Extrafloral Nectar Production Research Articles

Rhizobia-legume symbiosis mediates direct and indirect interactions between plants, herbivores and their parasitoids

Microorganisms associated with plant roots significantly impact the quality and quantity of plant defences. However, the bottom-up effects of soil microbes on the aboveground multitrophic interactions remain largely under studied. To address this gap, we investigated the chemically-mediated effects of nitrogen-fixing rhizobia on legume-herbivore-parasitoid multitrophic interactions. To address this, we initially examined the cascading effects of the rhizobia bean association on herbivore caterpillars, their parasitoids, and subsequently investigated how rhizobia influence on plant volatiles and extrafloral nectar. Our goal was to understand how these plant-mediated effects can affect parasitoids. Lima bean plants (Phaseoulus lunatus) inoculated with rhizobia exhibited better growth, and the number of root nodules positively correlated with defensive cyanogenic compounds. Despite increase of these chemical defences, Spodoptera latifascia caterpillars preferred to feed and grew faster on rhizobia-inoculated plants. Moreover, the emission of plant volatiles after leaf damage showed distinct patterns between inoculation treatments, with inoculated plants producing more sesquiterpenes and benzyl nitrile than non-inoculated plants. Despite these differences, Euplectrus platyhypenae parasitoid wasps were similarly attracted to rhizobia- or no rhizobia-treated plants. Yet, the oviposition and offspring development of E. platyhypenae was better on caterpillars fed with rhizobia-inoculated plants. We additionally show that rhizobia-inoculated common bean plants (Phaseolus vulgaris) produced more extrafloral nectar, with higher hydrocarbon concentration, than non-inoculated plants. Consequently, parasitoids performed better when fed with extrafloral nectar from rhizobia-inoculated plants. While the overall effects of bean-rhizobia symbiosis on caterpillars were positive, rhizobia also indirectly benefited parasitoids through the caterpillar host, and directly through the improved production of high quality extrafloral nectar. This study underscores the importance of exploring diverse facets and chemical mechanisms that influence the dynamics between herbivores and predators. This knowledge is crucial for gaining a comprehensive understanding of the ecological implications of rhizobia symbiosis on these interactions.

Temporal and spatial variation in extrafloral nectar and associated ants in Canavalia rosea (Fabaceae) on coastal dunes along the Gulf of Mexico

ABSTRACT Extrafloral nectar production can vary depending on the structure of the plant in which it is found, as well as environmental conditions. This can in turn influence plant-ant interactions. We evaluated the role of extrafloral nectaries of Canavalia rosea as ant attractants in a coastal zone of the Gulf of Mexico. We compared the productivity of the extrafloral nectaries present on leaves and inflorescences, as well as between day and night, and described the associated ant community. Nectar volume and concentration were higher in inflorescences than in leaves, and the rate of ant visitation was higher in inflorescences during the day. These results suggest congruence with the Optimal Defense Hypothesis. Nectar volume was generally greater during the night in both structures, but sugar concentration did not differ between day and night. We found a turnover in the ant species visiting C. rosea, with S. geminata being the most abundant species. The extrafloral nectar of C. rosea attracts several ant species and probably other arthropods, providing a valuable resource in a stressful system.

Soil salinization disrupts plant-plant signaling effects on extra-floral nectar induction in wild cotton.

Plant-plant interactions via volatile organic compounds (VOCs) have received much attention, but how abiotic stresses affect these interactions is poorly understood. We tested the effect of VOCs exposure from damaged conspecifics on the production of extra-floral nectar (EFN) in wild cotton plants (Gossypium hirsutum), a coastal species in northern Yucatan (Mexico), and whether soil salinization affected these responses. We placed plants in mesh cages, and within each cage assigned plants as emitters or receivers. We exposed emitters to either ambient or augmented soil salinity to simulate a salinity shock, and within each group subjected half of the emitters to no damage or artificial leaf damage with caterpillar regurgitant. Damage increased the emission of sesquiterpenes and aromatic compounds under ambient but not under augmented salinity. Correspondingly, exposure to VOCs from damaged emitters had effect on receiver EFN induction, but this effect was contingent on salinization. Receivers produced more EFN in response to damage after being exposed to VOCs from damaged emitters when the latter were grown under ambient salinity, but not when they were subjected to salinization. These results suggest complex effects of abiotic factors on VOC-mediated plant interactions.

Time course of inducibility of indirect responses in an ant-defended plant.

Plants have evolved inducible defenses that allow them to minimize costs associated with the production of constitutive defenses when herbivores are not present. However, as a consequence, some plants might experience a period of vulnerability between damage and the onset of defense and/or between the cessation of damage and relaxation of defense. Few studies have examined the time course in the inducible protective mutualism between ants and extrafloral nectary (EFN)-bearing plants. None has compared the inducibility of EFNs on vegetative versus reproductive parts or in response to different levels of herbivore damage. Here, we disentangle the inducibility process by evaluating extrafloral nectar production and ant attendance over time, the time course of inducibility on different plant parts, and the time course of inducibility in response to different levels of foliar damage in a Brazilian tree, Qualea multiflora (Vochysiaceae). Using simulated herbivory on leaves and flowers, we found that (a) the production of extrafloral nectar from foliar and floral EFNs, as well as ant attendance, exhibited a lag between the moment of damage and the peak of response, followed by a response peak (usually 24 h after damage) at which the defense remains at its maximum level, then declines to prestimulus levels; (b) the time course of inducibility and the peak activity did not differ between EFNs located in vegetative versus reproductive parts, except for sugar concentration, which was higher in EFNs on vegetative parts; and (c) the time course of inducibility of foliar EFNs depended on damage level. Although considered a cost-saving strategy, inducible defenses can be disadvantageous since they can leave plants vulnerable to attack for extended periods. Our results illuminate the dynamics of the induced response and the underlying mechanisms that might mediate it, ultimately providing new insights into defense strategies employed by plants.

Defoliation in Perennial Plants: Predictable and Surprising Results in Senna spp.

When some plants are defoliated, they may suffer by reaching a smaller final size than if they had not been damaged. Other plants may compensate for damage, ending up the same size as if they had not been damaged. Still, others may overcompensate, ending up larger after defoliation than if they had been spared from damage. We investigated the response of Senna species (Fabaceae) to defoliation, comparing two native and several ornamental congeners, all of which grow locally in southern Florida. Many Senna spp. bear foliar nectaries as nutritional resources for beneficial insects that may, in exchange, protect them from herbivores. We grew five species from seed and subjected them to three levels of defoliation for a period of several months to measure effects of leaf area removal on plant height, number of leaves, and number of extrafloral nectaries. Only three of five species displayed shorter plant heights with greater levels of damage. Two species produced fewer new leaves with moderate to severe defoliation. In only one species, the number of extrafloral nectaries decreased with defoliation, suggesting that while extrafloral nectar production may be an inducible defense in some species, producing more nectaries in response to damage does not occur in these Senna species.

Genetic architecture of multiple mutualisms and mating system in Turnera ulmifolia.

Plants often associate with multiple arthropod mutualists. These partners provide important services to their hosts, but multiple interactions can constrain a plant's ability to respond to complex, multivariate selection. Here, we quantified patterns of genetic variance and covariance among rewards for pollination, biotic defence and seed dispersal mutualisms in multiple populations of Turnera ulmifolia to better understand how the genetic architecture of multiple mutualisms might influence their evolution. We phenotyped plants cultivated from 17 Jamaican populations for several mutualism and mating system-related traits. We then fit genetic variance-covariance (G) matrices for the island metapopulation and the five largest individual populations. At the metapopulation level, we observed significant positive genetic correlations among stigma-anther separation, floral nectar production and extrafloral nectar production. These correlations have the potential to significantly constrain or facilitate the evolution of multiple mutualisms in T.ulmifolia and suggest that pollination, seed dispersal and defence mutualisms do not evolve independently. In particular, we found that positive genetic correlations between floral and extrafloral nectar production may help explain their stable coexistence in the face of physiological trade-offs and negative interactions between pollinators and ant bodyguards. Locally, we found only small differences in G among our T.ulmifolia populations, suggesting that geographic variation in G may not shape the evolution of multiple mutualisms.

Extrafloral nectar production induced by simulated herbivory does not improve ant bodyguard attendance and ultimately plant defence

Abstract Highly competitive and aggressive ant species are efficient bodyguards that monopolize the more attractive plants bearing extrafloral nectaries. Given that herbivory often increases the quality of extrafloral nectar, we hypothesized that plants damaged by herbivory would be more prone to interact with high-quality ant bodyguards and be better defended against herbivores. We performed an experiment with Chamaecrista nictitans plants. We induced anti-herbivore responses by applying jasmonic acid to a group of plants while keeping another group unmanaged. We measured extrafloral nectar production, censused ants visiting extrafloral nectaries and, subsequently, added herbivore mimics to measure the efficiency of ant anti-herbivore defence in both conditions. Induction increased the volume of extrafloral nectar and the mass of sugar per nectary without affecting the sugar concentration or the patterns of plant attendance and defence by ants. Thus, we found no evidence that defence-induced C. nictitans plants are more prone to interact with high-quality bodyguards or to receive better anti-herbivore defence. These findings highlight that increases in extrafloral nectar production are not always rewarded with increases in the biotic defences; instead, these rewards might be dependent on the traits of the nectar induced by herbivory events and/or on the ecological context in which the interaction is embedded. Consequently, herbivory might increase the costs of this induced biotic defence to plants bearing extrafloral nectaries when the induced defence does not increase the attractiveness of the plants to ants.

Contrasting effects of herbivore damage type on extrafloral nectar production and ant attendance

The ant-plant mutualism mediated by extrafloral nectaries (EFNs) is based on the provision of extrafloral nectar (a sugar-rich liquid) in exchange for ant defense against herbivores. The extrafloral nectar production can vary depending on the intensity or types of leaf damage caused by different mouthparts of insect herbivores (i.e., sucking or chewing insects). However, it is poorly known how the type of herbivore damage changes the outcomes of tri-trophic interactions. Here, we address how the damage inflicted by different herbivore groups affects extrafloral nectar production and ants’ attendance and aggressiveness. Through an experimental field study, we tested whether the type of leaf damage, sucking or chewing, changes extrafloral nectar volume and sugar concentration, and subsequently ant composition and behavior. We evaluated the extrafloral nectar production and ant attendance and aggressiveness before and after simulated herbivory in Qualea parviflora (Vochysiaceae), a Brazilian savanna EFN-bearing plant. We found that only plants with sucking damage increased the sugar concentration of their extrafloral nectar, only plants with chewing damage increased ant abundance, and surrogate herbivore predation differed before and after simulated herbivory only for sucking damage. We provided the first evidence that the type of damage can influence ant aggressiveness via changes in extrafloral nectar production, translating to changes in the outcomes of this ant-plant mutualism. Ultimately, studies should consider the effects of herbivore damage type when studying insect-plant interactions.

Plant protection services mediated by extrafloral nectaries decline with aridity but are not influenced by chronic anthropogenic disturbance in Brazilian Caatinga

Abstract Most terrestrial species occur in human‐modified landscapes that are experiencing climate change. In addition to direct impacts on species, both anthropogenic disturbance and climate change can have important effects through changes in species interactions, including the disruption of ecological services provided by them. Here we investigate how chronic anthropogenic disturbance (CAD) and aridity affect the effectiveness of plant protection services provided by ants to plants bearing extrafloral nectaries (EFNs). The study was conducted across 13 01‐ha plots distributed along CAD and aridity gradients in Caatinga vegetation of northeastern Brazil. We focused on Pityrocarpa moniliformis, the most abundant and widely distributed EFN‐bearing tree species occurring in our study area, and we used experimental attack rates on termites as a measure of effectiveness of ant protection services. We investigated the relative roles of nectar production (volume and concentration) and ant species composition in mediating the effects of CAD and aridity on the effectiveness of protection services. Attack rates by ants declined with increasing aridity but were not related to CAD. The volume of extrafloral nectar declined with increasing CAD but was not affected by aridity, whereas the concentration was not related to either CAD or aridity. The composition of attendant ant species varied with aridity but not with CAD. Synthesis. Our findings suggest that CAD does not affect plant‐protection services mediated by EFNs in Brazilian Caatinga. However, ant‐protection services declined with increased aridity, and this occurred through changes in the composition of attendant ant species rather than by changes in the production of extrafloral nectar. Such a response to increasing aridity highlights the vulnerability of EFN‐bearing plants to climate change through decreased predation of herbivores.

Optimal Defense Theory in an ant–plant mutualism: Extrafloral nectar as an induced defence is maximized in the most valuable plant structures

Abstract Plants allocate defences in order to decrease costs and maximize benefits against herbivores. The Optimal Defense Theory (ODT) predicts that continuously expressed (i.e. constitutive) defences are expected in structures of high value, whereas defences that are expressed or that increase their expression only after damage or upon risk of damage (i.e. induced defences) are expected in structures of low value. Although there are several studies evaluating ODT predictions, few studies have successfully tested them as a way of measuring ecological investment in extrafloral nectary (EFN)‐mediated ant–plant interactions. Here we compared extrafloral nectar production and ant attractiveness to EFNs located on vegetative versus reproductive plant structures on Qualea multiflora plants subjected to different levels of simulated herbivory. We then addressed the following predictions emerging from the ODT: (a) extrafloral nectar produced in inflorescence EFNs will have higher volumes and calories and will attract more ants than extrafloral nectar produced in leaf EFNs; (b) extrafloral nectar production (volume and calories) and ant attendance will increase after simulated herbivory in leaf EFNs but not in inflorescence EFNs; (c) higher simulated leaf herbivory will induce higher extrafloral nectar production in EFNs on leaves and (d) more attractive extrafloral nectar (higher volume and calories) will attract more ants. Extrafloral nectar volume and calorie content, as well as ant abundance, were higher in EFNs of inflorescences compared to EFNs of leaves both before and after simulated herbivory, consistent with one of our predictions. However, EFNs on both leaves and inflorescences, not on leaves only, were induced by simulated herbivory, a pattern opposite to our prediction. Plants subjected to higher levels of leaf damage produced more and higher calorie extrafloral nectar, but showed similar ant abundance. Finally, more attractive extrafloral nectar attracted more ants. Synthesis. Our results show that extrafloral nectar production before and after simulated herbivory, as well as ant recruitment, varies according to the plant structure on which EFNs are located. Our study is the first to show that ant recruitment via extrafloral nectar follows predictions from Optimal Defense Theory, and that the ant foraging patterns may be shaped by the plant part attacked and the level of damage it receives.

Functional Role of Extrafloral Nectar in Boreal Forest Ecosystems under Climate Change

Carbohydrate-rich extrafloral nectar (EFN) is produced in nectaries on the leaves, stipules, and stems of plants and provides a significant energy source for ants and other plant mutualists outside of the flowering period. Our review of literature on EFN indicates that only a few forest plant species in cool boreal environments bear EFN-producing nectaries and that EFN production in many boreal and subarctic plant species is poorly studied. Boreal forest, the world’s largest land biome, is dominated by coniferous trees, which, like most gymnosperms, do not produce EFN. Notably, common deciduous tree species that can be dominant in boreal forest stands, such as Betula and Alnus species, do not produce EFN, while Prunus and Populus species are the most important EFN-producing tree species. EFN together with aphid honeydew is known to play a main role in shaping ant communities. Ants are considered to be keystone species in mixed and conifer-dominated boreal and mountain forests because they transfer a significant amount of carbon from the canopy to the soil. Our review suggests that in boreal forests aphid honeydew is a more important carbohydrate source for ants than in many warmer ecosystems and that EFN-bearing plant species might not have a competitive advantage against herbivores. However, this hypothesis needs to be tested in the future. Warming of northern ecosystems under climate change might drastically promote the invasion of many EFN-producing plants and the associated insect species that consume EFN as their major carbohydrate source. This may result in substantial changes in the diet preferences of ant communities, the preventative roles of ants against insect pest outbreaks, and the ecosystem services they provide. However, wood ants have adapted to using tree sap that leaks from bark cracks in spring, which may mitigate the effects of improved EFN availability.

Production of extrafloral nectar in the Neotropical shrub Turnera subulata mediated by biotic and abiotic factors

Interactions among plants secreting extrafloral nectar (EFN) and arthropods, mainly ants, have traditionally been considered as a classic example of mutualism. However, the net balance of these interactions can be highly variable according to biotic and abiotic factors. In the present study, we investigated whether the production of EFN in the Neotropical shrub Turnera subulata (Turneraceae) is induced by ecological context (i.e. biotic and abiotic factors). For this, we tested the hypotheses that the production of EFN in T. subulata plants is increased according to the biotic (phonological stage, herbivore and its simulation, presence of associated ants) and abiotic factors (water stress) in order to protect the most valuable and vulnerable parts. Manipulative experiments were carried out under greenhouse conditions. First, the production of EFN was analyzed in plants of T. subulata in vegetative and reproductive stages with and without water stress. Then, EFN production in response to herbivory and presence of associated ants were tested in plants in the reproductive with and without water stress. EFN volume was estimated with microcapillaries tubes. In general, plants under water stress produced less EFN than plants without stress. The production of EFN was increased in the reproductive stage and under herbivory. Plants under herbivory and with water stress produced a higher amount of EFN only when experience damages caused by the specialist herbivore. On the other hand, plants did not respond to the removal of EFN by ants or manually. Our results demonstrate that T. subulata can modulate the production of EFN according to the ecological context. The plastic response in the production of EFN by T. subulata seems to minimize the costs in defense.

Effects of amount and recurrence of leaf herbivory on the induction of direct and indirect defences in wild cotton.

The induction of defences in response to herbivory is a key mechanism of plant resistance. While a number ofstudies have investigated the time course and magnitude of plant induction in response to a single event of herbivory, few have looked at the effects of recurrent herbivory. Furthermore, studies measuring the effects of the total amount and recurrence of herbivory on both direct and indirect plant defences are lacking. To address this gap,here we asked whether insect leaf herbivory induced changes in the amount and concentration of extrafloral nectar (an indirect defence) and concentration of leaf phenolic compounds (a direct defence) in wild cotton (Gossypium hirsutum). We conducted a greenhouse experiment where we tested single event or recurrent herbivory effects on defence induction by applying mechanical leaf damage and caterpillar (Spodoptera frugiperda) regurgitant. Single events of 25% and 50% leaf damagedid not significantly influence extrafloral nectar production or concentration. Extrafloral nectar traits did, however, increase significantly relative to controls when plants were exposed to recurrent herbivory (two episodes of 25% damage). In contrast, phenolic compounds increased significantly in response to single events of leaf damage but not to recurrent damage. In addition, we found. that local induction of extrafloral nectar productionwas stronger than systemic induction, whereas the reverse pattern was observed for phenolics. Together, these results reveal seemingly inverse patterns of induction of direct and indirect defences in response to herbivory in wild cotton.

Cotton extrafloral nectaries as indirect defence against insect pests

Knowledge about indirect defences that mediate the ecological interactions between plants and their beneficial and harmful insects are needed to better incorporate pest resistance traits in breeding programs. “Top-down” pest control by natural enemies has not been considered in most plant breeding strategies and, in many cases, beneficial interactions of crops with the third trophic level have been reduced through breeding processes. Extrafloral nectaries (EFNs) are plant structures that provide indirect defences to plants through nectar rewards attracting arthropods that may act as predators of pests. Recent studies have demonstrated that EFNs may play an important role in herbivore suppression in agricultural systems. The main objectives of this study were to quantify the leaf extrafloral nectar production of cotton plants, to identify natural enemies of crop pests visiting these EFNs, and to assess the biological control and occurrence of various cotton pests. To address these objectives, we performed both greenhouse and field experiments in which we compared three wild varieties of cotton plants with EFNs, two cultivated varieties with EFNs and two nectariless varieties. Results showed that wild cotton varieties produced greater quantity of nectar per leaf than cultivated varieties. In addition, wild varieties bearing EFNs enhanced the diversity of natural enemies (especially ants) which could, in turn, increase biological control under certain conditions. However, the association between the community of insects visiting EFNs and pest suppression was not clear and depended on the pest species. Indirect defences associated to EFN need to be better explored to enhance selection of pest-resistant cotton cultivars.

Root-associated fungi increase male fitness, while high simulated herbivory decreases indirect defenses in Croton lachnostachyus plants

Plants interact with a diverse array of organisms below and above ground; some interactions with ants allow plants to be protected against herbivorous insects, influencing their growth or reproduction. In addition, indirect plant defenses—such as those mediated by extrafloral nectaries (EFNs)—could be affected by plant root symbionts. However, it is not clear how the suppression of root symbionts might affect extrafloral nectar (EFN) production and plant reproductive output. We made an experimental study with a shrub species with EFNs. Firstly, we tested if root-associated fungi (i.e., comparing plants with and without fungicide) increased the production of pollen (male function) and EFN (volume, nectar concentration, and total sugar content) in Croton lachnostachyus. Subsequently, we implemented a second experiment on the same plants, adding different levels of simulated herbivory (none, low, high) to assess the combined effects of root-associated fungi and herbivory. While we found high levels of mycorrhizal colonization, we found no signs of pathogenic fungi and negligible values of dark septate endophytes in roots so we attributed our results mostly to arbuscular mycorrhizal fungi (AMF). The first experiment showed that plants without the fungicide treatment increased pollen production and secreted a lower mean volume of EFN with higher concentration of dissolved soluble solids. In the second experiment, high levels of simulated herbivory showed a diminution on EFN variables; also, we detected a lower shoot dry mass on plants with low levels of herbivory and no interactions with AMF. Our results suggest complex ecological responses related to the root-associated fungal community and simulated herbivory.

Neomycin: An Effective Inhibitor of Jasmonate-Induced Reactions in Plants

Jasmonates are important phytohormones involved in both plant developmental processes as well as defense reactions. Many JA-mediated plant defense responses have been studied in model plants using mutants of the jasmonate signaling pathway. However, in plant species where JA-signaling mutants are not accessible, the availability of a tool targeting JA signaling is crucial to investigate jasmonate-dependent processes. Neomycin is a poly-cationic aminoglycoside antibiotic that blocks the release of Ca2+ from internal stores. We examined the inhibitory activities of neomycin on different jasmonate-inducible responses in eight different plant species: Intracellular calcium measurements in Nicotiana tabacum cell culture, Sporamin gene induction in Ipomoea batatas, PDF2.2 gene expression in Triticum aestivum, Nepenthesin protease activity measurement in Nepenthes alata, extrafloral nectar production in Phaseolus lunatus, nectary formation in Populus trichocarpa, terpene accumulation in Picea abies, and secondary metabolite generation in Nicotiana attenuata. We are able to show that neomycin, an easily manageable and commercially available compound, inhibits JA-mediated responses across the plant kingdom.

Response of extrafloral nectar production to elevated atmospheric carbon dioxide

Extrafloral nectar attracts ants, whose presence provides protection for the plant against herbivores. Extrafloral nectar is thus a critical component of many plant–insect mutualisms worldwide, so environmental perturbations that alter extrafloral nectar production or composition could be disruptive. The carbon–nutrient balance hypothesis predicts that under elevated CO2 the total volume of extrafloral nectar will increase but the proportion of the foliar carbohydrate pool secreted as extrafloral nectar will decrease, without any change in the sugar composition of the extrafloral nectar. We investigated the impact of elevated atmospheric CO2 on extrafloral nectar in an Australian wild cotton species, Gossypium sturtianum J.H.Willis. Under elevated CO2 there was an increase in the proportion of leaves actively producing nectar and a decrease in the nectar volume per active leaf. Elevated CO2 did not affect the total volume or composition of extrafloral nectar, but there was a change in how the nectar was distributed within the leaf canopy, as well as evidence of increased turnover of leaves and earlier onset of flowering. By the end of the study, there was no difference in the total resources allocated to extrafloral nectar under elevated CO2, which contrasts with the predictions of the carbon-nutrient balance hypothesis. Developmental changes, however, could affect the timing of extrafloral nectar production which could, in turn, alter the foraging patterns of ants and their defence of plants.

Pea aphids (Acyrthosiphon pisum Harris) reduce secretion of extrafloral nectar in broad bean (Vicia faba)

1. Herbivores sometimes suppress plant defences. This study tested whether the presence of pea aphids (Acyrthosiphon pisum Harris) on broad bean (Vicia faba) led to decreased secretion of extrafloral nectar (EFN) which functions as an indirect plant defence against herbivores.2. To determine effects of aphid infestation on EFN secretion, a comparison was done between EFN secretion in uninfested plants and that in plants infested by A. pisum and another aphid species (Aphis craccivora Koch).3. When broad bean plants were infested by A. pisum, they secreted significantly smaller amounts of EFN than did uninfested plants and A. craccivora‐infested plants. There was no significant difference in EFN secretion between uninfested plants and A. craccivora‐infested plants. The number of extrafloral nectaries did not differ among the three treatments.4. These results suggest that A. pisum reduced EFN production in broad bean plants.

The arms race between heliconiine butterflies and Passiflora plants - new insights on an ancient subject.

Heliconiines are called passion vine butterflies because they feed exclusively on Passiflora plants during the larval stage. Many features of Passiflora and heliconiines indicate that they have radiated and speciated in association with each other, and therefore this model system was one of the first examples used to exemplify coevolution theory. Three major adaptations of Passiflora plants supported arguments in favour of their coevolution with heliconiines: unusual variation of leaf shape within the genus; the occurrence of yellow structures mimicking heliconiine eggs; and their extensive diversity of defence compounds called cyanogenic glucosides. However, the protection systems of Passiflora plants go beyond these three features. Trichomes, mimicry of pathogen infection through variegation, and production of extrafloral nectar to attract ants and other predators of their herbivores, are morphological defences reported in this plant genus. Moreover, Passiflora plants are well protected chemically, not only by cyanogenic glucosides, but also by other compounds such as alkaloids, flavonoids, saponins, tannins and phenolics. Heliconiines can synthesize cyanogenic glucosides themselves, and their ability to handle these compounds was probably one of the most crucial adaptations that allowed the ancestor of these butterflies to feed on Passiflora plants. Indeed, it has been shown that Heliconius larvae can sequester cyanogenic glucosides and alkaloids from their host plants and utilize them for their own benefit. Recently, it was discovered that Heliconius adults have highly accurate visual and chemosensory systems, and the expansion of brain structures that can process such information allows them to memorize shapes and display elaborate pre-oviposition behaviour in order to defeat visual barriers evolved by Passiflora species. Even though the heliconiine-Passiflora model system has been intensively studied, the forces driving host-plant preference in these butterflies remain unclear. New studies have shown that host-plant preference seems to be genetically controlled, but in many species there is some plasticity in this choice and preferences can even be induced. Although much knowledge regarding the coevolution of Passiflora plants and heliconiine butterflies has accumulated in recent decades, there remain many exciting unanswered questions concerning this model system.

Methyl jasmonate induction of cotton: a field test of the 'attract and reward' strategy of conservation biological control.

Natural or synthetic elicitors can affect plant physiology by stimulating direct and indirect defence responses to herbivores. For example, increased production of plant secondary metabolites, a direct response, can negatively affect herbivore survival, development and fecundity. Indirect responses include increased emission of plant volatiles that influence herbivore and natural enemy behaviour, and production of extrafloral nectar that serves as a food source for natural enemies after their arrival on induced plants. Therefore, the use of elicitors has potential for the study of basic aspects of tritrophic interactions, as well as application in biorational pest control, i.e. an 'attract and reward' strategy. We conducted a field study to investigate the effects of methyl jasmonate, an elicitor of plant defence responses, on three trophic levels: the plant, herbivores and natural enemies. We made exogenous applications of methyl jasmonate to transgenic cotton and measured volatile emission, extrafloral nectar production and plant performance (yield). We also assessed insect abundance, insect performance, and parasitism and predation of brown stink bug, Euschistus servus, eggs in methyl jasmonate-treated and untreated control plots. Application of methyl jasmonate increased emission of volatiles, in particular, (+)-limonene and (3E)-4,8-dimethyl-1,3,7-nonatriene, and production of extrafloral nectar, but not yield, compared with the control treatment. Despite increased volatile and extrafloral nectar production, methyl jasmonate application did not affect plant bug performance, or mortality of E. servus egg masses, and only marginally influenced insect abundance. Mortality of E. servus eggs varied over the course of the study. Overall, methyl jasmonate treatment affected cotton plant-induced responses, but not the insects that inhabit the plants. Our results were probably influenced by reduced natural enemy colonization of cotton from adjacent non-crop habitats, and subsequent low within-field population recruitment. Much remains to be learned about the effects of exogenous application of plant-produced 'enhancers' on the behaviour of natural enemies before crop physiology can be manipulated to enhance pest control.