Electrophysiological and behavioral responses of a parasitic wasp to plant volatiles induced by two leaf miner species

In the present study, Y-tube olfactometric assays demonstrated that headspace volatile extracts collected from leaf miner-damaged, or artificially damaged, bean plants were more attractive to naive females of the parasitoid insect Opius dissitus than those collected from healthy plants. Headspace extracts from both Liriomyza huidobrensis and Liriomyza sativae second-instar larvae-damaged beans were analyzed by coupled gas chromatography-electroantennographic detection (GC-EAD). Of nine EAD-active volatiles identified, (3E)-4,8-dimethyl-1,3,7-nonatriene, (3Z)-hexenyl acetate, (syn)-2-methylpropanal oxime, and (syn)-2-methylbutanal oxime were the most abundant compounds that evoked significant electroantennogram (EAG) responses. Compounds (3Z)-hexen-1-ol, (anti)-2-methylbutanal oxime, linalool, beta-caryophyllene, and (3E,7E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene also elicited clear EAG responses but were present in smaller amounts. Choice experiments in a Y-tube olfactometer indicated that synthetic versions of (3Z)-hexen-1-ol, 2-methylpropanal oxime, 2-methylbutanal oxime, 3-methylbutanal oxime, linalool, (E,E)-alpha-farnesene, and (3E,7E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene were attractive individually, while (3Z)-hexenyl acetate and (3E)-4,8-dimethyl-1,3,7-nonatriene were unattractive at concentrations similar to those obtained from the headspace collection. Moreover, a blend of nine EAD-active volatiles was significantly more attractive relative to hexane (solvent). A mixture of oximes, tereponids, and green leaf volatiles may facilitate host location by female O. dissitus.     

De Novo Biosynthesis of Volatiles Induced by Insect Herbivory in Cotton Plants

In response to insect feeding on the leaves, cotton (Gossypium hirsutum L.) plants release elevated levels of volatiles, which can serve as a chemical signal that attracts natural enemies of the herbivore to the damaged plant. Pulse-labeling experiments with [13C]CO2 demonstrated that many of the volatiles released, including the acyclic terpenes (E,E)-[alpha]-farnesene, (E)-[beta]-farnesene, (E)-[beta]-ocimene, linalool, (E)-4,8-dimethyl-1,3,7-nonatriene, and (E/E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene, as well as the shikimate pathway product indole, are biosynthesized de novo following insect damage. However, other volatile constituents, including several cyclic terpenes, butyrates, and green leaf volatiles of the lipoxygenase pathway are released from storage or synthesized from stored intermediates. Analysis of volatiles from artificially damaged plants, with and without beet armyworm (Spodoptera exigua Hubner) oral secretions exogenously applied to the leaves, as well as volatiles from beet armyworm-damaged and -undamaged control plants, demonstrated that the application of caterpillar oral secretions increased both the production and release of several volatiles that are synthesized de novo in response to insect feeding. These results establish that the plant plays an active and dynamic role in mediating the interaction between herbivores and natural enemies of herbivores.     

Volatiles from apple (Malus domestica) eliciting antennal responses in female codling moth Cydia pomonella (L.) (Lepidoptera: Tortricidae): effect of plant injury and sampling technique

The antennal responses of codling moth females, Cydia pomonella, to volatiles from apple branches with green fruits were recorded by electroantennography coupled to gas chromatography. The antennae strongly responded to 4,8-dimethyl-1,3(E),7-nonatriene, linalool, beta-caryophyllene, (E)-beta-farnesene, germacrene D, (Z,E)-alpha-farnesene, (E,E)-alpha-farnesene and methyl salicylate. These compounds were all present in volatile collections on Porapak Q from both living and cut branches. Analysis by the solid phase microextraction technique (SPME) showed that the emission of some electrophysiologically active compounds increased after branches had been cut, especially 4,8-dimethyl-1,3(E),7-nonatriene, linalool and (E,E)-alpha-farnesene. The identification of apple volatiles eliciting antennal responses is the first step towards the identification of compounds mediating host-finding and oviposition in codling moth females.     

Emission of herbivore-induced volatiles in absence of a herbivore--response of Zea mays to green leaf volatiles and terpenoids

Green leaf volatiles (GLV), a series of saturated and monounsaturated six-carbon aldehydes, alcohols, and esters are emitted by plants upon mechanical damage. Evidence is increasing that intact plants respond to GLV by activating their own defense mechanisms, thus suggesting that they function in plant-plant communication. The present paper demonstrates that exposure of maize plants to naturally occurring GLV, including (Z)-3-, (E)-2- and saturated derivatives, induce the emission of volatile blends typically associated with herbivory. Position or configuration of a double bond, but not the functional group of the GLV influenced the strength of the emissions. (Z)-3-Configured compounds elicited stronger responses than (E)-2- and saturated derivatives. The response to (Z)-3-hexen-1-ol increased linearly with the dose between 200 and 1000 nmol per plant. Not only the naturally occurring (E)-2-hexenal, but also (E)-2-pentenal and (E)-2-heptenal induced maize plants, although to a lesser extent. Externally applied terpenoids [(3E)-4,8-dimethyl-1,3,7-nonatriene, beta-caryophyllene, and (E)-beta-farnesene] did not significantly increase the total amount of inducible volatiles in maize. Of three tested maize cultivars Delprim and Pactol responded much stronger than Attribut. Recovery experiments in the presence and absence of maize plants demonstrated that large proportions of externally applied GLV were assimilated by the plants, whereas (3E)-4,8-dimethyl-1,3,7-nonatriene was recovered in much higher amounts. The results furthermore suggested that plants converted a part of the assimilated leaf aldehydes and alcohols to the respective acetates. We propose that GLV not only can alert neighboring plants, but may facilitate intra-plant information transfer and can help mediate the systemic defense response in a plant.     

Olfactory preferences of Popillia japonica, Vanessa cardui, and Aphis glycines for Glycine max grown under elevated CO2

Levels of atmospheric CO(2) have been increasing steadily over the last century and are projected to increase even more dramatically in the future. Soybeans (Glycine max L.) grown under elevated levels of CO(2) have larger herbivore populations than soybeans grown under ambient levels of CO(2). Increased abundance could reflect the fact that these herbivores are drawn in by increased amounts of volatiles or changes in the composition of volatiles released by plants grown under elevated CO(2) conditions. To determine impacts of elevated CO(2) on olfactory preferences, Japanese beetles (Popillia japonica Newman) and soybean aphids (Aphis glycines Matsumura) were placed in Y-tube olfactometers with a choice between ambient levels of CO(2) gas versus elevated levels of CO(2) gas or damaged and undamaged leaves and plants grown under ambient levels of CO(2) versus damaged and undamaged plants grown under elevated levels of CO(2). All plants had been grown from seeds under ambient or elevated levels of CO(2). Painted lady butterflies (Vanessa cardui L.) were placed in an oviposition chamber with a choice between plants grown under ambient and elevated levels of CO(2). A. glycines and V. cardui showed no significant preference for plants in either treatment. P. japonica showed no significant preference between ambient levels and elevated levels of CO(2) gas. There was a significant P. japonica preference for damaged plants grown under ambient CO(2) versus undamaged plants but no preference for damaged plants grown under elevated CO(2) versus undamaged plants. P. japonica also preferred damaged plants grown under elevated levels of CO(2) versus damaged plants grown under ambient levels of CO(2). This lack of preference for damaged plants grown under elevated CO(2) versus undamaged plants could be the result of the identical elevated levels of a green leaf volatile (2-hexenal) present in all foliage grown under elevated CO(2) regardless of damage status. Green leaf volatiles are typically released from damaged leaves and are used as kairomones by many herbivorous insects for host plant location. An increase in production of volatiles in soybeans grown under elevated CO(2) conditions may lead to larger herbivore outbreaks in the future.     

Spider Mite-Induced (3S)-(E)-Nerolidol Synthase Activity in Cucumber and Lima Bean. The First Dedicated Step in Acyclic C11-Homoterpene Biosynthesis

Many plant species respond to herbivory with de novo production of a mixture of volatiles that attracts carnivorous enemies of the herbivores. One of the major components in the blend of volatiles produced by many different plant species in response to herbivory by insects and spider mites is the homoterpene 4,8-dimethyl-1,3(E), 7-nonatriene. One study (J. Donath, W. Boland [1995] Phytochemistry 39: 785-790) demonstrated that a number of plant species can convert the acyclic sesquiterpene alcohol (3S)-(E)-nerolidol to this homoterpene. Cucumber (Cucumis sativus L.) and lima bean (Phaseolus lunatus L.) both produce 4,8-dimethyl-1,3(E),7-nonatriene in response to herbivory. We report the presence in cucumber and lima bean of a sesquiterpene synthase catalyzing the formation of (3S)-(E)-nerolidol from farnesyl diphosphate. The enzyme is inactive in uninfested cucumber leaves, slightly active in uninfested lima bean leaves, and strongly induced by feeding of the two-spotted spider mite (Tetranychus urticae Koch) on both plant species, but not by mechanical wounding. The activities of the (3S)-(E)-nerolidol synthase correlated well with the levels of release of 4, 8-dimethyl-1,3(E),7-nonatriene from the leaves of the different treatments. Thus, (3S)-(E)-nerolidol synthase is a good candidate for a regulatory role in the release of the important signaling molecule 4,8-dimethyl-1,3(E),7-nonatriene.     

Emission of herbivore-induced volatile terpenoids from two hybrid aspen (Populus tremula×tremuloides) clones under ambient and elevated ozone concentrations in the field

Tropospheric ozone levels are continuously rising due to human activities in the 21st century. Although the phytotoxic impact of ozone on plants has been well documented, the effect of ozone on plant emissions has received little attention. We have conducted a field-based investigation utilizing two clones of hybrid aspen ( Populus tremula L. × P . tremuloides Michx.) in a free-air ozone concentration enrichment (FACE) facility. The effects of chronic exposure to moderately increased concentrations of ozone on insect-induced terpene emissions by these trees were investigated. We used two herbivore species, Phyllobius piri , and Epirrita autumnata , both of which can reach outbreak levels on deciduous trees in Northern Europe. Our results indicated only very small changes in emissions due to increased ozone levels, but showed induction of some terpenes, particularly the monoterpene trans- β -ocimene and the homoterpene ( E )-4,8-dimethyl-1,3,7-nonatriene, in response to insect feeding. Here, we consider the positive aspects of conducting this type of study in the field and consider the possible influences of other field-based environmental factors.     

Volatile Semiochemicals Released from Undamaged Cotton Leaves (A Systemic Response of Living Plants to Caterpillar Damage)

Cotton plants (Gossypium hirsutum L.), attacked by herbivorous insects release volatile semiochemicals (chemical signals) that attract natural enemies of the herbivores to the damaged plants. We found chemical evidence that volatiles are released not only at the damaged site but from the entire cotton plant. The release of volatiles was detected from upper, undamaged leaves after 2 to 3 d of continuous larval damage on lower leaves of the same plant. Compounds released systemically were (Z)-3-hexenyl acetate, (E)-[beta]-ocimene, linalool, (E)-4,8-dimethyl-1,3,7-nonatriene, (E)-[beta]-farnesene, (E,E)-[alpha]-farnesene, and (E,E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene. All systemically released compounds are known to be induced by caterpillar damage and are not released in significant amounts by undamaged plants. Other compounds, specifically indole, isomeric hexenyl butyrates, and 2-methylbutyrates, known to be released by cotton in response to caterpillar damage, were not released systemically. However, when upper, undamaged leaves of a caterpillar-damaged plant were damaged with a razor blade, they released isomeric hexenyl butyrates, 2-methylbutyrates, and large amounts of constitutive compounds in addition to the previously detected induced compounds. Control plants, damaged with a razor blade in the same way, did not release isomeric hexenyl butyrates or 2-methylbutyrates and released significantly smaller amounts of constitutive compounds. Indole was not released systemically, even after artificial damage.     

Deuterium labeling for investigating de novo synthesis of terpene volatiles in Achyranthes bidentata

Purpose of work

Plants synthesize and accumulate secondary metabolites as defensive volatiles against diverse stresses. We aim to unravel the jasmonate-inducible volatile de novo synthetic metabolites in plants using a deuterium-labeling technique. Jasmonic acid and its methyl ester (MeJA) are well-documented for inducing defensive volatiles. Here, we have developed an efficient deuterium oxide (D₂O)-based labeling approach to determine the extent of de novo synthetic metabolites in a model plant A. bidentata bidentata. The labeling approach was demonstrated on quantitative profiling of terpene volatile organic compounds (VOCs) elicited by airborne MeJA in Achyranthes plants. We show, for the first time that airborne MeJA-elicited terpene VOCs are predominantly and differentially de novo synthesized except for a homoterpene, (3E)-4,8-dimethyl-1,3,7-nonatriene, which is weakly and least labelled with deuterium. D₂O is therefore an efficient labeling source for investigating de novo synthetic metabolites of terpene VOCs in planta.     

Effect of needle damage on the release rate of Masson pine (<Emphasis Type="Italic">Pinus massoniana</Emphasis>) volatiles

The effect of needle damage on the release rate of Masson pine (Pinus massoniana Lamb.) volatiles was examined. Needles were continuously damaged by mechanical damage (MDP) or by feeding of pine caterpillar (Dendrolimus punctatus) larvae (LFP); undamaged pine was used as a control (UDP). Volatiles were collected before damage, and at 16, 24, 40, 48, 64, 72, 88 and 96 h post-damage, and analyzed. The analyses revealed that 19 compounds identified as constitutive volatiles from UDP were terpenes and green leaf odors. The release rate of volatiles from MDP or LFP was higher than that from UDP. At 96 h post-damage, emission from MDP or LFP returned to the same level as that of UDP. Some volatiles, including sabinene, ocimene, limonene-1,2-epoxide, linalool, linalool acetate, germacrene d-4-ol, farnesol, and (E)-4,8-dimethyl-1,3,7-nonatriene were induced by mechanical damage and/or larval attack. Furthermore, the release rate of linalool acetate, farnesol, or (E)-4,8-dimethyl-1,3,7-nonatriene from LFP was higher than that from MDP. Based on an exact estimation of the proportion of damaged pine needles, a significant linear correlation between the release rate of total volatiles identified and the proportion of damaged needles was found in the case of LFP but not MDP.     

Field attraction of the vine weevil Otiorhynchus sulcatus to kairomones

Root weevils in the genus Otiorhynchus are cited as one of the most important pests in the major nursery and small fruit production areas throughout the United States, western Canada, and northern Europe. A major problem in combating weevil attack is monitoring and timing of control measures. Because of the night-activity of the adult weevils growers do not observe the emerging weevils in a timely manner and oviposition often starts before effective control measures are taken. Several vine weevil electroantennogram-active plant volatiles were identified from a preferred host plant, Euonymus fortunei. Main compounds evoking antennal responses on the weevils' antennae were (Z)-2-pentenol, (E)-2-hexenol, (Z)-3-hexenol, methyl benzoate, linalool, (E)-4,8-dimethyl-1,3,7-nonatriene, methyl eugenol, and (E, E)-alpha-farnesene. Several of these compounds were tested alone and in mixtures on attractiveness for the vine weevil Otiorhynchus sulcatus (F.) in field-grown strawberry in Oregon. O. sulcatus were attracted to (Z)-2-pentenol (approximately 3 x more than control) and a 1:1 ratio mixture of (Z)-2-pentenol and methyl eugenol (4.5 x more than control). This is the first report of field-active attractants for O. sulcatus which holds promise for the development of new monitoring strategies for growers in the near future.     

The role of volatiles from cruciferous plants and pre-flight experience in the foraging behaviour of the specialist parasitoid Cotesia plutellae

The braconid Cotesia plutellae is an important larval parasitoid of the diamondback moth (Plutella xylostella), a major pest of crucifers in the tropics and sub-tropics. The in-flight searching behaviour of C. plutellae was investigated in a wind tunnel and the close-range attack behaviour observed in cages. The relative importance of volatile stimuli emanating from the plant-host-complex, oilseed rape (Brassica napus) – P. xylostella, in the long-range attraction of C. plutellae was investigated. Plants that were mechanically damaged, or damaged by P. xylostella larvae, were attractive to the parasitoid. Host-damaged leaves remained attractive to the parasitoid after removal of the host larvae. These results indicate that C. plutellae predominantly uses plant derived stimuli in its in-flight searching behaviour. An oviposition experience or contact with a host-damaged leaf prior to the bioassay significantly increased the response to these volatile cues. The foraging behaviour of C. plutellae is compared with other braconid larval parasitoids attacking lepidopteran hosts on crucifers.     

Growth at elevated CO(2) delays the adverse effects of drought stress on leaf photosynthesis of the C(4) sugarcane

Sugarcane (Saccharum officinarum L. cv. CP72-2086) was grown in sunlit greenhouses at daytime [CO(2)] of 360 (ambient) and 720 (elevated)mumolmol(-1). Drought stress was imposed for 13d when plants were 4 months old, and various photosynthetic parameters and levels of nonstructural carbohydrates were determined for uppermost fully expanded leaves of well-watered (control) and drought stress plants. Control plants at elevated [CO(2)] were 34% and 25% lower in leaf stomatal conductance (g(s)) and transpiration rate (E) and 35% greater in leaf water-use efficiency (WUE) than their counterparts at ambient [CO(2)]. Leaf CO(2) exchange rate (CER) and activities of Rubisco, NADP-malate dehydrogenase, NADP-malic enzyme and pyruvate P(i) dikinase were marginally affected by elevated [CO(2)], but were reduced by drought, whereas activity of PEP carboxylase was reduced by elevated [CO(2)], but not by drought. At severe drought developed at day 12, leaf g(s) and WUE of ambient-[CO(2)] stress plants declined to 5% and 7%, while elevated-[CO(2)] stress plants still maintained g(s) and WUE at 20% and 74% of their controls. In control plants, elevated [CO(2)] did not enhance the midday levels of starch, sucrose, or reducing sugars. For both ambient- and elevated-[CO(2)] stress plants, severe drought did not affect the midday level of sucrose but substantially reduced that of starch. Nighttime starch decomposition in control plants was 55% for ambient [CO(2)] and 59% for elevated [CO(2)], but was negligible for stress plants of both [CO(2)] treatments. For both ambient-[CO(2)] control and stress plants, midday sucrose level at day 12 was similar to the predawn value at day 13. In contrast, sucrose levels of elevated-[CO(2)] control and stress plants at predawn of day 13 were 61-65% of the midday values of day 12. Levels of reducing sugars were much greater for both ambient- and elevated-[CO(2)] stress plants, implying an adaptation to drought stress. Sugarcane grown at elevated [CO(2)] had lower leaf g(s) and E and greater leaf WUE, which helped to delay the adverse effects of drought and, thus, allowed the stress plants to continue photosynthesis for at least an extra day during episodic drought cycles.     

From plants to birds: higher avian predation rates in trees responding to insect herbivory

BACKGROUND: An understanding of the evolution of potential signals from plants to the predators of their herbivores may provide exciting examples of co-evolution among multiple trophic levels. Understanding the mechanism behind the attraction of predators to plants is crucial to conclusions about co-evolution. For example, insectivorous birds are attracted to herbivore-damaged trees without seeing the herbivores or the defoliated parts, but it is not known whether birds use cues from herbivore-damaged plants with a specific adaptation of plants for this purpose. METHODOLOGY: We examined whether signals from damaged trees attract avian predators in the wild and whether birds could use volatile organic compound (VOC) emissions or net photosynthesis of leaves as cues to detect herbivore-rich trees. We conducted a field experiment with mountain birches (Betula pubescens ssp. czerepanovii), their main herbivore (Epirrita autumnata) and insectivorous birds. Half of the trees had herbivore larvae defoliating trees hidden inside branch bags and half had empty bags as controls. We measured predation rate of birds towards artificial larvae on tree branches, and VOC emissions and net photosynthesis of leaves. PRINCIPAL FINDINGS AND SIGNIFICANCE: The predation rate was higher in the herbivore trees than in the control trees. This confirms that birds use cues from trees to locate insect-rich trees in the wild. The herbivore trees had decreased photosynthesis and elevated emissions of many VOCs, which suggests that birds could use either one, or both, as cues. There was, however, large variation in how the VOC emission correlated with predation rate. Emissions of (E)-DMNT [(E)-4,8-dimethyl-1,3,7-nonatriene], beta-ocimene and linalool were positively correlated with predation rate, while those of highly inducible green leaf volatiles were not. These three VOCs are also involved in the attraction of insect parasitoids and predatory mites to herbivore-damaged plants, which suggests that plants may not have specific adaptations to signal only to birds.     

Volatiles released from bean plants in response to agromyzid flies

Liriomyza sativae Blanchard and Liriomyza huidobrensis (Blanchard) (Diptera: Agromyzidae) are two invasive flies in China that have caused economical damage on vegetables and ornamental plants. In this article, we report the profiles of emitted volatiles from healthy, mechanically damaged, and leafminer-damaged bean, Phaseolus vulgaris L., plants. Among 25 emitted volatiles identified, (E)-2-hexen-1-al, (3E)-4,8-dimethyl-1,3,7-nonatriene (DMNT), (Z)-3-hexenyl acetate, (Z)-3-hexen-1-ol, (syn)- and (anti)-2-methylpropanal oxime, (syn)-2-methylbutanal oxime, linalool, and (E,E)-α-farnesene were consistently released from damaged bean plants. Combined amounts of these nine compounds made up more than 70% of the total volatiles emitted from each treatment. No qualitative differences in volatile emission were found between bean plants damaged by the two fly species; however, amounts of several major compounds induced by L. huidobrensis damage were significantly higher than those from plants damaged by L. sativae. The mechanically damaged plants released a higher proportion of green leaf volatiles than plants in the other treatments, whereas leafminer-damaged plants produced more terpenoids and oximes. Furthermore, the volatile profiles emitted from plants, damaged by adult leafminers, by second instar larvae, and even the plants with empty mines left by leafminer larvae (the pupal stage) were significantly different. The identification of volatile oximes released from damaged plants was confirmed and is discussed in a behavioral and biological control context.Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.     

The influence of different nutrient levels on insect-induced plant volatiles in Bt and conventional oilseed rape plants

Transgenic Bt (expressing the cry1Ac endotoxin gene) and conventional oilseed rape plants grown in different soils were used to study nutrient uptake and emission of volatiles after herbivore damage. All plants were greenhouse-grown in soils representing low-, medium- and high-nutrient levels. The concentrations of N, P, K, Mg and Zn were significantly affected by the transgene, while the main effect of soil type appeared in N, P, Ca, Mg, B, Mn and Zn concentrations in the plants. Plants with four to five leaves were infested with the third instar larvae of Bt-susceptible Plutella xylostella for 48 h, and samples of volatiles were collected and analysed. In the first experiment, the soil nutrient level had a significant effect on the emissions of (Z)-3-hexen-1-ol, (Z)-3-hexenyl acetate, hexyl acetate, (E)-4,8-dimethyl-1,3,7-non-atriene (DMNT), beta-elemene, gamma-bisabolene, alpha-bisabolene and (E)-nerolidol. The induction of these volatiles was significantly higher in infested conventional plants grown at a high-soil nutrient level compared to infested conventional plants at a low-soil nutrient level. In the second experiment, the soil nutrient level had a significant effect on the emissions of (Z)-3-hexen-1-ol, (Z)-3-hexenyl acetate and beta-elemene and, again, this was significantly higher in infested conventional plants grown at high-soil nutrient levels in comparison with infested plants at a low-soil nutrient level. In both experiments, the transgene effect was significant on the emissions of DMNT and (E,E)-alpha-farnesene. The differences in emissions between the two separate experiments suggest that growth conditions (particularly daylength) and sampling procedure may affect the ratio of compounds detected in the emission blend, even though the response to herbivory, nutrient availability and the transgene were similar.