Feeding-induced rearrangement of green leaf volatiles reduces moth oviposition

The olfactory system of Pieris brassicae caterpillars: from receptors to glomeruli

The olfactory system of adult lepidopterans is among the best described neuronal circuits. However, comparatively little is known about the organization of the olfactory system in the larval stage of these insects. Here, we explore the expression of olfactory receptors and the organization of olfactory sensory neurons in caterpillars of Pieris brassicae, a significant pest species in Europe and a well-studied species for its chemical ecology. To describe the larval olfactory system in this species, we first analyzed the head transcriptome of third-instar larvae (L3) and identified 16 odorant receptors (ORs) including the OR coreceptor (Orco), 13 ionotropic receptors (IRs), and 8 gustatory receptors (GRs). We then quantified the expression of these 16 ORs in different life stages, using qPCR, and found that the majority of ORs had significantly higher expression in the L4 stage than in the L3 and L5 stages, indicating that the larval olfactory system is not static throughout caterpillar development. Using an Orco-specific antibody, we identified all olfactory receptor neurons (ORNs) expressing the Orco protein in L3, L4, and L5 caterpillars and found a total of 34 Orco-positive ORNs, distributed among three sensilla on the antenna. The number of Orco-positive ORNs did not differ among the three larval instars. Finally, we used retrograde axon tracing of the antennal nerve and identified a mean of 15 glomeruli in the larval antennal center (LAC), suggesting that the caterpillar olfactory system follows a similar design as the adult olfactory system, although with a lower numerical redundancy. Taken together, our results provide a detailed analysis of the larval olfactory neurons in P. brassicae, highlighting both the differences as well as the commonalities with the adult olfactory system. These findings contribute to a better understanding of the development of the olfactory system in insects and its life-stage-specific adaptations.

A salivary GMC oxidoreductase of Manduca sexta re-arranges the green leaf volatile profile of its host plant

Green leaf volatiles (GLVs) are short-chain oxylipins that are emitted from plants in response to stress. Previous studies have shown that oral secretions (OS) of the tobacco hornworm Manduca sexta, introduced into plant wounds during feeding, catalyze the re-arrangement of GLVs from Z-3- to E-2-isomers. This change in the volatile signal however is bittersweet for the insect as it can be used by their natural enemies, as a prey location cue. Here we show that (3Z):(2E)-hexenal isomerase (Hi-1) in M. sexta's OS catalyzes the conversion of the GLV Z-3-hexenal to E-2-hexenal. Hi-1 mutants that were raised on a GLV-free diet showed developmental disorders, indicating that Hi-1 also metabolizes other substrates important for the insect's development. Phylogenetic analysis placed Hi-1 within the GMCβ-subfamily and showed that Hi-1 homologs from other lepidopterans could catalyze similar reactions. Our results indicate that Hi-1 not only modulates the plant's GLV-bouquet but also functions in insect development.

A green leaf volatile, (Z)-3-hexenyl-acetate, mediates differential oviposition by Spodoptera frugiperda on maize and rice

BACKGROUND

Insects rely on chemosensory perception, mainly olfaction, for the location of mates, food sources, and oviposition sites. Plant-released volatile compounds guide herbivorous insects to search for and locate their host plants, further helping them to identify suitable positions for oviposition. The fall armyworm Spodoptera frugiperda (S. frugiperda) was found to invade China in 2019 and has since seriously threatened multiple crops, particularly maize and rice. However, the chemical and molecular mechanisms underlying oviposition preference in this pest are not fully understood. Here, the oviposition preference of S. frugiperda on maize and rice plants was investigated.

RESULTS

GC-EAD and GC-MS/MS techniques were used to identify the antennally active volatiles from maize and rice plants. The attraction and oviposition stimulation of identified components to female adults were tested in both laboratory and field settings. The odorant receptors (ORs) on female antennae were expressed in Xenopus oocytes, and their functions evaluated by RNAi. Ten and eleven compounds of maize and rice plants, respectively, were identified to possess electrophysiological activity from headspace volatiles. Among these compounds, (Z)-3-hexenyl-acetate specifically presented in maize volatiles was found to play a critical role in attracting females and stimulating oviposition compared to rice volatiles. Among the cloned ORs on the antennae of both sexes, SfruOR23 with highly female-biased expression mediated the responses of females to (Z)-3-hexenyl-acetate. Knockdown of SfruOR23 using RNAi markedly reduced the electrophysiological response of female antennae and oviposition preference to the compound.

CONCLUSIONS

(Z)-3-Hexenyl-acetate is a key volatile mediating the host and oviposition preference of S. frugiperda on maize. The olfactory receptor of (Z)-3-hexenyl-acetate was identified to be SfruOR23, which is mainly expressed in the antennae of S. frugiperda.

Understanding of formation and change of chiral aroma compounds from tea leaf to tea cup provides essential information for tea quality improvement

Abundant secondary metabolites endow tea with unique quality characteristics, among which aroma is the core component of tea quality. The ratio of chiral isomers of aroma compounds greatly affects the flavor of tea leaves. In this paper, we review the progress of research on chiral aroma compounds in tea. With the well-established GC-MS methods, the formation of, and changes in, the chiral configuration of tea aroma compounds during the whole cycle of tea leaves from the plant to the tea cup has been studied in detail. The ratio of aroma chiral isomers varies among different tea varieties and finished teas. Enzymatic reactions involving tea aroma synthases and glycoside hydrolases participate the formation of aroma compound chiral isomers during tea tree growth and tea processing. Non-enzymatic reactions including environmental factors such as high temperature and microbial fermentation involve in the change of aroma compound chiral isomers during tea processing and storage. In the future, it will be interesting to determine how changes in the proportions of chiral isomers of aroma compounds affect the environmental adaptability of tea trees; and to determine how to improve tea flavor by modifying processing methods or targeting specific genes to alter the ratio of chiral isomers of aroma compounds.

Root volatile profiles and herbivore preference are mediated by maize domestication, geographic spread, and modern breeding

Domestication affected the abundances and diversity of maize root volatiles more than northward spread and modern breeding, and herbivore preference for roots was correlated with volatile diversity and herbivore resistance. Studies show that herbivore defenses in crops are mediated by domestication, spread, and breeding, among other human-driven processes. They also show that those processes affected chemical communication between crop plants and herbivores. We hypothesized that (i) preference of the herbivore (Diabrotica virgifera virgifera) larvae for embryonic roots of maize (Zea mays mays) would increase and (ii) root volatile diversity would decrease with the crop's domestication, northward spread to present-day USA, and modern breeding. We used Balsas teosinte (Zea mays parviglumis), Mexican and USA landrace maizes, and US inbred maize lines to test these hypotheses. We found that herbivore preference and volatile diversity increased with maize domestication and northward spread but decreased with modern breeding. Additionally, we found that the abundances of single volatiles did not consistently increase or decrease with maize domestication, spread, and breeding; rather, volatiles grouped per their abundances were differentially affected by those processes, and domestication had the greatest effects. Altogether, our results suggested that: the herbivore's preference for maize roots is correlated with volatile diversity and herbivore resistance; changes in abundances of individual volatiles are evident at the level of volatile groups; and maize domestication, but not spread and breeding, affected the abundances of some green leaf volatiles and sesquiterpenes/sesquiterpenoids. In part, we discussed our results in the context of herbivore defense evolution when resources for plant growth and defense vary across environments. We suggested that variability in relative abundance of volatiles may be associated with their local, functional relevance across wild and agricultural environments.

Mutualism has its limits: consequences of asymmetric interactions between a well-defended plant and its herbivorous pollinator

Concern for pollinator health often focuses on social bees and their agricultural importance at the expense of other pollinators and their ecosystem services. When pollinating herbivores use the same plants as nectar sources and larval hosts, ecological conflicts emerge for both parties, as the pollinator's services are mitigated by herbivory and its larvae are harmed by plant defences. We tracked individual-level metrics of pollinator health-growth, survivorship, fecundity-across the life cycle of a pollinating herbivore, the common hawkmoth, Hyles lineata, interacting with a rare plant, Oenothera harringtonii, that is polymorphic for the common floral volatile (R)-(-)-linalool. Linalool had no impact on floral attraction, but its experimental addition suppressed oviposition on plants lacking linalool. Plants showed robust resistance against herbivory from leaf-disc to whole-plant scales, through poor larval growth and survivorship. Higher larval performance on other Oenothera species indicates that constitutive herbivore resistance by O. harringtonii is not a genus-wide trait. Leaf volatiles differed among populations of O. harringtonii but were not induced by larval herbivory. Similarly, elagitannins and other phenolics varied among plant tissues but were not herbivore-induced. Our findings highlight asymmetric plant-pollinator interactions and the importance of third parties, including alternative larval host plants, in maintaining pollinator health. This article is part of the theme issue 'Natural processes influencing pollinator health: from chemistry to landscapes'.

Unique neural coding of crucial versus irrelevant plant odors in a hawkmoth

The sense of smell is pivotal for nocturnal moths to locate feeding and oviposition sites. However, these crucial resources are often rare and their bouquets are intermingled with volatiles emanating from surrounding ‘background’ plants. Here, we asked if the olfactory system of female hawkmoths, Manduca sexta, could differentiate between crucial and background cues. To answer this question, we collected nocturnal headspaces of numerous plants in a natural habitat of M. sexta. We analyzed the chemical composition of these headspaces and used them as stimuli in physiological experiments at the antenna and in the brain. The intense odors of floral nectar sources evoked strong responses in virgin and mated female moths, most likely enabling the localization of profitable flowers at a distance. Bouquets of larval host plants and most background plants, in contrast, were subtle, thus potentially complicating host identification. However, despite being subtle, antennal responses and brain activation patterns evoked by the smell of larval host plants were clearly different from those evoked by other plants. Interestingly, this difference was even more pronounced in the antennal lobe of mated females, revealing a status-dependent tuning of their olfactory system towards oviposition sites. Our study suggests that female moths possess unique neural coding strategies to find not only conspicuous floral cues but also inconspicuous bouquets of larval host plants within a complex olfactory landscape.

The herbivore-induced plant volatile tetradecane enhances plant resistance to Holotrichia parallela larvae in maize roots

BACKGROUND

Many herbivore-induced volatiles have been proven to act as signaling compounds to regulate nearby plant defense responses. However, the precise roles of key volatiles produced by maize roots after Holotrichia parallela larva feeding remain largely unknown.

RESULTS

We investigated changes in phytohormones and volatiles in maize roots after H. parallela larval infestation. Marked increases in the phytohormone jasmonic acid (JA) and the volatiles jasmone and tetradecane were induced by herbivores, whereas the salicylic acid content decreased. In addition, pre-exposure to tetradecane markedly increased the levels of the stress hormone JA, its precursors and derivatives, and related gene expression. In addition, pre-exposure altered the production of defensive benzoxazinoid secondary metabolites, resulting in increased plant resistance to H. parallela larvae. Plants pre-exposed to jasmone did not differ from control plants. In addition, bioassays showed that H. parallela larval growth was suppressed by feeding maize roots after pre-exposure to tetradecane.

CONCLUSION

These results demonstrate that tetradecane may function as a potent defense induction signal that prepares neighboring plants for incoming attacks. © 2021 Society of Chemical Industry.

Elicitation of (E)-2-Hexenal and 2,3-Butanediol on the Bioactive Compounds in Adventitious Roots of Astragalus membranaceus var. mongholicus

This study aimed to investigate the elicitation of volatile organic compounds (E)-2-hexenal and 2,3-butanediol on bioactive metabolites in Astragalus membranaceus var. mongholicus adventitious root cultures by adding them into the medium. The experiment was performed for 72 h and the roots were dynamically sampled for quantification of representative astragaloside IV, calycosin-7-O-β-d-glucoside (CG), ononin, and the gene expression. Compared with the controls, the combination of 2,3-butanediol and (E)-2-hexenal advanced the peak accumulation of astragaloside IV and was the most effective, but their individual application delayed it. Meanwhile, 2,3-butanediol and (E)-2-hexenal had no obviously promoting effect on the production of CG and ononin but chronologically changed their accumulation patterns. The underlying mechanism was uncovered by the correlation analysis between the metabolites and the gene expression, as did the identification of the target genes. Collectively, 2,3-butanediol and (E)-2-hexenal were important cues shaping the production of bioactive products in the herbal plant.

Evolution of olfactory receptors tuned to mustard oils in herbivorous Drosophilidae

The diversity of herbivorous insects is attributed to their propensity to specialize on toxic plants. In an evolutionary twist, toxins betray the identity of their bearers when herbivores coopt them as cues for host-plant finding, but the evolutionary mechanisms underlying this phenomenon are poorly understood. We focused on Scaptomyza flava, an herbivorous drosophilid specialized on isothiocyanate (ITC)-producing (Brassicales) plants, and identified Or67b paralogs that were triplicated as mustard-specific herbivory evolved. Using in vivo heterologous systems for the expression of olfactory receptors, we found that S. flava Or67bs, but not the homologs from microbe-feeding relatives, responded selectively to ITCs, each paralog detecting different ITC subsets. Consistent with this, S. flava was attracted to ITCs, as was Drosophila melanogaster expressing S. flava Or67b3 in the homologous Or67b olfactory circuit. ITCs were likely coopted as olfactory attractants through gene duplication and functional specialization (neofunctionalization and subfunctionalization) in S. flava, a recently derived herbivore.

Detecting the Conspecific: Herbivory-Induced Olfactory Cues in the Fall Armyworm (Lepidoptera: Noctuidae)

The fall armyworm (FAW), Spodoptera frugiperda (Smith), is a polyphagous pest whose larval feeding threatens several economically important crops worldwide with especially severe damage to corn (Zea mays L.). Field-derived resistance to several conventional pesticides and Bt toxins have threatened the efficacy of current management strategies, necessitating the development of alternative pest management methods and technologies. One possible avenue is the use of volatile organic compounds (VOCs) and other secondary metabolites that are produced and sequestered by plants as a response to larval feeding. The effects of conspecific larval feeding on fall armyworm oviposition preferences and larval fitness were examined using two-choice oviposition experiments, larval feeding trials, targeted metabolomics, and VOC analyses. There was a significant preference for oviposition on corn plants that lacked larval feeding damage, and larvae fed tissue from damaged plants exhibited reduced weights and head capsule widths. All larval feeding promoted significantly increased metabolite and VOC concentrations compared to corn plants without any feeding. Metabolite differences were driven primarily by linoleic acid (which is directly toxic to fall armyworm) and tricarboxylic acids. Several VOCs with significantly increased concentrations in damaged corn plants were known oviposition deterrents that warrant further investigation in an integrated pest management context.

Natural Pest Regulation and Its Compatibility with Other Crop Protection Practices in Smallholder Bean Farming Systems

Simple Summary

Bean production by smallholder farmers in sub-Saharan Africa is frequently constrained by insect pests, two of the most serious being Maruca vitrata and Aphis fabae. For many bean farmers, the options available to control these pests are limited. A few can access synthetic insecticides, but these have negative consequences for their health and the environment. Natural pest regulation (NPR) offers environmentally benign approaches for smallholders to manage bean pests. For example, here, we focus on biological control whereby beneficial organisms predate or parasitize the pests. Field studies show this is a feasible strategy for controlling M. vitrata and A. fabae. In particular, we highlight how compatible biological control is with other NPR options, such as the use of biopesticides (including plant extracts), resistant varieties, and cultural control. We recommend that smallholder farmers consider biological control alongside other NPR strategies for reducing the populations of A. fabae and M. vitrata in the common bean, increasing the yields and reducing the negative impacts of the synthetic pesticides.

Abstract

Common bean (Phaseolus vulgaris) production and storage are limited by numerous constraints. Insect pests are often the most destructive. However, resource-constrained smallholders in sub-Saharan Africa (SSA) often do little to manage pests. Where farmers do use a control strategy, it typically relies on chemical pesticides, which have adverse effects on the wildlife, crop pollinators, natural enemies, mammals, and the development of resistance by pests. Nature-based solutions —in particular, using biological control agents with sustainable approaches that include biopesticides, resistant varieties, and cultural tools—are alternatives to chemical control. However, significant barriers to their adoption in SSA include a lack of field data and knowledge on the natural enemies of pests, safety, efficacy, the spectrum of activities, the availability and costs of biopesticides, the lack of sources of resistance for different cultivars, and spatial and temporal inconsistencies for cultural methods. Here, we critically review the control options for bean pests, particularly the black bean aphid (Aphis fabae) and pod borers (Maruca vitrata). We identified natural pest regulation as the option with the greatest potential for this farming system. We recommend that farmers adapt to using biological control due to its compatibility with other sustainable approaches, such as cultural tools, resistant varieties, and biopesticides for effective management, especially in SSA.

Apparent inhibition of induced plant volatiles by a fungal pathogen prevents airborne communication between potato plants

Plant communication in response to insect herbivory has been increasingly studied, whereas that involving pathogen attack has received much less attention. We tested for communication between potato (Solanum tuberosum) plants in response to leaf infection by the fungal pathogen Sclerotinia sclerotiorum. To this end, we measured the total amount and composition of volatile organic compounds (VOCs) produced by control and infected emitter plants, as well as tested for induced resistance of receiver plants exposed to VOCs from emitters. We further tested for changes in the expression of defensive genes due to pathogen infection. Fungal infection did not significantly affect the total amount or composition of VOCs produced by emitter plants. Correspondingly, we found no evidence of higher resistance to the pathogen in receiver plants exposed to VOCs from infected emitters relative to control emitters. Molecular analyses indicated that pathogen infection drove a down-regulation of genes coding for VOC precursors, potentially explaining the absence of pathogen effects on VOC emissions and thus of communication. Overall, these results indicate no evidence of airborne communication between potato plants in response to fungal infection and point at pathogen inhibition of VOC emissions as a likely explanation for this result.

Leaf Induction Impacts Behavior and Performance of a Pollinating Herbivore

Flowering plants use volatiles to attract pollinators while deterring herbivores. Vegetative and floral traits may interact to affect insect behavior. Pollinator behavior is most likely influenced by leaf traits when larval stages interact with plants in different ways than adult stages, such as when larvae are leaf herbivores but adult moths visit flowers as pollinators. Here, we determine how leaf induction and corresponding volatile differences in induced plants influence behavior in adult moths and whether these preferences align with larval performance. We manipulated vegetative induction in four Nicotiana species. Using paired induced and control plants of the same species with standardized artificial flowers, we measured foraging and oviposition choices by their ecologically and economically important herbivore/pollinator, Manduca sexta. In parallel, we measured growth rates of M. sexta larvae fed leaves from control or induced plants to determine if this was consistent with female oviposition preference. Lastly, we used plant headspace collections and gas chromatography to quantify volatile compounds from both induced and control leaves to link changes in plant chemistry with moth behavior. In the absence of floral chemical cues, vegetative defensive status influenced adult moth foraging preference from artificial flowers in one species (N. excelsior), where females nectared from induced plants more often than control plants. Plant vegetative resistance consistently influenced oviposition choice such that moths deposited more eggs on control plants than on induced plants of all four species. This oviposition preference for control plants aligned with higher larval growth rates on control leaves compared with induced leaves. Control and induced plants of each species had similar leaf volatile profiles, but induced plants had higher emission levels. Leaves of N. excelsior produced the most volatile compounds, including some inducible compounds typically associated with floral scent. We demonstrate that vegetative plant defensive volatiles play a role in host plant selection and that insects assess information from leaves differently when choosing between nectaring and oviposition locations. These results underscore the complex interactions between plants, their pollinators, and herbivores.

Diamondback Moth Larvae Trigger Host Plant Volatiles that Lure Its Adult Females for Oviposition

The diamondback moth (DBM) is a destructive pest of crucifer crops. In this study, DBM larvae shown to herbivore induced plant volatiles (HIPVs) that were attractive to adult females exposed in a Y-tube olfactometer. Our results showed that olfactory responses of adult females to HIPVs induced by third instar larvae feeding on Barbarea vulgaris were significantly higher (20.40 ± 1.78; mean moths (%) ± SD) than those induced by first instar larvae (14.80 ± 1.86; mean moths (%) ± SD). Meanwhile, a significant concentration of Sulphur-containing isothiocyanate, 3-methylsulfinylpropyl isothiocyanate, and 4-methylsulfinyl-3-butenyl isothiocyanate were detected in HIPVs released by third instar larvae compared to those released by first instar larvae while feeding on B. vulgaris. When the DBM females were exposed to synthetic chemicals, singly and in blend form, a similar response was observed as to natural HIPVs. Our study demonstrated that the relationship between isothiocyanates acting as plant defense compounds, host plant cues emission and regulation of the DBM adult female behavior due to key volatile triggered by the DBM larvae feeding on B. vulgaris.

Previous herbivory alerts conspecific gravid sawflies to avoid unsuitable host plants

The willow sawfly, Nematus oligospilus (Förster), is a pest in Salix commercial forests and has been reported worldwide. Female adults must recognize a suitable host plant to oviposit, since her offspring lack the ability to move to another host. We evaluated the effect of conspecific herbivory on the oviposition choices of N. oligospilus females by providing damaged (DP) and undamaged (UP) plants of Salix humboldtiana, a native willow from South America, as oviposition substrates. Local and systemic effects were studied. For the local treatment, a twig from the DP with damaged leaves was contrasted to a twig from a UP in dual choice experiments. For systemic treatment, a twig from the DP with intact leaves was contrasted to a twig from a UP. We estimated the use of olfactory and contact cues by comparing volatile emission of DP and UP, and by analysing the behaviour of the females during host recognition after landing on the leaf surface. In the context of the preference-performance hypothesis (PPH), we also tested if oviposition site selection maximizes offspring fitness by evaluating neonate hatching, larval performance and survival of larvae that were born and bred on either DP or UP. Our results demonstrate that previous conspecific herbivory on S. humboldtiana has a dramatic impact on female oviposition choices and offspring performance of the sawfly N. oligospilus. Females showed a marked preference for laying eggs on UP of S. humboldtiana. This preference was found for both local and systemic treatments. Volatile emission was quantitatively changed after conspecific damage suggesting that it could be related to N. oligospilus avoidance. In the dual choice preference experiments, the analysis of the behaviour of the females once landing on the leaf surface suggested the use of contact cues triggering egg laying on leaves from UP and avoidance of leaves from DP. Furthermore, 48 h of previous conspecific feeding was sufficient to dramatically impair neonate hatching, as well as larval development and survival, suggesting a rapid and effective reaction of the induced resistance mechanisms of the tree. In agreement with the PPH, these results support the idea that decisions made by colonizing females may result in optimal outcomes for their offspring in a barely studied insect model, and also opens the opportunity for studying tree-induced defences in the unexplored South American willow S. humboldtiana.

Female adult puncture-induced plant volatiles promote mating success of the pea leafminer via enhancing vibrational signals

Herbivore-induced plant volatiles (HIPVs) synergize with the sex pheromones of herbivorous insects to facilitate mate location. However, the synergism of HIPVs and acoustic signals for sexual communication remains unknown. Here, we investigated the synergy between HIPVs and vibrational duets for sexual communication and mating in the pea leafminer ( Liriomyza huidobrensis). Our results indicated that adult leafminers do not produce species-specific pheromone, and female-puncture-induced plant volatiles facilitate the attraction of both sexes to host plant leaves and sexual encounters. Insect-derived cues do not participate in mate locations. Both sexes do not produce qualitatively different cuticular hydrocarbons (CHCs), and CHCs from females cannot elicit the antennal and behavioural responses of males. By contrast, induced green leaf volatiles, terpenoids and oximes elicit dramatic antennal responses in both sexes. Electrophysiological and behavioural tests consistently showed that the volatiles (Z)-3-hexenol and (Z)-3-hexenyl-acetate elicited the most intense gas chromatographic-electroantennographic responses, and attracted males and females. Remarkably, these volatiles significantly promoted the occurrence of vibrational duets between the sexes, thereby increasing the mating success of leafminers. Therefore, the synergism of HIPVs and vibrational signals largely promoted the mating success of leafminers, suggesting an alternative control strategy through precision trapping for non-pheromone-producing insects. This article is part of the theme issue 'Biotic signalling sheds light on smart pest management'.

Spodoptera frugiperda Caterpillars Suppress Herbivore-Induced Volatile Emissions in Maize

The vast spectrum of inducible plant defenses can have direct negative effects on herbivores, or indirect effects, for instance in the form of herbivore-induced plant volatiles (HIPVs) that attract natural enemies. Various arthropods have evolved ways to suppress plant defenses. To test whether this is the case for caterpillar-induced HIPVs, we compared the volatile induction by Spodoptera frugiperda (Lepidoptera: Noctuidae), which is particularly well adapted to feed on maize (Zea mays), with the induction by three more generalist noctuid larvae. We tested the hypothesis that S. frugiperda suppresses HIPV emissions in maize, and thereby reduces attractiveness to natural enemies. HIPV emissions triggered by S. frugiperda when feeding on maize were indeed found to be significantly weaker than by Spodoptera littoralis, Spodoptera exigua, and Helicoverpa armigera. The suppression seems specific for maize, as we found no evidence for this when S. frugiperda caterpillars fed on cotton (Gossypium herbaceum). Artificially damaged maize plants treated with larval regurgitant revealed that HIPV suppression may be related to factors in the caterpillars' oral secretions. We also found evidence that differential physical damage that the caterpillars inflict on maize leaves may play a role. The suppressed induction of HIPVs had no apparent consequences for the attraction of a common parasitoid of S. frugiperda, Cotesia marginiventris (Hymenoptera: Braconidae). Nevertheless, the ability to manipulate the defenses of its main host plant may have contributed to the success of S. frugiperda as a major pest of maize, especially in Africa and Asia, which it has recently invaded.

Using natural variation to achieve a whole-plant functional understanding of the responses mediated by jasmonate signaling

The dramatic advances in our understanding of the molecular biology and biochemistry of jasmonate (JA) signaling have been the subject of several excellent recent reviews that have highlighted the phytohormonal function of this signaling pathway. Here, we focus on the responses mediated by JA signaling which have consequences for a plant's Darwinian fitness, i.e. the organism-level function of JA signaling. The most diverse module in the signaling cascade, the JAZ proteins, and their interactions with other proteins and transcription factors, allow this canonical signaling cascade to mediate a bewildering array of traits in different tissues at different times; the functional coherence of these diverse responses are best appreciated in an organismal/ecological context. From published work, it appears that jasmonates can function as the 'Swiss Army knife' of plant signaling, mediating many different biotic and abiotic stress and developmental responses that allow plants to contextualize their responses to their frequently changing local environments and optimize their fitness. We propose that a deeper analysis of the natural variation in both within-plant and within-population JA signaling components is a profitable means of attaining a coherent whole-plant functional perspective of this signaling cascade, and provide examples of this approach from the Nicotiana attenuata system.

Mutagenesis of odorant coreceptor Orco fully disrupts foraging but not oviposition behaviors in the hawkmoth Manduca sexta

The hawkmoth Manduca sexta and one of its preferred hosts in the North American Southwest, Datura wrightii, share a model insect-plant relationship based on mutualistic and antagonistic life-history traits. D. wrightii is the innately preferred nectar source and oviposition host for M. sexta Hence, the hawkmoth is an important pollinator while the M. sexta larvae are specialized herbivores of the plant. Olfactory detection of plant volatiles plays a crucial role in the behavior of the hawkmoth. In vivo, the odorant receptor coreceptor (Orco) is an obligatory component for the function of odorant receptors (ORs), a major receptor family involved in insect olfaction. We used CRISPR-Cas9 targeted mutagenesis to knock out (KO) the MsexOrco gene to test the consequences of a loss of OR-mediated olfaction in an insect-plant relationship. Neurophysiological characterization revealed severely reduced antennal and antennal lobe responses to representative odorants emitted by D. wrightii In a wind-tunnel setting with a flowering plant, Orco KO hawkmoths showed disrupted flight orientation and an ablated proboscis extension response to the natural stimulus. The Orco KO gravid female displayed reduced attraction toward a nonflowering plant. However, more than half of hawkmoths were able to use characteristic odor-directed flight orientation and oviposit on the host plant. Overall, OR-mediated olfaction is essential for foraging and pollination behaviors, but plant-seeking and oviposition behaviors are sustained through additional OR-independent sensory cues.

Herbivorous Caterpillars Can Utilize Three Mechanisms to Alter Green Leaf Volatile Emission

Green plants emit green leaf volatiles (GLVs) as a general damage response. These compounds act as signals for the emitter plant, neighboring plants, and even for insects in the ecosystem. However, when oral secretions from certain caterpillars are applied to wounded leaves, GLV emissions are significantly decreased or modified. We examined four caterpillar species representing two lepidopteran families for their capacity to decrease GLV emissions from Zea mays leaf tissue. We also investigated the source of the GLV modifying components in the alimentary tract of the various caterpillars. In Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae), Spodoptera frugiperda (Smith) (Lepidoptera: Noctuidae), Trichoplusia ni (Hübner) (Lepidoptera: Noctuidae), and Manduca sexta (Linnaeus) (Lepidoptera: Sphingidae), we found three distinct mechanisms to modify GLV emission: a heat-stable compound in the gut, a heat-labile enzyme in salivary gland homogenate (previously described in Bombyx mori (Linnaeus) (Lepidoptera: Bombycidae), and an isomerase in the salivary gland homogenate, which catalyzes the conversion of (Z)-3-hexenal to (E)-2-hexenal (previously described in M. sexta). These mechanisms employed by caterpillars to suppress or modify GLV emission suggest a counteraction against the induced indirect volatile defenses of a plant and provides further insights into the ecological functions of GLVs.

Volatiles from Maruca vitrata (Lepidoptera, Crambidae) host plants influence olfactory responses of the parasitoid Therophilus javanus (Hymenoptera, Braconidae, Agathidinae)

Plants damaged by herbivores are known to release odors attracting parasitoids. However, there is currently no information how leguminous plants damaged by the pod borer Maruca vitrata attract the exotic larval parasitoid Therophilus javanus, which was imported into Benin from the putative area of origin of the pod borer in tropical Asia for assessing its potential as a biological control agent. In this study, we used Y-tube olfactometer bioassays to investigate T. javanus response towards odors emitted by four M. vitrata-damaged host plants: cowpea Vigna unguiculata, the most important cultivated host, and the naturally occurring legumes Lonchocarpus sericeus, Sesbania rostrata and Tephrosia platycarpa. Olfactory attraction of T. javanus was influenced by the species of plant damaged by the pod borer. Moreover, odors released from M. vitrata-infested host plant organs (flowers and pods) were discriminated over non-infested organs in cowpea and T. platycarpa, respectively. These results are discussed in the context of the possible impact of M. vitrata host plants on T. javanus foraging activity and subsequent establishment in natural environments following experimental releases.

Green leaf volatile production by plants: a meta-analysis

666 I. Introduction 667 II. Biosynthesis 667 III. Meta-analysis 669 IV. The type of stress influences the total amount of GLVs released 669 V. Herbivores can modulate the wound-induced release of GLVs 669 VI. Fungal infection greatly induces GLV production 672 VII. Monocots and eudicots respond differentially to different types of stress 673 VIII. The type of stress does not influence the proportion of GLVs per chemical class 673 IX. The type of stress does influence the isomeric ratio within each chemical class 674 X. GLVs: from signal perception to signal transduction 676 XI. GLVs influence the C/N metabolism 677 XII. Interaction with plant hormones 678 XIII. General conclusions and unanswered questions 678 Acknowledgements 679 References 679 SUMMARY: Plants respond to stress by releasing biogenic volatile organic compounds (BVOCs). Green leaf volatiles (GLVs), which are abundantly produced across the plant kingdom, comprise an important group within the BVOCs. They can repel or attract herbivores and their natural enemies; and they can induce plant defences or prime plants for enhanced defence against herbivores and pathogens and can have direct toxic effects on bacteria and fungi. Unlike other volatiles, GLVs are released almost instantly upon mechanical damage and (a)biotic stress and could thus function as an immediate and informative signal for many organisms in the plant's environment. We used a meta-analysis approach in which data from the literature on GLV production during biotic stress responses were compiled and interpreted. We identified that different types of attackers and feeding styles add a degree of complexity to the amount of emitted GLVs, compared with wounding alone. This meta-analysis illustrates that there is less variation in the GLV profile than we presumed, that pathogens induce more GLVs than insects and wounding, and that there are clear differences in GLV emission between monocots and dicots. Besides the meta-analysis, this review provides an update on recent insights into the perception and signalling of GLVs in plants.

Silkworms suppress the release of green leaf volatiles by mulberry leaves with an enzyme from their spinnerets

In response to herbivory, plants emit a blend of volatile organic compounds that includes green leaf volatiles (GLVs) and terpenoids. These volatiles are known to attract natural enemies of herbivores and are therefore considered to function as an indirect defense. Selection should favor herbivores that are able to suppress these volatile emissions, and thereby make themselves less conspicuous to natural enemies. We tested this possibility for silkworms, which were observed to leave secretions from their spinnerets while feeding on mulberry leaves. When we ablated the spinnerets of silkworms, no secretions were observed. Leaves infested by intact silkworms released smaller amounts of GLVs than leaves infested by ablated silkworms, indicating that the spinneret secretion suppressed GLV production. This difference in GLV emissions was also reflected in the behavioral response of Zenillia dolosa (Tachinidae), a parasitoid fly of silkworms. The flies laid fewer eggs when exposed to the volatiles from intact silkworm-infested leaves than when exposed to the volatiles from ablated silkworm-infested leaves. We identified a novel enzyme in the secretion from the spinneret that is responsible for the GLV suppression. The enzyme converted 13(S)-hydroperoxy-(9Z,11E,15Z)-octadecatrienoic acid, an intermediate in the biosynthetic pathway of GLVs, into its keto-derivative in a stereospecific manner. Taken together, this study shows that silkworms are able to feed on mulberry in a stealthy manner by suppressing GLV production with an enzyme in secretions of their spinnerets, which might be a countermeasure against induced indirect defense by mulberry plants.

Field studies reveal functions of chemical mediators in plant interactions

Plants are at the trophic base of most ecosystems, embedded in a rich network of ecological interactions in which they evolved. While their limited range and speed of motion precludes animal-typical behavior, plants are accomplished chemists, producing thousands of specialized metabolites which may function to convey information, or even to manipulate the physiology of other organisms. Plants' complex interactions and their underlying mechanisms are typically dissected within the controlled environments of growth chambers and glasshouses, but doing so introduces conditions alien to plants evolved in natural environments, such as being pot-bound, and receiving artificial light with a spectrum very different from sunlight. The mechanistic understanding gained from a reductionist approach provides the tools required to query and manipulate plant interactions in real-world settings. The few tests conducted in natural ecosystems and agricultural fields have highlighted the limitations of studying plant interactions only in artificial environments. Here, we focus on three examples of known or hypothesized chemical mediators of plants' interactions: the volatile phytohormone ethylene (ET), more complex plant volatile blends, and as-yet-unknown mediators transferred by common mycorrhizal networks (CMNs). We highlight how mechanistic knowledge has advanced research in all three areas, and the critical importance of field work if we are to put our understanding of chemical ecology on rigorous experimental and theoretical footing, and demonstrate function.

Two Odorant-Binding Proteins of the Dark Black Chafer (Holotrichia parallela) Display Preferential Binding to Biologically Active Host Plant Volatiles

The dark black chafer (DBC), Holotrichia parallela, is an important pest of multiple crops. Insect host-searching behaviors are regulated by host plant volatiles. Therefore, a better understanding of the mechanism linking the chemosensory system to plant volatiles at the molecular level will benefit DBC control strategies. Based on antenna transcriptome data, two highly expressed antenna-specific odorant-binding proteins (HparOBP20 and 49) were selected to identify novel DBC attractants using reverse chemical ecology methods. We expressed these proteins, mapped their binding specificity, and tested the activity of the plant volatiles in the field. The ligands used in the binding specificity assays included 31 host-plant-associated volatiles and two sex pheromone components. The results showed that (1) HparOBP20 and 49 are involved in odor recognition; (2) these proteins bind attractive plant volatiles strongly and can therefore be employed to develop environmentally friendly DBC management strategies; and (3) the green-leaf volatile (Z)-3-hexenyl acetate shows a high binding affinity to HparOBP20 (Ki = 18.51 μM) and HparOBP49 (Ki = 39.65 μM) and is highly attractive to DBC adults, especially females. In the field test, a (Z)-3-hexenyl acetate trap caught an average of 13 ± 1.202 females per day, which was significantly greater than the corresponding male catch (F2,6 = 74.18, P < 0.0001). (Z)-3-Hexenyl acetate may represent a useful supplement to the known sex pheromone for DBC attraction. In the present study, the binding characteristics of two HparOBPs with host plant volatiles were screened, providing behaviourally active compounds that might be useful for DBC control, based on reverse chemical ecology.

Electrophysiological and Oviposition Responses of Tuta absoluta Females to Herbivore-Induced Volatiles in Tomato Plants

In response to attack by herbivorous insects, plants produce semiochemicals for intra- and interspecific communication. The perception of these semiochemicals by conspecifics of the herbivore defines their choice for oviposition and feeding. We aimed to investigate the role of herbivore-induced plant volatiles (HIPVs) by Tuta absoluta larvae on the oviposition choice of conspecific females on tomato plants. We performed two- choice and non-choice bioassays with plants damaged by larvae feeding and intact control plants. We also collected headspace volatiles of those plants and tested the response of female antennae on those blends with Gas Chromatography- Electro-Antennographical Detection (GC-EAD). In total 55 compounds were collected from the headspace of T. absoluta larvae-infested plants. Our results show that female moths preferred to oviposit on intact control plants instead of damaged ones. Herbivory induced the emission of hexanal, (Ζ)-3-hexen-1-ol, (E)-β-ocimene, linalool, (Z)-3-hexenyl butanoate, methyl salicylate, indole, nerolidol, guaidiene-6,9, β-pinene, β-myrcene, α-terpinene, hexenyl hexanoate, β-elemene, β-caryophyllene and (Ε-Ε)- 4,8,12-trimethyl-1,3,7,11-tridecatetraene (TMTT), one unidentified sesquiterpene and three unknown compounds. In Electroantennographic (EAG) assays, the antennae of T. absoluta females responded to hexanal, (Ζ)-3-hexen-1-ol, methyl salicylate and indole. The antennae of T. absoluta females exhibited a dose-response in EAG studies with authentic samples. Strong EAG responses were obtained for compounds induced on damaged tomato plants, as well as in nonanal, a compound emitted by both infested and control plants. These compounds could be utilized in integrated pest management of T. absoluta.

Tritrophic Interactions Mediated by Herbivore-Induced Plant Volatiles: Mechanisms, Ecological Relevance, and Application Potential

Tritrophic interactions between plants, herbivores, and their natural enemies are an integral part of all terrestrial ecosystems. Herbivore-induced plant volatiles (HIPVs) play a key role in these interactions, as they can attract predators and parasitoids to herbivore-attacked plants. Thirty years after this discovery, the ecological importance of the phenomena is widely recognized. However, the primary function of HIPVs is still subject to much debate, as is the possibility of using these plant-produced cues in crop protection. In this review, we summarize the current knowledge on the role of HIPVs in tritrophic interactions from an ecological as well as a mechanistic perspective. This overview focuses on the main gaps in our knowledge of tritrophic interactions, and we argue that filling these gaps will greatly facilitate efforts to exploit HIPVs for pest control.

Identification and Characterization of (3Z):(2E)-Hexenal Isomerases from Cucumber

E-2-hexenal is a volatile compound that is commonly emitted by wounded or stressed plants. It belongs to the group of so-called green leaf volatiles (GLVs), which play an important role in transferring information to plants and insects. While most biosynthetic enzymes upstream of E-2-hexenal have been studied extensively, much less is known about the enzyme responsible for the conversion from Z-3- to E-2-hexenal. In this study we have identified two (3Z):(2E)-hexenal isomerases (HIs) from cucumber fruits by classical biochemical fractionation techniques and we were able to confirm their activity by heterologous expression. Recombinant protein of the HIs did not only convert the leaf aldehyde Z-3-hexenal to E-2-hexenal, but also (Z,Z)-3,6-nonadienal to (E,Z)-2,6-nonadienal, these last two representing major flavor volatiles of cucumber fruits. Transient expression of the cucumber HIs in Nicotiana benthamiana leaves drastically changed the GLV bouquet of damaged plants from a Z-3- to an E-2-enriched GLV profile. Furthermore, transcriptional analysis revealed that the two HIs showed distinct expression patterns. While HI-1 was specifically expressed in the flesh of cucumber fruits HI-2 was expressed in leaves as well. Interestingly, wounding of cucumber leaves caused only a slight increase in HI-2 transcript levels. These results demonstrate that cucumber HIs are responsible for the rearrangement of Z-3-aldehydes in both leaves and fruits. Future research will reveal the physiological importance of an increased conversion to E-2-aldehydes for plants and insects.

Sensory genes identification with head transcriptome of the migratory armyworm, Mythimna separata

Sensory system plays important roles in a wide array of insect’s behavior and physiological events, including the host landing and locating, feeding, flying, sex responding, mating and oviposition which happen independently and in sequence. The armyworm Mythimna separata (Lepidoptera: Noctuidae) of migratory insect is destructive for alimentarn crop and economic crop throughout the world. Here we present the high throughput sequencing of the head transcriptome and identify members of the major sensory genes which are crucial for armyworm’s success worldwide, including 8 opsins, 22 chemosensory proteins, 50 odorant binding proteins, 60 odorant receptors, 8 gustatory receptors, 24 ionotropic receptors, and 2 sensory neuron membrane proteins. It is worth noting that a duplication of the LW opsin gene exists in this insect. Several genes were clustered with functionally validated genes, such as Co-receptors of OR and IR, PBPs, PRs, CO2 GRs, bitter GRs and sweet GRs, were also identified. The transcriptome gene library provided the basis for further studies that elucidate the fundamental molecular mechanism of biology and control in M. separata. Our research exhibits the first comprehensive catalogue of the sensory genes fundamental for success and distribution in M. separata, which are potential novel targets for pest control strategies.

Plant Volatiles Increase Sex Pheromone Attraction of Holotrichia parallela (Coleoptera: Scarabaeoidea)

Holotrichia parallela (Coleoptera: Scarabaeoidea) is a notorious pest of many crops. To improve the effectiveness of its female-produced sex pheromone (L-isoleucine methyl ester:(R)-(-)-linalool = 6:1), 14 plant volatiles, including dodecanoic acid, dodecanal, farnesol, α-farnesene, (Z)-3-hexen-1-ol, (E)-2-hexen-1-ol, (Z)-3-hexenyl acetate, (E)-2-hexenyl acetate, (R)-(+)-limonene, α-phellandrene, α-pinene, ocimene, methyl benzoate, and benzaldehyde, were individually evaluated using electroantennography and olfactometer assays. (E)-2-Hexenyl acetate and (Z)-3-hexenyl acetate were found to elicit the strongest responses in both males and females. Further testing of these two compounds in mixtures with the sex pheromone indicated that (E)-2-hexenyl acetate had a stronger synergistic effect than (Z)-3-hexenyl acetate. Field evaluations showed that mixtures of (E)-2-hexenyl acetate and the sex pheromone resulted in significantly higher catches than the sex pheromone alone. Using a 5:1 mixture of the sex pheromone and (E)-2-hexenyl acetate, the maximum number of females per trap per day was 14, showing a synergistic effect of a factor of four. For males, a 3:1 mixture of the sex pheromone and (E)-2-hexenyl acetate yielded a maximum number of 310 individuals per trap per day, equivalent to a synergistic effect of 175%. These results may provide the basis for the development of efficient pest management systems against H. parallela using plant volatiles and insect sex pheromones.

Learning in Insect Pollinators and Herbivores

The relationship between plants and insects is influenced by insects' behavioral decisions during foraging and oviposition. In mutualistic pollinators and antagonistic herbivores, past experience (learning) affects such decisions, which ultimately can impact plant fitness. The higher levels of dietary generalism in pollinators than in herbivores may be an explanation for the differences in learning seen between these two groups. Generalist pollinators experience a high level of environmental variation, which we suggest favors associative learning. Larval herbivores employ habituation and sensitization-strategies useful in their less variable environments. Exceptions to these patterns based on habitats, mobility, and life history provide critical tests of current theory. Relevant plant traits should be under selection to be easily learned and remembered in pollinators and difficult to learn in herbivores. Insect learning thereby has the potential to have an important, yet largely unexplored, role in plant-insect coevolution.

Identification and testing of oviposition attractant chemical compounds for Musca domestica

Oviposition attractants for the house fly Musca domestica have been investigated using electrophysiological tests, behavioural assays and field tests. Volatiles were collected via head space absorption method from fermented wheat bran, fresh wheat bran, rearing substrate residue and house fly maggots. A Y-tube olfactometer assay showed that the odor of fermented wheat bran was a significant attractant for female house flies. Bioactive compounds from fermented wheat bran for house fly females were identified by electrophysiology and mass spectrophotometry and confirmed with standard chemicals. Four electrophysiologically active compounds including ethyl palmitate, ethyl linoleate, methyl linoleate, and linoleic acid were found at a proportion of 10:24:6:0.2. Functional imaging in the female antennal lobes revealed an overlapped active pattern for all chemicals. Further multiple-choice behavioural bioassays showed that these chemicals, as well as a mixture that mimicked the naturally occurring combination, increased the attractiveness of non-preferred rearing substrates of cotton and maize powder. Finally, a field demonstration test revealed that, by adding this mimic blend into a rearing substrate used to attract and breed house flies in West Africa, egg numbers laid by females were increased. These chemicals could be utilized to improve house fly production systems or considered for lure traps.

The Olfactory Neuroecology of Herbivory, Hostplant Selection and Plant-Pollinator Interactions

Plants experience often opposing energetic demands and selective pressures-for instance, where plants need to attract an insect that is both the pollinator and herbivore, or alternately, where plants attract prey (due to limited resources) and pollinators. Together, these selective pressures can modify the volatile signals available to the plant's mutualistic and antagonistic partners. Nevertheless, it remains an open question how changes in the information content of volatile signals modify behavioral responses in mutualists and antagonists, and what the underlying neural bases of these behaviors are. This review focuses on two systems to explore the impact of herbivory and resource availability on plant-pollinator interactions: hawkmoth-pollinated hostplants (where herbivory is common), and carnivorous bee-pollinated pitcher plants (where the plants differentially attract bee pollinators and other insect prey). We focus on (1) the volatile signals emitted from these plants because these volatiles operate as long-distance signals to attract, or deter, insect partners, (2) how this information is processed in the hawkmoth olfactory system, and (3) how volatile information changes spatiotemporally. In both the plants and their respective insect partner(s), volatile signaling, reception and behavior are dynamic and plastic, providing flexibility an ever-changing environment.

Scaling of photosynthesis and constitutive and induced volatile emissions with severity of leaf infection by rust fungus (Melampsora larici-populina) in Populus balsamifera var. suaveolens

Fungal infections result in decreases in photosynthesis, induction of stress and signaling volatile emissions and reductions in constitutive volatile emissions, but the way different physiological processes scale with the severity of infection is poorly known. We studied the effects of infection by the obligate biotrophic fungal pathogen Melampsora larici-populina Kleb., the causal agent of poplar leaf rust disease, on photosynthetic characteristics, and constitutive isoprene and induced volatile emissions in leaves of Populus balsamifera var. suaveolens (Fisch.) Loudon. exhibiting different degrees of damage. The degree of fungal damage, quantified by the total area of chlorotic and necrotic leaf areas, varied between 0 (noninfected control) and ∼60%. The rates of all physiological processes scaled quantitatively with the degree of visual damage, but the scaling with damage severity was weaker for photosynthetic characteristics than for constitutive and induced volatile release. Over the whole range of damage severity, the net assimilation rate per area (AA) decreased 1.5-fold, dry mass per unit area 2.4-fold and constitutive isoprene emissions 5-fold, while stomatal conductance increased 1.9-fold and dark respiration rate 1.6-fold. The emissions of key stress and signaling volatiles (methanol, green leaf volatiles, monoterpenes, sesquiterpenes and methyl salicylate) were in most cases nondetectable in noninfested leaves, and increased strongly with increasing the spread of infection. The moderate reduction in AA resulted from the loss of photosynthetically active biomass, but the reduction in constitutive isoprene emissions and the increase in induced volatile emissions primarily reflected changes in the activities of corresponding biochemical pathways. Although all physiological alterations in fungal-infected leaves occurred in a stress severity-dependent manner, modifications in primary and secondary metabolic pathways scaled differently due to contrasting operational mechanisms.

Electrophysiological Responses and Reproductive Behavior of Fall Webworm Moths (Hyphantria cunea Drury) are Influenced by Volatile Compounds from Its Mulberry Host (Morus alba L.)

Hyphantria cunea (Drury) is an invasive pest of Morus alba L. in China. β-ocimene and cis-2-penten-1-ol among eleven electro-physiologically active leaf volatiles from M. alba have been reported to influence captures of Hyphantria cunea moths when added into sex pheromone traps. This study further investigated influences of volatile types and their dosages on the electro-physiological responses in the antennae of male and female moths, as well as on mating and oviposition behaviors. Females were, regardless of dosages, more sensitive to β-ocimene and cis-2-penten-1-ol in electro-physiological response tests than males. For males, a dose response was detected, i.e., a dosage of 10 μg and 100 μg of either chemical stimulated higher electric response in their antennae than 1 μg. Moth pairs either exposed respectively to a herbivore-induced M. alba volatile blend (HIPV), to a mechanically-damaged M. alba volatile blend (MDV), to β-ocimene, to cis-2-penten-1-ol, or to pentane as a control showed that pairs exposed to β-ocimene most likely mated, followed by HIPV blends and least by the other volatiles or the control. In contrast, β-ocimene induced about 70% of the female oviposition behaviors and was nearly 4.5 times the oviposition rate than cis-2-penten-1-ol and 2 times than the control. However, none of the chemicals had any effect on the 48 h fecundity or on egg sizes. In conclusion, β-ocimene from mulberry plants alone could promote mating and oviposition in H. cunea at a dosage of 1 mg. The results indicate that reproductive behaviors of H. cunea moths can be enhanced through HIPV blends and β-ocimene induced by feeding of larvae. This contra phenomenon has revealed a different ecology in this moth during colonizing China as local pests would commonly be repelled by herbivore induced chemicals. These chemicals can be used for the development of biological control approaches such as being used together with sex pheromone traps.

The Layers of Plant Responses to Insect Herbivores

Plants collectively produce hundreds of thousands of specialized metabolites that are not required for growth or development. Each species has a qualitatively unique profile, with variation among individuals, growth stages, and tissues. By the 1950s, entomologists began to recognize the supreme importance of these metabolites in shaping insect herbivore communities. Plant defense theories arose to address observed patterns of variation, but provided few testable hypotheses because they did not distinguish clearly among proximate and ultimate causes. Molecular plant-insect interaction research has since revealed the sophistication of plant metabolic, developmental, and signaling networks. This understanding at the molecular level, rather than theoretical predictions, has driven the development of new hypotheses and tools and pushed the field forward. We reflect on the utility of the functional perspective provided by the optimal defense theory, and propose a conceptual model of plant defense as a series of layers each at a different level of analysis, illustrated by advances in the molecular ecology of plant-insect interactions.

Identification of Host-Plant Volatiles and Characterization of Two Novel General Odorant-Binding Proteins from the Legume Pod Borer, Maruca vitrata Fabricius (Lepidoptera: Crambidae)

Chemoreception is a key feature in selection of host plant by phytophagous insects, and odorant-binding proteins (OBPs) are involved in chemical communication of both insects and vertebrates. The legume pod borer, Maruca vitrata Fabricius (Lepidoptera: Crambidae) is one of the key pest species of cowpea and widely distributed throughout tropical and subtropical regions, causing up to 80% of yield loss. In this study, we investigated the electrophysiological responses of female M. vitrata to floral volatiles from V. unguiculata. Seventeen electroantennogram-active compounds were identified from floral volatiles of V. unguiculata by coupled gas chromatography-electroantennography (GC-EAD) and gas chromatography-mass spectrometry (GC-MS). Then, we cloned two novel full-length GOBP genes (MvitGOBP1 and MvitGOBP2) from the antennae of M. vitrata using reverse transcription PCR. Protein sequence analysis indicated that they shared high sequence similarity with other Pyralididae insect GOBPs and had the typical six-cysteine signature. Real-time PCR analysis indicated that MvitGOBP1-2 mRNA was highly expressed in the antennae of female adult with several thousands-fold difference compare to other tissue. Next, the recombinant MvitGOBP1-2 was expressed in Escherichia coli and purified using Ni ion affinity chromatography. Fluorescence binding assays demonstrated that MvitGOBP1-2 had different binding affinities with 17 volatile odorant molecules including butanoic acid butyl ester, limonene, 4-ethylpropiophenone, 1H-indol-4-ol, butanoic acid octyl ester and 2-methyl-3-phenylpropanal. In the field trapping experiment, these six floral volatiles could effectively attract female moths and showed significant difference compared with the blank lure. These results suggested that MvitGOBPs and the seventeen floral volatiles are likely to function in the olfactory behavior response of female moths, which may have played crucial roles in the selection of oviposition sites. The six compounds that we have identified from the volatiles of V. unguiculata may provide useful information for exploring efficiency monitoring and integrated pest management strategies of this legume pod borer in the field.

A major host plant volatile, 1-octen-3-ol, contributes to mating in the legume pod borer, Maruca vitrata (Fabricius) (Lepidoptera: Crambidae)

Previous studies on the legume pod borer, Maruca vitrata Fabricius (Lepidoptera: Crambidae), a serious pest of cowpea, Vigna unguiculata (L.) Walp. (Fabales: Fabaceae), in sub-Saharan Africa have focused on sex pheromones, but the role of the host plant on sexual behavior has not been explored. We investigated this interaction in the laboratory using behavioral assays and chemical analyses. We found that the presence of cowpea seedlings and a dichloromethane extract of the leaf increased coupling in the legume pod borer by 33 and 61 %, respectively, compared to the control, suggesting the involvement of both contact and olfactory cues. We used coupled gas chromatography-electroantennographic detection (GC/EAD) and GC-mass spectrometry (GC/MS) to identify compounds from the cowpea leaf extract, detected by M. vitrata antenna. We found that the antennae of the insect consistently detected four components, with 1-octen-3-ol identified as a common and dominant component in both the volatiles released by the intact cowpea plant and leaf extract. We therefore investigated its role in the coupling of M. vitrata. In dose-response assays, 1-octen-3-ol increased coupling in M. vitrata with increasing dose of the compound compared to the control. Our results suggest that the cowpea volatile 1-octen-3-ol contributes to M. vitrata sexual behavior.

Herbivore-induced plant volatiles in natural and agricultural ecosystems: open questions and future prospects

Herbivore-induced plant volatiles (HIPV) have been shown to convey ecologically relevant information to other organisms, including carnivorous and herbivorous arthropods and neighboring plants. However, many questions about the evolutionary and ecological functions of HIPV remain unanswered. In particular, a current lack of information about the ways in which environmental factors-including habitat structure and atmospheric conditions-influence HIPV mediated interactions in real-world settings limits our ability to anticipate the ways in which HIPV-mediated ecological interactions may be altered or disrupted by anthropogenic environmental change, including atmospheric pollution and climate change. Understanding these influences thus has significant implications for the sustainable management of natural and agricultural ecosystems and should be a priority for future research.

Jasmonic acid and its precursor 12-oxophytodienoic acid control different aspects of constitutive and induced herbivore defenses in tomato

The jasmonate family of growth regulators includes the isoleucine (Ile) conjugate of jasmonic acid (JA-Ile) and its biosynthetic precursor 12-oxophytodienoic acid (OPDA) as signaling molecules. To assess the relative contribution of JA/JA-Ile and OPDA to insect resistance in tomato (Solanum lycopersicum), we silenced the expression of OPDA reductase3 (OPR3) by RNA interference (RNAi). Consistent with a block in the biosynthetic pathway downstream of OPDA, OPR3-RNAi plants contained wild-type levels of OPDA but failed to accumulate JA or JA-Ile after wounding. JA/JA-Ile deficiency in OPR3-RNAi plants resulted in reduced trichome formation and impaired monoterpene and sesquiterpene production. The loss of these JA/JA-Ile -dependent defense traits rendered them more attractive to the specialist herbivore Manduca sexta with respect to feeding and oviposition. Oviposition preference resulted from reduced levels of repellant monoterpenes and sesquiterpenes. Feeding preference, on the other hand, was caused by increased production of cis-3-hexenal acting as a feeding stimulant for M. sexta larvae in OPR3-RNAi plants. Despite impaired constitutive defenses and increased palatability of OPR3-RNAi leaves, larval development was indistinguishable on OPR3-RNAi and wild-type plants, and was much delayed compared with development on the jasmonic acid-insensitive1 (jai1) mutant. Apparently, signaling through JAI1, the tomato ortholog of the ubiquitin ligase CORONATINE INSENSITIVE1 in Arabidopsis (Arabidopsis thaliana), is required for defense, whereas the conversion of OPDA to JA/JA-Ile is not. Comparing the signaling activities of OPDA and JA/JA-Ile, we found that OPDA can substitute for JA/JA-Ile in the local induction of defense gene expression, but the production of JA/JA-Ile is required for a systemic response.

Responses to Pheromones in a Complex Odor World: Sensory Processing and Behavior

Insects communicating with pheromones, be it sex- or aggregation pheromones, are confronted with an olfactory environment rich in a diversity of volatile organic compounds of which plants are the main releaser. Certain of these volatiles can represent behaviorally relevant information, such as indications about host- or non-host plants; others will provide essentially a rich odor background out of which the behaviorally relevant information needs to be extracted. In an attempt to disentangle mechanisms of pheromone communication in a rich olfactory environment, which might underlie interactions between intraspecific signals and a background, we will summarize recent literature on pheromone/plant volatile interactions. Starting from molecular mechanisms, describing the peripheral detection and central nervous integration of pheromone-plant volatile mixtures, we will end with behavioral output in response to such mixtures and its plasticity.

A robust, simple, high-throughput technique for time-resolved plant volatile analysis in field experiments

Plant volatiles (PVs) mediate interactions between plants and arthropods, microbes and other plants, and are involved in responses to abiotic stress. PV emissions are therefore influenced by many environmental factors, including herbivore damage, microbial invasion, and cues from neighboring plants, and also light regime, temperature, humidity and nutrient availability. Thus, an understanding of the physiological and ecological functions of PVs must be based on measurements reflecting PV emissions under natural conditions. However, PVs are usually sampled in the artificial environments of laboratories or climate chambers. Sampling of PVs in natural environments is difficult, being limited by the need to transport, maintain and provide power to instruments, or use expensive sorbent devices in replicate. Ideally, PVs should be measured in natural settings with high replication, spatio-temporal resolution and sensitivity, and modest costs. Polydimethylsiloxane (PDMS), a sorbent commonly used for PV sampling, is available as silicone tubing for as little as 0.60 € m(-1) (versus 100-550 € each for standard PDMS sorbent devices). Small pieces of silicone tubing (STs) of various lengths from millimeters to centimeters may be added to any experimental setting and used for headspace sampling, with little manipulation of the organism or headspace. STs have sufficiently fast absorption kinetics and large capacity to sample plant headspaces over a timescale of minutes to hours, and thus can produce biologically meaningful 'snapshots' of PV blends. When combined with thermal desorption coupled to GC-MS (a 40-year-old widely available technology), use of STs yields reproducible, sensitive, spatio-temporally resolved quantitative data from headspace samples taken in natural environments.

Green leaf volatiles: a plant's multifunctional weapon against herbivores and pathogens

Plants cannot avoid being attacked by an almost infinite number of microorganisms and insects. Consequently, they arm themselves with molecular weapons against their attackers. Plant defense responses are the result of a complex signaling network, in which the hormones jasmonic acid (JA), salicylic acid (SA) and ethylene (ET) are the usual suspects under the magnifying glass when researchers investigate host-pest interactions. However, Green Leaf Volatiles (GLVs), C₆ molecules, which are very quickly produced and/or emitted upon herbivory or pathogen infection by almost every green plant, also play an important role in plant defenses. GLVs are semiochemicals used by insects to find their food or their conspecifics. They have also been reported to be fundamental in indirect defenses and to have a direct effect on pests, but these are not the only roles of GLVs. These volatiles, being probably one of the fastest weapons exploited, are also able to directly elicit or prime plant defense responses. Moreover, GLVs, via crosstalk with phytohormones, mostly JA, can influence the outcome of the plant’s defense response against pathogens. For all these reasons GLVs should be considered as co-protagonists in the play between plants and their attackers.

Feeding-induced rearrangement of green leaf volatiles reduces moth oviposition

The ability to decrypt volatile plant signals is essential if herbivorous insects are to optimize their choice of host plants for their offspring. Green leaf volatiles (GLVs) constitute a widespread group of defensive plant volatiles that convey a herbivory-specific message via their isomeric composition: feeding of the tobacco hornworm Manduca sexta converts (Z)-3- to (E)-2-GLVs thereby attracting predatory insects. Here we show that this isomer-coded message is monitored by ovipositing M. sexta females. We detected the isomeric shift in the host plant Datura wrightii and performed functional imaging in the primary olfactory center of M. sexta females with GLV structural isomers. We identified two isomer-specific regions responding to either (Z)-3- or (E)-2-hexenyl acetate. Field experiments demonstrated that ovipositing Manduca moths preferred (Z)-3-perfumed D. wrightii over (E)-2-perfumed plants. These results show that (E)-2-GLVs and/or specific (Z)-3/(E)-2-ratios provide information regarding host plant attack by conspecifics that ovipositing hawkmoths use for host plant selection.

DOI:http://dx.doi.org/10.7554/eLife.00421.001

Plants have developed a variety of strategies to defend themselves against herbivorous animals, particularly insects. In addition to mechanical defences such as thorns and spines, plants also produce compounds known as secondary metabolites that keep insects and other herbivores at bay by acting as repellents or toxins. Some of these metabolites are produced on a continuous basis by plants, whereas others—notably compounds called green-leaf volatiles—are only produced once the plant has been attacked. Green-leaf volatiles—which are also responsible for the smell of freshly cut grass—have been observed to provide plants with both direct protection, by inhibiting or repelling herbivores, and indirect protection, by attracting predators of the herbivores themselves.

The hawkmoth Manduca sexta lays its eggs on various plants, including tobacco plants and sacred Datura plants. Once the eggs have hatched into caterpillars, they start eating the leaves of their host plant, and if present in large numbers, these caterpillars can quickly defoliate and destroy it. In an effort to defend itself, the host plant releases green-leaf volatiles to attract various species of Geocoris, and these bugs eat the eggs.

One of the green-leaf volatiles released by tobacco plants is known as (Z)-3-hexenal, but enzymes released by M. sexta caterpillars change some of these molecules into (E)-2-hexenal, which has the same chemical formula but a different structure. The resulting changes in the ‘volatile profile’ alerts Geocoris bugs to the presence of M. sexta eggs and caterpillars on the plant.

Now Allmann et al. show that adult female M. sexta moths can also detect similar changes in the volatile profile emitted by sacred Datura plants that have been damaged by M. sexta caterpillars. This alerts the moths to the fact that Geocoris bugs are likely to be attacking eggs and caterpillars on the plant, or on their way to the plant, so they lay their eggs on other plants. This reduces competition for resources and also reduces the risk of newly laid eggs being eaten by predators. Allmann et al. also identified the neural mechanism that allows moths to detect changes in the volatile profile of plants—the E- and Z- odours lead to different activation patterns in the moth brain.

DOI:http://dx.doi.org/10.7554/eLife.00421.002