Variation in the guild composition of herbivorous insect assemblages among co‐occurring plant species

Variation in plant traits among plant species may promote the development of a characteristic functional assemblage of insect herbivores associated with each plant species. However, only a small number of studies have detailed the representation of several herbivore guilds among co‐occurring plant species to determine whether the functional structure of herbivorous insect assemblages varies widely and consistently among plant species. The present study provides one of the few published data sets reporting on the density of several guilds of insect herbivores among numerous plant species. Variation in guild associations with plant phenology and season are also described. Insect herbivores were divided into 10 guilds, and the representation of these guilds was examined for 18 co‐occurring plant species. Guild densities and assemblage composition varied significantly among plant species, even when variation over time was taken into account. Variation in guild densities and assemblage composition were not strongly related to the taxonomic relationships of the plants. The highest densities of several guilds occurred in spring and summer, although other guilds were not strongly seasonal. Certain guilds were strongly associated with the presence of new leaves, whereas other guilds appeared to prefer mature leaves. This resulted in assemblage differences between samples containing new and mature leaves and samples containing mature leaves only. Even though the timing and duration of leaf and flower production varied among plant species, this did not explain all variation in guild densities among plant species. It is suggested that additional factors, including plant traits, are contributing to the wide and consistent variation in herbivore assemblage composition among plant species.     

Influence of presence and spatial arrangement of belowground insects on host‐plant selection of aboveground insects: a field study

Abstract 1. Several studies have shown that above‐ and belowground insects can interact by influencing each others growth, development, and survival when they feed on the same host‐plant. In natural systems, however, insects can make choices on which plants to oviposit and feed. A field experiment was carried out to determine if root‐feeding insects can influence feeding and oviposition preferences and decisions of naturally colonising foliar‐feeding insects. 2. Using the wild cruciferous plant Brassica nigra and larvae of the cabbage root fly Delia radicum as the belowground root‐feeding insect, naturally colonising populations of foliar‐feeding insects were monitored over the course of a summer season. 3. Groups of root‐infested and root‐uninfested B. nigra plants were placed in a meadow during June, July, and August of 2006 for periods of 3 days. The root‐infested and the root‐uninfested plants were either dispersed evenly or placed in clusters. Once daily, all leaves of each plant were carefully inspected and insects were removed and collected for identification. 4. The flea beetles Phyllotreta spp. and the aphid Brevicoryne brassicae were significantly more abundant on root‐uninfested (control) than on root‐infested plants. However, for B. brassicae this was only apparent when the plants were placed in clusters. Host‐plant selection by the generalist aphid M. persicae and oviposition preference by the specialist butterfly P. rapae, however, were not significantly influenced by root herbivory. 5. The results of this study show that the presence of root‐feeding insects can affect feeding and oviposition preferences of foliar‐feeding insects, even under natural conditions where many other interactions occur simultaneously. The results suggest that root‐feeding insects play a role in the structuring of aboveground communities of insects, but these effects depend on the insect species as well as on the spatial distribution of the root‐feeding insects.     

Helianthus maximiliani and species fine‐scale spatial pattern affect diversity interactions in reconstructed tallgrass prairies

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Plant–plant interactions as a mechanism structuring plant diversity in a Mediterranean semi‐arid ecosystem

Plant–plant interactions are among the fundamental processes that shape structure and functioning of arid and semi‐arid plant communities. Despite the large amount of studies that have assessed the relationship between plant–plant interactions (i.e., facilitation and competition) and diversity, often researchers forget a third kind of interaction, known as allelopathy. We examined the effect of plant–plant interactions of three dominant species: the perennial grass Lygeum spartum, the allelopathic dwarf shrub Artemisia herba‐alba, and the nurse shrub Salsola vermiculata, on plant diversity and species composition in a semi‐arid ecosystem in NE Spain. Specifically, we quantified the interaction outcome (IO) based on species co‐occurrence, we analyzed diversity by calculation of the individual species–area relationship (ISAR), and compositional changes by calculation of the Chao‐Jaccard similarity index. We found that S. vermiculata had more positive IO values than L. spartum, and A. herba‐alba had values between them. Lygeum spartum and A. herba‐alba acted as diversity repellers, whereas S. vermiculata acted as a diversity accumulator. As aridity increased, A. herba‐alba transitioned from diversity repeller to neutral and S. vermiculata transitioned from neutral to diversity accumulator, while L. spartum remained as diversity repeller. Artemisia herba‐alba had more perennial grass species in its local neighborhood than expected by the null model, suggesting some tolerance of this group to its “chemical neighbor”. Consequently, species that coexist with A. herba‐alba were very similar among different A. herba‐alba individuals. Our findings highlight the role of the nurse shrub S. vermiculata as ecosystem engineer, creating and maintaining patches of diversity, as well as the complex mechanism that an allelopathic plant may have on diversity and species assemblage. Further research is needed to determine the relative importance of allelopathy and competition in the overall interference of allelopathic plants.     

Plant odour plumes as mediators of plant–insect interactions

Insect olfactory orientation along odour plumes has been studied intensively with respect to pheromonal communication, whereas little knowledge is available on how plant odour plumes (POPs) affect olfactory searching by an insect for its host plants. The primary objective of this review is to examine the role of POPs in the attraction of insects. First, we consider parameters of an odour source and the environment which determine the size, shape and structure of an odour plume, and we apply that knowledge to POPs. Second, we compare characteristics of insect pheromonal plumes and POPs. We propose a ‘POP concept’ for the olfactory orientation of insects to plants. We suggest that: (i) an insect recognises a POP by means of plant volatile components that are encountered in concentrations higher than a threshold detection limit and that occur in a qualitative and quantitative blend indicating a resource; (ii) perception of the fine structure of a POP enables an insect to distinguish a POP from an unspecific odorous background and other interfering plumes; and (iii) an insect can follow several POPs to their sources, and may leave the track of one POP and switch to another one if this conveys a signal with higher reliability or indicates a more suitable resource. The POP concept proposed here may be a useful tool for research in olfactory‐mediated plant–insect interactions.     

Echinacea angustifolia and its specialist ant‐tended aphid: a multi‐year study of manipulated and naturally‐occurring aphid infestation

1. The impact of herbivores on plant fitness depends on multiple ecological mechanisms, including interactions between herbivore guilds. 2. This study assessed the effects of a specialist aphid (Aphis echinaceae) on performance and foliar herbivore damage of a long‐lived perennial plant (Echinacea angustifolia) native to the North American tallgrass prairie. A 2‐year field experiment manipulating aphid infestation on 100 plants was compared with concurrent and past observations of unmanipulated plants in the same outdoor experimental plot. Because ants co‐occur with aphids, the experiment tested the combined effects of aphids and ants. 3. Neither manipulated nor naturally‐occurring aphid infestations led to measurable declines in plant performance. Results for foliar herbivore damage differed between experimental and observational studies: the occurrence of foliar herbivore damage decreased with aphid infestation in the first year of the experiment and increased with aphid infestation over 5 years in unmanipulated plants. 4. While the experimental results concur with other experiments of ant–hemipteran–herbivore relationships, the observational results suggest that ant–aphid interactions do not naturally play a major role in determining patterns of foliar herbivory in this system. This result demonstrates the value of using field observations to interpret the relevance of experimental results.     

Root volatiles in plant–plant interactions II: Root volatiles alter root chemistry and plant–herbivore interactions of neighbouring plants

Volatile organic compounds (VOCs) emitted by plant roots can influence the germination and growth of neighbouring plants. However, little is known about the effects of root VOCs on plant–herbivore interactions of neighbouring plants. The spotted knapweed (Centaurea stoebe) constitutively releases high amounts of sesquiterpenes into the rhizosphere. Here, we examine the impact of C. stoebe root VOCs on the primary and secondary metabolites of sympatric Taraxacum officinale plants and the resulting plant‐mediated effects on a generalist root herbivore, the white grub Melolontha melolontha. We show that exposure of T. officinale to C.stoebe root VOCs does not affect the accumulation of defensive secondary metabolites but modulates carbohydrate and total protein levels in T. officinale roots. Furthermore, VOC exposure increases M. melolontha growth on T. officinale plants. Exposure of T. officinale to a major C. stoebe root VOC, the sesquiterpene (E)‐β‐caryophyllene, partially mimics the effect of the full root VOC blend on M. melolontha growth. Thus, releasing root VOCs can modify plant–herbivore interactions of neighbouring plants. The release of VOCs to increase the susceptibility of other plants may be a form of plant offense.     

Parasitic wasp‐associated symbiont affects plant‐mediated species interactions between herbivores

Microbial mutualistic symbiosis is increasingly recognised as a hidden driving force in the ecology of plant–insect interactions. Although plant‐associated and herbivore‐associated symbionts clearly affect interactions between plants and herbivores, the effects of symbionts associated with higher trophic levels has been largely overlooked. At the third‐trophic level, parasitic wasps are a common group of insects that can inject symbiotic viruses (polydnaviruses) and venom into their herbivorous hosts to support parasitoid offspring development. Here, we show that such third‐trophic level symbionts act in combination with venom to affect plant‐mediated interactions by reducing colonisation of subsequent herbivore species. This ecological effect correlated with changes induced by polydnaviruses and venom in caterpillar salivary glands and in plant defence responses to herbivory. Because thousands of parasitoid species are associated with mutualistic symbiotic viruses in an intimate, specific relationship, our findings may represent a novel and widespread ecological phenomenon in plant–insect interactions.     

The effects of drought and herbivory on plant–herbivore interactions across 16 soybean genotypes in a field experiment

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Intraspecific variation in herbivore‐induced plant volatiles influences the spatial range of plant–parasitoid interactions

Chemical information influences the behaviour of many animals, thus affecting species interactions. Many animals forage for resources that are heterogeneously distributed in space and time, and have evolved foraging behaviour that utilizes information related to these resources. Herbivore‐induced plant volatiles (HIPVs), emitted by plants upon herbivore attack, provide information on herbivory to various animal species, including parasitoids. Little is known about the spatial scale at which plants attract parasitoids via HIPVs under field conditions and how intraspecific variation in HIPV emission affects this spatial scale. Here, we investigated the spatial scale of parasitoid attraction to two cabbage accessions that differ in relative preference of the parasitoid Cotesia glomerata when plants were damaged by Pieris brassicae caterpillars. Parasitoids were released in a field experiment with plants at distances of up to 60 m from the release site using intervals between plants of 10 or 20 m to assess parasitism rates over time and distance. Additionally, we observed host‐location behaviour of parasitoids in detail in a semi‐field tent experiment with plant spacing up to 8 m. Plant accession strongly affected successful host location in field set‐ups with 10 or 20 m intervals between plants. In the semi‐field set‐up, plant finding success by parasitoids decreased with increasing plant spacing, differed between plant accessions, and was higher for host‐infested plants than for uninfested plants. We demonstrate that parasitoids can be attracted to herbivore‐infested plants over large distances (10 m or 20 m) in the field, and that stronger plant attractiveness via HIPVs increases this distance (up to at least 20 m). Our study indicates that variation in plant traits can affect attraction distance, movement patterns of parasitoids, and ultimately spatial patterns of plant–insect interactions. It is therefore important to consider plant‐trait variation in HIPVs when studying animal foraging behaviour and multi‐trophic interactions in a spatial context.     

Above‐ and belowground biotic interactions facilitate relocation of plants into cooler environments

One important but largely unanswered question about floristic responses to climate change is how interactions such as competition, facilitation and plant–soil feedbacks will influence the ability of species to track shifting climates. In a rugged and moisture‐limited region that has recently warmed by 2° (Siskiyou Mountains, OR, USA), we planted three species into cooler aspects and elevations than those they currently inhabit, with and without removal of neighbouring plants, and tracked them over 2 years. Two species had higher success in cooler topographic locations, and this success was enhanced by neighbouring plants, which appeared to modulate minimum growing season temperatures. One species' success was also facilitated by the higher soil organic matter found in cooler sites. These results are a novel experimental demonstration of two important factors that may buffer climate change impacts on plants: rugged topography and plant–plant facilitation.     

Reevaluating the conceptual framework for applied research on host‐plant resistance

Applied research on host‐plant resistance to arthropod pests has been guided over the past 60 years by a framework originally developed by Reginald Painter in his 1951 book, Insect Resistance in Crop Plants. Painter divided the “phenomena” of resistance into three “mechanisms,” nonpreference (later renamed antixenosis), antibiosis, and tolerance. The weaknesses of this framework are discussed. In particular, this trichotomous framework does not encompass all known mechanisms of resistance, and the antixenosis and antibiosis categories are ambiguous and inseparable in practice. These features have perhaps led to a simplistic approach to understanding arthropod resistance in crop plants. A dichotomous scheme is proposed as a replacement, with a major division between resistance (plant traits that limit injury to the plant) and tolerance (plant traits that reduce amount of yield loss per unit injury), and the resistance category subdivided into constitutive/inducible and direct/indirect subcategories. The most important benefits of adopting this dichotomous scheme are to more closely align the basic and applied literatures on plant resistance and to encourage a more mechanistic approach to studying plant resistance in crop plants. A more mechanistic approach will be needed to develop novel approaches for integrating plant resistance into pest management programs.     

Oviposition and feeding behaviour by the vine weevil Otiorhynchus sulcatus on red raspberry: effects of cultivars and plant nutritional status

• 1 The vine weevil Otiorhynchus sulcatus is a major pest of horticultural crops worldwide, with root‐feeding larvae causing most damage. Adult oviposition aboveground may therefore influence levels of damage as the larvae are relatively immobile after oviposition.
• 2 The present study investigated feeding and oviposition behaviour on red raspberry Rubus idaeus using intact plants, ensuring that choices reflected the realistic differences in cultivar appearance and chemical composition. Previous studies investigating vine weevil feeding and oviposition on other crops have used excised plant material, which may inadvertently influence behaviour.
• 3 Adult weevils significantly preferred to feed on particular cultivars in the choice experiment (e.g. Tulameen), although they consumed significantly more foliage (0.22–1.03 cm²/day) on different raspberry cultivars (e.g. Glen Moy, Glen Rosa and a wild accession) in no‐choice situations.
• 4 In choice experiments, weevils tended to avoid laying eggs on some cultivars (e.g. Glen Moy and the wild accession). The number of eggs laid (1.91–4.32 eggs per day) did not, however, differ significantly between the cultivars in a no‐choice situation. Foliar nitrogen and magnesium concentrations were positively, although weakly, correlated with the total number of eggs laid.
• 5 The present study highlights the importance of considering both choice and no‐choice tests when assessing crop susceptibility to attack because weevils may avoid feeding on certain cultivars (e.g. Glen Moy) when given a choice, although this would cause significant damage to such cultivars if they were grown in monoculture (i.e. when there is no alternative).

     

Plant and insect genetic variation mediate the impact of arbuscular mycorrhizal fungi on a natural plant–herbivore interaction

1. While both arbuscular mycorrhizal (AM) fungi and plant and insect genotype are well known to influence plant and herbivore growth and performance, information is lacking on how these factors jointly influence the relationship between plants and their natural herbivores. 2. The aim of the present study was to investigate how a natural community of arbuscular mycorrhizal fungi affects the growth of the perennial herb Plantago lanceolata L. (Plantaginaceae), as well as its interaction with the Glanville fritillary butterfly [Melitaea cinxia L. (Nymphalidae)]. For this, a multifactorial experiment was conducted using plant lines originating from multiple plant populations in the Åland Islands, Finland, grown either with or without mycorrhizal fungi. For a subset of plant lines, the impact of mycorrhizal inoculation, plant line, and larval family on the performance of M. cinxia larvae were tested. 3. Arbuscular mycorrhizal inoculation did not have a consistently positive or negative impact on plant growth or herbivore performance. Instead, plant genetic variation mediated the impact of arbuscular mycorrhizal fungi on plant growth, and both plant genetic variation and herbivore genetic variation mediated the response of the herbivore. For both the plant and insect, the impact of the arbuscular mycorrhizal community ranged from mutualistic to antagonistic. Overall, the present findings illustrate that genetic variation in response to mycorrhizal fungi may play a key role in the ecology and evolution of plant–insect interactions.     

Effect of local spatial plant distribution and conspecific density on bumble bee foraging behaviour

1. Size variations in pollinator populations may modify competitive interactions among foragers. Competition among pollinators has been shown to lead to one of two contrasting behaviours: either specialisation to the most profitable plant species or generalisation to several species. When foraging, pollinators are also confronted with heterogeneity in the spatial distribution of plant resources. Because variations in both the forager density and plant spatial distribution can affect pollinator behaviour, focus was on the interactive effect of these two factors. 2. Bumble bee (Bombus terrestris L.) individuals were trained on a focal species (Lotus corniculatus L.) and experimentally tested whether variations in the forager density (two or six bumble bees foraging together), plant community spatial distribution (two plant species: L. corniculatus and Medicago sativa, which were either patchily or randomly distributed), and their interaction modified bumble bee foraging behaviour. 3. It was shown that when confronted with a high forager density, bumble bees focused their visits towards the most familiar species, especially when foraging under a random plant distribution. These modifications affected the fruiting of the focal plant species, with a significantly lower reproductive success under low density/patchy conditions. 4. This study demonstrates that the foraging decisions of bumble bees are influenced by variations in both the conspecific density and plant spatial distribution. Given the increasing impact of human activities on plant‐pollinator communities, this raises the question of the potential implications of these results for plant communities in natural conditions when confronted with variations in the pollinator density and spatial distribution of plants.     

Linking plant genes to insect communities: Identifying the genetic bases of plant traits and community composition

Community genetics aims to understand the effects of intraspecific genetic variation on community composition and diversity, thereby connecting community ecology with evolutionary biology. Thus far, research has shown that plant genetics can underlie variation in the composition of associated communities (e.g., insects, lichen and endophytes), and those communities can therefore be considered as extended phenotypes. This work, however, has been conducted primarily at the plant genotype level and has not identified the key underlying genes. To address this gap, we used genome‐wide association mapping with a population of 445 aspen (Populus tremuloides) genets to identify the genes governing variation in plant traits (defence chemistry, bud phenology, leaf morphology, growth) and insect community composition. We found 49 significant SNP associations in 13 Populus genes that are correlated with chemical defence compounds and insect community traits. Most notably, we identified an early nodulin‐like protein that was associated with insect community diversity and the abundance of interacting foundation species (ants and aphids). These findings support the concept that particular plant traits are the mechanistic link between plant genes and the composition of associated insect communities. In putting the “genes” into “genes to ecosystems ecology”, this work enhances understanding of the molecular genetic mechanisms that underlie plant–insect associations and the consequences thereof for the structure of ecological communities.     

Variation in host plant has no effect on the performance and fitness‐related traits of the specialist herbivore Pareuchaetes insulata

The effect of host plant dissimilarity on insect preference and performance was tested using two morphological forms of Chromolaena odorata (L.) King & Robinson (Asteraceae) (one from Florida, USA, another from South Africa), and a specialist herbivore, Pareuchaetes insulata (Walker) (Lepidoptera: Erebidae: Arctiinae) from Florida, USA, that was introduced as a biological control agent in South Africa. Although this insect did establish at one site, of some 21 sites at which over 800 000 individuals were released, its population level in the field has remained low after an initial outbreak in 2006. To explain the poor performance of P. insulata, we hypothesised that P. insulata larvae prefer Floridian C. odorata to the southern African C. odorata, which is morphologically and probably genetically distinct, and that larvae reared on Floridian C. odorata should have higher fitness and performance. We tested this by comparing insect performance metrics on each of the two plant forms in laboratory experiments. Apart from pupal mass, which was significantly greater on southern African C. odorata, P. insulata performance metrics were similar on both plant forms; there were no significant differences in total leaf area consumed, egg and larval development, immature survival rates, feeding index, host suitability index, growth index, and fecundity between the Floridian and southern African C. odorata plants. In sum, we could not demonstrate that differences in plant forms in C. odorata are responsible for the poor performance of P. insulata in South Africa.