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Plant insects interactions
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.
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Plant insects interactions
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- 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 ofinteractions 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 Drainsplants 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 relativesize 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.,,, Insectsare 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.,,, Thehuge 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 hugenumber 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 planttaxa 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 Theway 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 responseschange 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 responseschange 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 responseschange 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 Plantshave 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 Defencescan 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, insecthormone 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 Thesecompounds 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, includequinones 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, Insectdevelopment 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 hormoneanalogues 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 Arean 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 Thesecompounds 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 endotoxinsproduced 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, Area 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 Someplants 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. AntipredatorDefences. 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 Petersonet 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 Thesecompounds (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 andhaemolytic 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 responseschange 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 Insectoral 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 Aphidhoneydew 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 recognizeinsects 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 recognizeinsects 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 recognizeinsects 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 recognizeinsects 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 insectsand 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 insectsand 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 insectsand 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 interactionsbetween 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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