular plants (lignins)
 Compose pigments that determine flower color for angiosperms,
 Play a role in plant nutrient acquisition by affecting soil chemistry.
 Phenolics include the hydrolyzeable tannins, derivatives of simple phenolic acids, and
condensed tannins, polymers of higher molecular weight hydroxyflavenol units.
 Polymerized tannins are highly resistant to decomposition, eventually composing the
humic materials that largely determine soil properties.
 Tannins are distasteful, usually bitter and astringent, and act as feeding deterrents for
many herbivores.
 When ingested, tannins chelate N-bearing molecules to form indigestible
complexes
 Insects incapable of catabolizing tannins or preventing chelation suffer gut
damage and are unable to assimilate nitrogen from their food.
 Some flavonoids, such as rotenone, are directly toxic to insects and other
animals.

21. B. Terpenoids
 These compounds are synthesized by linking isoprene subunits.
 The lower molecular weight monoterpenes and sesquiterpenes are highly volatile
compounds that function as floral scents that attract pollinators and other plant
scents that herbivores or their predators and parasites use to find hosts.
 Some insects modify plant terpenes for use as pheromones.
 Terpenoids with higher molecular weights include plant resins, cardiac
glycosides, and saponins.
 Terpenoids usually are distasteful or toxic to herbivores.
 In addition, they are primary resin components of pitch, produced by many
plants to seal wounds.

22. C. Photooxidants,
 include quinones and furanocoumarins.
 increase epidermal sensitivity to solar radiation.
 Assimilation of these compounds can result in severe sunburn, necrosis of the
skin, and other epidermal damage on exposure to sunlight.
 Feeding on furanocoumarin-producing plants in daylight can cause 100%
mortality to insects, whereas feeding in the dark causes only 60% mortality.
 Insect herbivores can circumvent this defence by becoming leaf rollers or
nocturnal feeders or by sequestering antioxidants

23. C. Photooxidants,
 Insect development and reproduction are governed primarily by two hormones,
molting hormone (ecdysone) and juvenile hormone
 The relative concentrations of these two hormones dictate the timing of ecdysis
and the subsequent stage of development.
 A large number of phytoecdysones have been identified, primarily from ferns and
gymnosperms.
 Some of the phytoecdysones are 20 times more active than the ecdysones
produced by insects and resist inactivation by insects
 It has been shown that that spinach, Spinacia oleracea, produces 20-
hydroxyecdysone in roots in response to root damage or root herbivory.
 larvae preferred a diet with a low concentration of 20-hydroxyecdysone and
showed significantly reduced survival when reared on a diet with a high
concentration of 20-hydroxyecdysone.
 20-hydroxyecdysone. Confises molting; speeds upmplting n cuticle shedding
early co

24. D. juvenile hormone analogues
 These are primarily juvabione and compounds that interfere with juvenile hormone
activity, primarily precocene.-mimics juvemile
 The ant juvenile hormones usually cause precocious development.
 Plant-derived hormone analogues are highly disruptive to insect development, usually
preventing maturation or producing imperfect and sterile adult
 Some plants produce insect alarm pheromones that induce rapid departure of colonizing
insects.
 The wild potato, Solanum berthaultii, produces (E)-b-farnesene, the major component of
alarm pheromones for many aphid species.
 This compound is released from glandular hairs on the foliage at sufficient quantities to
induce departure of settled colonies of aphids and avoidance by host seeking aphids.
 Track keeping
 Aggregate
 Sex pheromone
 Alarm pheromone

25. E. Pyrethroids
 Are an important group of plant toxins.
 Many synthetic Pyrethroids are widely used as contact insecticides
(i.e., absorbed through the exoskeleton) because of their rapid effect on
insect pests.
 Aflatoxins are toxic compounds produced by fungi.
 Many are highly toxic to vertebrates
 Higher plants may augment their own defenses through mutualistic
associations with endophytic or mycorrhizal fungi that produce
aflatoxins

26. Nitrogenous Defences
 These compounds are highly toxic as a result of their interference with protein
function or physiological processes.
Nonprotein amino acids are analogues of essential amino acids.
 Their substitution for essential amino acids in proteins results in improper
configuration, loss of enzyme function, and inability to maintain physiological
processes critical to survival.
 Some non-protein amino acids interfere with tyrosinase (an enzyme critical to
hardening of the insect cuticle) by 3,4-dihydrophenylalanine (L-DOPA).
 Toxic or other defensive proteins are produced by many organisms.
Proteinase inhibitors, produced by a variety of plants, interfere with insect digestive
enzymes .

27. Nitrogenous Defences
The endotoxins produced by the bacterium Bacillus thuringiensis (Bt) have been widely
used for control of several Lepidoptera, Coleoptera, and mosquito pests.
 Because of their effectiveness, the genes coding for these toxins have been introduced
into a number of crop plant species, including corn, sorghum, soybean, potato, and
cotton, to control crop pests
Cyanogenic glycosides are distributed widely among plant families
 These compounds are inert in plant cells.
 Plants also produce specific enzymes to control hydrolysis of the glycoside.
 When crushed plant cells enter the herbivore gut, the glycoside is hydrolyzed into
glucose and a cyanohydrin that spontaneously decomposes into a ketone or aldehyde
and hydrogen cyanide.
 Hydrogen cyanide is toxic to most organisms because of its inhibition of
cytochromes in the electron transport system
 Cassava root has tese glycosides

28. Nitrogenous Defences
Glucosinolates,
 Are a characteristic of the Brassicaceae,
 shown to deter feeding and reduce growth in a variety of herbivores
 The young larvae of the cabbage white butterfly, Pieris rapae, a specialized
herbivore, have shown reduced growth with increasing glucosinolate concentration
in Brassica napus hosts, but that older larvae were relatively tolerant of
glucosinolates.

29. Elemental defences
 Some plants accumulate and tolerate high concentrations of toxic elements,
including Se, Mn, Cu, Ni, Zn, Cd, Cr, Pb, Co, Al,
 In some cases, foliage concentrations of these metals can exceed 2%
 Although the function of such hyper accumulation remains unclear, some
plants benefit from protection against herbivores
 Boyd and Martens (1994) found that larvae of the cabbage white butterfly
fed Thlaspi montanum grown in high Ni soil showed 100% mortality after
12 days, compared to 21% mortality for larvae fed on plants grown in low
Ni soil.

30. Arthropod Defences
1. Antipredator Defences.
 Physical defenses include hardened exoskeleton, spines, claws, and mandibles.
 Chemical defenses are nearly as varied as plant defences.
 The compounds used by arthropods, including predaceous species, generally belong to
the same categories of compounds described previously for plants.
 Many insect herbivores sequester plant defences for their own defence
 The relatively inert exoskeleton provides an ideal site for storage of toxic compounds.
 Toxins can be stored in scales on the wings of Lepidoptera (e.g., cardiac glycosides in
the wings of monarch butterflies).
 Some insects make more than such passive use of their sequestered defences.

31. Arthropod Defences
 Peterson et al. (2003) reported that grasshoppers and spiders, and other invertebrates,
all had elevated Ni concentrations at sites where the Ni-accumulating plant, Alyssum
pintodasilvae, was present but not at sites where this plant was absent, indicating
spread of Ni through trophic interactions.
 Accumulation of Ni from Thlaspi montanum by an adapted mirid plant bug,
Melanotrichus boydi, protected it against some predators (Boyd and Wall 2001) but not
against entomopathogens
 Concentrations of Ni in invertebrate tissues approached levels that have toxic effects
on birds and mammals, suggesting that using hyper accumulating plant species for
bioremediation may, instead, spread toxic metals through food chains at hazardous
concentrations.
 A number of Orthoptera, Heteroptera, and Coleoptera exude noxious, irritating, or
repellent fluids or froths when disturbed
 Blister beetles (Meloidae) synthesize the terpenoid, cantharidin, and ladybird beetles
(Coccinellidae), synthesize the alkaloid, coccinelline.

32. Arthropod Defences
 These compounds (coccinelline.cantharidin ) occur in the hemolymph and are exuded by
reflex bleeding from leg joints.
 They deter both invertebrate and vertebrate predators.
 Whiptail scorpions spray acetic acid from their “tail,” and the millipede, Harpaphe,
sprays cyanide.
 The bombardier beetle, Brachynus, sprays a hot (100°C) cloud of benzoquinone produced
by mixing, at the time of discharge, a phenolic substrate (hydroquinone), peroxide, and
an enzyme catalase
 Several arthropod groups produce venoms, primarily peptides, including phospholipases,
histamines, proteases, and esterases, for defence as well as predation

33. Arthropod Defences
neurotoxic and haemolytic venoms
 Phospholipases are particularly well-known because of their high toxicity and their
strong antigen activity capable of inducing life-threatening allergy.
 Larvae of several families of Lepidoptera, especially the Saturniidae and Limacodidae
deliver venoms passively through urticating spines
 A number of Heteroptera, Diptera, Neuroptera, and Coleoptera produce orally derived
venoms that facilitate prey capture, as well as defense
 Venoms are particularly well-known among the Hymenoptera and consist of a variety
of enzymes, biogenic amines (such as histamine and dopamine), epinephrine,
norepinephrine, and acetylcholine.
 Melittin, found in bee venom, disrupts erythrocyte membranes
 This combination produces severe pain and effects cardiovascular, central nervous, and
endocrine systems in vertebrates
 Some venoms include nonpeptide components. For example, venom of the red
imported fire ant, Solenopsis invicta, contains piperidine alkaloids, with hemolytic,
insecticidal, and antibiotic effects.

34. How plant responses change over time
 Although many plant secondary metabolites have evolved as plant defence, insects may
overcome the defences by coevolving adaptations such as cytochrome P450
monooxygenases (P450s) that metabolize plant toxins
 Specialist insects may even use the plant secondary metabolites to defend themselves
against their own attackers at the third trophic level (Boppré, 1978).
 The molecular basis of resistance to toxic cardenolides involves an amino acid change on
the transmembrane sodium channel, which is the target site of the toxin.
 There has been convergent evolution with several insect species evolving the same amino
acid change
 Insights into the evolutionary process have been obtained from studies of the recent host
shift to tobacco (Nicotiana tabacum) by the peach-potato aphid, Myzus persicae.
 Tobacco-adapted aphid races were found to overexpress a cytochrome P450 enzyme
(CYP6CY3) that allows them to detoxify nicotine (Bass et al., 2013

35. Insect effectors
 Insect oral secretions contain specific proteins and chemicals as effectors to inhibit
plant defences but, with time, some plants have adapted to recognize some of these
substances so that they may even trigger defence responses
 Salivary protein C002 was shown to play a crucial role in pea aphid survival and,
when knocked down by RNAi, reduced time spent by aphids in contact with phloem
sap when feeding on broad bean
 Candidate effectors were identified from the aphid Myzus persicae by Bos et al.
(2010) and of these Mp10 and Mp42 reduced aphid fecundity whereas MpC002
enhanced aphid fecundity when overexpressed in Nicotiana benthamiana.
 Although there may be differences when these proteins are expressed by the aphid
instead of being continuously expressed in the plant it appears that Mp10 and Mp42
benefit the plant rather than the aphid.
 Phloem-feeding insects need to overcome plant physical defence mechanisms based
on plugging the sieve tubes with callose or proteins and require effectors for this.

36. Insect effectors
 Aphid honeydew has also been shown to suppress induced plant defence.
 Highly polyphagous species, like Helicoverpa zea, are more likely to possess
relatively high levels of salivary glucose oxidase (GOX) for suppression of plant
defences, compared to species with a more limited host range
 Intricate adaptations have evolved with specialist herbivores;
 Velvetbean caterpillar (Anticarsia gemmatalis) evades detection by cowpea by
converting fragments of chloroplastic ATP synthase gamma-subunit proteins, termed
inceptin-related peptides, that usually function as an elicitor of plant defence into an
antagonist effector

37. How plants recognize insects
 All living organisms face the shared challenge of detecting and responding to
chemical stimuli from their external environment.
 Detection of molecules associated with attacking organisms is crucial for eliciting
behavioural, physiological, and biochemical responses to ensure survival.
 Being unable to flee from attack, plants have had to evolve sophisticated ways of
detecting attackers and it is becoming increasingly clear that they can detect and
respond to a wide range of molecules.
 Pattern recognition is a fundamental process in the immune responses of both
plants and animals
 It is becoming increasingly clear that molecular recognition via ligand–receptor
binding phenomena plays important roles in plants and that this plays a role in
insect–plant interactions

38. How plants recognize insects
 The identification of receptors and ligands is crucial to understand specificity in
plant immunity to herbivores
 Plants possess surveillance systems that are able to detect highly specific
herbivore-associated cues as well as general patterns of cellular damage, thus
allowing them to mount defences.
 Molecular recognition mechanisms underpin this process with receptors tuned to
herbivore-associated molecular patterns (HAMPs) or damaged-self compounds
produced after insect attack
 miRNAs have also been implicated in insect– plant interactions
 Sattar et al. (2012) found that Aphis gossypii miRNAs were differentially
regulated during resistant and susceptible interactions with different melon lines,
some possessing the Vat resistance gene and others not.

39. How plants recognize insects
 Recognizing the herbivore challenge to allow precise timing of appropriate plant
metabolic responses is important so that metabolites and energy are efficiently
allocated and correctly timed (Mithoefer and Boland, 2012).
 However, for most insect–plant interactions, relatively little is currently known
about the molecular basis of insect perception by plants, the signalling
mechanisms directly associated with this perception, or how plants differentially
discriminate between different species of attacking insects
 Plant–pathogen interactions have been better defined in this respect and effector-
based models of insect–plant interactions are now being put forward
 Thus plants not only respond directly to molecules from attacking organisms but
can also respond to volatiles released by other plants which are under attack
 Putative receptors are known but their ligands have not yet been identified.

40. How plants recognize insects
 For example, three genes conferring resistance to insects have been identified in plants
and are all members of the NB-LRR family: the Mi-1 gene in tomato confers
resistance to Macrosiphum euphorbiae
 mechanism of resistance is thought to involve the putative receptors binding to as yet
unidentified insect effectors.
 The pests involved are all in the insect order Hemiptera, which are stealthy herbivores
with a sucking mode of feeding, and it seems likely that the HAMP is a small
molecule or protein contained in the insect’s saliva.
 It is possible that the detergent-like properties of fatty acid conjugates could disrupt
plasma membranes and cause influx of Ca2+ thus triggering responses.
 However, radiolabelled volicitin has been shown to bind rapidly, reversibly, and
saturatably to plasma membranes suggesting that there is an interaction with a
receptor.
 HAMPs have also been identified from insect egg ovipositional fluid

41. Interactions between insects and other organisms
associated with plants
 Nature is more complicated because plants are exposed to multiple attacking and beneficial
organisms
 Much less is known about the effect of multiple, co-occurring stress factors than individual
biotic and abiotic stresses, despite the fact that multiple stresses are probably the rule under
natural conditions .
 Negative crosstalk between plant defence pathways means that time can have an impact on
these multi-species interactions due to differences in the sequence in which plants are
exposed to different organisms.
 Thus, the chronological order in which attackers arrive at a plant matters: later arrivals will
perform better or worse according to the types of defence that have been induced or primed
by the earlier arrivals.
 Soler et al. (2013) proposed that the outcome of intra-feeding guild interactions is generally
negative due to induction of similar phytohormonal pathways, whereas between-guild
interactions are often positive due to negative signal crosstalk.
 However, each interaction should be considered individually because it also depends
whether the previous attacker managed to suppress plant defences against it or whether it
activated them.

42. Interactions between insects and other
organisms associated with plants
 Interactions with the third trophic level can also change the outcome of insect–
plant interactions.
 In an experiment, it was found that cereal aphids preferred larvipositing on
nutritionally superior wheat cultivars, but in the presence of the harlequin ladybird,
Harmonia axyridis, they changed their preference to nutritionally inferior cultivars
apparently because the risk of predation was lower on these.
 HIPVs are important in tritrophic interactions.
 Any negative effects of HIPVs on pollinator visitation rates are likely also to exert
selection pressure on HIPV emission (Lucas-Barbosa et al., 2011).
 Attraction of natural enemies may be compromised if their hyperparasisoids are
also attracted to the HIPVs (Poelman et al., 2012).
 An example is the use of symbiotic bacteria by Colorado potato beetle to evade
ant herbivore defences of its host.

43. Interactions between insects and other
organisms associated with plants
 These beetles can secrete symbiotic bacteria into wounded plants that elicit SA-
regulated defences (Chung et al., 2013).
 Due to negative crosstalk with jasmonate-regulated defences this makes plants more
suitable for the chewing herbivore.
 By sharing the same host plant above-ground and belowground insects can influence
each other even though they are not in direct contact (Bruce and Pickett, 2007).
 Robert et al. (2012) found that Diabrotica virgifera larvae showed stronger growth on
roots previously attacked by conspecific larvae, but performed more poorly on roots
of plants whose leaves had been attacked by larvae of the moth S. littoralis.

44. Conclusions
 Ecological interactions between insects and plants are complicated and dynamic.
 What occurs in one system at one snapshot in time may not occur again at another
snapshot at a different time and each insect–plant system has its own unique features.
 Both the insect and the plant can change over time: the insect changes because of
learning behaviour in the short term and by gene mutations in the longer term; the plant
changes due to induced defence processes in the short term, epigenetic changes in the
medium term, and gene mutations in the longer term.
 There is variation between different strains of both insects and plants.
 The genetic and temporal variability of biological material allows survival in an
environment which is also dynamic and not entirely predictable.
 Interactions are complicated even further because the history of exposure to other
associated insects can change the suitability of a plant to the insect being considered

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 THANKS ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,