Cross-Kingdom Effects of Plant-Plant Signaling via Volatile Organic Compounds Emitted by Tomato (Solanum lycopersicum) Plants Infested by the Greenhouse Whitefly (Trialeurodes vaporariorum)

Herbivore-induced volatiles reduce the susceptibility of neighboring tomato plants to transmission of a whitefly-borne begomovirus

Plant viruses exist in a broader ecological community that includes non-vector herbivores that can impact vector abundance, behavior, and virus transmission within shared host plants. However, little is known about the effects of non-vector herbivore infestation on virus transmission by vector insects on neighboring plants through inter-plant airborne chemicals. In this study, we investigated how volatiles emitted from tomato plants infested with the two-spotted spider mite (Tetranychus urticae) affect the infection of neighboring plants by tomato yellow leaf curl virus (TYLCV) transmitted by whitefly (Bemisia tabaci). Exposure of neighboring tomato plants to volatiles released from T. urticae-infested tomato plants reduced subsequent herbivory as well as TYLCV transmission and infection, and the jasmonic acid signaling pathway was essential for generation of the inter-plant defense signals. We also demonstrated that (E)-β-ocimene and methyl salicylic acid were two volatiles induced by T. urticae that synergistically attenuated TYLCV transmission and infection in tomato. Thus, our findings suggest that plant-plant communication via volatiles likely represents a widespread defensive mechanism that substantially contributes to plant fitness. Understanding such phenomena may help us to predict the occurrence and epidemics of multiple herbivores and viruses in agroecosystems, and ultimately to manage pest and virus outbreaks.

What Do We Know About Capsicum Volatilome?

The Capsicum genus includes several cultivated species that release complex blends of volatile organic compounds (VOCs) associated with their unique aroma. These VOCs are essential info-chemicals in ecological interactions. In this review, we describe how the volatilomic profiling naturally varies based on specific plant organs and genotypes as well as how non-beneficial organisms affect VOCs biosynthesis and accumulation in pepper plants. Also, we show evidence about VOCs variation under the pressure of different abiotic factors such as water stress, soil type and nutrient availability. The contribution of specific metabolic pathways and gene expression related to the biosynthesis of particular VOCs is addressed. We highlighted the utility of VOCs as chemical markers for quality control in the food industry, breeding programs to generate resistant plants and to improve aroma innovation. Herein we present a database containing 2734 VOCs, revealing 113 as the basic core of the volatilome from five Capsicum species.

Effects of far-red light on the behaviour and reproduction of the zoophytophagous predator Macrolophus pygmaeus and its interaction with a whitefly herbivore

Plants can detect neighbouring plants through a reduction in the ratio between red and far-red light (R:FR). This provides a signal of plant-plant competition and induces rapid plant growth while inhibiting defence against biotic stress, two interlinked responses designated as the shade avoidance syndrome (SAS). Consequently, the SAS can influence plant-herbivore interactions that could cascade to higher trophic levels. However, little is known about how the expression of the SAS can influence tritrophic interactions. We investigated whether changes in R:FR affect the emission of herbivore-induced plant volatiles (HIPVs), and whether these changes influence the attraction of the zoophytophagous predator Macrolophus pygmaeus. We also studied how the expression of the SAS and subsequent inhibition of plant defences affects the reproduction of M. pygmaeus in both the presence and absence of the greenhouse whitefly (WF) (Trialeurodes vaporariorum) as arthropod prey. The results show that changes in R:FR have little effect on HIPV emissions and predator attraction. However, a reduction in R:FR leads to increased reproduction of both the predator and the WFs. We discuss that shade avoidance responses can increase the population development of M. pygmaeus through a combination of reduced plant defences and increased herbivore densities.

Better Together: Volatile-Mediated Intraguild Effects on the Preference of Tuta absoluta and Trialeurodes vaporariorum for Tomato Plants

Plant-herbivore interactions have been extensively studied in tomato plants and their most common pests. Tomato plant chemical defenses, both constitutive and inducible, play a role in mediating these interactions. Damaged tomato plants alter their volatile profiles, affecting herbivore preferences between undamaged and damaged plants. However, previous studies on tomato volatiles and herbivore preferences have yielded conflicting results, both in the volatile chemistry itself as well as in the attraction/repellent herbivore response. This study revisits the volatile-mediated interactions between tomato plants and two of their main herbivores: the leafminer Tuta absoluta and the whitefly Trialeurodes vaporariorum. Tomato plant volatiles were analyzed before and after damage by each of these herbivores, and the preference for oviposition (T. absoluta) and settling (T. vaporariorum) on undamaged and damaged plants was assessed both after conspecific and heterospecific damage. We found that both insects consistently preferred damaged plants over undamaged plants. The emission of herbivore-induced plant volatiles (HIPVs) increased after T. absoluta damage but decreased after T. vaporariorum damage. While some of our findings are in line with previous reports, T. absoluta preferred to oviposit on plants damaged by conspecifics, which differs from earlier studies. A comparison of HIPVs emitted after damage by T. absoluta and T. vaporariorum revealed differences in up- or down-regulation, as well as significant variations in specific compounds (12 for T. absoluta and 26 for T. vaporariorum damaged-plants). Only two compounds, β-caryophyllene and tetradecane, significantly varied because of damage by either herbivore, in line with the overall variation of the HIPV blend. Differences in HIPVs and herbivore preferences may be attributed to the distinct feeding habits of both herbivores, which activate different defensive pathways in plants. The plant's challenge in simultaneously activating both defensive pathways may explain the preference for heterospecific damaged plants found in this study, which are also in line with our own observations in greenhouses.

Dynamic distress calls: volatile info chemicals induce and regulate defense responses during herbivory

Plants are continuously threatened by a plethora of biotic stresses caused by microbes, pathogens, and pests, which often act as the major constraint in crop productivity. To overcome such attacks, plants have evolved with an array of constitutive and induced defense mechanisms- morphological, biochemical, and molecular. Volatile organic compounds (VOCs) are a class of specialized metabolites that are naturally emitted by plants and play an important role in plant communication and signaling. During herbivory and mechanical damage, plants also emit an exclusive blend of volatiles often referred to as herbivore-induced plant volatiles (HIPVs). The composition of this unique aroma bouquet is dependent upon the plant species, developmental stage, environment, and herbivore species. HIPVs emitted from infested and non-infested plant parts can prime plant defense responses by various mechanisms such as redox, systemic and jasmonate signaling, activation of mitogen-activated protein (MAP) kinases, and transcription factors; mediate histone modifications; and can also modulate the interactions with natural enemies via direct and indirect mechanisms. These specific volatile cues mediate allelopathic interactions leading to altered transcription of defense-related genes, viz., proteinase inhibitors, amylase inhibitors in neighboring plants, and enhanced levels of defense-related secondary metabolites like terpenoids and phenolic compounds. These factors act as deterrents to feeding insects, attract parasitoids, and provoke behavioral changes in plants and their neighboring species. This review presents an overview of the plasticity identified in HIPVs and their role as regulators of plant defense in Solanaceous plants. The selective emission of green leaf volatiles (GLVs) including hexanal and its derivatives, terpenes, methyl salicylate, and methyl jasmonate (MeJa) inducing direct and indirect defense responses during an attack from phloem-sucking and leaf-chewing pests is discussed. Furthermore, we also focus on the recent developments in the field of metabolic engineering focused on modulation of the volatile bouquet to improve plant defenses.

Abiotic and Herbivory Combined Stress in Tomato: Additive, Synergic and Antagonistic Effects and Within-Plant Phenotypic Plasticity

Background: Drought, N deficiency and herbivory are considered the most important stressors caused by climate change in the agro- and eco-systems and varied in space and time shaping highly dynamic and heterogeneous stressful environments. This study aims to evaluate the tomato morpho-physiological and metabolic responses to combined abiotic and herbivory at different within-plant spatial levels and temporal scales. Methods: Leaf-level morphological, gas exchange traits and volatile organic compounds (VOCs) profiles were measured in tomato plants exposed to N deficiency and drought, Tuta absoluta larvae and their combination. Additive, synergistic or antagonistic effects of the single stress when combined were also evaluated. Morpho-physiological traits and VOCs profile were also measured on leaves located at three different positions along the shoot axes. Results: The combination of the abiotic and biotic stress has been more harmful than single stress with antagonistic and synergistic but non-additive effects for the morpho-physiological and VOCs tomato responses, respectively. Combined stress also determined a high within-plant phenotypic plasticity of the morpho-physiological responses. Conclusions: These results suggested that the combined stress in tomato determined a “new stress state” and a higher within-plant phenotypic plasticity which could permit an efficient use of the growth and defense resources in the heterogeneous and multiple stressful environmental conditions.

Efficacy of Metarhizium anisopliae and (E)-2-hexenal combination using autodissemination technology for the management of the adult greenhouse whitefly, Trialeurodes vaporariorum Westwood (Hemiptera: Aleyrodidae)

The efficiency of an autodissemination technique in controlling adult whiteflies, Trialeurodes vaporariorum Westwood (Hemiptera: Aleyrodidae) on tomato, Solunum lycopersicum was investigated with previously identified potent fungal isolates of Metarhizium anisopliae ICIPE 18, ICIPE 62 and ICIPE 69 under screenhouse or semi-field conditions. The autodissemination device was inoculated with dry conidia of the M. anisopliae isolates, while control insects were exposed to a fungus-free device. Sampling for conidia uptake, conidial viability and persistence, and insect mortality was done at 1, 2, 3, 5 and 8 days post-exposure, and collected insects were monitored for mortality over ten days. Overall, mortality was higher in insects exposed to ICIPE 18 (62.8%) and ICIPE 69 (61.8%) than in those exposed to ICIPE 62 (42.6%), with median lethal times, (LT50) ranging between 6.73-8.54 days. The control group recorded the lowest mortality rates (18.9%). A general linear reduction in conidial viability with exposure time was observed, although this was more pronounced with M. anisopliae ICIPE 62. Insects exposed to M. anisopliae ICIPE 69 also recorded the highest conidia uptake, hence selected for further evaluation with a T. vaporariorum attractant volatile organic compound, (E)-2-hexenal. The volatile inhibited fungal germination in laboratory compatibility tests, therefore, spatial separation of M. anisopliae ICIPE 69 and (E)-2-hexenal in the autodissemination device was conducted. The inhibitory effects of the volatile were significantly reduced by spatial separation at a distance of 5 cm between the fungus and the volatile, which was found to be more suitable and chosen for the subsequent experiments. Results showed that (E)-2-hexenal did not influence conidia uptake by the insects, while fungal viability and the subsequent mortality variations were more related to duration of exposure. The fungus-volatile compatibility demonstrated with spatial separation provides a basis for the optimisation of the volatile formulation to achieve better T. vaporariorum suppression with an excellent autodissemination efficiency when used in the management of whiteflies under screenhouse conditions.

Improved plant heat shock resistance is introduced differently by heat and insect infestation: the role of volatile emission traits

Heat stress is one of the most important abiotic stresses confronted by plants under global climate change. Plant exposure to abiotic or biotic stress can improve its tolerance to subsequent severe episodes of the same or different stress (stress priming), but so far there is limited comparative information about how pre-exposures to different abiotic and biotic elicitors alter plant resistance to severe heat stress. We exposed the perennial herb Melilotus albus Medik., a species rich in secondary metabolites, to moderate heat stress (35 °C) and greenhouse whitefly (Trialeurodes vaporariorum West.) infestation to comparatively determine whether both pre-treatments could enhance plant tolerance to the subsequent heat shock (45 °C) stress. Plant physiological responses to stress were characterized by photosynthetic traits and volatile organic compound emissions through 72 h recovery. Heat shock treatment reduced net assimilation rate (A) and stomatal conductance in all plants, but heat-primed plants had significantly faster rates of recovery of A than other plants. By the end of the recovery period, A in none of the three heat shock-stressed groups recovered to the control level, but in whitefly-infested plants it reached the pre-heat shock level. In heat-primed plants, the heat shock treatment was associated with a fast rise of monoterpene emissions, and in whitefly-infested plants with benzenoid emissions and an increase in total phenolic content.

Repellency Potential of Tomato Herbivore-Induced Volatiles Against the Greenhouse Whitefly (Trialeurodes vaporariorum) (Hemiptera: Aleyrodidae)

The greenhouse whitefly, Trialeurode vaporariorum, is among the key pests of tomato (Solanum lycopersicum) in sub-Saharan Africa with Tuta absoluta, spider mite, thrips, and fruitworms. To understand the interaction between the pest and the plant's herbivory-induced plant volatile (HIPVs), we investigated the repellency of four tomato cultivars (Kilele F1, Assila F1, Red Beauty F1, and Nemonneta F1) upon infestation by Trialeurode vaporariorum. We analyzed the behavioral response of T. vaporariorum to infested and uninfested tomato plants of these cultivars using olfactory bioassays followed by gas chromatography-mass spectrometry (GC-MS) analyses of emitted volatiles. Trialeurode vaporariorum was attracted to uninfested plants of all four tomato cultivars. However, two cultivars Kilele F1 and Red Beauty F1 were no longer attractive to the whitefly when they were already infested by the pest. GC-MS analyses identified 25 compounds, 18 monoterpenes, 3 sesquiterpenes, 2 xylenes, 1 aldehyde, and 1 carboxylic compound in the 4 uninfested and infested cultivars. Based on the insects' behavioral response, 1,8-cineole, p-cymene, and limonene did not attract T. vaporariorum at varying concentrations when combined with Red Beauty F1, the most attractive tomato cultivar. This repellence behavioral response can be used as a basis for improvement of other vegetable crops for the management of arthropod pests as for odor masking technique.

Chemical Cues From Honeydew and Cuticular Extracts of Trialeurodes Vaporariorum Serve as Kairomones for The Parasitoid Encarsia Formosa

Kairomones are semiochemicals that are emitted by an organism and which mediate interspecific interaction that is of benefit to an organism of another species that receives these chemical substances. Parasitoids find and recognize their hosts through eavesdropping on the kairomones emitted from the by-products or the body of the host. Hemipteran insect pests feed on plant sap and excrete the digested plant materials as honeydew. Honeydew serves as a nutritional food source for parasitoids and a medium for micro-organisms whose activity induces the release of volatiles exploited by parasitoids for host location. The parasitoid Encarsia formosa preferentially parasitizes its host, the greenhouse whitefly, Trialeurodes vaporariorum, on tomato Solanum lycopersicum, but little is known about the chemicals that mediate these interactions. We investigated the olfactory responses of the parasitoid E. formosa to odours from honeydew and nymphs of T. vaporariorum in a Y-tube olfactometer. Arrestment behaviour of the parasitoid to honeydew and nymph extracts, as well as to synthetic hydrocarbons, was also observed in Petri-dish bioassays. We found that T. vaporariorum honeydew volatiles attracted the parasitoid E. formosa but odours from the whitefly nymphs did not. We also found that the parasitoid spent more time searching on areas treated with extracts of honeydew and nymphs than on untreated areas. Gas-chromatography-mass spectrometric analysis revealed that the honeydew volatiles contained compounds such as (Z)-3-hexenol, δ-3-carene, 3-octanone, α-phellandrene, methyl salicylate, β-ocimene, β-myrcene, and (E)-β-caryophyllene which are known to be attractive to E. formosa. The cuticular extracts of the nymphs predominantly contained alkanes, alkenes, and esters. Among the alkanes, synthetic nonacosane arrested the parasitoid. Our findings are discussed in relation to how the parasitoid E. formosa uses these chemicals to locate its host, T. vaporariorum.

Integrated management of residues from tomato production: Recovery of value-added compounds and biogas production in the biorefinery context

The biorefinery approach must be boosted in the management of agro-residues in the future. The present study aims to investigate the valorization of tomato production residues, namely rotten tomato (unfit for consumption - RT), green tomato (GT), and tomato branches (TB). The assessment involves the recovery of value-added compounds through the extraction process followed by biogas production through anaerobic digestion. A thorough characterization of the three residues (RT, GT, and TB) was carried out, including the identification of volatile compounds by solid-phase microextraction (SPME) and gas chromatography/mass spectrometry (GC/MS). The volatiles analysis revealed the presence of flavor enhancer compounds and molecules with insecticidal properties. A solid-liquid extraction with ethanol allowed the recovery of value-added compounds in the extracts, in particular phenolic compounds, β-carotene, and lycopene, which contributed to the antioxidant activity. RT and TB extracts were found to be richer in total phenolic compounds (~27 mg GAE/g_db dry basis) and exhibited higher antioxidant activity (IC₅₀ = 0.911 and 0.745 mg/mL). The tomato branches extract had the highest concentration of carotenoids with 37.23 and 3.08 mg/kg_db of β-carotene and lycopene, respectively. The biochemical methane potential (BMP) was assessed in sealed reactors operating in anaerobic conditions for all the raw (RT, GT, and TB) and extracted substrates waste (RTe, GTe, and TBe). While the BMP of RT and GT was in the range of 232-285 mL CH₄/g VS, a lower value of 141 mL CH₄/g VS was obtained for TB. The methane production for each pair of raw and extracted substrates (RT/RTe, GT/GTe, and TB/TBe) was considered statistically similar at a 95 % confidence level. Overall, the value-added compounds recovery through ethanolic extraction did not compromise the methane production of the materials.

Development of Portable E-Nose System for Fast Diagnosis of Whitefly Infestation in Tomato Plant in Greenhouse

An electronic nose (E-nose) system equipped with a gas sensor array and real-time control panel was developed for a fast diagnosis of whitefly infestation in tomato plants. Profile changes of volatile organic compounds (VOCs) released from tomato plants under different treatments (i.e., whitefly infestation, mechanical damage, and no treatment) were successfully determined by the developed E-nose system. A rapid sensor response with high sensitivity towards whitefly-infested tomato plants was observed in the E-nose system. Results of principal component analysis (PCA) and hierarchical clustering analysis (HCA) indicated that the E-nose system was able to provide accurate distinguishment between whitefly-infested plants and healthy plants, with the first three principal components (PCs) accounting for 87.4% of the classification. To reveal the mechanism of whitefly infestation in tomato plants, VOC profiles of whitefly-infested plants and mechanically damaged plants were investigated by using the E-nose system and GC-MS. VOCs of 2-nonanol, oxime-, methoxy-phenyl, and n-hexadecanoic acid were only detected in whitefly-infested plants, while compounds of dodecane and 4,6-dimethyl were only found in mechanically damaged plant samples. Those unique VOC profiles of different tomato plant groups could be considered as bio-markers for diagnosing different damages. Moreover, the E-nose system was demonstrated to have the capability to differentiate whitefly-infested plants and mechanically damaged plants. The relationship between sensor performance and VOC profiles confirmed that the developed E-nose system could be used as a fast and smart device to detect whitefly infestation in greenhouse cultivation.

Changes in plant responses induced by an arthropod influence the colonization behavior of a subsequent herbivore

BACKGROUND

Plants in nature can be sequentially attacked by different arthropod herbivores. Feeding by one arthropod species may induce plant-defense responses that might affect the performance of a later-arriving herbivorous species. Understanding these interactions can help in developing pest-management strategies. In tomato, the sweet-potato whitefly Bemisia tabaci and the two-spotted spider mite Tetranychus urticae are key pests that frequently cohabit on the same plant. We studied whether colonization by one species can either facilitate or impede later colonization of tomato plants by conspecific or heterospecific individuals.

RESULTS

B. tabaci females showed a strong preference for and increased oviposition on plants previously colonized by conspecifics. In contrast, plants infested with T. urticae repelled B. tabaci females and reduced their oviposition rate by 86%. Although females of T. urticae showed no preference between conspecific-infested or uninfested plants, we observed a 50% reduction in the number of eggs laid on conspecific-infested plants. Both herbivorous arthropods up-regulated the expression of genes involving the jasmonic acid and abscisic acid pathways, increasing emissions of fatty-acid derivatives, but only B. tabaci increased the expression of genes related to the salicylic acid pathway and the total amount of phenylpropanoids released. Terpenoids were the most abundant compounds in the volatile blends; many terpenoids were emitted at different rates, which might have influenced the arthropods' host selection.

CONCLUSION

Our results indicate that B. tabaci infestation facilitated subsequent infestations by conspecifics and mites, while T. urticae infestation promoted herbivore-induced resistance. Based on both the molecular and behavioral findings, a novel sustainable pest-management strategy is discussed.

Seed treatment and foliar application of methyl salicylate (MeSA) as a defense mechanism in rice plants against the pathogenic bacterium, Xanthomonas oryzae pv. oryzae

Methyl salicylate (MeSA) is a volatile biological compound synthesized from salicylic acid (SA) and is a plant hormone that helps defend against pests and pathogens. A major bacterial pathogen of rice, Xanthomonas oryzae pv. oryzae (Xoo) causes severe disease. Seed and plant treatments with MeSA can stimulate the defense enzyme peroxidase (POD) in plants. Response of peroxidase activity in rice (Oryza sativa L) cultivars IR 20, IR 50, IR 64, ASD 16, ASD 19 and ADT 46 to MeSA were measured under greenhouse conditions. Treatments of rice seedlings with MeSA at 50 and 100 mg L^-1 significantly upregulated POD expression in the plants. The activity of POD was also significantly upregulated when plants were inoculated with bacterial blight. Effects were stronger in ASD 16, ASD 19 and ADT 46 and were more pronounced in high dose treatment (100 mg L^-1) when inoculated with bacterial blight condition and the effects were dose dependent, although the relationship between dose and rice varieties were not always linear. The pathogenic related (PR) protein bands at 33 kDa and 14 kDa were identified in treatments of 100 mg L^-1 MeSA in the presence of bacterial blight disease. Band intensity was estimated to be twice that of those from pathogen induce MeSA levels in rice plants. These results suggest that treatment with MeSA can significantly increase the POD defense related enzyme by altering the plant physiology in ways that may be beneficial for crop protection.

Prior Experience with Food Reward Influences the Behavioral Responses of the Honeybee Apis mellifera and the Bumblebee Bombus lantschouensis to Tomato Floral Scent

Simple Summary

Bees are important pollinators for many agricultural crops. Compared with bumblebees, honeybees are less attracted to tomato flowers. Floral scent usually plays an important role in mediating the foraging behavior of bees, and tomato flowers release special scents. However, little is known about how tomato floral scent regulates the foraging behaviors of these two bee taxa. In the current study, we investigated the foraging behaviors of the widely used pollinator honeybee Apis mellifera and a native bumblebee, Bombus lantschouensis, on tomato flowers to evaluate the potential application of these two bee species for tomato pollination in solar greenhouses. Moreover, we determined whether honeybees and bumblebees show different responses to tomato floral scent and how innate biases and prior experience influence bee choice behavior. We found that naïve bees showed no preference for tomato floral scent but could develop such a preference after learning to associate tomato floral scent with a food reward on the basis of foraging experience or scent-learning procedures. We conclude that scent-learning experiences with food reward can change the innate bias of bees and could be utilized to improve the pollination service efficiency of bees for commercial crops.

Abstract

Bee responses to floral scent are usually influenced by both innate biases and prior experience. Honeybees are less attracted than bumblebees to tomato flowers. However, little is known about how tomato floral scent regulates the foraging behaviors of honeybees and bumblebees. In this study, the foraging behaviors of the honeybee Apis mellifera and the bumblebee Bombus lantschouensis on tomato flowers in greenhouses were investigated. Whether the two bee species exhibit different responses to tomato floral scent and how innate biases and prior experience influence bee choice behavior were examined. In the greenhouses, honeybees failed to collect pollen from tomato flowers, and their foraging activities decreased significantly over days. Additionally, neither naïve honeybees nor naïve bumblebees showed a preference for tomato floral scent in a Y-tube olfactometer. However, foraging experience in the tomato greenhouses helped bumblebees develop a strong preference for the scent, whereas honeybees with foraging experience continued to show aversion to tomato floral scent. After learning to associate tomato floral scent with a sugar reward in proboscis extension response (PER) assays, both bee species exhibited a preference for tomato floral scent in Y-tube olfactometers. The findings indicated that prior experience with a food reward strongly influenced bee preference for tomato floral scent.

Volatile Organic Compounds as Insect Repellents and Plant Elicitors: an Integrated Pest Management (IPM) Strategy for Glasshouse Whitefly (Trialeurodes vaporariorum)

The glasshouse whitefly (Trialeurodes vaporariorum Westwood) is a polyphagous arthropod pest that is of particular detriment to glasshouse grown tomato (Solanum lycopersicum) across temperate regions of the world. Control of whiteflies with synthetic pesticides has resulted in the evolution of resistant genotypes and a reduction in natural enemies, thus highlighting the need for environmentally sound control strategies. Volatile organic compounds (VOCs) offer an environmentally benign alternative to synthetic chemical sprays and this study explored the use of VOCs as insect repellents and plant defence elicitors to control whiteflies on tomato in a commercial glasshouse setting. Limonene in the form of a volatile dispenser system was found to successfully repel whitefly from the target crop and increased fruit yield by 32% during a heavy whitefly infestation. Analysis of tomato herbivore induced plant volatiles (HIPVs) led us to select methyl salicylate (MeSA) as the plant elicitor and application of MeSA to un-infested tomato plants was found to successfully reduce whitefly population development and increase yield by 11%, although this difference was marginally statistically significant. Combination of these two methods was also effective but whitefly abundance in combined plots was similar to the standalone limonene treatment across the course of the experiment. All of the VOC based control methods we used had a negative impact on whitefly performance, with more pronounced effects during the first few weeks of infestation. In subsequent laboratory experiments, we found elevated peroxidase (POD) activity and a significant increase in TPX1 and PR1 transcripts in MeSA treated plants. This led us to deduce that MeSA immediately induced plant defences, rather than priming them. We did however see evidence for residual priming, as plants treated with MeSA and infested with whiteflies produced significantly higher levels of POD activity than whitefly infestation alone. Despite the fact that our treatments failed to synergise, our methods can be optimised further, and the effectiveness of the standalone treatments is promising for future studies. In particular, our repellent limonene dispensers were extremely effective at deterring whiteflies and offer a low economic cost and easy to implement whitefly control option. The methods we have used here could be incorporated into current integrated pest management (IPM) systems, a sustainable approach to pest control which will be central to our efforts to manage whitefly populations under glass in the future.

Predatory functional response and fitness parameters of Orius strigicollis Poppius when fed Bemisia tabaci and Trialeurodes vaporariorum as determined by age-stage, two-sex life table

Background

The polyphagous predatory bug O. strigicollis is an active predator used to control thrips and aphids. The whitefly species Bemisia tabaci and Trialeurodes vaporariorum are voracious pests of different economic agricultural crops and vegetables.

Method

In this study, the Holling disc equation and the age-stage, two-sex life table technique were used to investigate the functional response and biological traits of third instar nymphs and adult female O. strigicollis when presented third instar nymphs of both whitefly species as prey.

Results

The results showed a type II functional response for each life stage of O. strigicollis when fed each whitefly species. The calculated prey handling time for different O. strigicollis life stages were shorter when fed T. vaporariorum than when fed B. tabaci nymphs. In contrast, the nymphal development of O. strigicollis was significantly shorter when fed B. tabaci than T. vaporariorum nymphs. Additionally, the total pre-oviposition period of adult females was statistically shorter when fed B. tabaci nymphs than T. vaporariorum nymphs. Furthermore, the survival rates and total fecundity of O. strigicollis were higher when fed B. tabaci than T. vaporariorum. There were no significant differences in any population parameters of O. strigicollis when fed either whitefly species. These results show that O. strigicollis could survive and maintain its populations on both species of whitefly and could therefore serve as a biological control agent in integrated pest management (IPM).

Oviposition-Induced Volatiles Affect Electrophysiological and Behavioral Responses of Egg Parasitoids

In response to an attack by herbivores, plants emit a variety of compounds that may act as semiochemicals. Oviposition-induced volatiles (OIPVs) have been shown to mediate interactions between plants and natural enemies. Here, we investigated the role of OIPVs by Tuta absoluta towards two egg parasitoids, Trichogramma cordubense and T. achaeae. We collected headspace volatiles from tomato plants at 24, 48, and 72 h after oviposition by T. absoluta females and tested the antennographic response of Trichogramma parasitoids to them by means of gas chromatography- electro-antennographical detection (GC-EAD). The response of the parasitoids was also tested in behavioral experiments using a Y-tube olfactometer. Oviposition by T. absoluta females induced qualitative and quantitative changes in the volatiles emitted by tomato plants. Antennae of Trichogramma parasitoids responded to several of the induced volatiles in GC-EAD. T. cordubense females were attracted to tomato plants with T. absoluta eggs 24 h after oviposition. The elucidation of the behavior of egg parasitoids towards OIPVs enhances the development of sustainable management strategies either by selecting species that exploit OIPVs or by manipulating their foraging behavior by utilizing specific OIPVs that are used by parasitoids as a host location.

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.

Attraction of Three Mirid Predators to Tomato Infested by Both the Tomato Leaf Mining Moth Tuta absoluta and the Whitefly Bemisia tabaci

Plants emit volatile compounds in response to insect herbivory, which may play multiple roles as defensive compounds and mediators of interactions with other plants, microorganisms and animals. Herbivore-induced plant volatiles (HIPVs) may act as indirect plant defenses by attracting natural enemies of the attacking herbivore. We report here the first evidence of the attraction of three Neotropical mirid predators (Macrolophus basicornis, Engytatus varians and Campyloneuropsis infumatus) toward plants emitting volatiles induced upon feeding by two tomato pests, the leaf miner Tuta absoluta and the phloem feeder Bemisia tabaci, in olfactometer bioassays. Subsequently, we compared the composition of volatile blends emitted by insect-infested tomato plants by collecting headspace samples and analyzing them with GC-FID and GC-MS. Egg deposition by T. absoluta did not make tomato plants more attractive to the mirid predators than uninfested tomato plants. Macrolophus basicornis is attracted to tomato plants infested with either T. absoluta larvae or by a mixture of B. tabaci eggs, nymphs and adults. Engytatus varians and C. infumatus responded to volatile blends released by tomato plants infested with T. absoluta larvae over uninfested plants. Also, multiple herbivory by T. absoluta and B. tabaci did not increase the attraction of the mirids compared to infestation with T. absoluta alone. Terpenoids represented the most important class of compounds in the volatile blends and there were significant differences between the volatile blends emitted by tomato plants in response to attack by T. absoluta, B. tabaci, or by both insects. We, therefore, conclude that all three mirids use tomato plant volatiles to find T. absoluta larvae. Multiple herbivory did neither increase, nor decrease attraction of C. infumatus, E. varians and M. basicornis. By breeding for higher rates of emission of selected terpenes, increased attractiveness of tomato plants to natural enemies may improve the effectiveness of biological control.

Phytoplasma infection of a tropical root crop triggers bottom-up cascades by favoring generalist over specialist herbivores

Global interest on plant-microbe-insect interactions is rapidly growing, revealing the multiple ways in which microorganisms mediate plant-herbivore interactions. Phytopathogens regularly alter whole repertoires of plant phenotypic traits, and bring about shifts in key chemical or morphological characteristics of plant hosts. Pathogens can also cause cascading effects on higher trophic levels, and eventually shape entire plant-associated arthropod communities. We tested the hypothesis that a Candidatus Phytoplasma causing cassava witches' broom (CWB) on cassava (Manihot esculenta Grantz) is altering species composition of invasive herbivores and their associated parasitic hymenopterans. We conducted observational studies in cassava fields in eastern Cambodia to assess the effect of CWB infection on abundance of specialist and generalist mealybugs (Homoptera: Pseudococcidae), and associated primary and hyper-parasitoid species. CWB infection positively affects overall mealybug abundance and species richness at a plant- and field-level, and disproportionately favors a generalist mealybug over a specialist feeder. CWB phytoplasma infection led to increased parasitoid richness and diversity, with richness of 'comparative' specialist taxa being the most significantly affected. Parasitism rate did not differ among infected and uninfected plants, and mealybug host suppression was not impacted. CWB phytoplasma modifies host plant quality for sap-feeding homopterans, differentially affects success rates of two invasive species, and generates niche opportunities for higher trophic orders. By doing so, a Candidatus phytoplasma affects broader food web structure and functioning, and assumes the role of an ecosystem engineer. Our work unveils key facets of phytoplasma ecology, and sheds light upon complex multi-trophic interactions mediated by an emerging phytopathogen. These findings have further implications for invasion ecology and management.

GC-MS analysis of volatile compounds of Perilla frutescens Britton var. Japonica accessions: Morphological and seasonal variability

OBJECTIVE

To investigate the composition of volatile compounds in the different accessions of Perilla frutescens (P. frutescens) collected from various habitats of China and Japan.

METHODS

In the present study, the essential oil from the leaves of P. frutescens cultivars from China and Japan was extracted by hydro-distillation and the chemical composition and concentration of the volatile components present in the oils were determined by gas chromatography-mass spectrometry (GC-MS) analysis.

RESULTS

Among the volatile components, the major proportion was of perilla ketone, which was followed by elemicin and beta-caryophyllene in the Chinese Perilla cultivars. The main component in the oil extracted from the Japanese accessions was myristicin, which was followed by perilla ketone and beta-caryophyllene. We could distinguish seven chemotypes, namely the perilla ketone (PK) type, perilla ketone, myristicin (PM) type, perilla ketone, unknown (PU) type, perilla ketone, beta-caryophyllene, myristicine (PB) type, perilla ketone, myristicin, unknown (PMU) type, perilla ketone, elemicine, myristicin, beta-caryophyllene (PEMB) type, and the perilla ketone, limonene, beta-cryophyllene, myristicin (L) type. Most of the accessions possessed higher essential oil content before the flowering time than at the flowering stage. The average plant height, leaf length, leaf width of the Chinese accessions was higher than those of the Japanese accessions.

CONCLUSION

The results revealed that the harvest time and geographical origin caused polymorphisms in the essential oil composition and morphological traits in the Perilla accessions originating from China and Japan. Therefore, these chemotypes with desirable characters might be useful for industrial exploitation and for determining the harvest time.

Comparative host selection responses of specialist (Helicoverpa assulta) and generalist (Helicoverpa armigera) moths in complex plant environments

We tested the behavioral responses of ovipositing females and natal larvae of two sibling species, a generalist Helicoverpa armigera (Hübner) and a specialist Helicoverpa assulta (Guenée), to odor sources emitted from different combinations of six plant species (tobacco, Nicotiana tabacum; hot pepper, Capsicum annuum; tomato, Solanum esculentum; cotton, Gossypium hirsutum; peanut, Arachis hypogaea; maize, Zea mays). Under the conditions of plant materials versus corresponding controls, both stages of both species could find their corresponding host plants. However, H. assulta females and larvae exhibited a supersensitive and an insensitive response, respectively. Under the conditions of tobacco paired with each plant species, H. assulta females exhibited more specialized ovipositional response to tobacco than its sibling. When each plant species were combined with tobacco and tested against tobacco reference, peanut played an opposite role in the two species in their ovipositional responses to tobacco, and cotton can enhance the approaching response of H. armigera larvae when combined with tobacco. It seems that two attractive host plants also can act antagonistically with respect to host selection of the generalist via volatile exchange. Tomato should better be excluded from host list of H. assulta.

Qualitative and Quantitative Differences in Herbivore-Induced Plant Volatile Blends from Tomato Plants Infested by Either Tuta absoluta or Bemisia tabaci

Plants release a variety of volatile organic compounds that play multiple roles in the interactions with other plants and animals. Natural enemies of plant-feeding insects use these volatiles as cues to find their prey or host. Here, we report differences between the volatile blends of tomato plants infested with the whitefly Bemisia tabaci or the tomato borer Tuta absoluta. We compared the volatile emission of: (1) clean tomato plants; (2) tomato plants infested with T. absoluta larvae; and (3) tomato plants infested with B. tabaci adults, nymphs, and eggs. A total of 80 volatiles were recorded of which 10 occurred consistently only in the headspace of T. absoluta-infested plants. Many of the compounds detected in the headspace of the two herbivory treatments were emitted at different rates. Plants damaged by T. absoluta emitted at least 10 times higher levels of many compounds compared to plants damaged by B. tabaci and intact plants. The multivariate separation of T. absoluta-infested plants from those infested with B. tabaci was due largely to the chorismate-derived compounds as well as volatile metabolites of C₁₈-fatty acids and branched chain amino acids that had higher emission rates from T. absoluta-infested plants, whereas the cyclic sesquiterpenes α- and β-copaene, valencene, and aristolochene were emitted at significantly higher levels from B. tabaci-infested plants. Our findings imply that feeding by T. absoluta and B. tabaci induced emission of volatile blends that differ quantitatively and qualitatively, providing a chemical basis for the recently documented behavioral discrimination by two generalist predatory mirid species, natural enemies of T. absoluta and B. tabaci employed in biological control.

Headspace-Solid Phase Microextraction Approach for Dimethylsulfoniopropionate Quantification in Solanum lycopersicum Plants Subjected to Water Stress

Dimethylsulfoniopropionate (DMSP) and dimethyl sulphide (DMS) are compounds found mainly in marine phytoplankton and in some halophytic plants. DMS is a globally important biogenic volatile in regulating of global sulfur cycle and planetary albedo, whereas DMSP is involved in the maintenance of plant-environment homeostasis. Plants emit minute amounts of DMS compared to marine phytoplankton and there is a need for hypersensitive analytic techniques to enable its quantification in plants. Solid Phase Micro Extraction from Head Space (HS-SPME) is a simple, rapid, solvent-free and cost-effective extraction mode, which can be easily hyphenated with GC-MS for the analysis of volatile organic compounds. Using tomato (Solanum lycopersicum) plants subjected to water stress as a model system, we standardized a sensitive and accurate protocol for detecting and quantifying DMSP pool sizes, and potential DMS emissions, in cryoextracted leaves. The method relies on the determination of DMS free and from DMSP pools before and after the alkaline hydrolysis via Headspace-Solid Phase Micro Extraction-Gas Chromatography-Mass Spectrometry (HS-SPME-GC-MS). We found a significant (2.5 time) increase of DMSP content in water-stressed leaves reflecting clear stress to the photosynthetic apparatus. We hypothesize that increased DMSP, and in turn DMS, in water-stressed leaves are produced by carbon sources other than direct photosynthesis, and function to protect plants either osmotically or as antioxidants. Finally, our results suggest that SPME is a powerful and suitable technique for the detection and quantification of biogenic gasses in trace amounts.

A portion of plant airborne communication is endorsed by uptake and metabolism of volatile organic compounds

Plants have the ability to sense volatile organic compounds (VOCs) so as to efficiently adapt to their environment. The mechanisms underlying such plant 'olfactory' systems are largely unknown. Here I would like to propose that the metabolism of VOCs in plant tissues is one of the mechanisms by which plants sense VOCs. During the gas-exchange that is essential for photosynthesis, VOCs in the atmosphere are taken into the intercellular spaces of leaves. Each VOC is partitioned between the gas phase (intercellular space) and liquid phase (cell wall) at a certain ratio determined by Henry's law. The VOCs in the cell wall diffuse through the plasma membrane to the cytosol depending on their oil/water partition coefficients. Plants detoxify some VOCs, especially those that are oxidized, through glycosylation, glutathionylation, and reduction. These metabolic processes lower the concentration of VOCs in the cytosol, which facilitates further cytosolic uptake. As a result, vigorous metabolism of VOCs in the cytosol can lead to a substantial accumulation of VOC metabolites and the depletion of glutathione or NADPH. One such metabolite (a VOC glycoside) is known to mount a direct defense against herbivores, whilst deprivation of glutathione and NADPH can fortify plants with responses similar to the oxidative stress response.

Single- versus Multiple-Pest Infestation Affects Differently the Biochemistry of Tomato (Solanum lycopersicum 'Ailsa Craig')

Tomato is susceptible to pest infestations by both spider mites and aphids. The effects of each individual pest on plants are known, whereas multiple-pest infestations have received little interest. We studied the effects of single- versus multiple-pest infestation by Tetranychus urticae and Myzus persicae on tomato biochemistry (Solanum lycopersicum) by combining a metabolomic approach and analyses of carotenoids using UHPLC-ToF-MS and volatiles using GC-MS. Plants responded differently to aphids and mites after 3 weeks of infestation, and a multiple infestation induced a specific metabolite composition in plants. In addition, we showed that volatiles emissions differed between the adaxial and abaxial leaf epidermes and identified compounds emitted particularly in response to a multiple infestation (cyclohexadecane, dodecane, aromadendrene, and β-elemene). Finally, the carotenoid concentrations in leaves and stems were more affected by multiple than single infestations. Our study highlights and discusses the interplay of biotic stressors within the terpenoid metabolism.

Population structure of the greenhouse whitefly, Trialeurodes vaporariorum (Westwood), an invasive species from the Americas, 60 years after invading China

Though the greenhouse whitefly, Trialeurodes vaporariorum (Westwood) (Hemiptera: Aleyrodidae) was introduced into China more than 60 years ago, the genetic diversity and structure of this exotic insect pest and virus vector have not been studied. To investigate the population genetic characteristics of this invasive species and to identify potential invasion routes, the genetic diversity and population structure of 17 collections of T. vaporariorum from nine provinces in China were analyzed using seven microsatellite loci. The results of the analyses indicated that the genetic diversity for the populations examined from the four provinces: Jilin, Ningxia, Guizhou and Qinghai, was lower than the genetic diversity of populations from the five provinces: Yunnan, Shandong, Shanxi, Liaoning, and Gansu. The T. vaporariorum populations analyzed in this study grouped as two distinct genetic clusters based on the analysis using STRUCTURE, whereas, 8 clusters were identified based on the BAPS analysis. Of the 136 genetic distance (Fst) values, 128 (94%) were associated with a significant exact test. However, there was no significant relationship between Fst and geographical distance. These results demonstrate that populations of T. vaporariorum in China exhibit significant genetic differentiation, indicating the likelihood that multiple introductions of T. vaporariorum into China have occurred. Also, the populations collected from the provinces of Jilin, Ningxia, Guizhou and Qinghai appear to represent secondary introductions originating from other Chinese provinces.

Intraguild predation on the whitefly parasitoid Eretmocerus eremicus by the generalist predator Geocoris punctipes: a behavioral approach

Intraguild predation (IGP) takes place when natural enemies that use similar resources attack each other. The impact of IGP on biological control can be significant if the survival of natural enemy species is disrupted. In the present study, we assessed whether Geocoris punctipes (Hemiptera: Lygaeidae) engages in IGP on Eretmocerus eremicus (Hymenoptera: Aphelinidae) while developing on whitefly nymphs of Trialeurodes vaporariorum (Hemiptera: Aleyrodidae). In choice and non-choice tests, we exposed G. punctipes to parasitized and non-parasitized whitefly nymphs. We found that G. punctipes does practice IGP on E. eremicus. However, choice tests assessing G. punctipes consumption revealed a significant preference for non-parasitized T. vaporariorum nymphs. Subsequently, we investigated whether E. eremicus females modify their foraging behavior when exposed to conditions involving IGP risk. To assess this, we analyzed wasp foraging behavior under the following treatments: i) whitefly nymphs only (control = C), ii) whitefly nymphs previously exposed to a predator ( = PEP) and, iii) whitefly nymphs and presence of a predator ( = PP). In non-choice tests we found that E. eremicus did not significantly modify its number of attacks, attack duration, oviposition duration, or behavior sequences. However, E. eremicus oviposited significantly more eggs in the PEP treatment. In the PP treatment, G. punctipes also preyed upon adult E. eremicus wasps, significantly reducing their number of ovipositions and residence time. When the wasps were studied under choice tests, in which they were exposed simultaneously to all three treatments, the number of attacks and frequency of selection were similar under all treatments. These results indicate that under IGP risk, E. eremicus maintains several behavioral traits, but can also increase its number of ovipositions in the presence of IG-predator cues. We discuss these findings in the context of population dynamics and biological control.

Where do herbivore-induced plant volatiles go?

Herbivore induced plant volatiles (HIPVs) are specific volatile organic compounds (VOC) that a plant produces in response to herbivory. Some HIPVs are only produced after damage, while others are also produced by intact plants, but in lower quantities. Among the known functions of HIPVs are within plant volatile signaling to activate systemic plant defenses, the priming and activation of defenses in neighboring plants and the attraction of natural enemies of herbivores. When released into the atmosphere a plant's control over the produced compounds ends. However, many of the HIPVs are highly reactive with atmospheric oxidants and their atmospheric life times could be relatively short, often only a few minutes. We summarise the potential ecological and atmospheric processes that involve the reaction products of HIPVs in their gaseous, liquid and solid secondary organic aerosol (SOA) forms, both in the atmosphere and after deposition on plant surfaces. A potential negative feedback loop, based on the reactions forming SOA from HIPVs and the associated stimulation of sun screening cloud formation is presented. This hypothesis is based on recent field surveys in the geographical areas facing the greatest degree of global warming and insect outbreaks. Furthermore, we discuss how these processes could benefit the individual plant or conspecifics that originally released the HIPVs into the atmosphere. Further ecological studies should aim to elucidate the possible reasons for biosynthesis of short-lived volatile compounds to have evolved as a response to external biotic damage to plants.