Herbivore-mediated effects of glucosinolates on different natural enemies of a specialist aphid

Insects' perception and behavioral responses to plant semiochemicals

Insect-plant interactions are shaped by the exchange of chemical cues called semiochemicals, which play a vital role in communication between organisms. Plants release a variety of volatile organic compounds in response to environmental cues, such as herbivore attacks. These compounds play a crucial role in mediating the interactions between plants and insects. This review provides an in-depth analysis of plant semiochemicals, encompassing their classification, current understanding of extraction, identification, and characterization using various analytical techniques, including gas chromatography-mass spectrometry (GC-MS), liquid chromatography-mass spectrometry (LC-MS), nuclear magnetic resonance (NMR) spectroscopy, and infrared (IR) spectroscopy. The article also delves into the manner in which insects perceive and respond to plant semiochemicals, as well as the impact of environmental factors on plant odor emission and insect orientation. Furthermore, it explores the underlying mechanisms by which insects perceive and interpret these chemical cues, and how this impacts their behavioral responses, including feeding habits, oviposition patterns, and mating behaviors. Additionally, the potential applications of plant semiochemicals in integrated pest management strategies are explored. This review provides insight into the intricate relationships between plants and insects mediated by semiochemicals, highlighting the significance of continued research in this field to better understand and leverage these interactions for effective pest control.

Defense strategies and associated phytohormonal regulation in Brassica plants in response to chewing and sap-sucking insects

Plants have evolved distinct defense strategies in response to a diverse range of chewing and sucking insect herbivory. While chewing insect herbivores, exemplified by caterpillars and beetles, cause visible tissue damage and induce jasmonic acid (JA)-mediated defense responses, sucking insects, such as aphids and whiteflies, delicately tap into the phloem sap and elicit salicylic acid (SA)-mediated defense responses. This review aims to highlight the specificity of defense strategies in Brassica plants and associated underlying molecular mechanisms when challenged by herbivorous insects from different feeding guilds (i.e., chewing and sucking insects). To establish such an understanding in Brassica plants, the typical defense responses were categorized into physical, chemical, and metabolic adjustments. Further, the impact of contrasting feeding patterns on Brassica is discussed in context to unique biochemical and molecular modus operandi that governs the resistance against chewing and sucking insect pests. Grasping these interactions is crucial to developing innovative and targeted pest management approaches to ensure ecosystem sustainability and Brassica productivity.

Induction of resistance, enzyme activity, and phytochemicals in canola plants treated with abscisic acid elevated based on nutrient availability: a case study on Brevicoryne brassicae L. (Hemiptera: Aphididae)

The cabbage aphid, Brevicoryne brassicae L. (Hemiptera: Aphididae), is one of the important pests of cruciferous plants throughout the world including Iran. In the present study, we grew cultivated canola plants under different fertilizers or distilled water and sprayed them with 100 µM abscisic acid (ABA) or a control solution (NaOH dissolved in water) to study (i) the antibiosis parameters of B. brassicae on these plants; (ii) the antixenosis of B. brassicae adults on these plants; (iii) the plant's peroxidase (POD), polyphenol oxidase (PPO), and phenylalanine ammonia-lyase (PAL) activity; and (iv) the plant's total phenolic and glucosinolate content. The results of antibiosis experiments showed that ABA and fertilizers have a profound and negative effect on the performance of B. brassicae. In the antixenosis experiment, control plants attracted a significantly higher number of adult females in comparison to treated plants. Also, B. brassicae had lower performance and preference when they were reared on the ABA-treated fertilized plants with higher levels of phenolic and glucosinolate content. These results prompted us to hypothesize that fertilizers enable canola plants to trigger a higher level of secondary metabolites. Our findings reveal that the type and level of nutrient availability may have different impacts on how the plant regulates its defense mechanisms.

Cereal Aphid Parasitoids in Europe (Hymenoptera: Braconidae: Aphidiinae): Taxonomy, Biodiversity, and Ecology

Cereals are very common and widespread crops in Europe. Aphids are a diverse group of herbivorous pests on cereals and one of the most important limiting factors of cereal production. Here, we present an overview of knowledge about the taxonomy, biodiversity, and ecology of cereal aphid parasitoids in Europe, an important group of natural enemies contributing to cereal aphid control. We review the knowledge obtained from the integrative taxonomy of 26 cereal aphid primary parasitoid species, including two allochthonous species (Lysiphlebus testaceipes and Trioxys sunnysidensis) and two recently described species (Lipolexis labialis and Paralipsis brachycaudi). We further review 28 hyperparasitoid species belonging to three hymenopteran superfamilies and four families (Ceraphronoidea: Megaspillidae; Chalcidoidea: Pteromalidae, Encyrtidae; Cynipoidea: Figitidae). We also compile knowledge on the presence of secondary endosymbionts in cereal aphids, as these are expected to influence the community composition and biocontrol efficiency of cereal aphid parasitoids. To study aphid-parasitoid-hyperparasitoid food webs more effectively, we present two kinds of DNA-based approach: (i) diagnostic PCR (mainly multiplex PCR), and (ii) DNA sequence-based methods. Finally, we also review the effects of landscape complexity on the different trophic levels in the food webs of cereal aphids and their associated parasitoids, as well as the impacts of agricultural practices and environmental variation.

Concurrent anthropogenic air pollutants enhance recruitment of a specialist parasitoid

Air pollutants-such as nitrogen oxides, emitted in diesel exhaust, and ozone (O3)-disrupt interactions between plants, the insect herbivore pests that feed upon them and natural enemies of those herbivores (e.g. parasitoids). Using eight field-based rings that emit regulated quantities of diesel exhaust and O3, we investigated how both pollutants, individually and in combination, altered the attraction and parasitism rate of a specialist parasitoid (Diaeretiella rapae) on aphid-infested and un-infested Brassica napus plants. Individual effects of O3 decreased D. rapae abundance and emergence by 37% and 55%, respectively, compared with ambient (control) conditions. When O3 and diesel exhaust were emitted concomitantly, D. rapae abundance and emergence increased by 79% and 181%, respectively, relative to control conditions. This attraction response occurred regardless of whether plants were infested with aphids and was associated with an increase in the concentration of aliphatic glucosinolates, especially gluconapin (3-butenyl-glucosinolate), within B. napus leaves. Plant defensive responses and their ability to attract natural aphid enemies may be beneficially impacted by pollution exposure. These results demonstrate the importance of incorporating multiple air pollutants when considering the effects of air pollution on plant-insect interactions.

Perspectives for integrated insect pest protection in oilseed rape breeding

In the past, breeding for incorporation of insect pest resistance or tolerance into cultivars for use in integrated pest management schemes in oilseed rape/canola (Brassica napus) production has hardly ever been approached. This has been largely due to the broad availability of insecticides and the complexity of dealing with high-throughput phenotyping of insect performance and plant damage parameters. However, recent changes in the political framework in many countries demand future sustainable crop protection which makes breeding approaches for crop protection as a measure for pest insect control attractive again. At the same time, new camera-based tracking technologies, new knowledge-based genomic technologies and new scientific insights into the ecology of insect-Brassica interactions are becoming available. Here we discuss and prioritise promising breeding strategies and direct and indirect breeding targets, and their time-perspective for future realisation in integrated insect pest protection of oilseed rape. In conclusion, researchers and oilseed rape breeders can nowadays benefit from an array of new technologies which in combination will accelerate the development of improved oilseed rape cultivars with multiple insect pest resistances/tolerances in the near future.

Plant metabolism and defence strategies in the flowering stage: Time-dependent responses of leaves and flowers under attack

Plants developing into the flowering stage undergo major physiological changes. Because flowers are reproductive tissues and resource sinks, strategies to defend them may differ from those for leaves. Thus, this study investigates the defences of flowering plants by assessing processes that sustain resistance (constitutive and induced) and tolerance to attack. We exposed the annual plant Brassica nigra to three distinct floral attackers (caterpillar, aphid and bacterial pathogen) and measured whole-plant responses at 4, 8 and 12 days after the attack. We simultaneously analysed profiles of primary and secondary metabolites in leaves and inflorescences and measured dry biomass of roots, leaves and inflorescences as proxies of resource allocation and regrowth. Regardless of treatments, inflorescences contained 1.2 to 4 times higher concentrations of primary metabolites than leaves, and up to 7 times higher concentrations of glucosinolates, which highlights the plant's high investment of resources into inflorescences. No induction of glucosinolates was detected in inflorescences, but the attack transiently affected the total concentration of soluble sugars in both leaves and inflorescences. We conclude that B. nigra evolved high constitutive rather than inducible resistance to protect their flowers; plants additionally compensated for damage by attackers via the regrowth of reproductive parts. This strategy may be typical of annual plants.

The Evolution of Glycoside Hydrolase Family 1 in Insects Related to Their Adaptation to Plant Utilization

Insects closely interact with plants with multiple genes involved in their interactions. β-glucosidase, constituted mainly by glycoside hydrolase family 1 (GH1), is a crucial enzyme in insects to digest plant cell walls and defend against natural enemies with sequestered plant metabolites. To gain more insights into the role of this enzyme in plant-insect interactions, we analyzed the evolutionary history of the GH1 gene family with publicly available insect genomes. We found that GH1 is widely present in insects, while the gene numbers are significantly higher in insect herbivores directly feeding on plant cell walls than in other insects. After reconciling the insect GH1 gene tree with a species tree, we found that the patterns of duplication and loss of GH1 genes differ among insect orders, which may be associated with the evolution of their ecology. Furthermore, the majority of insects' GH1 genes were tandem-duplicated and subsequently went through neofunctionalization. This study shows the evolutionary history of an important gene family GH1 in insects and facilitates our understanding of the evolution of insect-plant interactions.

Flowers prepare thyselves: leaf and root herbivores induce specific changes in floral phytochemistry with consequences for plant interactions with florivores

The phenotypic plasticity of flowering plants in response to herbivore damage to vegetative tissues can affect plant interactions with flower-feeding organisms. Such induced systemic responses are probably regulated by defence-related phytohormones that signal flowers to alter secondary chemistry that affects resistance to florivores. Current knowledge on the effects of damage to vegetative tissues on plant interactions with florivores and the underlying mechanisms is limited. We compared the preference and performance of two florivores on flowering Brassica nigra plants damaged by one of three herbivores feeding from roots or leaves. To investigate the underlying mechanisms, we quantified expression patterns of marker genes for defence-related phytohormonal pathways, and concentrations of phytohormones and glucosinolates in buds and flowers. Florivores displayed contrasting preferences for plants damaged by herbivores feeding on roots and leaves. Chewing florivores performed better on plants damaged by folivores, but worse on plants damaged by the root herbivore. Chewing root and foliar herbivory led to specific induced changes in the phytohormone profile of buds and flowers. This resulted in increased glucosinolate concentrations for leaf-damaged plants, and decreased glucosinolate concentrations for root-damaged plants. The outcome of herbivore-herbivore interactions spanning from vegetative tissues to floral tissues is unique for the inducing root/leaf herbivore and receiving florivore combination.

Aphid Species and Feeding Location on Canola Influences the Impact of Glucosinolates on a Native Lady Beetle Predator

Aphids that attack canola (Brassica napus L.) exhibit feeding preferences for different parts of canola plants, which may be associated with brassica-specific glucosinolates. However, this idea remains untested. Furthermore, canola aphid species employ different strategies for tolerating glucosinolates. While the green peach aphid, Myzus persicae (Sulzer) (Hemiptera: Aphididae), excretes glucosinolates, the cabbage aphid Brevicoryne brassicae (L.) (Hemiptera: Aphididae) sequesters them. Given the different detoxification mechanisms, we predicted that both aphid species and aphid feeding location would affect prey suitability for larvae of the predator, Hippodamia convergens (Guérin-Méneville) (Coleoptera: Coccinellidae). We hypothesized that aphids, specifically glucosinolate-sequestering cabbage aphid, reared on reproductive structures that harbor higher glucosinolates concentrations would have greater negative effects on predators than those reared on vegetative structures which have lower levels of glucosinolates, and that the impact of aphid feeding location would vary depending on the prey detoxification mechanism. To test these predictions, we conducted experiments to compare 1) glucosinolates profiles between B. brassicae and M. persicae reared on reproductive and vegetative canola structures, 2) aphid population growth on each structure, and 3) their subsequent impact on fitness traits of H. convergens. Results indicate that the population growth of both aphids was greater on reproductive structures, with B. brassicae having the highest population growth. B. brassicae reared on reproductive structures had the highest concentrations of glucosinolates, and the greatest adverse effects on H. convergens. These findings suggest that both aphid-prey species and feeding location on canola could influence populations of this predator and, thus, its potential for biological control of canola aphids.

Feeding guild determines strength of top-down forces in multitrophic system experiencing bottom-up constraints

Nitrogen (N) and water are crucial in crop production but increasingly scarce environmental resources. Reducing their inputs can affect the whole plant-arthropod community including biocontrol agents. In a multitrophic system, we studied the interaction of the bottom-up effects of moderately reduced N concentration and/or water supply as well as the top-down effects of pests of different feeding guilds on plant nutritional quality (N and carbon concentration), direct defense (alkaloids and phenolics), and indirect defense (plant volatile organic compounds); on herbivore performance and host quality (N and carbon) to parasitoids and the latter's performance. Studied organisms were tomato plants, the sap feeders Macrosiphum euphorbiae and Bemisia tabaci, the leaf chewers Tuta absoluta and Spodoptera littoralis, and the parasitic wasps Aphelinus abdominalis and Necremnus tutae. Resource limitation affected plant quality, triggering bottom-up effects on herbivore and parasitoid performance, except for T. absoluta and N. tutae. Feeding guild had a major influence: bottom-up effects were stronger on sap feeders; N effects were stronger on sap feeders while water effects were stronger with leaf chewers (S. littoralis). Top-down effects of leaf chewer herbivory partly attenuated bottom-up effects and partly suppressed plant defenses. Bottom-up effects weakened when cascading up trophic levels. In summary, the interaction between plants, pests, and beneficial insects was modulated by abiotic factors, affecting insect performance. Simultaneous abiotic and biotic impact shaped plant biochemistry depending on the feeding guild: the biotic top-down effect of leaf chewer herbivory attenuated the bottom-up effects of plant nutrition and hence dominated the plant biochemical profile whereas in sap feeder infested leaves, it corresponded to the abiotic impact. This study highlights the plant's finely tuned regulatory system facilitating response prioritization. It offers perspectives on how smart manipulation of plant nutrient solutions might save resources while maintaining efficient biocontrol in crop production.

Capsella bursa-pastoris Is a Key Overwintering Plant for Aphids in the Mediterranean Region

The reproduction of aphids depends to a great extent on their host plants, an integration that impacts on the successful expansion of overwintering populations. Therefore, a survey was conducted to evaluate the globally distributed Capsella bursa-pastoris as an overwintering host of economically important aphid species, their parasitoids and hyperparasitoids in the southern and western regions of Turkey from November to March in 2006 to 2013. During this survey, 395 samples of C. bursa-pastoris were collected with 25 aphid species recorded. Among aphids that feed on this host, Myzus persicae, Aphis gossypii, Rhopalosiphum padi, Aphis fabae, Aphis craccivora, Lipaphis erysimi, and Brevicoryne brassicae were the most frequently recorded. In total, 10,761 individual parasitoids were identified. Binodoxys angelicae, Aphidius colemani, Aphidius matricariae, Diaeretiella rapae, Ephedrus persicae, and Lysiphlebus confusus were the most abundant aphidiines that emerged from the aphids collected from C. bursa-pastoris. Alloxysta spp. (Hymenoptera: Cynipoidea), Chalcidoidea (unidentified at genus level), and Dendrocerus spp. (Hymenoptera: Ceraphronoidea) were identified as hyperparasitoids on the parasitoids. These findings indicate that C. bursa-pastoris is a key non-agricultural plant that significantly contributes to the overwintering of numerous aphids and their parasitoids, which should be given serious consideration when biological control strategies are designed.

Using plant chemistry to improve interactions between plants, herbivores and their natural enemies: challenges and opportunities

Plant secondary (or specialized) metabolites determine multitrophic interaction dynamics. Herbivore natural enemies exploit plant volatiles for host location and are negatively affected by plant defense chemicals that are transferred through herbivores. Recent work shows that herbivore natural enemies can evolve resistance to plant defense chemicals, and that generating plant defense resistance through forward evolution enhances their capacity to prey on herbivores. Here, we discuss how this knowledge can be used to engineer better biocontrol agents. We argue that herbivore natural enemies which are adapted to plant chemistry will likely enhance the efficacy of future pest control efforts. Detailed phenotyping and field experiments will be necessary to quantify costs and benefits of optimizing chemical links between plants and higher trophic levels.

Quantitative Proteomics and Phosphoproteomics Support a Role for Mut9-Like Kinases in Multiple Metabolic and Signaling Pathways in Arabidopsis

Protein phosphorylation is one of the most prevalent posttranslational modifications found in eukaryotic systems. It serves as a key molecular mechanism that regulates protein function in response to environmental stimuli. The Mut9-like kinases (MLKs) are a plant-specific family of Ser/Thr kinases linked to light, circadian, and abiotic stress signaling. Here we use quantitative phosphoproteomics in conjunction with global proteomic analysis to explore the role of the MLKs in daily protein dynamics. Proteins involved in light, circadian, and hormone signaling, as well as several chromatin-modifying enzymes and DNA damage response factors, were found to have altered phosphorylation profiles in the absence of MLK family kinases. In addition to altered phosphorylation levels, mlk mutant seedlings have an increase in glucosinolate metabolism enzymes. Subsequently, we show that a functional consequence of the changes to the proteome and phosphoproteome in mlk mutant plants is elevated glucosinolate accumulation and increased sensitivity to DNA damaging agents. Combined with previous reports, this work supports the involvement of MLKs in a diverse set of stress responses and developmental processes, suggesting that the MLKs serve as key regulators linking environmental inputs to developmental outputs.

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MUT9-LIKE KINASE mutant quantitative proteome and phosphoproteome measured.

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Changes to proteome and phosphoproteome are specific to genotype and environment.

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Loss of MLKs alters glucosinolate enzyme abundance and metabolism.

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Loss of MLKs increases plant sensitivity to UV radiation and DNA damage agents.

EgJUB1 and EgERF113 transcription factors as potential master regulators of defense response in Elaeis guineensis against the hemibiotrophic Ganoderma boninense

BACKGROUND

Hemibiotrophic pathogen such as the fungal pathogen Ganoderma boninense that is destructive to oil palm, manipulates host defense mechanism by strategically switching from biotrophic to necrotrophic phase. Our previous study revealed two distinguishable expression profiles of oil palm genes that formed the basis in deducing biotrophic phase at early interaction which switched to necrotrophic phase at a later stage of infection.

RESULTS

The present report is a continuing study from our previous published transcriptomic profiling of oil palm seedlings against G. boninense. We focused on identifying differentially expressed genes (DEGs) encoding transcription factors (TFs) from the same RNA-seq data; resulting in 106 upregulated and 108 downregulated TFs being identified. The DEGs are involved in four established defense-related pathways responsible for cell wall modification, reactive oxygen species (ROS)-mediated signaling, programmed cell death (PCD) and plant innate immunity. We discovered upregulation of JUNGBRUNNEN 1 (EgJUB1) during the fungal biotrophic phase while Ethylene Responsive Factor 113 (EgERF113) demonstrated prominent upregulation when the palm switches to defense against necrotrophic phase. EgJUB1 was shown to have a binding activity to a 19 bp palindromic SNBE1 element, WNNYBTNNNNNNNAMGNHW found in the promoter region of co-expressing EgHSFC-2b. Further in silico analysis of promoter regions revealed co-expression of EgJUB1 with TFs containing SNBE1 element with single nucleotide change at either the 5th or 18th position. Meanwhile, EgERF113 binds to both GCC and DRE/CRT elements promoting plasticity in upregulating the downstream defense-related genes. Both TFs were proven to be nuclear-localized based on subcellular localization experiment using onion epidermal cells.

CONCLUSION

Our findings demonstrated unprecedented transcriptional reprogramming of specific TFs potentially to enable regulation of a specific set of genes during different infection phases of this hemibiotrophic fungal pathogen. The results propose the intricacy of oil palm defense response in orchestrating EgJUB1 during biotrophic and EgERF113 during the subsequent transition to the necrotrophic phase. Binding of EgJUB1 to SNBE motif instead of NACBS while EgERF113 to GCC-box and DRE/CRT motifs is unconventional and not normally associated with pathogen infection. Identification of these phase-specific oil palm TFs is important in designing strategies to tackle or attenuate the progress of infection.

Isothiocyanate Derivatives of Glucosinolates as Efficient Natural Fungicides

Fungal pathogens on crops account for losses that exceed US$200 billion annually. At present, chemical fungicides are widely used in the agricultural industry. Many of these products have a detrimental effect on human and animal health and are consequently forbidden postharvest, especially in Europe. Despite efforts to develop natural crop protection, very few have been commercialized. We explored the physicochemical characteristics of (i) glucosinolate derivatives from the present study and previously published papers in the light of their known biological roles and (ii) fungitoxic glucosinolate derivatives compared with natural and chemical fungicides. We found that 13 out of 31 tested natural and semisynthetic isothiocyanates are efficient fungicides against widespread species of plant pathogens alone and in a synergistic manner. Interestingly, physicochemical characteristics of fungitoxic glucosinolate derivatives differ from those showing no activities or known for their insecticidal or insect-attractive properties. The comparison of physicochemical characteristics of natural and semisynthetic fungitoxic glucosinolate derivatives with other fungicides (natural, semisynthetic, and synthetic) revealed that isothiocyanate glucosinolate derivatives clustered with nonglucosinolate derivatives plant fungicides as well as with some synthetic ones. Most of the fungicides show high bioaccumulation potential and lipophilic properties that most likely allow them to go through membranes.

[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

Antixenosis, Antibiosis, and Potential Yield Compensatory Response in Barley Cultivars Exposed to Wheat Stem Sawfly (Hymenoptera: Cephidae) Under Field Conditions

Wheat stem sawfly, Cephus cinctus Norton, is an economically serious pest of cereals grown in North America. Barley cultivars were previously planted as resistant crops in rotations to manage C. cinctus, but due to increasing levels of injury to this crop, this is no longer a valid management tactic in Montana. Therefore, we aimed to understand antixenosis (behavioral preference), antibiosis (mortality), and potential yield compensation (increased productivity in response to stem injuries) in barley exposed to C. cinctus. We examined these traits in eight barley cultivars. Antixenosis was assessed by counting number of eggs per stem and antibiosis was assessed by counting infested stems, dead larvae, and stems cut by mature larvae. Potential yield compensation was evaluated by comparing grain yield from three categories of stem infestation: 1) uninfested, 2) infested with dead larva, and 3) infested cut by mature larva at crop maturity. We found the greatest number of eggs per infested stem (1.80 ± 0.04), the highest proportion of infested stems (0.63 ± 0.01), and the highest proportion of cut stems (0.33 ± 0.01) in 'Hockett'. Seven out of eight cultivars had greater grain weight for infested stems than for uninfested stems. These cultivars may have compensatory responses to larval feeding injury. Overall, these barley cultivars contain varying levels of antixenosis, antibiosis, and differing levels of yield compensation. Our results provide foundational knowledge on barley traits that will provide a framework to further develop C. cinctus resistant or tolerant barley cultivars.

Plant phenolics mediated bottom-up effects of elevated CO2 on Acyrthosiphon pisum and its parasitoid Aphidius avenae

Elevated concentrations of atmospheric CO₂ can alter plant secondary metabolites, which play important roles in the interactions among plants, herbivorous insects and natural enemies. However, few studies have examined the cascading effects of host plant secondary metabolites on tri-trophic interactions under elevated CO₂ (eCO₂ ). In this study, we determined the effects of eCO₂ on the growth and foliar phenolics of Medicago truncatula and the cascading effects on two color genotypes of Acyrthosiphon pisum (pink vs. green) and their parasitoid Aphidius avenae in the field open-top chambers. Our results showed that eCO₂ increased photosynthetic rate, nodule number, yield and the total phenolic content of M. truncatula. eCO₂ had contrasting effects on two genotypes of A. pisum; the green genotype demonstrated increased population abundance, fecundity, growth and feeding efficiency, while the pink genotype showed decreased fitness and these were closely associated with the foliar genstein content. Furthermore, eCO₂ decreased the parasitic rate of A. avenae independent of aphid genotypes. eCO₂ prolonged the emergence time and reduced the emergence rate and percentage of females when associated with the green genotype, but little difference, except for increased percentage of females, was observed in A. avenae under eCO₂ when associated with the pink genotype, indicating that parasitoids can perceive and discriminate the qualities of aphid hosts. We concluded that eCO₂ altered plant phenolics and thus the performance of aphids and parasitoids. Our results indicate that plant phenolics vary by different abiotic and biotic stimuli and could potentially deliver the cascading effects of eCO₂ to the higher trophic levels. Our results also suggest that the green genotype is expected to perform better in future eCO₂ because of decreased plant resistance after its infestation and decreased parasitic rate.

Impact of Mutations in Arabidopsis thaliana Metabolic Pathways on Polerovirus Accumulation, Aphid Performance, and Feeding Behavior

During the process of virus acquisition by aphids, plants respond to both the virus and the aphids by mobilizing different metabolic pathways. It is conceivable that the plant metabolic responses to both aggressors may be conducive to virus acquisition. To address this question, we analyze the accumulation of the phloem-limited polerovirus Turnip yellows virus (TuYV), which is strictly transmitted by aphids, and aphid's life traits in six Arabidopsis thaliana mutants (xth33, ss3-2, nata1, myc234, quad, atr1D, and pad4-1). We observed that mutations affecting the carbohydrate metabolism, the synthesis of a non-protein amino acid and the glucosinolate pathway had an effect on TuYV accumulation. However, the virus titer did not correlate with the virus transmission efficiency. Some mutations in A. thaliana affect the aphid feeding behavior but often only in infected plants. The duration of the phloem sap ingestion phase, together with the time preceding the first sap ingestion, affect the virus transmission rate more than the virus titer did. Our results also show that the aphids reared on infected mutant plants had a reduced biomass regardless of the mutation and the duration of the sap ingestion phase.

Dual-guild herbivory disrupts predator-prey interactions in the field

Plant defenses often mediate whether competing chewing and sucking herbivores indirectly benefit or harm one another. Dual-guild herbivory also can muddle plant signals used by specialist natural enemies to locate prey, further complicating the net impact of herbivore-herbivore interactions in naturally diverse settings. While dual-guild herbivore communities are common in nature, consequences for top-down processes are unclear, as chemically mediated tri-trophic interactions are rarely evaluated in field environments. Combining observational and experimental approaches in the open field, we test a prediction that chewing herbivores interfere with top-down suppression of phloem feeders on Brassica oleracea across broad landscapes. In a two-year survey of 52 working farm sites, we found that parasitoid and aphid densities on broccoli plants positively correlated at farms where aphids and caterpillars rarely co-occurred, but this relationship disappeared at farms where caterpillars commonly co-occurred. In a follow-up experiment, we compared single and dual-guild herbivore communities at four local farm sites and found that caterpillars (P. rapae) caused a 30% reduction in aphid parasitism (primarily by Diaeretiella rapae), and increased aphid colony (Brevicoryne brassicae) growth at some sites. Notably, in the absence of predators, caterpillars indirectly suppressed, rather than enhanced, aphid growth. Amid considerable ecological noise, our study reveals a pattern of apparent commensalism: herbivore-herbivore facilitation via relaxed top-down suppression. This work suggests that enemy-mediated apparent commensalism may override constraints to growth induced by competing herbivores in field environments, and emphasizes the value of placing chemically mediated interactions within their broader environmental and community contexts.

The preference-performance relationship as a means of classifying parasitoids according to their specialization degree

Host range in parasitoids could be described by the preference-performance hypothesis (PPH) where preference is defined as host acceptance and performance is defined as the sum of all species on which parasitoid offspring can complete their life cycle. The PPH predicts that highly suitable hosts will be preferred by ovipositing females. However, generalist parasitoids may not conform to this hypothesis if they attack a large range of hosts of varying suitability. Under laboratory conditions, we tested the PPH relationship of three aphid parasitoids currently considered as generalist species (Aphelinus abdominalis, Aphidius ervi, Diaeretiella rapae). As expected, the three parasitoids species showed low selectivity, i.e., females stung all aphid species encountered (at least in some extent). However, depending on the parasitoid species, only 42%-58% of aphid species enabled producing parasitoid offspring. We did not find a correlation between the extent of preference and the performance of three generalist aphid parasitoids. For A. ervi, host phylogeny is also important as females showed higher attack and developmental rates on hosts closely related to the most suitable one. In addition, traits such as (a) the presence of protective secondary endosymbionts, for example, Hamiltonella defensa detected in Aphis fabae and Metopolophium dirhodum and (b) the sequestration of plant toxins as defense mechanism against parasitism, for example, in Aphis nerii and Brevicoryne brassicae, were likely at play to some extent in narrowing parasitoid host range. The lack of PPH relationship involved a low selectivity leading to a high adaptability, as well as selection pressure; the combination of which enabled the production of offspring in a new host species or a new environment. Testing for PPH relationships in parasitoids may provide useful cues to classify parasitoids in terms of specialization degree.

The plastidial metabolite 2-C-methyl-D-erythritol-2,4-cyclodiphosphate modulates defence responses against aphids

Feeding by insect herbivores such as caterpillars and aphids induces plant resistance mechanisms that are mediated by the phytohormones jasmonic acid (JA) and salicylic acid (SA). These phytohormonal pathways often crosstalk. Besides phytohormones, methyl-D-erythriol-2,4-cyclodiphosphate (MEcPP), the penultimate metabolite in the methyl-D-erythritol-4-phosphate pathway, has been speculated to regulate transcription of nuclear genes in response to biotic stressors such as aphids. Here, we show that MEcPP uniquely enhances the SA pathway without attenuating the JA pathway. Arabidopsis mutant plants that accumulate high levels of MEcPP (hds3) are highly resistant to the cabbage aphid (Brevicoryne brassicae), whereas resistance to the large cabbage white caterpillar (Pieris brassicae) remains unaltered. Thus, MEcPP is a distinct signalling molecule that acts beyond phytohormonal crosstalk to induce resistance against the cabbage aphid in Arabidopsis. We dissect the molecular mechanisms of MEcPP mediating plant resistance against the aphid B. brassicae. This shows that MEcPP induces the expression of genes encoding enzymes involved in the biosynthesis of several primary and secondary metabolic pathways contributing to enhanced resistance against this aphid species. A unique ability to regulate multifaceted molecular mechanisms makes MEcPP an attractive target for metabolic engineering in Brassica crop plants to increase resistance to cabbage aphids.

Volatile β-Ocimene Can Regulate Developmental Performance of Peach Aphid Myzus persicae Through Activation of Defense Responses in Chinese Cabbage Brassica pekinensis

In nature, plants have evolved sophisticated defense mechanisms against the attack of pathogens and insect herbivores. Plant volatile-mediated plant-to-plant communication has been assessed in multitrophic systems in different plant species and different pest species. β-ocimene is recognized as an herbivore-induced plant volatile that play an important role in the chemical communication between plants and pests. However, it is still unclear whether β-ocimene can active the defense mechanism of Chinese cabbage Brassica pekinensis against the peach aphid Myzus persicae. In this study, we found that treatment of Chinese cabbage with β-ocimene inhibited the growth of M. persicae in terms of weight gain and reproduction. Moreover, β-ocimene treatment negatively influenced the feeding behavior of M. persicae by shortening the total feeding period and phloem ingestion and increasing the frequency of stylet puncture. When given a choice, winged aphids preferred to settle on healthy Chinese cabbage compared with β-ocimene-treated plants. In addition, performance of the parasitoid Aphidius gifuensis in terms of Y-tube olfaction and landings was better on β-ocimene-treated Chinese cabbage than on healthy plants. Furthermore, β-ocimene significantly increased the expression levels of salicylic acid and jasmonic acid marker genes and the accumulation of glucosinolates. Surprisingly, the transcriptional levels of detoxifying enzymes (CYP6CY3, CYP4, and GST) in aphids reared on β-ocimene-treated Chinese cabbage were significantly higher than those of aphids reared on healthy plants. In summary, our results indicated that β-ocimene can activate the defense response of Chinese cabbage against M. persicae, and that M. persicae can also adjust its detoxifying enzymes machinery to counter the host plant defense reaction.

Caterpillars induce jasmonates in flowers and alter plant responses to a second attacker

In nature, herbivorous insects and plant pathogens are generally abundant when plants are flowering. Thus, plants face a diversity of attackers during their reproductive phase. Plant responses to one attacker can interfere with responses to a second attacker, and phytohormones that orchestrate plant reproduction are also involved in resistance to insect and pathogen attack. We quantified phytohormonal responses of flowering plants exposed to single or dual attack and studied resistance mechanisms of plants in the flowering stage. Flowering Brassica nigra were exposed to either a chewing caterpillar, a phloem-feeding aphid or a bacterial pathogen, and plant hormonal responses were compared with dual attack situations. We quantified phytohormones in inflorescences and leaves, and determined the consequences of hormonal changes for components of direct and indirect plant resistance. Caterpillars were the main inducers of jasmonates in inflorescences, and the phytohormonal profile of leaves was not affected by either insect or pathogen attack. Dual attack increased plant resistance to caterpillars, but compromised resistance to aphids. Parasitoid performance was negatively correlated with the performance of their hosts. We conclude that plants prioritize resistance of reproductive tissues over vegetative tissues, and that a chewing herbivore species is the main driver of responses in flowering B. nigra.

Host Plant-Herbivore-Predator Interactions in Chrysoperla carnea (Neuroptera: Chrysopidae) and Myzus persicae (Homoptera: Aphididae) on Four Plant Species Under Laboratory Conditions

The common green lacewing, Chrysoperla carnea (Stephens) (Neuroptera: Chrysopidae), is a well-known biocontrol agent. The current study examined host plant-herbivore-predator interactions with C. carnea and Myzus persicae on four host plants (peach, almond, pepper, and potato). The experiments were carried out at 25 ± 1°C and 65 ± 5% RH at a photoperiod of 16:8 (L:D) h). Duration of the preadult growth period, adult longevity, fecundity, and population growth parameters were analyzed based on the age-stage, two-sex life table theory. The shortest and longest preadult developmental times of the predator were observed on the peach and potato, respectively. The highest and lowest predation rate, oviposition period, and male and female longevity of predator were also observed on the peach and potato, respectively. The lowest intrinsic rate of increase (r) and finite rate of increase (λ) were observed on the potato (0.1087 and 1.11 d-1, respectively) and the highest on the peach (0.1460 and 1.15 d-1, respectively). The maximum and minimum mean generation times (T) were 41.84 and 35.59 d in the potato and peach, respectively. Overall, peach was found to be a more appropriate host than the other host plants for development and predation fitness of C. carnea. These findings reveal that information on tritrophic interactions and subsequent life table evaluation of natural enemies improves integrated pest management programs.

Effects of intraspecific and intra-individual differences in plant quality on preference and performance of monophagous aphid species

Plant chemistry is one of the main drivers of herbivore distribution. Monophagous herbivore species are highly specialized, but even within their only host species the chemistry varies. The herbivore's choice is initially mainly guided by volatile plant compounds. Once on the plant, particularly for aphids the phloem quality affects their performance. However, little is known about the intraspecific and intra-individual variation in phloem sap and their influences on monophagous aphids. To determine potential mechanisms involved in aphid colonization, we tested the effects of intraspecific chemical variation in Tanacetum vulgare, which produces different chemotypes, on the preference of two monophagous aphid species. Moreover, we measured the performance of the aphids on different plant parts (stem close to the inflorescence, young and old leaves) of these chemotypes and analyzed their phloem sap composition. Both species preferred the β-thujone (THU) over the trans-carvyl acetate (CAR) chemotype in dual-choice assays. Survival of Macrosiphoniella tanacetaria was neither affected by intraspecific nor intra-individual variation, whereas the reproduction was highest on stems. In contrast, Uroleucon tanaceti survived and reproduced best on old leaves of the preferred chemotype. The sugar, organic acid and amino acid composition pronouncedly differed between phloem exudates of different plant parts, but less between chemotypes. Unexpectedly, high concentrations of amino acids did not necessarily enhance aphid performance. These different performance optima may cause niche differentiation and, therefore, enable co-existence. In conclusion, the tremendous variation in plant chemistry even within one species can affect the distribution of highly specialized aphids at various scales aphid species-specifically.

Both heavy metal-amendment of soil and aphid-infestation increase Cd and Zn concentrations in phloem exudates of a metal-hyperaccumulating plant

Plants that are able to hyperaccumulate heavy metals show increased concentrations of these metals in their leaf tissue. However, little is known about the concentrations of heavy metals and of organic defence metabolites in the phloem sap of these plants in response to either heavy metal-amendment of the soil or biotic challenges such as aphid-infestation. In this study, we investigated the effects of heavy metal-exposure and of aphid-infestation on phloem exudate composition of the metal hyperaccumulator species Arabidopsis halleri L. O'Kane & Al-Shehbaz (Brassicaceae). The concentrations of elements and of organic defence compounds, namely glucosinolates, were measured in phloem exudates of young and old (mature) leaves of plants challenged either by amendment of the soil with cadmium and zinc and/or by an infestation with the generalist aphid Myzus persicae. Metal-amendment of the soil led to increased concentrations of Cd and Zn, but also of two other elements and one indole glucosinolate, in phloem exudates. This enhanced defence in the phloem sap of heavy metal-hyperaccumulating plants can thus potentially act as effective protection against aphids, as predicted by the elemental defence hypothesis. Aphid-infestation also caused enhanced Cd and Zn concentrations in phloem exudates. This result provides first evidence that metal-hyperaccumulating plants can increase heavy metal concentrations tissue-specifically in response to an attack by phloem-sucking herbivores. Overall, the concentrations of most elements, including the heavy metals, and glucosinolates were higher in phloem exudates of young leaves than in those of old leaves. This defence distribution highlights that the optimal defence theory, which predicts more valuable tissue to be better defended, is applicable for both inorganic and organic defences.

Comparative venomics of Psyttalia lounsburyi and P. concolor, two olive fruit fly parasitoids: a hypothetical role for a GH1 β-glucosidase

Venom composition of parasitoid wasps attracts increasing interest - notably molecules ensuring parasitism success on arthropod pests - but its variation within and among taxa is not yet understood. We have identified here the main venom proteins of two braconid wasps, Psyttalia lounsburyi (two strains from South Africa and Kenya) and P. concolor, olive fruit fly parasitoids that differ in host range. Among the shared abundant proteins, we found a GH1 β-glucosidase and a family of leucine-rich repeat (LRR) proteins. Olive is extremely rich in glycoside compounds that are hydrolyzed by β-glucosidases into defensive toxic products in response to phytophagous insect attacks. Assuming that Psyttalia host larvae sequester ingested glycosides, the injected venom GH1 β-glucosidase could induce the release of toxic compounds, thus participating in parasitism success by weakening the host. Venom LRR proteins are similar to truncated Toll-like receptors and may possibly scavenge the host immunity. The abundance of one of these LRR proteins in the venom of only one of the two P. lounsburyi strains evidences intraspecific variation in venom composition. Altogether, venom intra- and inter-specific variation in Psyttalia spp. were much lower than previously reported in the Leptopilina genus (Figitidae), suggesting it might depend upon the parasitoid taxa.

How specialized volatiles respond to chronic and short-term physiological and shock heat stress in Brassica nigra

Brassicales release volatile glucosinolate breakdown products upon tissue mechanical damage, but it is unclear how the release of glucosinolate volatiles responds to abiotic stresses such as heat stress. We used three different heat treatments, simulating different dynamic temperature conditions in the field to gain insight into stress-dependent changes in volatile blends and photosynthetic characteristics in the annual herb Brassica nigra (L.) Koch. Heat stress was applied by either heating leaves through temperature response curve measurements from 20 to 40 °C (mild stress), exposing plants for 4 h to temperatures 25-44 °C (long-term stress) or shock-heating leaves to 45-50 °C. Photosynthetic reduction through temperature response curves was associated with decreased stomatal conductance, while the reduction due to long-term stress and collapse of photosynthetic activity after heat shock stress were associated with non-stomatal processes. Mild stress decreased constitutive monoterpene emissions, while long-term stress and shock stress resulted in emissions of the lipoxygenase pathway and glucosinolate volatiles. Glucosinolate volatile release was more strongly elicited by long-term stress and lipoxygenase product released by heat shock. These results demonstrate that glucosinolate volatiles constitute a major part of emission blend in heat-stressed B. nigra plants, especially upon chronic stress that leads to induction responses.

Glucose enhances indolic glucosinolate biosynthesis without reducing primary sulfur assimilation

The effect of glucose as a signaling molecule on induction of aliphatic glucosinolate biosynthesis was reported in our former study. Here, we further investigated the regulatory mechanism of indolic glucosinolate biosynthesis by glucose in Arabidopsis. Glucose exerted a positive influence on indolic glucosinolate biosynthesis, which was demonstrated by induced accumulation of indolic glucosinolates and enhanced expression of related genes upon glucose treatment. Genetic analysis revealed that MYB34 and MYB51 were crucial in maintaining the basal indolic glucosinolate accumulation, with MYB34 being pivotal in response to glucose signaling. The increased accumulation of indolic glucosinolates and mRNA levels of MYB34, MYB51, and MYB122 caused by glucose were inhibited in the gin2-1 mutant, suggesting an important role of HXK1 in glucose-mediated induction of indolic glucosinolate biosynthesis. In contrast to what was known on the function of ABI5 in glucose-mediated aliphatic glucosinolate biosynthesis, ABI5 was not required for glucose-induced indolic glucosinolate accumulation. In addition, our results also indicated that glucose-induced glucosinolate accumulation was due to enhanced sulfur assimilation instead of directed sulfur partitioning into glucosinolate biosynthesis. Thus, our data provide new insights into molecular mechanisms underlying glucose-regulated glucosinolate biosynthesis.

How herbivores coopt plant defenses: natural selection, specialization, and sequestration

We review progress in understanding sequestration by herbivorous insects, the use of plant chemical defenses for their own defense. We incorporate sequestration into the framework of plant-insect coevolution by integrating three hierarchical issues: (1) the relationship between dietary specialization and sequestration of plant defenses, (2) the physiological mechanisms involved in sequestration, and (3) how sequestration evolves via interactions between trophic levels. Sequestration is often associated with specialization, but even specialized sequestration is not an evolutionary dead-end. Despite considerable progress in understanding physiological mechanisms, detailed knowledge of how plant toxins cross the insect gut epithelium is still largely lacking. Sequestration is likely a major vehicle for coevolutionary escalation in speciose plant-insect-predator interactions, suggesting that a strictly bitrophic view is untenable.

Macroevolutionary patterns of glucosinolate defense and tests of defense-escalation and resource availability hypotheses

We explored macroevolutionary patterns of plant chemical defense in Streptanthus (Brassicaceae), tested for evolutionary escalation of defense, as predicted by Ehrlich and Raven's plant-herbivore coevolutionary arms-race hypothesis, and tested whether species inhabiting low-resource or harsh environments invest more in defense, as predicted by the resource availability hypothesis (RAH). We conducted phylogenetically explicit analyses using glucosinolate profiles, soil nutrient analyses, and microhabitat bareness estimates across 30 species of Streptanthus inhabiting varied environments and soils. We found weak to moderate phylogenetic signal in glucosinolate classes and no signal in total glucosinolate production; a trend toward evolutionary de-escalation in the numbers and diversity of glucosinolates, accompanied by an evolutionary increase in the proportion of aliphatic glucosinolates; some support for the RAH relative to soil macronutrients, but not relative to serpentine soil use; and that the number of glucosinolates increases with microhabitat bareness, which is associated with increased herbivory and drought. Weak phylogenetic signal in chemical defense has been observed in other plant systems. A more holistic approach incorporating other forms of defense might be necessary to confidently reject escalation of defense. That defense increases with microhabitat bareness supports the hypothesis that habitat bareness is an underappreciated selective force on plants in harsh environments.

Enemy-free space promotes maintenance of host races in an aphid species

The enormous biodiversity of herbivorous insects may arise from ecological speciation via continuous host-plant switches. Whether such switches are successful depends on the trade-off between different selection pressures that act on herbivores. Decreased herbivore performance due to suboptimal nutrition might be compensated for by a reduced natural enemy pressure. As a consequence, an "enemy-free space" on a certain plant might facilitate host-plant switches and maintain biotypes. To test this hypothesis, we used the pea aphid (Acyrthosiphon pisum) complex, which consists of at least 11 genetically distinct host races that are native to specific legume host plants but can all develop on the universal host plant Vicia faba. Three A. pisum host races native to Trifolium pratense, Pisum sativum, and Medicago sativa were investigated in experiments on their respective host plants and on the universal host plant V. faba. We found that hoverflies preferred to oviposit on P. sativum and the universal host V. faba. Since feeding by hoverfly larvae suppressed aphid population growth on these host plants, the native hosts M. sativa and T. pratense provided enemy-free space for the respective A. pisum races. Mobile predators, such as ants and ladybird beetles, preferred Pisum race aphids on V. faba over P. sativum. Thus, all three of the native host plants studied supply enemy-free space for A. pisum compared to the universal host V. faba. Reducing encounters between aphid races on V. faba would reduce gene flow among them and could contribute to maintaining the host races.

Aphids Pick Their Poison: Selective Sequestration of Plant Chemicals Affects Host Plant Use in a Specialist Herbivore

In some plant-insect interactions, specialist herbivores exploit the chemical defenses of their food plant to their own advantage. Brassica plants produce glucosinolates that are broken down into defensive toxins when tissue is damaged, but the specialist aphid, Brevicoryne brassicae, uses these chemicals against its own natural enemies by becoming a "walking mustard-oil bomb". Analysis of glucosinolate concentrations in plant tissue and associated aphid colonies reveals that not only do aphids sequester glucosinolates, but they do so selectively. Aphids specifically accumulate sinigrin to high concentrations while preferentially excreting a structurally similar glucosinolate, progoitrin. Surveys of aphid infestation in wild populations of Brassica oleracea show that this pattern of sequestration and excretion maps onto host plant use. The probability of aphid infestation decreases with increasing concentrations of progoitrin in plants. Brassica brassicae, therefore, appear to select among food plants according to plant secondary metabolite profiles, and selectively store only some compounds that are used against their own enemies. The results demonstrate chemical and behavioral mechanisms that help to explain evidence of geographic patterns and evolutionary dynamics in Brassica-aphid interactions.

Antennal gustatory perception and behavioural responses in Trissolcus brochymenae females

Trissolcus brochymenae (Hymenoptera: Platygastridae) is an egg parasitoid that could be used to control stink bugs like Murgantia histrionica (Heteroptera: Pentatomidae), a pest of brassicaceous crops. Before laying their eggs, parasitoid females spend considerable time examining the substrate with their antennae, which are also used during feeding and mating behaviours. This suggests that contact chemoreception plays a prominent role in many aspects of parasitoid ecology. Therefore, we examined the sensitivity of antennal taste sensilla chaetica to several classical tastants including those that are appetitive or aversive. In addition we studied the taste preferences of wasps when presented with these chemicals alone or mixed. The sensilla chaetica of T. brochymenae responded to serial concentrations of sucrose, salts, and quinine, but no concentration-dependent effect was observed when testing sinigrin, a secondary metabolite found in many brassicaceae. However, both sinigrin and quinine inhibited responses to 0.1 M sucrose when mixed with this sugar. Behavioural taste preference assays confirmed that wasps showed a dose dependent preference for sucrose over agarose. In addition, a behavioural avoidance of sucrose solutions containing quinine was observed. This effect was not observed when sinigrin was used as a feeding deterrent. In the two-choice tests the wasp did not discriminate between sucrose solutions mixed with salts and sucrose alone. Further no preference for salts or sinigrin compared to agarose alone was observed. This work represents the first step towards the identification of gustatory receptor neurons implicated in the detection of different types of chemical cues in egg parasitoids.

Ecological Interactions Affecting the Efficacy of Aphidius colemani in Greenhouse Crops

Aphidius colemani Viereck (Hymenoptera: Braconidae) is a solitary endoparasitoid used for biological control of many economically important pest aphids. Given its widespread use, a vast array of literature on this natural enemy exists. Though often highly effective for aphid suppression, the literature reveals that A. colemani efficacy within greenhouse production systems can be reduced by many stressors, both biotic (plants, aphid hosts, other natural enemies) and abiotic (climate and lighting). For example, effects from 3rd and 4th trophic levels (fungal-based control products, hyperparasitoids) can suddenly decimate A. colemani populations. But, the most chronic negative effects (reduced parasitoid foraging efficiency, fitness) seem to be from stressors at the first trophic level. Negative effects from the 1st trophic level are difficult to mediate since growers are usually constrained to particular plant varieties due to market demands. Major research gaps identified by our review include determining how plants, aphid hosts, and A. colemani interact to affect the net aphid population, and how production conditions such as temperature, humidity and lighting affect both the population growth rate of A. colemani and its target pest. Decades of research have made A. colemani an essential part of biological control programs in greenhouse crops. Future gains in A. colemani efficacy and aphid biological control will require an interdisciplinary, systems approach that considers plant production and climate effects at all trophic levels.

Mechanisms and ecological consequences of plant defence induction and suppression in herbivore communities

BACKGROUND

Plants are hotbeds for parasites such as arthropod herbivores, which acquire nutrients and energy from their hosts in order to grow and reproduce. Hence plants are selected to evolve resistance, which in turn selects for herbivores that can cope with this resistance. To preserve their fitness when attacked by herbivores, plants can employ complex strategies that include reallocation of resources and the production of defensive metabolites and structures. Plant defences can be either prefabricated or be produced only upon attack. Those that are ready-made are referred to as constitutive defences. Some constitutive defences are operational at any time while others require activation. Defences produced only when herbivores are present are referred to as induced defences. These can be established via de novo biosynthesis of defensive substances or via modifications of prefabricated substances and consequently these are active only when needed. Inducibility of defence may serve to save energy and to prevent self-intoxication but also implies that there is a delay in these defences becoming operational. Induced defences can be characterized by alterations in plant morphology and molecular chemistry and are associated with a decrease in herbivore performance. These alterations are set in motion by signals generated by herbivores. Finally, a subset of induced metabolites are released into the air as volatiles and function as a beacon for foraging natural enemies searching for prey, and this is referred to as induced indirect defence.

SCOPE

The objective of this review is to evaluate (1) which strategies plants have evolved to cope with herbivores and (2) which traits herbivores have evolved that enable them to counter these defences. The primary focus is on the induction and suppression of plant defences and the review outlines how the palette of traits that determine induction/suppression of, and resistance/susceptibility of herbivores to, plant defences can give rise to exploitative competition and facilitation within ecological communities "inhabiting" a plant.

CONCLUSIONS

Herbivores have evolved diverse strategies, which are not mutually exclusive, to decrease the negative effects of plant defences in order to maximize the conversion of plant material into offspring. Numerous adaptations have been found in herbivores, enabling them to dismantle or bypass defensive barriers, to avoid tissues with relatively high levels of defensive chemicals or to metabolize these chemicals once ingested. In addition, some herbivores interfere with the onset or completion of induced plant defences, resulting in the plant's resistance being partly or fully suppressed. The ability to suppress induced plant defences appears to occur across plant parasites from different kingdoms, including herbivorous arthropods, and there is remarkable diversity in suppression mechanisms. Suppression may strongly affect the structure of the food web, because the ability to suppress the activation of defences of a communal host may facilitate competitors, whereas the ability of a herbivore to cope with activated plant defences will not. Further characterization of the mechanisms and traits that give rise to suppression of plant defences will enable us to determine their role in shaping direct and indirect interactions in food webs and the extent to which these determine the coexistence and persistence of species.

Plant-modified trophic interactions

Plants can modify the interactions between herbivorous insects and their natural enemies in various ways. Chemical defences from the plants against herbivores may in fact harm the latter's natural enemies, thereby weakening the trophic interaction. On the other hand, volatile chemicals produced by the plant in response to herbivory may attract natural enemies, thereby strengthening the interaction. Recent research shows that effects of plants on insect interactions are not curious phenomena confined to a few specialist species but rather that they are ubiquitous in terrestrial ecosystems and often involve complex interactions among many species. The major challenge now is to study how the commonly reported short-term effects of plants affect long term dynamics of insect interactions in the context of complex natural communities.

Direct and indirect chemical defences against insects in a multitrophic framework

Plant secondary metabolites play an important role in mediating interactions with insect herbivores and their natural enemies. Metabolites stored in plant tissues are usually investigated in relation to herbivore behaviour and performance (direct defence), whereas volatile metabolites are often studied in relation to natural enemy attraction (indirect defence). However, so-called direct and indirect defences may also affect the behaviour and performance of the herbivore's natural enemies and the natural enemy's prey or hosts, respectively. This suggests that the distinction between these defence strategies may not be as black and white as is often portrayed in the literature. The ecological costs associated with direct and indirect chemical defence are often poorly understood. Chemical defence traits are often studied in two-species interactions in highly simplified experiments. However, in nature, plants and insects are often engaged in mutualistic interactions with microbes that may also affect plant secondary chemistry. Moreover, plants are challenged by threats above- and belowground and herbivory may have consequences for plant-insect multitrophic interactions in the alternative compartment mediated by changes in plant secondary chemistry. These additional associations further increase the complexity of interaction networks. Consequently, the effect of a putative defence trait may be under- or overestimated when other interactions are not considered.

Plutella xylostella (L.) infestations at varying temperatures induce the emission of specific volatile blends by Arabidopsis thaliana (L.) Heynh

The effect of combined abiotic and biotic factors on plant volatile organic compound (VOC) emissions is poorly understood. This study evaluated the VOC emissions produced by Arabidopsis thaliana (L.) Col-0 subjected to 3 temperature regimes (17, 22, and 27°C) in the presence and absence of Plutella xylostella larvae over 2 time intervals (0-4 and 4-8 h), in comparison to control plants. The analyses of VOCs emitted by Arabidopsis plants were made by headspace solid phase microextraction (HS-SPME) coupled with gas chromatography-mass spectrometry (GC-MS). It was found that certain volatile groups (e.g., alcohols, ketones, aldehydes, and terpenes) are induced by both single factors (temperature or larval infestation) and combined factors (temperature and larvae interactions), whereas other volatile groups (e.g., isothiocyanates [ITCs] and nitrile) were specific to the experimental conditions. ITCs (mainly 4-methylpentyl isothiocyanate) were emitted from plants subjected to larval infestation at 17 and 27°C after the 2 time intervals. The proportions of sulfides (mainly dimethyl disulfide) and 4-(methylthio) butanenitrile were significantly higher on herbivore-infested plants at 22°C compared to the other treatments. Overall, our findings indicate that changes in all experimental conditions caused significant changes to the VOC emissions of Arabidopsis plants. Therefore, the interaction between temperature and larval feeding may represent an important factor determining the variability of volatile emissions by plants subjected to multiple simultaneous factors.

AtMYB44 regulates resistance to the green peach aphid and diamondback moth by activating EIN2-affected defences in Arabidopsis

Recently we showed that the transcription activator AtMYB44 regulates expression of EIN2, a gene essential for ethylene signalling and insect resistance, in Arabidopsis thaliana (Arabidopsis). To link the transactivation with insect resistance, we investigated the wild-type and atmyb44 mutant plants, genetically Complemented atmyb44 (Catmyb44) and AtMYB44-Overexpression Transgenic Arabidopsis (MYB44OTA). We found that AtMYB44 played a critical role in Arabidopsis resistance to the phloem-feeding generalist green peach aphid (Myzus persicae Sulzer) and leaf-chewing specialist caterpillar diamondback moth (Plutella xylostella L.). AtMYB44 was required not only for the development of constitutive resistance but also for the induction of resistance by both herbivorous insects. Levels of constitutive and herbivore-induced resistance were consistent with corresponding amounts of the AtMYB44 protein constitutively produced in MYB44OTA and induced by herbivory in Catmyb44. In both cases, AtMYB44 promoted EIN2 expression to a greater extent in MYB44OTA than in Catmyb44. However, AtMYB44-promoted EIN2 expression was arrested with reduced resistance levels in the EIN2-deficient Arabidopsis mutant ein2-1 and the MYB44OTA ein2-1 hybrid. In the different plant genotypes, only MYB44OTA constitutively displayed phloem-based defences, which are specific to phloem-feeding insects, and robust expression of genes involved in the biosynthesis of glucosinolates, which are the secondary plant metabolites known as deterrents to generalist herbivores. Phloem-based defences and glucosinolate-related gene expression were not detected in ein2-1 and MYB44OTA ein2-1. These results establish a genetic connection between the regulatory role of AtMYB44 in EIN2 expression and the development of Arabidopsis resistance to insects.

Heterodera schachtii nematodes interfere with aphid-plant relations on Brassica oleracea

Aboveground and belowground herbivore species modify plant defense responses differently. Simultaneous attack can lead to non-additive effects on primary and secondary metabolite composition in roots and shoots. We previously found that aphid (Brevicoryne brassicae) population growth on Brassica oleracea was reduced on plants that were infested with nematodes (Heterodera schachtii) prior (4 weeks) to aphid infestation. Here, we examined how infection with root-feeding nematodes affected primary and secondary metabolites in the host plant and whether this could explain the increase in aphid doubling time from 3.8 to 6.7 days. We hypothesized that the effects of herbivores on plant metabolites would depend on the presence of the other herbivore and that nematode-induced changes in primary metabolites would correlate with reduced aphid performance. Total glucosinolate concentration in the leaves was not affected by nematode presence, but the composition of glucosinolates shifted, as gluconapin concentrations were reduced, while gluconapoleiferin concentrations increased in plants exposed to nematodes. Aphid presence increased 4-methoxyglucobrassicin concentrations in leaves, which correlated positively with the number of aphids per plant. Nematodes decreased amino acid and sugar concentrations in the phloem. Aphid population doubling time correlated negatively with amino acids and glucosinolate levels in leaves, whereas these correlations were non-significant when nematodes were present. In conclusion, the effects of an herbivore on plant metabolites were independent of the presence of another herbivore. Nematode presence reduced aphid population growth and disturbed feeding relations between plants and aphids.

Can caterpillar density or host-plant quality explain host-plant-related parasitism of a generalist forest caterpillar assemblage?

Herbivore-carnivore interactions are influenced by the plants on which herbivores feed. Accordingly, dietary generalist herbivores have been shown to experience differential risk of mortality from carnivores on different host-plant species. Here, we investigate whether caterpillar density and host-plant quality play a role in driving variation in generalist forest caterpillar mortality from insect parasitoids using a large-scale, multi-year observational study. A total of 4,500 polyphagous caterpillars were collected from eight host-tree species in Connecticut deciduous forests over 5 years, and frequencies of mortality from insect parasitoids (flies and wasps) were compared across the eight host-plant species for the entire generalist caterpillar assemblage (76 species). Separate comparisons were made using seven numerically dominant generalist species, allowing us to account for variation in caterpillar species-specific parasitism risk. We find significant variation in parasitism frequencies of generalist caterpillars across the eight host-plant species when accounting for variation in caterpillar density. We find no support for an influence of caterpillar density on parasitism and no clear evidence for an effect of host-plant quality on parasitism. Therefore, the results of this study discount the hypotheses that variation in caterpillar density and host-plant quality are responsible for variation in parasitism frequencies across host-plant species. Instead, our findings point to other plant-related characteristics, such as plant-derived parasitoid attractants, which may have robust, community-wide effects.

Genetic engineering of plant volatile terpenoids: effects on a herbivore, a predator and a parasitoid

BACKGROUND

Most insect-resistant transgenic crops employ toxins to control pests. A novel approach is to enhance the effectiveness of natural enemies by genetic engineering of the biosynthesis of volatile organic compounds (VOCs). Before the commercialisation of such transgenic plants can be pursued, detailed fundamental studies of their effects on herbivores and their natural enemies are necessary. The linalool/nerolidol synthase gene FaNES1 was constitutively expressed from strawberry in three Arabidopsis thaliana accessions, and the behaviour of the aphid Brevicoryne brassicae L., the parasitoid Diaeretiella rapae McIntosh and the predator Episyrphus balteatus de Geer was studied.

RESULTS

Transgenic FaNES1-expressing plants emitted (E)-nerolidol and larger amounts of (E)-DMNT and linalool. Brevicoryne brassicae was repelled by the transgenic lines of two of the accessions, whereas its performance was not affected. Diaeretiella rapae preferred aphid-infested transgenic plants over aphid-infested wild-type plants for two of the accessions. In contrast, female E. balteatus predators did not differentiate between aphid-infested transgenic or wild-type plants.

CONCLUSION

The results indicate that the genetic engineering of plants to modify their emission of VOCs holds considerable promise for facilitating biological control of herbivores. Validation for crop plants is a necessary next step to assess the usefulness of modified volatile emission in integrated pest management.

Crosstalk between above- and belowground herbivores is mediated by minute metabolic responses of the host Arabidopsis thaliana

Plants are frequently under attack by multiple herbivores and can be infested at their shoots as well as their roots. As a consequence, plant metabolites are readily induced, mediated by phytohormones such as salicylic acid and jasmonic acid. Thereby, cross-talk between signal transduction pathways may occur if different herbivores attack the plant simultaneously. In turn, modifications in the plant metabolic pattern can affect herbivores infesting local and systemic tissue. Here, an integrative approach combining metabolomics and performance experiments was used to study the induction of plant metabolites in Arabidopsis thaliana by the specialist aphid Brevicoryne brassicae feeding on shoots and the generalist nematode Heterodera schachtii infesting root tissue. In contrast to most other studies, low infestation rates typical for the decisive early stages of infestation were used. Moreover, the consequences of induction responses on plant-mediated indirect interactions between these herbivores were investigated. In aphid-treated plants, several metabolites including glucosinolates, important defence compounds of Brassicaceae, were reduced in the shoot, but only minute changes took part in the systemic root tissue. Primary metabolites as well as phytohormones were not altered 3 days post infestation. In contrast, nematodes did not evoke significant metabolic alterations locally or systemically. In accordance, nematode presence did not affect aphid population growth, whereas aphids mediated a considerably reduced nematode infestation. These results demonstrate that plants respond in a very fine-tuned way to different challenges. Although they show only minute systemic responses to low herbivore stress, these changes can have pronounced effects on plant-mediated interactions between herbivores.