Defense Suppression through Interplant Communication Depends on the Attacking Herbivore Species

In response to herbivory, plants emit volatile compounds that play important roles in plant defense. Herbivore-induced plant volatiles (HIPVs) can deter herbivores, recruit natural enemies, and warn other plants of possible herbivore attack. Following HIPV detection, neighboring plants often respond by enhancing their anti-herbivore defenses, but a recent study found that herbivores can manipulate HIPV-interplant communication for their own benefit and suppress defenses in neighboring plants. Herbivores induce species-specific blends of HIPVs and how these different blends affect the specificity of plant defense responses remains unclear. Here we assessed how HIPVs from zucchini plants (Cucurbita pepo) challenged with different herbivore species affect resistance in neighboring plants. Volatile “emitter” plants were damaged by one of three herbivore species: saltmarsh caterpillars (Estigmene acrea), squash bugs (Anasa tristis), or striped cucumber beetles (Acalymma vittatum), or were left as undamaged controls. Neighboring “receiver” plants were exposed to HIPVs or control volatiles and then challenged by the associated herbivore species. As measures of plant resistance, we quantified herbivore feeding damage and defense-related phytohormones in receivers. We found that the three herbivore species induced different HIPV blends from squash plants. HIPVs induced by saltmarsh caterpillars suppressed defenses in receivers, leading to greater herbivory and lower defense induction compared to controls. In contrast, HIPVs induced by cucumber beetles and squash bugs did not affect plant resistance to subsequent herbivory in receivers. Our study shows that herbivore species identity affects volatile-mediated interplant communication in zucchini, revealing a new example of herbivore defense suppression through volatile cues.

     

The Impact of Herbivore-Induced Plant Volatiles on Parasitoid Foraging Success: A General Deterministic Model

Parasitoids respond to volatiles that plants produce when injured by herbivores. A considerable body of literature addresses the chemical pathways of herbivore-induced volatile production. However, there is almost no theory or data on how timing of volatile release in relationship to host availability for parasitization impacts the utility of these cues to parasitoids and on the extent that this volatile release timing might increase or decrease the percent of herbivores that become parasitized. This kind of information is critical in judging the benefits that might accrue from a breeding program aimed at enhancing herbivore-responsive volatile production. We developed a general model to begin examining this issue by using available parameters from two tritrophic systems. The model uses herbivore oviposition, development, and mortality rates, linked to a range of plant volatile induction and cessation periods for calculating the proportion of plants in a field that are (1) not producing volatiles but occupied by suitable herbivore hosts, (2) producing volatiles and occupied by suitable herbivore hosts, (3) producing volatiles but not occupied by suitable herbivore hosts, and (4) not producing volatiles and not occupied by suitable herbivore hosts. The impact of the plant volatiles on parasitoid foraging success is then determined by comparing the expected number of hosts parasitized when the parasitoid focuses solely on the volatile-producing plants to when it forages randomly among all plants. Under some conditions, parasitoids can attack three times more herbivores if they focus on volatile-producing plants. However, when we simulate plants that take several days to cease volatile production after pupation or death of the herbivore, parasitization rate does not increase when parasitoids use volatiles as cues. The utility of the volatile cues is consistently greater when a smaller proportion of plants is occupied by herbivores, indicating that their usefulness may be reduced to zero in fields saturated with volatiles.     

Variation in Herbivore and Methyl Jasmonate-Induced Volatiles Among Genetic Lines of <Emphasis Type="Italic">Datura wrightii</Emphasis>

Many plant species produce volatile organic compounds after being damaged by herbivores. The production of volatiles also may be induced by exposing plants to the plant hormone, jasmonic acid, or its volatile ester, methyl jasmonate. This study addresses the induction of the production volatile organic compounds among genetic lines of Datura wrightii. Within populations, some plants produce glandular trichomes, whereas others produce nonglandular trichomes, and trichome phenotype is controlled by a single dominant gene. Glandular trichomes not only confer resistance to some herbivorous insects, but they also inhibit many natural enemies of those herbivores. Because of the potential benefit of natural enemies that use volatile cues to find individuals of the non-glandular phenotype, it is reasonable to ask if plants of D. wrightii that differ in trichome morphology might produce different blends of volatile compounds. Volatile compounds were collected from eight genetic lines of plants that had been backcrossed for three generations. Volatiles were collected from pairs of sibling plants before and after insect damage or treatment with methyl jasmonate. Within each pair, one sib expressed glandular trichomes and the other expressed nonglandular trichomes. Overall, plants produced an array of at least 17 compounds, most of which were sesquiterpenes. Total production of volatiles increased from 3.9- to 16.2-fold among genetic lines after insect damage and from 3.6- to 32-fold in plants treated with methyl jasmonate. The most abundant compound was (E)-β-caryophyllene. This single compound comprised from 17 to 59% of the volatiles from insect-damaged plants and from 24 to 88% of the volatiles from plants treated with methyl jasmonate, depending upon genetic line. The production of (E)-β-caryophyllene by the original male parents of the eight genetic lines was significantly related to the mean production of their third-generation backcross progeny indicating that the variation in the production of (E)-β-caryophyllene was inherited. Blends did not differ qualitatively or quantitatively between sibs expressing glandular or nonglandular trichomes.     

Season-Long Volatile Emissions from Peach and Pear Trees In Situ, Overlapping Profiles, and Olfactory Attraction of an Oligophagous Fruit Moth in the Laboratory

Insect herbivores that have more than one generation per year and reproduce on different host plants are confronted with substantial seasonal variation in the volatile blends emitted by their hosts. One way to deal with such variation is to respond to a specific set of compounds common to all host plants. The oriental fruit moth Cydia (=Grapholita) molesta is a highly damaging invasive pest. The stone fruit peach (Prunus persica) is its primary host, whereas pome fruits such as pear (Pyrus communis) are considered secondary hosts. In some parts of their geographic range, moth populations switch from stone to pome fruit orchards during the growing season. Here, we tested whether this temporal switch is facilitated by female responses to plant volatiles. We collected volatiles from peach and pear trees in situ and characterized their seasonal dynamics by gas chromatography–mass spectrometry. We also assessed the effects of the natural volatile blends released by the two plant species on female attraction by using Y-tube olfactometry. Finally, we related variations in volatile emissions to female olfactory responses. Our results indicate that the seasonal host switch from peach to pear is facilitated by the changing olfactory effect of the natural volatile blends being emitted. Peach volatiles were only attractive early and mid season, whereas pear volatiles were attractive from mid to late season. Blends from the various attractive stages shared a common set of five aldehydes, which are suggested to play an essential role in female attraction to host plants. Particular attention should be given to these aldehydes when designing candidate attractants for oriental fruit moth females.     

Differential activity and degradation of plant volatile elicitors in regurgitant of tobacco hornworm (Manduca sexta) larvae

Plants respond to insect herbivory by emitting volatile compounds that attract natural enemies of the herbivores. Biosynthesis of many of these volatiles in plants is induced by herbivore-produced compounds. Components of tobacco hornworm (THW) regurgitant were investigated for their efficacy as elicitors of corn seedling volatiles. Two components that elicited the strongest release of volatiles were isolated and identified as N-linolenoyl-L-glutamine (18:3-GLN) and N-linolenoyl-L-glutamic acid (18:3-GLU). The approximately 10 times more active 18:3-GLN, which also is found in the regurgitant of several other Lepidopteran larvae, was rapidly degraded when THW regurgitant was left at room temperature, while 18:3-GLU degraded at a much slower rate. Different dietary sources of THW and tobacco bud worm larvae, including both host and nonhost plants, did not affect the amino acid composition of the fatty acid–amino acid conjugates in the regurgitant.     

Foliar Terpene Chemotypes and Herbivory Determine Variation in Plant Volatile Emissions

Plants that synthesize and store terpenes in specialized cells accumulate large concentrations of these compounds while avoiding autotoxicity. Stored terpenes may influence the quantity and profile of volatile compounds that are emitted into the environment and the subsequent role of those volatiles in mediating the activity of herbivores. The Australian medicinal tea tree, Melaleuca alternifolia, occurs as several distinct terpene chemotypes. We studied the profile of its terpene emissions to understand how variations in stored foliar terpenes influenced emissions, both constitutive and when damaged either by herbivores or mechanically. We found that foliar chemistry influenced differences in the composition of terpene emissions, but those emissions were minimal in intact plants. When plants were damaged by herbivores or mechanically, the emissions were greatly increased and the composition corresponded to the constitutive terpenes and the volatility of each compound, suggesting the main origin of emissions is the stored terpenes and not de novo biosynthesized volatiles. However, herbivores modified the composition of the volatile emissions in only one chemotype, probably due to the oxidative metabolism of 1,8-cineole by the beetles. We also tested whether the foliar terpene blend acted as an attractant for the specialized leaf beetles Paropsisterna tigrina and Faex sp. and a parasitoid fly, Anagonia zentae. None of these species responded to extracts of young leaves in an olfactometer, so we found no evidence that these species use plant odor cues for host location in laboratory conditions.     

Prey and Non-prey Arthropods Sharing a Host Plant: Effects on Induced Volatile Emission and Predator Attraction

It is well established that plants infested with a single herbivore species can attract specific natural enemies through the emission of herbivore-induced volatiles. However, it is less clear what happens when plants are simultaneously attacked by more than one species. We analyzed volatile emissions of lima bean and cucumber plants upon multi-species herbivory by spider mites (Tetranychus urticae) and caterpillars (Spodoptera exigua) in comparison to single-species herbivory. Upon herbivory by single or multiple species, lima bean and cucumber plants emitted volatile blends that comprised mostly the same compounds. To detect additive, synergistic, or antagonistic effects, we compared the multi-species herbivory volatile blend with the sum of the volatile blends induced by each of the herbivore species feeding alone. In lima bean, the majority of compounds were more strongly induced by multi-species herbivory than expected based on the sum of volatile emissions by each of the herbivores separately, potentially caused by synergistic effects. In contrast, in cucumber, two compounds were suppressed by multi-species herbivory, suggesting the potential for antagonistic effects. We also studied the behavioral responses of the predatory mite Phytoseiulus persimilis, a specialized natural enemy of spider mites. Olfactometer experiments showed that P. persimilis preferred volatiles induced by multi-species herbivory to volatiles induced by S. exigua alone or by prey mites alone. We conclude that both lima bean and cucumber plants effectively attract predatory mites upon multi-species herbivory, but the underlying mechanisms appear different between these species.     

Innate and Learned Prey-Searching Behavior in a Generalist Predator

Early colonization by Zyginidia scutellaris leafhoppers might be a key factor in the attraction and settling of generalist predators, such as Orius spp., in maize fields. In this paper, we aimed to determine whether our observations of early season increases in field populations of Orius spp. reflect a specific attraction to Z. scutellaris-induced maize volatiles, and how the responses of Orius predators to herbivore-induced volatiles (HIPVs) might be affected by previous experiences on plants infested by herbivorous prey. Therefore, we examined the innate and learned preferences of Orius majusculus toward volatiles from maize plants attacked by three potential herbivores with different feeding strategies: the leafhopper Z. scutellaris (mesophyll feeder), the lepidopteran Spodoptera littoralis (chewer), and another leafhopper Dalbulus maidis (phloem feeder). In addition, we examined the volatile profiles emitted by maize plants infested by the three herbivores. Our results show that predators exhibit a strong innate attraction to volatiles from maize plants infested with Z. scutellaris or S. littoralis. Previous predation experience in the presence of HIPVs influences the predator’s odor preferences. The innate preference for plants with cell or tissue damage may be explained by these plants releasing far more volatiles than plants infested by the phloem-sucking D. maidis. However, a predation experience on D. maidis-infested plants increased the preference for D. maidis-induced maize volatiles. After O. majusculus experienced L3-L4 larvae (too large to serve as prey) on S. littoralis-infested plants, they showed reduced attraction toward these plants and an increased attraction toward D. maidis-infested plants. When offered young larvae of S. littoralis, which are more suitable prey, preference toward HIPVs was similar to that of naive individuals. The HIPVs from plants infested by herbivores with distinctly different feeding strategies showed distinguishable quantitative differences in (Z)-3-hexenal, (E)-2-hexenal, and methyl salicylate. These compounds might serve as reliable indicators of prey presence and identity for the predator. Our results support the idea that feeding by Z. scutellaris results in the emission of maize’s HIPVs that initially recruit Orius spp. into maize fields.     

Herbivore-Induced Plant Volatiles Can Serve as Host Location Cues for a Generalist and a Specialist Egg Parasitoid

Herbivore-induced plant volatiles are important host finding cues for larval parasitoids, and similarly, insect oviposition might elicit the release of plant volatiles functioning as host finding cues for egg parasitoids. We hypothesized that egg parasitoids also might utilize HIPVs of emerging larvae to locate plants with host eggs. We, therefore, assessed the olfactory response of two egg parasitoids, a generalist, Trichogramma pretiosum (Tricogrammatidae), and a specialist, Telenomus remus (Scelionidae) to HIPVs. We used a Y-tube olfactometer to tests the wasps’ responses to volatiles released by young maize plants that were treated with regurgitant from caterpillars of the moth Spodoptera frugiperda (Noctuidae) or were directly attacked by the caterpillars. The results show that the generalist egg parasitoid Tr. pretiosum is innately attracted by volatiles from freshly-damaged plants 0–1 and 2–3 h after regurgitant treatment. During this interval, the volatile blend consisted of green leaf volatiles (GLVs) and a blend of aromatic compounds, mono- and homoterpenes, respectively. Behavioral assays with synthetic GLVs confirmed their attractiveness to Tr. pretiosum. The generalist learned the more complex volatile blends released 6–7 h after induction, which consisted mainly of sesquiterpenes. The specialist T. remus on the other hand was attracted only to volatiles emitted from fresh and old damage after associating these volatiles with oviposition. Taken together, these results strengthen the emerging pattern that egg and larval parasitoids behave in a similar way in that generalists can respond innately to HIPVs, while specialists seems to rely more on associative learning.     

Attractiveness of Constitutive and Herbivore-Induced Sesquiterpene Blends of Maize to the Parasitic Wasp <Emphasis Type="Italic">Cotesia marginiventris</Emphasis> (Cresson)

Plant volatile compounds induced by herbivore attack have been demonstrated to provide a signal to herbivore enemies such as parasitic wasps that use these volatiles to locate their hosts. However, in addition to herbivore-induced volatiles, plants often release volatiles constitutively. We assessed the interaction between herbivore-induced and constitutively released volatiles of maize in the attraction of the wasp Cotesia marginiventris that parasitizes herbivorous lepidopteran larvae feeding on maize. Experiments were carried out with olfactometers in which the sources of volatiles were transgenic Arabidopsis thaliana plants overexpressing maize sesquiterpene synthases that produce blends of herbivore-induced or constitutive compounds. We found that the constitutive volatiles of maize terpene synthase 8 (TPS8) were attractive to C. marginiventris, just like the herbivore-induced volatiles of TPS10 studied earlier. A mixture of both the TPS8 and TPS10 volatile blends, however, was more effective in parasitoid attraction, indicating that constitutively released sesquiterpenes enhance the attraction of those induced by herbivores. While C. marginiventris did not distinguish among the volatiles of TPS8, TPS10, nor those of another maize sesquiterpene synthase (TPS5), when these blends were combined, their attractiveness to the wasp appeared to increase with the complexity of the blend.     

Exogenous Application of Jasmonic Acid Induces Volatile Emissions in Rice and Enhances Parasitism of <Emphasis Type="Italic">Nilaparvata lugens</Emphasis> Eggs by theParasitoid <Emphasis Type="Italic">Anagrus nilaparvatae</Emphasis>

Jasmonate signaling pathway plays an important role in induced plant defense against herbivores and pathogens, including the emission of volatiles that serve as attractants for natural enemies of herbivores. We studied the volatiles emitted from rice plants that were wounded and treated with jasmonic acid (JA) and their effects on the host-searching behavior of the rice brown planthopper, Nilaparvata lugens (Stål), and its mymarid egg parasitoid Anagrus nilaparvatae Pang et Wang. Female adults of N. lugens significantly preferred to settle on JA-treated rice plants immediately after release. The parasitoid A. nilaparvatae showed a similar preference and was more attracted to the volatiles emitted from JA-treated rice plants than to volatiles from control plants. This was also evident from greenhouse and field experiments in which parasitism of N. lugens eggs by A. nilaparvatae on plants that were surrounded by JA-treated plants was more than twofold higher than on control plants. Analyses of volatiles collected from rice plants showed that JA treatment dramatically increased the release of volatiles, which included aliphatic aldehydes and alcohols, monoterpenes, sesquiterpenes, methyl salicylate, n-heptadecane, and several as yet unidentified compounds. These results confirm an involvement of the JA pathway in induced defense in rice plants and demonstrate that the egg parasitoid A. nilaparvatae exploits plant-provided cues to locate hosts. We explain the use of induced plant volatiles by the egg parasitoid by a reliable association between planthopper feeding damage and egg presence.     

Manipulation of Chemically Mediated Interactions in Agricultural Soils to Enhance the Control of Crop Pests and to Improve Crop Yield

In most agro-ecosystems the organisms that feed on plant roots have an important impact on crop yield and can impose tremendous costs to farmers. Similar to aboveground pests, they rely on a broad range of chemical cues to locate their host plant. In their turn, plants have co-evolved a large arsenal of direct and indirect defense to face these attacks. For instance, insect herbivory induces the synthesis and release of specific volatile compounds in plants. These volatiles have been shown to be highly attractive to natural enemies of the herbivores, such as parasitoids, predators, or entomopathogenic nematodes. So far few of the key compounds mediating these so-called tritrophic interactions have been identified and only few genes and biochemical pathways responsible for the production of the emitted volatiles have been elucidated and described. Roots also exude chemicals that directly impact belowground herbivores by altering their behavior or development. Many of these compounds remain unknown, but the identification of, for instance, a key compound that triggers nematode egg hatching to some plant parasitic nematodes has great potential for application in crop protection. These advances in understanding the chemical emissions and their role in ecological signaling open novel ways to manipulate plant exudates in order to enhance their natural defense properties. The potential of this approach is discussed, and we identify several gaps in our knowledge and steps that need to be taken to arrive at ecologically sound strategies for belowground pest management.     

Systemic release of herbivore-induced plant volatiles by turnips infested by concealed root-feeding larvae Delia radicum L

When attacked by herbivorous insects, many plants emit volatile compounds that are used as cues by predators and parasitoids foraging for prey or hosts. While such interactions have been demonstrated in several host–plant complexes, in most studies, the herbivores involved are leaf-feeding arthropods. We studied the long-range plant volatiles involved in host location in a system based on a very different interaction since the herbivore is a fly whose larvae feed on the roots of cole plants in the cabbage root fly, Delia radicum L. (Diptera: Anthomyiidae). The parasitoid studied is Trybliographa rapae Westwood (Hymenoptera: Figitidae), a specialist larval endoparasitoid of D. radicum. Using a four-arm olfactometer, the attraction of naive T. rapae females toward uninfested and infested turnip plants was investigated. T. rapae females were not attracted to volatiles emanating from uninfested plants, whether presented as whole plants, roots, or leaves. In contrast, they were highly attracted to volatiles emitted by roots infested with D. radicum larvae, by undamaged parts of infested roots, and by undamaged leaves of infested plants. The production of parasitoid-attracting volatiles appeared to be systemic in this particular tritrophic system. The possible factors triggering this volatile emission were also investigated. Volatiles from leaves of water-stressed plants and artificially damaged plants were not attractive to T. rapae females, while volatiles emitted by leaves of artificially damaged plants treated with crushed D. radicum larvae were highly attractive. However, T. rapae females were not attracted to volatiles emitted by artificially damaged plants treated only with crushed salivary glands from D. radicum larvae. These results demonstrate the systemic production of herbivore-induced volatiles in this host-plant complex. Although the emission of parasitoid attracting volatiles is induced by factors present in the herbivorous host, their exact origin remains unclear. The probable nature of the volatiles involved and the possible origin of the elicitor of volatiles release are discussed.     

Identification of Volatiles That Are Used in Discrimination Between Plants Infested with Prey or Nonprey Herbivores by a Predatory Mite

Carnivorous arthropods can use herbivore-induced plant volatiles to locate their herbivorous prey. In the field, carnivores are confronted with information from plants infested with herbivores that may differ in their suitability as prey. Discrimination by the predatory mite Phytoseiulus persimilis between volatiles from lima bean plants infested with the prey herbivore Tetranychus urticae, or plants infested with the nonprey caterpillar Spodoptera exigua, depends on spider mite density. In this article, we analyzed the chemical composition of the volatile blends from T. urticae-infested lima bean plants at different densities of spider mites, and from S. exigua-infested plants. Based on the behavioral preferences of P. persimilis and the volatile profiles, we selected compounds that potentially enable the mite to discriminate between T. urticae-induced and S. exigua-induced volatiles. Subsequently, we demonstrated in Y-tube olfactometer assays that the relatively large amounts of methyl salicylate and (3E, 7E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene emitted by T. urticae-infested bean plants compared to S. exigua-infested plants enable the predators to discriminate. Our data show that specific compounds from complex herbivore-induced volatile blends can play an important role in the selective foraging behavior of natural enemies of herbivorous arthropods.     

Impact of Herbivore-induced Plant Volatiles on Parasitoid Foraging Success: A Spatial Simulation of the <Emphasis Type="Italic">Cotesia rubecula,Pieris rapae,</Emphasis> and <Emphasis Type="Italic">Brassica oleracea</Emphasis> System

Many parasitoids are known to use herbivore-induced plant volatiles as cues to locate hosts. However, data are lacking on how much of an advantage a parasitoid can gain from following these plant cues and which factors can limit the value of these cues to the parasitoid. In this study, we simulate the Cotesia rubecula-Pieris rapae-Brassica oleracea system, and ask how many more hosts can a parasitoid attack in a single day of foraging by following plant signals versus randomly foraging. We vary herbivore density, plant response time, parasitoid flight distance, and available host stages to see under which conditions parasitoids benefit from herbivore-induced plant cues. In most of the parameter combinations studied, parasitoids that responded to cues attacked more hosts than those that foraged randomly. Parasitoids following plant cues attacked up to ten times more hosts when they were able to successfully attack herbivores older than first instar; however, if parasitoids were limited to first instar hosts, those following plant cues were at a disadvantage when plants took longer than a day to respond to herbivory. At low herbivore densities, only parasitoids with a larger foraging radius could take advantage of plant cues. Although preference for herbivore-induced volatiles was not always beneficial for a parasitoid, under the most likely natural conditions, the model predicts that C. rubecula gains fitness from following plant cues.     

Genetic Variation in Jasmonic Acid- and Spider Mite-Induced Plant Volatile Emission of Cucumber Accessions and Attraction of the Predator <Emphasis Type="Italic">Phytoseiulus persimilis</Emphasis>

Cucumber plants (Cucumis sativus L.) respond to spider–mite (Tetranychus urticae) damage with the release of specific volatiles that are exploited by predatory mites, the natural enemies of the spider mites, to locate their prey. The production of volatiles also can be induced by exposing plants to the plant hormone jasmonic acid. We analyzed volatile emissions from 15 cucumber accessions upon herbivory by spider mites and upon exposure to jasmonic acid using gas chromatography—mass spectrometry. Upon induction, cucumber plants emitted over 24 different compounds, and the blend of induced volatiles consisted predominantly of terpenoids. The total amount of volatiles was higher in plants treated with jasmonic acid than in those infested with spider mites, with (E)-4,8-dimethyl-1,3,7-nonatriene, (E,E)-α-farnesene, and (E)-β-ocimene as the most abundant compounds in all accessions in both treatments. Significant variation among the accessions was found for the 24 major volatile compounds. The accessions differed strongly in total amount of volatiles emitted, and displayed very different odor profiles. Principal component analysis performed on the relative quantities of particular compounds within the blend revealed clusters of highly correlated volatiles, which is suggestive of common metabolic pathways. A number of cucumber accessions also were tested for their attractiveness to Phytoseiulus persimilis, a specialist predator of spider mites. Differences in the attraction of predatory mites by the various accessions correlated to differences in the individual chemical profiles of these accessions. The presence of genetic variation in induced plant volatile emission in cucumber shows that it is possible to breed for cucumber varieties that are more attractive to predatory mites and other biological control agents.     

Oral Secretions Affect HIPVs Induced by Generalist (<Emphasis Type="Italic">Mythimna loreyi</Emphasis>) and Specialist (<Emphasis Type="Italic">Parnara guttata</Emphasis>) Herbivores in Rice

Plants synthesize variable mixtures of herbivore-induced plant volatiles (HIPVs) as part of their evolutionary conserved defense. To elucidate the impact of chewing herbivores with different level of adaptation on HIPV profiles in rice, we measured HIPVs released from rice seedlings challenged by either the generalist herbivore Mythimna loreyi (MYL) or the specialist Parnara guttata (PAG). Both herbivores markedly elicited the emission of HIPVs, mainly on the second and third days after attack compared to control plants. In addition, side-by-side HIPV comparisons using MYL and PAG caterpillars revealed that generalist feeding induced comparably more HIPVs relative to specialist, particularly on day two as highlighted by multivariate analysis (PLS-DA) of emitted HIPVs, and further confirmed in mimicked herbivory experiments. Here, mechanically wounded plants treated with water (WW) released more VOCs than untreated controls, and on top of this, oral secretions (OS) from both herbivores showed differential effects on volatile emissions from the wounded plants. Similar to actual herbivory, MYL OS promoted higher amounts of HIPVs relative to PAG OS, thus supporting disparate induction of rice indirect defenses in response to generalist and specialist herbivores, which could be due to the differential composition of their OS. (196 words).     

Production of Induced Volatiles by <Emphasis Type="Italic">Datura wrightii</Emphasis> in Response to Damage by Insects: Effect of Herbivore Species and Time

Natural enemies of herbivorous insects utilize numerous chemical cues to locate and identify their prey. Among these, volatile plant compounds produced after attack by herbivores may play a significant role (hereafter herbivore-induced plant volatiles or HIPVs). One unresolved question is whether the composition of the volatile cue blends induced by different herbivore species differ consistently enough to indicate not only that the plants are damaged by herbivores but also the identity of the herbivore species causing the damage. We studied HIPV production in the undomesticated plant species Datura wrightii in the laboratory when damaged by either of two leaf-chewing herbivore species, Lema daturaphila or Manduca sexta, or when damaged by L. daturaphila and the piercing-sucking bug, Tupiocoris notatus, or both L. daturaphila and T. notatus, for 24 hr. HIPV production was monitored 1 d before induction, the day of induction, and for 7 d after induction. In all experiments, both the quantities and composition of the HIPV blends varied with the time since induction as different components reached peak production at different times after induction. HIPV blends did not differ consistently with the herbivore species causing the damage. For plants damaged by both L. daturaphila and T. notatus, greater amounts of HIPVs were produced than by plants damaged by either species alone, but the amounts did not differ from that predicted as the sum from damage inflicted by each herbivore species independently. The HIPVs of D. wrightii are a general rather than specific indicator of damage by herbivores. Because generalist predators are the most abundant natural enemies in this system, general cues of herbivore damage may be all that are required to facilitate the discovery by predators of plants damaged by any of several suitable prey species.     

Herbivore-induced Plant Volatiles Trigger Sporulation in Entomopathogenic Fungi: The Case of <Emphasis Type="Italic">Neozygites tanajoae</Emphasis> Infecting the Cassava Green Mite

A large body of evidence shows that plants release volatile chemicals upon attack by herbivores. These volatiles influence the performance of natural enemies. Nearly all the evidence on the effect of plant volatiles on natural enemies of herbivores concerns predators, parasitoids, and entomophagous nematodes. However, other entomopathogens, such as fungi, have not been studied yet for the way they exploit the chemical information that the plant conveys on the presence of herbivores. We tested the hypothesis that volatiles emanating from cassava plants infested by green mites (Mononychellus tanajoa) trigger sporulation in three isolates of the acaropathogenic fungus Neozygites tanajoae. Tests were conducted under climatic conditions optimal to fungal conidiation, such that the influence of the plant volatiles could only alter the quantity of conidia produced. For two isolates (Altal.brz and Colal.brz), it was found that, compared with clean air, the presence of volatiles from clean, excised leaf discs suppressed conidia production. This suppressive effect disappeared in the presence of herbivore-damaged leaves for the isolate Colal.brz. For the third isolate, no significant effects were observed. Another experiment differing mainly in the amount of volatiles showed that two isolates produced more conidia when exposed to herbivore-damaged leaves compared with clean air. Taken together, the results show that volatiles from clean plants suppress conidiation, whereas herbivore-induced plant volatiles promote conidiation of N. tanajoae. These opposing effects suggest that the entomopathogenic fungus tunes the release of spores to herbivore-induced plant signals indicating the presence of hosts.