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.     

Ontogeny and Season Constrain the Production of Herbivore-Inducible Plant Volatiles in the Field

Herbivores may induce plants to produce an array of volatile organic compounds (herbivore-induced plant volatiles, or HIPVs) after damage, and some natural enemies of herbivores are attracted by those HIPVs. The production of HIPVs by the undomesticated species Datura wrightii was quantified in response to damage by its natural community of herbivores or the plant hormone methyl jasmonate (MeJA) over plant’s 6-month growing season. Patterns of HIPV production were compared to the seasonal abundance of D. wrightii’s two most abundant herbivores, the chrysomelid beetle Lema daturaphila and the mirid bug Tupiocoris notatus, and their shared generalist predator, the lygaeid bug Geocoris pallens. HIPV production was especially high in the spring, when plants were growing vegetatively, but HIPV production declined after plants began to flower and produce fruit, and these volatiles no longer were inducible by September. The composition of the HIPV blends also changed seasonally. HIPV production and composition were partially restored by “rejuvenating” plants back to the vegetative growth stage independently of season by cutting them back and allowing them to resprout and regrow vegetatively. HIPV production of D. wrightii in the field is limited to the earlier ontogenetic stages of growth, despite the fact that both herbivores and their shared natural enemy inhabited plants throughout the full season. The adaptive value of HIPV production in D. wrightii may be constrained by plant ontogeny to the vegetative stages of plant growth.     

Differences in Volatile Profiles of Turnip Plants Subjected to Single and Dual Herbivory Above- and Belowground

Plants attacked by herbivorous insects emit volatile organic compounds that are used by natural enemies to locate their host or prey. The composition of the blend is often complex and specific. It may vary qualitatively and quantitatively according to plant and herbivore species, thus providing specific information for carnivorous arthropods. Most studies have focused on simple interactions that involve one species per trophic level, and typically have investigated the aboveground parts of plants. These investigations need to be extended to more complex networks that involve multiple herbivory above- and belowground. A previous study examined whether the presence of the leaf herbivore Pieris brassicae on turnip plants (Brassica rapa subsp. rapa) influences the response of Trybliographa rapae, a specialist parasitoid of the root feeder Delia radicum. It showed that the parasitoid was not attracted by volatiles emitted by plants under simultaneous attack. Here, we analyzed differences in the herbivore induced plant volatile (HIPV) mixtures that emanate from such infested plants by using Orthogonal Partial Least Squares-Discriminant Analysis (OPLS-DA). This multivariate model focuses on the differences between odor blends, and highlights the relative importance of each compound in an HIPV blend. Dual infestation resulted in several HIPVs that were present in both isolated infestation types. However, HIPVs collected from simultaneously infested plants were not the simple combination of volatiles from isolated forms of above- and belowground herbivory. Only a few specific compounds characterized the odor blend of each type of damaged plant. Indeed, some compounds were specifically induced by root herbivory (4-methyltridecane and salicylaldehyde) or shoot herbivory (methylsalicylate), whereas hexylacetate, a green leaf volatile, was specifically induced after dual herbivory. It remains to be determined whether or not these minor quantitative variations, within the background of more commonly induced odors, are involved in the reduced attraction of the root feeder’s parasitoid. The mechanisms involved in the specific modification of the odor blends emitted by dual infested turnip plants are discussed in the light of interferences between biosynthetic pathways linked to plant responses to shoot or root herbivory.     

Response of a Predatory ant to Volatiles Emitted by Aphid- and Caterpillar-Infested Cucumber and Potato Plants

In response to herbivory by insects, various plants produce volatiles that attract enemies of the herbivores. Although ants are important components of natural and agro-ecosystems, the importance of herbivore-induced plant volatiles (HIPVs) as cues for ants for finding food sources have received little attention. We investigated responses of the ant Formica pratensis to volatiles emitted by uninfested and insect-infested cucumber (Cucumis sativus) and potato (Solanum tuberosum) plants. Cucumber plants were infested by the phloem-feeding aphid Aphis gossypii, the leaf chewer Mamestra brassicae or simultaneously by both insects. Potato plants were infested by either Aphis gossypii, by the leaf chewer Chrysodeixis chalcites or both. In olfactometer experiments, ants preferred volatile blends emitted by cucumber plants infested with M. brassicae caterpillars alone or combined with A. gossypii to volatiles of undamaged plants or plants damaged by A. gossypii only. No preference was recorded in choice tests between volatiles released by aphid-infested plants over undamaged plants. Volatiles emitted by potato plants infested by either C. chalcites or A. gossypii were preferred by ants over volatiles released by undamaged plants. Ants did not discriminate between potato plants infested with aphids and caterpillars over plants infested with aphids only. Plant headspace composition showed qualitative and/or quantitative differences between herbivore treatments. Multivariate analysis revealed clear separation between uninfested and infested plants and among herbivore treatments. The importance of HIPVs in indirect plant defence by ants is discussed in the context of the ecology of ant-plant interactions and possible roles of ants in pest management.     

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.     

Herbivore-Induced Volatiles in the Perennial Shrub, <Emphasis Type="Italic">Vaccinium corymbosum</Emphasis>, and Their Role in Inter-branch Signaling

Herbivore feeding activates plant defenses at the site of damage as well as systemically. Systemic defenses can be induced internally by signals transported via phloem or xylem, or externally transmitted by volatiles emitted from the damaged tissues. We investigated the role of herbivore-induced plant volatiles (HIPVs) in activating a defense response between branches in blueberry plants. Blueberries are perennial shrubs that grow by initiating adventitious shoots from a basal crown, which produce new lateral branches. This type of growth constrains vascular connections between shoots and branches within plants. While we found that leaves within a branch were highly connected, vascular connectivity was limited between branches within shoots and absent between branches from different shoots. Larval feeding by gypsy moth, exogenous methyl jasmonate, and mechanical damage differentially induced volatile emissions in blueberry plants, and there was a positive correlation between amount of insect damage and volatile emission rates. Herbivore damage did not affect systemic defense induction when we isolated systemic branches from external exposure to HIPVs. Thus, internal signals were not capable of triggering systemic defenses among branches. However, exposure of branches to HIPVs from an adjacent branch decreased larval consumption by 70% compared to those exposed to volatiles from undamaged branches. This reduction in leaf consumption did not result in decreased volatile emissions, indicating that leaves became more responsive to herbivory (or “primed”) after being exposed to HIPVs. Chemical profiles of leaves damaged by gypsy moth caterpillars, exposed to HIPVs, or non-damaged controls revealed that HIPV-exposed leaves had greater chemical similarities to damaged leaves than to control leaves. Insect-damaged leaves and young HIPV-exposed leaves had higher amounts of endogenous cis-jasmonic acid compared to undamaged and non-exposed leaves, respectively. Our results show that exposure to HIPVs triggered systemic induction of direct defenses against gypsy moth and primed volatile emissions, which can be an indirect defense. Blueberry plants appear to rely on HIPVs as external signals for inter-branch communication.     

Pollinator and Herbivore Attraction to <Emphasis Type="Italic">Cucurbita</Emphasis> Floral Volatiles

Mutualists and antagonists may place conflicting selection pressures on plant traits. For example, the evolution of floral traits is typically studied in the context of attracting pollinators, but traits may incur fitness costs if they are also attractive to antagonists. Striped cucumber beetles (Acalymma vittatum) feed on cucurbits and are attracted to several volatiles emitted by Cucurbita blossoms. However, the effect of these volatiles on pollinator attraction is unknown. Our goal was to determine whether pollinators were attracted to the same or different floral volatiles as herbivorous cucumber beetles. We tested three volatiles previously found to attract cucumber beetles in a factorial design to determine attraction of squash bees (Peponapis pruinosa), the specialist pollinators of cucurbita species, as well as the specialist herbivore A. vittatum. We found that 1,2,4-trimethoxybenzene was attractive to both the pollinator and the herbivore, indole was attractive only to the herbivore, and (E)-cinnamaldehyde was attractive only to the pollinator. There were no interactions among volatiles on attraction of squash bees or cucumber beetles. Our results suggest that reduced indole emission could benefit plants by reducing herbivore attraction without loss of pollination, and that 1,2,4-trimethoxybenzene might be under conflicting selection pressure from mutualists and antagonists. By examining the attraction of both mutualists and antagonists to Cucurbita floral volatiles, we have demonstrated the potential for some compounds to influence only one type of interaction, while others may affect both interactions and possibly result in tradeoffs. These results shed light on the potential evolution of fragrance in native Cucurbita, and may have consequences for yield in agricultural settings.     

Multidisciplinary Approach to Unravelling the Relative Contribution of Different Oxylipins in Indirect Defense of <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis>

The oxylipin pathway is commonly involved in induced plant defenses, and is the main signal-transduction pathway induced by insect folivory. Herbivory induces the production of several oxylipins, and consequently alters the so-called ‘oxylipin signature’ in the plant. Jasmonic acid (JA), as well as pathway intermediates are known to induce plant defenses. Indirect defense against herbivorous insects comprises the production of herbivore-induced plant volatiles (HIPVs). To unravel the precise oxylipin signal-transduction underlying the production of HIPVs in Arabidopsis thaliana and the resulting attraction of parasitoid wasps, we used a multidisciplinary approach that includes molecular genetics, metabolite analysis, and behavioral analysis. Mutant plants affected in the jasmonate pathway (18:0 and/or 16:0 -oxylipin routes; mutants dde2-2, fad5, opr3) were studied to assess the effects of JA and its oxylipin intermediates 12-oxo-phytodienoate (OPDA) and dinor-OPDA (dnOPDA) on HIPV emission and parasitoid (Diadegma semiclausum) attraction. Interference with the production of the oxylipins JA and OPDA altered the emission of HIPVs, in particular terpenoids and the phenylpropanoid methyl salicylate, which affected parasitoid attraction. Our data show that the herbivore-induced attraction of parasitoid wasps to Arabidopsis plants depends on HIPVs that are induced through the 18:0 oxylipin-derivative JA. Furthermore, our study shows that the 16:0-oxylipin route towards dnOPDA does not play a role in HIPV induction, and that the role of 18:0 derived oxylipin-intermediates, such as OPDA, is either absent or limited.     

Body Odors of Parasitized Caterpillars Give Away the Presence of Parasitoid Larvae to Their Primary Hyperparasitoid Enemies

Foraging success of parasitoids depends on the utilization of reliable information on the presence of their often, inconspicuous hosts. These parasitic wasps use herbivore-induced plant volatiles (HIPVs) that provide reliable cues on host presence. However, host searching of hyperparasitoids, a group of parasitoids that parasitize the larvae and pupae of other parasitoids, is more constrained. Their hosts do not feed on plants, and often are even concealed inside the body of the herbivore host. Hyperparasitoids recently have been found to use HIPVs of plants damaged by herbivore hosts in which the parasitoid larvae develop. However, hyperparasitoids that search for these parasitoid larvae may be confronted with healthy and parasitized caterpillars on the same plant, further complicating their host location. In this study, we addressed whether the primary hyperparasitoid Baryscapus galactopus uses caterpillar body odors to discriminate between unparasitized herbivores and herbivores carrying larvae of parasitoid hosts. We show that the hyperparasitoids made faster first contact and spent a longer mounting time with parasitized caterpillars. Moreover, although the three parasitoid hosts conferred different fitness values for the development of B. galactopus, the hyperparasitoids showed similar behavioral responses to caterpillar hosts carrying different primary parasitoid hosts. In addition, a two-chamber olfactometer assay revealed that volatiles emitted by parasitized caterpillars were more attractive to the hyperparasitoids than those emitted by unparasitized caterpillars. Analysis of volatiles revealed that body odors of parasitized caterpillars differ from unparasitized caterpillars, allowing the hyperparasitoids to detect their parasitoid host.     

Plant–herbivore–carnivore Interactions in Cotton, <Emphasis Type="Italic">Gossypium hirsutum</Emphasis>: Linking Belowground and Aboveground

Most studies on plant–herbivore interactions focus on either root or shoot herbivory in isolation, but above- and belowground herbivores may interact on a shared host plant. Cotton (Gossypium spp.) produces gossypol and a variety of other gossypol-like terpenoids that exhibit toxicity to a wide range of herbivores and pathogens. Cotton plants also can emit herbivore-induced volatile compounds at the site of damage and systemically on all tissues above the site of damage. As these volatile compounds attract natural enemy species of the herbivore, they are thought to represent an indirect plant defense. Our study quantified gossypol and gossypol-like compounds in cotton plants with foliage feeding (Heliocoverpa zea), root feeding (Meloidogyne incognita), or their combination. Cotton plants with these treatments were studied also with respect to induced local and systemic volatile production and the attraction of the parasitic wasp Microplitis croceipes to those plants. We also evaluated whether foliage or root feeding affected foliar nitrogen levels in cotton. After 48 hr of leaf feeding and 5 wk of root feeding, local and systemic induction of volatiles (known to attract parasitoids such as M. croceipes) occurred with herbivore damage to leaves, and it increased in levels when root herbivory was added. Nevertheless, M. croceipes were equally attracted to plants with both leaf and root damage and leaf damage only. In contrast to previous studies in cotton, production of gossypol and gossypol-like compounds was not induced in leaf and root tissue following foliage or root herbivory, or their combination. We conclude that root feeding by M. incognita has little influence on direct and indirect defenses of Gossypium hirsutum against insect herbivory.     

Predatory Mite Attraction to Herbivore-induced Plant Odors is not a Consequence of Attraction to Individual Herbivore-induced Plant Volatiles

Predatory mites locate herbivorous mites, their prey, by the aid of herbivore-induced plant volatiles (HIPV). These HIPV differ with plant and/or herbivore species, and it is not well understood how predators cope with this variation. We hypothesized that predators are attracted to specific compounds in HIPV, and that they can identify these compounds in odor mixtures not previously experienced. To test this, we assessed the olfactory response of Phytoseiulus persimilis, a predatory mite that preys on the highly polyphagous herbivore Tetranychus urticae. The responses of the predatory mite to a dilution series of each of 30 structurally different compounds were tested. They mites responded to most of these compounds, but usually in an aversive way. Individual HIPV were no more attractive (or less repellent) than out-group compounds, i.e., volatiles not induced in plants fed upon by spider-mites. Only three samples were significantly attractive to the mites: octan-1-ol, not involved in indirect defense, and cis-3-hexen-1-ol and methyl salicylate, which are both induced by herbivory, but not specific for the herbivore that infests the plant. Attraction to individual compounds was low compared to the full HIPV blend from Lima bean. These results indicate that individual HIPV have no a priori meaning to the mites. Hence, there is no reason why they could profit from an ability to identify individual compounds in odor mixtures. Subsequent experiments confirmed that naive predatory mites do not prefer tomato HIPV, which included the attractive compound methyl salicylate, over the odor of an uninfested bean. However, upon associating each of these odors with food over a period of 15 min, both are preferred. The memory to this association wanes within 24 hr. We conclude that P. persimilis possesses a limited ability to identify individual spider mite-induced plant volatiles in odor mixtures. We suggest that predatory mites instead learn to respond to prey-associated mixtures of volatiles and, thus, to odor blends as a whole.     

Herbivore-Induced Volatile Production by Arabidopsis thaliana Leads to Attraction of the Parasitoid Cotesia rubecula: Chemical, Behavioral, and Gene-Expression Analysis

Many plant species defend themselves against herbivorous insects indirectly by producing volatiles in response to herbivory. These volatiles attract carnivorous enemies of the herbivores. Research on the model plant Arabidopsis thaliana (L.) Heynh. has contributed considerably to the unraveling of signal transduction pathways involved in direct plant defense mechanisms against pathogens. Here, we demonstrate that Arabidopsis is also a good candidate for studying signal transduction pathways involved in indirect defense mechanisms by showing that: (1) Adult females of Cotesia rubecula, a specialist parasitic wasp of Pieris rapae caterpillars, are attracted to P. rapae-infested Arabidopsis plants. (2) Arabidopsis infested by P. rapae emits volatiles from several major biosynthetic pathways, including terpenoids and green leaf volatiles. The blends from herbivore-infested and artificially damaged plants are similar. However, differences can be found with respect to a few components of the blend, such as two nitriles and the monoterpene myrcene, that were produced exclusively by caterpillar-infested plants, and methyl salicylate, that was produced in larger amounts by caterpillar-infested plants. (3) Genes from major biosynthetic pathways involved in volatile production are induced by caterpillar feeding. These include AtTPS10, encoding a terpene synthase involved in myrcene production, AtPAL1, encoding phenylalanine ammonia-lyase involved in methyl salicylate production, and AtLOX2 and AtHPL, encoding lipoxygenase and hydroperoxide lyase, respectively, both involved in the production of green leaf volatiles. AtAOS, encoding allene oxide synthase, involved in the production of jasmonic acid, also was induced by herbivory.     

Comparative Analysis of Headspace Volatiles from Different Caterpillar-Infested or Uninfested Food Plants of Pieris Species

Plants that are infested by herbivores emit volatile cues that can be used by the natural enemies of the herbivores in their search for hosts. Based on results from behavioral studies, we investigated to what extent intact and herbivore-infested plant species and varieties from the food plant range of Pieris herbivore species differ in the composition of the volatile blends. Parasitoids of Pieris species, Cotesia glomerata and C. rubecula, show differential responses towards various herbivore-infested food plants, whereas differences in responses to plants infested by other herbivore species were less clear. Chemical analysis of the headspace samples of red cabbage, white cabbage, and nasturtium plants that were infested by P. brassicae or P. rapae larvae, or that were intact, yielded 88 compounds including alcohols, ketones, aldehydes, esters, nitriles, terpenoids, sulfides, (iso)thiocyanates, carboxylic acids, and others. The analysis revealed that herbivore-infested plants emit the largest number of compounds in the highest amounts. The plant species affected the volatile blend more than did the herbivore species, and differences between plant varieties were less pronounced than differences between plant species. Differences in headspace composition between plants infested by P. brassicae or P. rapae were mainly of a quantitative nature. Herbivore-infested nasturtium differed considerably from the cabbage varieties in a qualitative way. Headspace compositions of red and white cabbage varieties were comparable to that of the food plant Brussels sprouts (Brassica oleracea gemmifera cv. Titurel) as determined in earlier studies in our laboratory. With respect to plant response to herbivory, nasturtium differed considerably from the cabbage varieties analyzed so far and shows resemblance with Lima bean, cucumber, and corn. These plant species produce a greater quantity and variety of volatiles under herbivore attack than intact plants. The results of this study are discussed in relation to behavioral observations on C. glomerata and C. rubecula.     

Direct Defense or Ecological Costs: Responses of Herbivorous Beetles to Volatiles Released by Wild Lima Bean (Phaseolus lunatus)

In response to feeding damage, Lima bean releases herbivore-induced plant volatiles (HIPV), which are generally assumed to attract carnivorous arthropods as an indirect defense. While many studies have focused on such tritrophic interactions, few have investigated effects of HIPV on herbivores. I used natural herbivores of wild Lima bean and studied their responses to jasmonic acid-induced plants in an olfactometer and in feeding trials. Both Cerotoma ruficornis and Gynandrobrotica guerreroensis (Chrysomelidae) significantly preferred control plants to induced ones in the olfactometer, and they avoided feeding on induced plants. In contrast, Curculionidae significantly preferred HIPV of the induced plant to those of the control in one plant pair and did not choose in the case of a second pair. In feeding trials, no choice occurred in the first plant pair, while control leaves were preferred in the second. Release of HIPV deterred Chrysomelid herbivores and, thus, acted as a direct defense. This may be an important addition to indirect defensive effects. Whether or not HIPV released by induced plants attracted herbivorous Curculionidae, thus incurring ecological costs, varied among plants. Such differences could be related to various HIPV blends released by individual plants.     

Does Aphid Infestation Interfere with Indirect Plant Defense against Lepidopteran Caterpillars in Wild Cabbage?

Attraction of parasitoids to plant volatiles induced by multiple herbivory depends on the specific combinations of attacking herbivore species, especially when their feeding modes activate different defense signalling pathways as has been reported for phloem feeding aphids and tissue feeding caterpillars. We studied the effects of pre-infestation with non-host aphids (Brevicoryne brassicae) for two different time periods on the ability of two parasitoid species to discriminate between volatiles emitted by plants infested by host caterpillars alone and those emitted by plants infested with host caterpillars plus aphids. Using plants originating from three chemically distinct wild cabbage (Brassica oleracea) populations, Diadegma semiclausum switched preference for dually infested plants to preference for plants infested with Plutella xylostella hosts alone when the duration of pre-aphid infestation doubled from 7 to 14 days. Microplitis mediator, a parasitoid of Mamestra brassicae caterpillars, preferred dually-infested plants irrespective of aphid-infestation duration. Separation of the volatile blends emitted by plants infested with hosts plus aphids or with hosts only was poor, based on multivariate statistics. However, emission rates of individual compounds were often reduced in plants infested with aphids plus hosts compared to those emitted by plants infested with hosts alone. This effect depended on host caterpillar species and plant population and was little affected by aphid infestation duration. Thus, the interactive effect of aphids and hosts on plant volatile production and parasitoid attraction can be dynamic and parasitoid specific. The characteristics of the multi-component volatile blends that determine parasitoid attraction are too complex to be deduced from simple correlative statistical analyses.     

Prevalence of Chemical Defenses among Freshwater Plants

Although macrophyte–herbivore interactions in freshwater systems were generally disregarded for many years, recent data suggest that herbivory can be intense and important in structuring freshwater communities. This has led to the hypothesis that chemical defenses should be common among freshwater plants, but few studies have reported such chemical defenses, and no previous studies have assessed the frequency of chemical defenses among a substantial number of freshwater plant species. In a study of 21 macrophyte species co-occurring with the omnivorous crayfish Procambarus acutus in a southeastern USA wetland environment, we found that extracts of 11 species (52%) deterred feeding by P. acutus when tested in artificial foods at natural concentrations. Of these 11 chemically defended species, one species, Eupatorium capillifolium, consistently had a more unpalatable extract following mechanical damage to plant tissue, indicative of an activated chemical defense. Because herbivores are commonly nitrogen-limited and select food based on several plant traits, including plant nutritional value, it might be expected that chemical defenses would be especially important for protein-rich plants. However, we found no relationship between soluble protein concentration and deterrence of plant extracts.     

Production of Herbivore-Induced Plant Volatiles is Constrained Seasonally in The Field but Predation on Herbivores is not

Natural enemies of herbivorous insects utilize numerous cues to locate and identify their prey. One class of such cues is volatile organic compounds (VOCs) often produced by plants after attack by herbivores (hereafter herbivore-induced plant volatiles or HIPVs). Under simplified laboratory conditions, natural enemies often make clear choices between different HIPV blends, but such clear choices may be more difficult in more complex field environments. We studied the impact of VOC production by the undomesticated species, Datura wrightii on predation of eggs and larvae of Lema daturaphila by the omnivore, Geocoris pallens in the field. HIPV production in D. wrightii is developmentally and seasonally constrained to the early stages of plant growth even though L. daturaphila and G. pallens inhabit plants throughout the plant’s growing season. We, therefore, asked if predation of L. daturaphila by G. pallens might be similarly constrained seasonally. Higher levels of VOC production were associated with higher levels of predation throughout the growing season, and the greater quantities of VOC production in May caused greater increases in predation than did VOC production later in the season (June–September). However, predation in the absence of VOC production ranged from 60–70% in June–September compared to only 14% in May, probably because plants were already colonized by predators later in the season. High levels of VOCs in response to herbivore damage by D. wrightii therefore may aid in the discovery of herbivore-damaged plants early in the season but the seasonal decline in VOC production does not limit predation of L. daturaphila by G. pallens later in the season.     

Comparative GC-EAD Responses of A Specialist (<Emphasis Type="Italic">Microplitis croceipes</Emphasis>) and A Generalist (<Emphasis Type="Italic">Cotesia marginiventris</Emphasis>) Parasitoid to Cotton Volatiles Induced by Two Caterpillar Species

Plants emit volatile blends that may be quantitatively and/or qualitatively different in response to attack by different herbivores. These differences may convey herbivore-specific information to parasitoids, and are predicted to play a role in mediating host specificity in specialist parasitoids. Here, we tested the above prediction by using as models two parasitoids (Hymenoptera: Braconidae) of cotton caterpillars with different degree of host specificity: Microplitis croceipes, a specialist parasitoid of Heliothis spp., and Cotesia marginiventris, a generalist parasitoid of caterpillars of several genera including Heliothis spp. and Spodoptera spp. We compared GC-EAD (coupled gas chromatography electroantennogram detection) responses of both parasitoid species to headspace volatiles of cotton plants damaged by H. virescens (a host species for both parasitoids) vs. S. exigua (a host species for C. marginiventris). Based on a recent study in which we reported differences in the EAG responses of both parasitoids to different types of host related volatiles, we hypothesized that M. croceipes (specialist) would show relatively greater GC-EAD responses to the herbivore-induced plant volatile (HIPV) components of cotton headspace, whereas C. marginiventris (generalist) would show greater response to the green leaf volatile (GLV) components. Thirty volatile components were emitted by cotton plants in response to feeding by either of the two caterpillars, however, 18 components were significantly elevated in the headspace of H. virescens damaged plants. Sixteen consistently elicited GC-EAD responses in both parasitoids. As predicted, C. marginiventris showed significantly greater GC-EAD responses than M. croceipes to most GLV components, whereas several HIPV components elicited comparatively greater responses in M. croceipes. These results suggest that differences in the ratios of identical volatile compounds between similar volatile blends may be used by specialist parasitoids to discriminate between host-plant and non-host-plant complexes.