Network motifs involving both competition and facilitation predict biodiversity in alpine plant communities.

Biological diversity depends on multiple, cooccurring ecological interactions. However, most studies focus on one interaction type at a time, leaving community ecologists unsure of how positive and negative associations among species combine to influence biodiversity patterns. Using surveys of plant populations in alpine communities worldwide, we explore patterns of positive and negative associations among triads of species (modules) and their relationship to local biodiversity. Three modules, each incorporating both positive and negative associations, were overrepresented, thus acting as "network motifs." Furthermore, the overrepresentation of these network motifs is positively linked to species diversity globally. A theoretical model illustrates that these network motifs, based on competition between facilitated species or facilitation between inferior competitors, increase local persistence. Our findings suggest that the interplay of competition and facilitation is crucial for maintaining biodiversity.

Effects of genotype and host environment on the cuticular hydrocarbon profiles of Lysiphlebus parasitoids and aggression by aphid-tending ants.

Parasitoids in the genus Lysiphlebus specialize on ant-tended aphids and have previously been reported to mimic the cuticular hydrocarbon (CHC) profiles of their aphid hosts to avoid detection by ants. However, the precise mechanisms that mediate reduced ant aggression toward Lysiphlebus spp. are not known, nor is it clear whether such mechanisms are broadly effective or specialized on particular aphid hosts. Here we explore the effects of wasp genotype and host environment on Lysiphlebus CHC profiles and ant aggression. Rearing asexual Lysiphlebus lines in different host aphid environments revealed effects of both wasp line and aphid host on wasp CHCs. However, variation in genotype and host affected different features of the CHC profile, with wasp genotype explaining most variation in linear and long-chain methyl alkanes, while aphid host environment primarily influenced short-chain methyl alkanes. Subsequent behavioural experiments revealed no effects of host environment on ant aggression, but strong evidence for genotypic effects. The influence of genotypic variation on experienced ant aggression and relevant chemical traits is particularly relevant in light of recent evidence for genetic divergence among Lysiphlebus parasitoids collected from different aphid hosts.

A sensory bias overrides learned preferences of bumblebees for honest signals in Mimulus guttatus.

Insect pollinators readily learn olfactory cues, and this is expected to select for 'honest signals' that provide reliable information about floral rewards. However, plants might alternatively produce signals that exploit pollinators' sensory biases, thereby relaxing selection for signal honesty. We examined the innate and learned preferences of Bombus impatiens for Mimulus guttatus floral scent phenotypes corresponding to different levels of pollen rewards in the presence and absence of the innately attractive floral volatile compound ß-trans-bergamotene. Bees learned to prefer honest signals after foraging on live M. guttatus flowers, but only exhibited this preference when presented floral scent phenotypes that did not include ß-trans-bergamotene. Our results suggest that a sensory bias for ß-trans-bergamotene overrides the ability of B. impatiens to use honest signals when foraging on M. guttatus. This may represent a deceptive pollination strategy that allows plants to minimize investment in costly rewards without incurring reduced rates of pollinator visitation.

Effects of Root-Colonizing Fluorescent Pseudomonas Strains on Arabidopsis Resistance to a Pathogen and an Herbivore.

Rhizobacteria in the genus Pseudomonas can enhance plant resistance to a range of pathogens and herbivores. However, resistance to these different classes of plant antagonists is mediated by different molecular mechanisms, and the extent to which induced systemic resistance by Pseudomonas can simultaneously protect plants against both pathogens and herbivores remains unclear. We screened 12 root-colonizing Pseudomonas strains to assess their ability to induce resistance in Arabidopsis thaliana against a foliar pathogen (Pseudomonas syringae DC3000) and a chewing herbivore (Spodoptera littoralis). None of our 12 strains increased plant resistance against herbivory; however, four strains enhanced pathogen resistance, and one of these (Pseudomonas strain P97-38) also made plants more susceptible to herbivory. Phytohormone analyses revealed stronger salicylic acid induction in plants colonized by P97-38 (versus controls) following subsequent pathogen infection but weaker induction of jasmonic acid (JA)-mediated defenses following herbivory. We found no effects of P97-38 inoculation on herbivore-relevant nutrients such as sugars and protein, suggesting that the observed enhancement of susceptibility to S. littoralis is due to effects on plant defense chemistry rather than nutrition. These findings suggest that Pseudomonas strains that enhance plant resistance to pathogens may have neutral or negative effects on resistance to herbivores and provide insight into potential mechanisms associated with effects on different classes of plant antagonists. Improved understanding of these effects has potentially important implications for the use of rhizobacteria inoculation in agriculture. IMPORTANCE Plant-associated microbes have significant potential to enhance agricultural production, for example, by enhancing plant resistance to pathogens and pests. Efforts to identify beneficial microbial strains typically focus on a narrow range of desirable plant traits; however, microbial symbionts can have complex effects on plant phenotypes, including susceptibility and resistance to different classes of plant antagonists. We examined the effects of 12 strains of Pseudomonas rhizobacteria on plant (Arabidopsis) resistance to a lepidopteran herbivore and a foliar pathogen. None of our strains increased plant resistance against herbivory; however, four strains enhanced pathogen resistance, and one of these made plants more susceptible to herbivory (likely via effects on plant defense chemistry). These findings indicate that microbial strains that enhance plant resistance to pathogens can have neutral or negative effects on resistance to herbivores, highlighting potential pitfalls in the application of beneficial rhizobacteria as biocontrol agents.

A plant parasite uses light cues to detect differences in host-plant proximity and architecture.

Sunlight filtered by green plant tissue becomes diminished in its ratio of red to far-red wavelengths (R:FR). Some parasitic plants exploit this change by growing towards regions of low R:FR to locate host plants. In principle, variation in R:FR can also convey ecologically relevant information about host proximity or architecture. Here, we demonstrate that the parasitic vine Cuscuta epilinum Weihe (Convolvulaceae) can distinguish fine-scale differences in R:FR associated with differences in the proximity and shape of potential host plants. We conducted dual-choice experiments by placing parasite seedlings between targets, including low R:FR fields manipulated via LED lighting and pairs of model plants exhibiting realistic R and FR reflectance but differing in proximity or shape. Seedlings consistently distinguished between low-R:FR fields of differing intensity. Furthermore, they exhibited preferences for nearer plant models versus identical models placed 4 cm further away and between same-sized models exhibiting shape differences. Our results indicate that parasites can discriminate minute differences in R:FR signatures corresponding to host factors (proximity and shape) that impact seedling survival. This keen sensory ability underpins the parasite's sophisticated foraging behaviour and highlights the broader importance of light cues in plant ecology.

Inbreeding in Solanum carolinense alters floral attractants and rewards and adversely affects pollinator visitation.

PREMISE: Inbreeding depression is well documented in flowering plants and adversely affects a wide range of fitness-related traits. Recent work has begun to explore the effects of inbreeding on ecological interactions among plants and other organisms, including insect herbivores and pathogens. However, the effects of inbreeding on floral traits, floral scents, and pollinator visitation are less well studied. METHODS: Using inbred and outbred maternal families of horsenettle (Solanum carolinense, Solanaceae), we examined the effects of inbreeding on traits associated with pollinator attraction and floral rewards. Specifically, we measured corolla size, counted pollen grains per flower, and analyzed floral volatile emissions via gas chromatography and mass spectrometry. We also examined pollinator visitation to experimental arrays of flowering inbred and outbred plants under field conditions. RESULTS: Compared to those of outbred plants, flowers of inbred plants exhibited reduced corolla size and pollen production, as well as significantly reduced emission of the two most abundant volatile compounds in the floral blend. Furthermore, bumblebees-the main pollinators of horsenettle-discriminated against inbred flowers in the field: bees were more likely to make initial visits to flowers on outbred plants, visited outbred flowers more often overall, and spent more time on outbred flowers. CONCLUSIONS: These results show that inbreeding can (1) alter floral traits that are known to mediate pollinator attraction; (2) reduce the production of floral rewards (pollen is the sole reward in horsenettle); and (3) adversely affect pollinator visitation under field conditions.

Volatile biomarkers of symptomatic and asymptomatic malaria infection in humans.

Malaria remains among the world's deadliest diseases, and control efforts depend critically on the availability of effective diagnostic tools, particularly for the identification of asymptomatic infections, which play a key role in disease persistence and may account for most instances of transmission but often evade detection by current screening methods. Research on humans and in animal models has shown that infection by malaria parasites elicits changes in host odors that influence vector attraction, suggesting that such changes might yield robust biomarkers of infection status. Here we present findings based on extensive collections of skin volatiles from human populations with high rates of malaria infection in Kenya. We report broad and consistent effects of malaria infection on human volatile profiles, as well as significant divergence in the effects of symptomatic and asymptomatic infections. Furthermore, predictive models based on machine learning algorithms reliably determined infection status based on volatile biomarkers. Critically, our models identified asymptomatic infections with 100% sensitivity, even in the case of low-level infections not detectable by microscopy, far exceeding the performance of currently available rapid diagnostic tests in this regard. We also identified a set of individual compounds that emerged as consistently important predictors of infection status. These findings suggest that volatile biomarkers may have significant potential for the development of a robust, noninvasive screening method for detecting malaria infections under field conditions.

Plant volatiles induced by herbivore eggs prime defences and mediate shifts in the reproductive strategy of receiving plants.

Plants can detect cues associated with the risk of future herbivory and modify defence phenotypes accordingly; however, our current understanding is limited both with respect to the range of early warning cues to which plants respond and the nature of the responses. Here we report that exposure to volatile emissions from plant tissues infested with herbivore eggs promotes stronger defence responses to subsequent herbivory in two Brassica species. Furthermore, exposure to these volatile cues elicited an apparent shift from growth to reproduction in Brassica nigra, with exposed plants exhibiting increased flower and seed production, but reduced leaf production, relative to unexposed controls. Our results thus document plant defence priming in response to a novel environmental cue, oviposition-induced plant volatiles, while also showing that plant responses to early warning cues can include changes in both defence and life-history traits.

Transgenerational impacts of herbivory and inbreeding on reproductive output in Solanum carolinense.

PREMISE: Plant maternal effects on offspring phenotypes are well documented. However, little is known about how herbivory on maternal plants affects offspring fitness. Furthermore, while inbreeding is known to reduce plant reproductive output, previous studies have not explored whether and how such effects may extend across generations. Here, we addressed the transgenerational consequences of herbivory and maternal plant inbreeding on the reproduction of Solanum carolinense offspring. METHODS: Manduca sexta caterpillars were used to inflict weekly damage on inbred and outbred S. carolinense maternal plants. Cross-pollinations were performed by hand to produce seed from herbivore-damaged outbred plants, herbivore-damaged inbred plants, undamaged outbred plants, and undamaged inbred plants. The resulting seeds were grown in the greenhouse to assess emergence rate and flower production in the absence of herbivores. We also grew offspring in the field to examine reproductive output under natural conditions. RESULTS: We found transgenerational effects of herbivory and maternal plant inbreeding on seedling emergence and reproductive output. Offspring of herbivore-damaged plants had greater emergence, flowered earlier, and produced more flowers and seeds than offspring of undamaged plants. Offspring of outbred maternal plants also had greater seedling emergence and reproductive output than offspring of inbred maternal plants, even though all offspring were outbred. Moreover, the effects of maternal plant inbreeding were more severe when plant offspring were grown in field conditions. CONCLUSIONS: This study demonstrates that both herbivory and inbreeding have fitness consequences that extend across generations even in outbred progeny.

Phytoplasma Infection of Cranberry Affects Development and Oviposition, but Not Host-Plant Selection, of the Insect Vector Limotettix vaccinii.

Vector-borne pathogens, such as phytoplasmas, are known to manipulate both host plants and insect vectors to enhance their own transmission. In cranberries, phytoplasma infection causes false blossom disease, which is vectored by blunt-nosed leafhoppers (Limotettix vaccinii Van Duzee). We explored how phytoplasma infection of vegetative cranberry tissues affects the developmental performance and host preferences of L. vaccinii, and compared volatile emissions and phytohormone levels of infected and uninfected plants. In no-choice performance assays, L. vaccinii survival was similar on infected and uninfected cranberry plants. However, nymphs on infected plants took longer to reach adulthood, and produced larger adults, than those on uninfected plants. In oviposition tests, L. vaccinii females laid more eggs on uninfected than phytoplasma-infected cranberry plants. Olfactometer studies revealed preferences of L. vaccinii nymphs and adults for odors of uninfected plants when compared to clean air but not when compared to infected plants. Uninfected plants emitted higher amounts of volatiles than infected plants, particularly (E)-4,8-dimethyl-1, 3, 7-nonatriene, dodecane, and germacrene-D; however, we observed no effect of infection on phytohormone levels. Overall, our data show an oviposition preference of L. vaccinii for healthy plants, whereas our attraction data show no preference between infected and uninfected plants. While slower development on phytoplasma-infected cranberry plants led to larger adult size, there was no evidence for any beneficial effects of infection on other L. vaccinii performance parameters. Results from these studies are of relevance to understand tripartite plant-phytoplasma-insect vector interactions and are discussed in the context of the vector manipulation hypothesis.

Costs of plant defense priming: exposure to volatile cues from a specialist herbivore increases short-term growth but reduces rhizome production in tall goldenrod (Solidago altissima).

BACKGROUND: By sensing environmental cues indicative of pathogens or herbivores, plants can "prime" appropriate defenses and deploy faster, stronger responses to subsequent attack. Such priming presumably entails costs-else the primed state should be constitutively expressed-yet those costs remain poorly documented, in part due to a lack of studies conducted under realistic ecological conditions. We explored how defence priming in goldenrod (Solidago altissima) influenced growth and reproduction under semi-natural field conditions by manipulating exposure to priming cues (volatile emissions of a specialist herbivore, Eurosta solidaginis), competition between neighbouring plants, and herbivory (via insecticide application). RESULTS: We found that primed plants grew faster than unprimed plants, but produced fewer rhizomes, suggesting reduced capacity for clonal reproduction. Unexpectedly, this effect was apparent only in the absence of insecticide, prompting a follow-up experiment that revealed direct effects of the pesticide esfenvalerate on plant growth (contrary to previous reports from goldenrod). Meanwhile, even in the absence of pesticide, priming had little effect on herbivore damage levels, likely because herbivores susceptible to the primed defences were rare or absent due to seasonality. CONCLUSIONS: Reduced clonal reproduction in primed plants suggest that priming can entail significant costs for plants. These costs, however, may only become apparent when priming cues fail to provide accurate information about prevailing threats, as was the case in this study. Additionally, our insecticide data indicate that pesticides or their carrier compounds can subtly, but significantly, affect plant physiology and may interact with plant defences.

Combined effects of mutualistic rhizobacteria counteract virus-induced suppression of indirect plant defences in soya bean.

It is increasingly clear that microbial plant symbionts can influence interactions between their plant hosts and other organisms. However, such effects remain poorly understood, particularly under ecologically realistic conditions where plants simultaneously interact with diverse mutualists and antagonists. Here, we examine how the effects of a plant virus on indirect plant defences against its insect vector are influenced by co-occurrence of other microbial plant symbionts. Using a multi-factorial design, we manipulated colonization of soya bean using three different microbes: a pathogenic plant virus (bean pod mottle virus (BPMV)), a nodule-forming beneficial rhizobacterium ( Bradyrhizobium japonicum) and a plant growth-promoting rhizobacterium ( Delftia acidovorans). We then assessed recruitment of parasitoids ( Pediobious foveolatus (Eulophidae)) and parasitism rates following feeding by the BPMV vector Epilachna varivestis (Coccinellidae). BPMV infection suppressed parasitoid recruitment, prolonged parasitoid foraging time and reduced parasitism rates in semi-natural foraging assays. However, simultaneous colonization of BPMV-infected hosts by both rhizobacteria restored parasitoid recruitment and rates of parasitism to levels similar to uninfected controls. Co-colonization by the two rhizobacteria also enhanced parasitoid recruitment in the absence of BPMV infection. These results illustrate the potential of plant-associated microbes to influence indirect plant defences, with implications for disease transmission and herbivory, but also highlight the potential complexity of such interactions.

Sorghum 3-Deoxyanthocyanidin Flavonoids Confer Resistance against Corn Leaf Aphid.

In this study we examined the role of sorghum flavonoids in providing resistance against corn leaf aphid (CLA) Rhopalosiphum maidis. In sorghum, accumulation of these flavonoids is regulated by a MYB transcription factor, yellow seed1 (y1). Functional y1 alleles accumulate 3-deoxyflavonoids (3-DFs) and 3-deoxyanthocyanidins (3-DAs) whereas null y1 alleles fail to accumulate these compounds. We found that significantly higher numbers of alate CLA adults colonized null y1 plants as compared to functional y1 plants. Controlled cage experiments and pairwise choice assays demonstrated that apterous aphids preferred to feed and reproduce on null y1 plants. These near-isogenic sorghum lines do not differ in their epicuticular wax content and were also devoid of any leaf trichomes. Significantly higher mortality of CLA was observed on artificial aphid diet supplemented with flavonoids obtained from functional y1 plants as compared to null y1 plants or the relevant controls. Our results demonstrate that the proximate mechanism underlying the deleterious effects on aphids is y1-regulated flavonoids which are important defense compounds against CLA.

Divergence in Glucosinolate Profiles between High- and Low-Elevation Populations of Arabidopsis halleri Correspond to Variation in Field Herbivory and Herbivore Behavioral Preferences.

Variation in local herbivore pressure along elevation gradients is predicted to drive variation in plant defense traits. Yet, the extent of intraspecific variation in defense investment along elevation gradients, and its effects on both herbivore preference and performance, remain relatively unexplored. Using populations of Arabidopsis halleri (Brassicaceae) occurring at different elevations in the Alps, we tested for associations between elevation, herbivore damage in the field, and constitutive chemical defense traits (glucosinolates) assayed under common-garden conditions. Additionally, we examined the feeding preferences and performance of a specialist herbivore, the butterfly Pieris brassicae, on plants from different elevations in the Alps. Although we found no effect of elevation on the overall levels of constitutive glucosinolates in leaves, relative amounts of indole glucosinolates increased significantly with elevation and were negatively correlated with herbivore damage in the field. In oviposition preference assays, P. brassicae females laid fewer eggs on plants from high-elevation populations, although larval performance was similar on populations from different elevations. Taken together, these results support the prediction that species distributed along elevation gradients exhibit genetic variation in chemical defenses, which can have consequences for interactions with herbivores in the field.

Non-glandular trichomes of Solanum carolinense deter feeding by Manduca sexta caterpillars and cause damage to the gut peritrophic matrix.

Plant trichomes constitute a first line of defence against insect herbivores. The pre- and post-ingestive defensive functions of glandular trichomes are well documented and include direct toxicity, adhesion, antinutrition and defence gene induction. By contrast, the defensive functions of non-glandular trichomes are less well characterized, although these structures are thought to serve as physical barriers that impede herbivore feeding and movement. We experimentally varied the density of stellate non-glandular trichomes in several ways to explore their pre- and post-ingestive effects on herbivores. Larvae of Manduca sexta (Sphingidae) initiated feeding faster and gained more weight on Solanum carolinense (Solanaceae) leaves having lower trichome densities (or experimentally removed trichomes) than on leaves having higher trichome densities. Adding trichomes to artificial diet also deterred feeding and adversely affected caterpillar growth relative to controls. Scanning electron and light microscopy revealed that the ingestion of stellate trichomes by M. sexta caterpillars caused extensive damage to the peritrophic membrane, a gut lining that is essential to digestion and pathogen isolation. These findings suggest that, in addition to acting as a physical barrier to deter feeding, trichomes can inhibit caterpillar growth and development via post-ingestive effects.

Glucosinolates from Host Plants Influence Growth of the Parasitic Plant Cuscuta gronovii and Its Susceptibility to Aphid Feeding.

Parasitic plants acquire diverse secondary metabolites from their hosts, including defense compounds that target insect herbivores. However, the ecological implications of this phenomenon, including the potential enhancement of parasite defenses, remain largely unexplored. We studied the translocation of glucosinolates from the brassicaceous host plant Arabidopsis (Arabidopsis thaliana) into parasitic dodder vines (Convolvulaceae; Cuscuta gronovii) and its effects on the parasite itself and on dodder-aphid interactions. Aliphatic and indole glucosinolates reached concentrations in parasite tissues higher than those observed in corresponding host tissues. Dodder growth was enhanced on cyp79B2 cyp79B3 hosts (without indole glucosinolates) but inhibited on atr1D hosts (with elevated indole glucosinolates) relative to wild-type hosts, which responded to parasitism with localized elevation of indole and aliphatic glucosinolates. These findings implicate indole glucosinolates in defense against parasitic plants. Rates of settling and survival on dodder vines by pea aphids (Acyrthosiphon pisum) were reduced significantly when dodder parasitized glucosinolate-producing hosts (wild type and atr1D) compared with glucosinolate-free hosts (cyp79B2 cyp79B3 myb28 myb29). However, settling and survival of green peach aphids (Myzus persicae) were not affected. M. persicae population growth was actually reduced on dodder parasitizing glucosinolate-free hosts compared with wild-type or atr1D hosts, even though stems of the former contain less glucosinolates and more amino acids. Strikingly, this effect was reversed when the aphids fed directly upon Arabidopsis, which indicates an interactive effect of parasite and host genotype on M. persicae that stems from host effects on dodder. Thus, our findings indicate that glucosinolates may have both direct and indirect effects on dodder-feeding herbivores.

Plant spines deter herbivory by restricting caterpillar movement.

The spines of flowering plants are thought to function primarily in defence against mammalian herbivores; however, we previously reported that feeding by Manduca sexta caterpillars on the leaves of horsenettle plants (Solanum carolinense) induces increased development of internode spines on new growth. To determine whether and how spines impact caterpillar feeding, we conducted assays with three Solanaceous plant species that vary in spine numbers (S. carolinense, S. atropurpureum and S. aethiopicum) and also manipulated spine numbers within each species. We found that M. sexta caterpillars located experimentally isolated target leaves much more quickly on plants with experimentally removed spines compared with plants with intact spines. Moreover, it took caterpillars longer to defoliate species with relatively high spine numbers (S. carolinense and particularly Satropurpureum) compared with S. aethiopicum, which has fewer spines. These findings suggest that spines may play a significant role in defence against insect herbivores by restricting herbivore movement and increasing the time taken to access feeding sites, with possible consequences including longer developmental periods and increased vulnerability or apparency to predators.

Exploring the Effects of Plant Odors, from Tree Species of Differing Host Quality, on the Response of Lymantria dispar Males to Female Sex Pheromones.

A widely accepted hypothesis for host-plant selection in herbivorous insects is that ovipositing females select host-plants that maximize the survival and performance of their offspring. However, numerous studies indicate that this is not always the case for polyphagous species. Lymantria dispar is a highly polyphagous forest defoliator and has flightless females in some subspecies, resulting in a limited capacity to make host-choices. Males of other Lepidopteran species utilize a combination of sexual pheromones and plant volatiles in their mating choices and exhibit preferences among plant species. We explored the behavior of L. dispar males towards sexual pheromone in the presence and absence of plant volatiles and their ability to discriminate between two plant species with different degrees of suitability for their offspring: a suboptimal host (Pinus sylvestris), and an optimal host (Quercus robur). In no-choice wind tunnel assays, we found that rates of male success in locating a pheromone source were not altered by the presence of plant odors; however, the time spent by males searching for the pheromone source after reaching the full length of the tunnel was reduced by more than 50% in the presence of plant volatiles. In dual choice assays, males exhibited a clear preference for a combination of pheromones and plant volatiles over the pheromone alone. However, we did not find evidence of an innate ability to discriminate between the odors of optimal and suboptimal host plants. We discuss possible ecological and evolutionary explanations for these observations.

Malaria-induced changes in host odors enhance mosquito attraction.

Vector-borne pathogens may alter traits of their primary hosts in ways that influence the frequency and nature of interactions between hosts and vectors. Previous work has reported enhanced mosquito attraction to host organisms infected with malaria parasites but did not address the mechanisms underlying such effects. Here we document malaria-induced changes in the odor profiles of infected mice (relative to healthy individuals) over the course of infection, as well as effects on the attractiveness of infected hosts to mosquito vectors. We observed enhanced mosquito attraction to infected mice during a key period after the subsidence of acute malaria symptoms, but during which mice remained highly infectious. This attraction corresponded to an overall elevation in the volatile emissions of infected mice observed during this period. Furthermore, data analyses--using discriminant analysis of principal components and random forest approaches--revealed clear differences in the composition of the volatile blends of infected and healthy individuals. Experimental manipulation of individual compounds that exhibited altered emission levels during the period when differential vector attraction was observed also elicited enhanced mosquito attraction, indicating that compounds being influenced by malaria infection status also mediate vector host-seeking behavior. These findings provide important insights into the cues that mediate vector attraction to hosts infected with transmissible stages of malaria parasites, as well as documenting characteristic changes in the odors of infected individuals that may have potential value as diagnostic biomarkers of infection.

Exposure of Solidago altissima plants to volatile emissions of an insect antagonist (Eurosta solidaginis) deters subsequent herbivory.

Recent work indicates that plants respond to environmental odors. For example, some parasitic plants grow toward volatile cues from their host plants, and other plants have been shown to exhibit enhanced defense capability after exposure to volatile emissions from herbivore-damaged neighbors. Despite such intriguing discoveries, we currently know relatively little about the occurrence and significance of plant responses to olfactory cues in natural systems. Here we explore the possibility that some plants may respond to the odors of insect antagonists. We report that tall goldenrod (Solidago altissima) plants exposed to the putative sex attractant of a closely associated herbivore, the gall-inducing fly Eurosta solidaginis, exhibit enhanced defense responses and reduced susceptibility to insect feeding damage. In a field study, egg-laying E. solidaginis females discriminated against plants previously exposed to the sex-specific volatile emissions of males; furthermore, overall rates of herbivory were reduced on exposed plants. Consistent with these findings, laboratory assays documented reduced performance of the specialist herbivore Trirhabda virgata on plants exposed to male fly emissions (or crude extracts), as well as enhanced induction of the key defense hormone jasmonic acid in exposed plants after herbivory. These unexpected findings from a classic ecological study system provide evidence for a previously unexplored class of plant-insect interactions involving plant responses to insect-derived olfactory cues.