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

The olfactory system of Pieris brassicae caterpillars: from receptors to glomeruli

The olfactory system of adult lepidopterans is among the best described neuronal circuits. However, comparatively little is known about the organization of the olfactory system in the larval stage of these insects. Here, we explore the expression of olfactory receptors and the organization of olfactory sensory neurons in caterpillars of Pieris brassicae, a significant pest species in Europe and a well-studied species for its chemical ecology. To describe the larval olfactory system in this species, we first analyzed the head transcriptome of third-instar larvae (L3) and identified 16 odorant receptors (ORs) including the OR coreceptor (Orco), 13 ionotropic receptors (IRs), and 8 gustatory receptors (GRs). We then quantified the expression of these 16 ORs in different life stages, using qPCR, and found that the majority of ORs had significantly higher expression in the L4 stage than in the L3 and L5 stages, indicating that the larval olfactory system is not static throughout caterpillar development. Using an Orco-specific antibody, we identified all olfactory receptor neurons (ORNs) expressing the Orco protein in L3, L4, and L5 caterpillars and found a total of 34 Orco-positive ORNs, distributed among three sensilla on the antenna. The number of Orco-positive ORNs did not differ among the three larval instars. Finally, we used retrograde axon tracing of the antennal nerve and identified a mean of 15 glomeruli in the larval antennal center (LAC), suggesting that the caterpillar olfactory system follows a similar design as the adult olfactory system, although with a lower numerical redundancy. Taken together, our results provide a detailed analysis of the larval olfactory neurons in P. brassicae, highlighting both the differences as well as the commonalities with the adult olfactory system. These findings contribute to a better understanding of the development of the olfactory system in insects and its life-stage-specific adaptations.

Parasitoid-induced changes in metabolic rate and feeding activity of the emerald ash borer (Coleoptera: Buprestidae): implications for biological control

Parasitoid-host interactions form the foundation of biological control strategies against many agriculture and forest insect pests. The emerald ash borer (EAB), Agrilus planipennis (Coleoptera: Buprestidae), is a serious invasive pest of ash (Fraxinus spp.) trees in North America. Tetrastichus planipennisi (Hymenoptera: Eulophidae) is a gregarious, koinobiont endoparasitoid, attacking late (3rd to 4th) instars of EAB larvae, which feed in the live phloem of ash trunks or branches, making serpentine-like galleries filled with larval frass. In the present study, we tested the hypothesis that T. planipennisi regulates the host metabolism and feeding activity to optimize its offspring development and fitness. We first compared the respiration rate of parasitized and unparasitized host larvae at different times after parasitism, and then measured feeding activity of both parasitized and unparasitized host larvae inside their feeding galleries. Although parasitized host larvae increased metabolic rate and feeding activity in the first few days of parasitism, T. planipennisi parasitism induced an overall reduction of the metabolic rate and decrease in feeding activity of parasitized host larvae over their development period. In addition, there was a negative relationship between feeding activity of parasitized hosts and brood sizes of the parasitoid progeny-i.e., the more parasitoid progeny a host larva received, the less feeding activity the host had. These findings suggest that T. planipennisi has limited ability to optimize its offspring development and fitness through regulations of the host metabolism and feeding activity and its parasitism reduces feeding damage of parasitized EAB larvae to infested ash trees.

Indirect plant-mediated interactions between heterospecific parasitoids that develop in different caterpillar species

Parasitoids induce physiological changes in their herbivorous hosts that affect how plants respond to herbivory. The signature of parasitoids on induced plant responses to feeding by parasitized herbivores indirectly impacts insect communities interacting with the plant. The effect may extend to parasitoids and cause indirect interaction between parasitoids that develop inside different herbivore hosts sharing the food plant. However, this type of interactions among parasitoid larvae has received very little attention. In this study, we investigated sequential and simultaneous plant-mediated interactions among two host-parasitoid systems feeding on Brassica oleracea plants: Mamestra brassicae parasitized by Microplitis mediator and Pieris rapae parasitized by Cotesia rubecula. We measured the mortality, development time, and weight of unparasitized herbivores and performance of parasitoids that had developed inside the two herbivore species when sharing the food plant either simultaneously or sequentially. Plant induction by parasitized or unparasitized hosts had no significant effect on the performance of the two herbivore host species. In contrast, the two parasitoid species had asymmetrical indirect plant-mediated effects on each other's performance. Cotesia rubecula weight was 15% higher on plants induced by M. mediator-parasitized hosts, compared to control plants. In addition, M. mediator development time was reduced by 30% on plants induced by conspecific but not heterospecific parasitoids, compared to plants induced by its unparasitized host. Contrary to sequential feeding, parasitoids had no effect on each other's performance when feeding simultaneously. These results reveal that indirect plant-mediated interactions among parasitoid larvae could involve any parasitoid species whose hosts share a food plant.

Sympatric Pieris butterfly species exhibit a high conservation of chemoreceptors

Sensory processes have often been argued to play a central role in the selection of ecological niches and in the formation of new species. Butterflies are among the best studied animal groups with regards to their evolutionary and behavioral ecology and thereby offer an attractive system to investigate the role of chemosensory genes in sympatric speciation. We focus on two Pieris butterflies with overlapping host-plant ranges: P. brassicae and P. rapae. Host-plant choice in lepidopterans is largely based on their olfactory and gustatory senses. Although the chemosensory responses of the two species have been well characterized at the behavioral and physiological levels, little is known about their chemoreceptor genes. Here, we compared the chemosensory genes of P. brassicae and P. rapae to investigate whether differences in these genes might have contributed to their evolutionary separation. We identified a total of 130 and 122 chemoreceptor genes in the P. brassicae genome and antennal transcriptome, respectively. Similarly, 133 and 124 chemoreceptors were identified in the P. rapae genome and antennal transcriptome. We found some chemoreceptors being differentially expressed in the antennal transcriptomes of the two species. The motifs and gene structures of chemoreceptors were compared between the two species. We show that paralogs share conserved motifs and orthologs have similar gene structures. Our study therefore found surprisingly few differences in the numbers, sequence identities and gene structures between the two species, indicating that the ecological differences between these two butterflies might be more related to a quantitative shift in the expression of orthologous genes than to the evolution of novel receptors as has been found in other insects. Our molecular data supplement the wealth of behavioral and ecological studies on these two species and will thereby help to better understand the role of chemoreceptor genes in the evolution of lepidopterans.

Parasitism causes changes in caterpillar odours and associated bacterial communities with consequences for host-location by a hyperparasitoid

Microorganisms living in and on macroorganisms may produce microbial volatile compounds (mVOCs) that characterise organismal odours. The mVOCs might thereby provide a reliable cue to carnivorous enemies in locating their host or prey. Parasitism by parasitoid wasps might alter the microbiome of their caterpillar host, affecting organismal odours and interactions with insects of higher trophic levels such as hyperparasitoids. Hyperparasitoids parasitise larvae or pupae of parasitoids, which are often concealed or inconspicuous. Odours of parasitised caterpillars aid them to locate their host, but the origin of these odours and its relationship to the caterpillar microbiome are unknown. Here, we analysed the odours and microbiome of the large cabbage white caterpillar Pieris brassicae in relation to parasitism by its endoparasitoid Cotesia glomerata. We identified how bacterial presence in and on the caterpillars is correlated with caterpillar odours and tested the attractiveness of parasitised and unparasitised caterpillars to the hyperparasitoid Baryscapus galactopus. We manipulated the presence of the external microbiome and the transient internal microbiome of caterpillars to identify the microbial origin of odours. We found that parasitism by C. glomerata led to the production of five characteristic volatile products and significantly affected the internal and external microbiome of the caterpillar, which were both found to have a significant correlation with caterpillar odours. The preference of the hyperparasitoid was correlated with the presence of the external microbiome. Likely, the changes in external microbiome and body odour after parasitism were driven by the resident internal microbiome of caterpillars, where the bacterium Wolbachia sp. was only present after parasitism. Micro-injection of Wolbachia in unparasitised caterpillars increased hyperparasitoid attraction to the caterpillars compared to untreated caterpillars, while no differences were found compared to parasitised caterpillars. In conclusion, our results indicate that host-parasite interactions can affect multi-trophic interactions and hyperparasitoid olfaction through alterations of the microbiome.

Chemical Ecology of Floral Resources in Conservation Biological Control

Conservation biological control aims to enhance populations of natural enemies of insect pests in crop habitats, typically by intentional provision of flowering plants as food resources. Ideally, these flowering plants should be inherently attractive to natural enemies to ensure that they are frequently visited. We review the chemical ecology of floral resources in a conservation biological control context, with a focus on insect parasitoids. We highlight the role of floral volatiles as semiochemicals that attract parasitoids to the food resources. The discovery that nectar-inhabiting microbes can be hidden players in mediating parasitoid responses to flowering plants has highlighted the complexity of the interactions between plants and parasitoids. Furthermore, because food webs in agroecosystems do not generally stop at the third trophic level, we also consider responses of hyperparasitoids to floral resources. We thus provide an overview of floral compounds as semiochemicals from a multitrophic perspective, and we focus on the remaining questions that need to be addressed to move the field forward.

Biological control needs evolutionary perspectives of ecological interactions

While ecological interactions have been identified as determinant for biological control efficiency, the role of evolution remains largely underestimated in biological control programs. With the restrictions on the use of both pesticides and exotic biological control agents (BCAs), the evolutionary optimization of local BCAs becomes central for improving the efficiency and the resilience of biological control. In particular, we need to better account for the natural processes of evolution to fully understand the interactions of pests and BCAs, including in biocontrol strategies integrating human manipulations of evolution (i.e., artificial selection and genetic engineering). In agroecosystems, the evolution of BCAs traits and performance depends on heritable phenotypic variation, trait genetic architecture, selection strength, stochastic processes, and other selective forces. Humans can manipulate these natural processes to increase the likelihood of evolutionary trait improvement, by artificially increasing heritable phenotypic variation, strengthening selection, controlling stochastic processes, or overpassing evolution through genetic engineering. We highlight these facets by reviewing recent studies addressing the importance of natural processes of evolution and human manipulations of these processes in biological control. We then discuss the interactions between the natural processes of evolution occurring in agroecosystems and affecting the artificially improved BCAs after their release. We emphasize that biological control cannot be summarized by interactions between species pairs because pests and biological control agents are entangled in diverse communities and are exposed to a multitude of deterministic and stochastic selective forces that can change rapidly in direction and intensity. We conclude that the combination of different evolutionary approaches can help optimize BCAs to remain efficient under changing environmental conditions and, ultimately, favor agroecosystem sustainability.

Evolution of koinobiont parasitoid host regulation and consequences for indirect plant defence

Tritrophic interactions among plants, herbivorous insects and their parasitoids have been well studied in the past four decades. Recently, a new angle has been uncovered: koinobiont parasitoids, that allow their host to keep feeding on the plant for a certain amount of time after parasitism, indirectly alter plant responses against herbivory via the many physiological changes induced in their herbivorous hosts. By affecting plant responses, parasitoids may indirectly affect the whole community of insects interacting with plants induced by parasitized herbivores and have extended effects on plant fitness. These important findings have renewed research interests on parasitoid manipulation of their host development. Parasitoids typically arrest their host development before the last instar, resulting in a lower final weight compared to unparasitized hosts. Yet, some parasitoids prolong their host development, leading to larger herbivores that consume more plant material than unparasitized ones. Furthermore, parasitoid host regulation is plastic and one parasitoid species may arrest or promote its host growth depending on the number of eggs laid, host developmental stage and species as well as environmental conditions. The consequences of plasticity in parasitoid host regulation for plant–insect interactions have received very little attention over the last two decades, particularly concerning parasitoids that promote their host growth. In this review, we first synthesize the mechanisms used by parasitoids to regulate host growth and food consumption. Then, we identify the evolutionary and environmental factors that influence the direction of parasitoid host regulation in terms of arrestment or promotion of host growth. In addition, we discuss the implication of different host regulation types for the parasitoid’s role as agent of plant indirect defence. Finally, we argue that the recent research interests about parasitoid plant-mediated interactions would strongly benefit from revival of research on the mechanisms, ecology and evolution of host regulation in parasitoids.

Impact of parasitoid-associated polydnaviruses on plant-mediated herbivore interactions

Insect herbivores interact via plant-mediated interactions in which one herbivore species induces changes in plant quality that affects the performance of a second phytophagous insect that shares the food plant. These interactions are often asymmetric due to specificity in induced plant responses to herbivore attack, amount of plant damage, elicitors in herbivore saliva and plant organ damaged by herbivores. Parasitoids and their symbiotic polydnaviruses alter herbivore physiology and behaviour and may influence how plants respond to parasitized herbivores. We argue that these phenomena affect plant-mediated interactions between herbivores. We identify that the extended phenotype of parasitoid polydnaviruses is an important knowledge gap in interaction networks of insect communities.

Influence of wild, local and cultivated tobacco varieties on the oviposition preference and offspring performance of Spodoptera litura

The influences of different plants on herbivores have recently attracted research interest; however, little is known regarding the effects of wild, local and cultivated varieties of the same plant from the same origin on herbivores. This study aimed to examine the effects of different tobacco varieties from the same origin on the oviposition preference and offspring performance of Spodoptera litura. We selected two wild ('Bishan wild tobacco' and 'Badan wild tobacco'), two local ('Liangqiao sun-cured tobacco' and 'Shuangguan sun-cured tobacco') and two cultivated ('Xiangyan No. 5' and 'Cunsanpi') tobacco varieties from Hunan Province, China. We found that female S. litura varied in oviposition preferences across the tobacco varieties. They preferred to lay eggs on the cultivated varieties, followed by the local varieties, with the wild varieties being the least preferred. Furthermore, different tobacco varieties significantly influenced the life history parameters of S. litura. Survival rate, pupal weight, emergence rate and adult dry weight decreased in the following order: cultivated varieties > local varieties > wild varieties. Conversely, the pupal stage and development period decreased in the following order: wild varieties > local varieties > cultivated varieties. Therefore, we conclude that wild tobacco varieties have higher resistance to S. litura than cultivated and local varieties, reflecting the evolutionary advantages of wild tobacco varieties.

Parasitoid Causes Cascading Effects on Plant-Induced Defenses Mediated Through the Gut Bacteria of Host Caterpillars

Koinobiont endoparasitoid wasps whose larvae develop inside a host insect alter several important facets of host physiology, potentially causing cascading effects across multiple trophic levels. For instance, the hijacking of the host immune responses may have effects on how insects interact with host plants and microbial associates. However, the parasitoid regulation of insect-plant-microbiome interactions is still understudied. In this study, we used the fall armyworm (FAW), Spodoptera frugiperda, and the braconid parasitoid Cotesia marginiventris to evaluate impacts of parasitism on the gut microbiome of FAW larvae, and respective maize plant defense responses. The level of reactive oxygen species and the microbial community in larval gut underwent significant changes in response to parasitism, leading to a significant reduction of Enterococcus, while elevating the relative abundance of Pseudomonas. FAW with parasitism had lower glucose oxidase (GOX) activity in salivary glands and triggered lower defense responses in maize plants. These changes corresponded to effects on plants, as Pseudomonas inoculated larvae had lower activity of salivary GOX and triggered lower defense responses in maize plants. Our results demonstrated that parasitism had cascading effects on microbial associates across trophic levels and also highlighted that insect gut bacteria may contribute to complex interrelationships among parasitoids, herbivores, and plants.

Plant-phenotypic changes induced by parasitoid ichnoviruses enhance the performance of both unparasitized and parasitized caterpillars

There is increasing awareness that interactions between plants and insects can be mediated by microbial symbionts. Nonetheless, evidence showing that symbionts associated with organisms beyond the second trophic level affect plant‐insect interactions are restricted to a few cases belonging to parasitoid‐associated bracoviruses. Insect parasitoids harbour a wide array of symbionts which, like bracoviruses, can be injected into their herbivorous hosts to manipulate their physiology and behaviour. Yet, the function of these symbionts in plant‐based trophic webs remains largely overlooked. Here, we provide the first evidence of a parasitoid‐associated symbiont belonging to the group of ichnoviruses which affects the strength of plant‐insect interactions. A comparative proteomic analysis shows that, upon parasitoid injection of calyx fluid containing ichnovirus particles, the composition of salivary glands of caterpillars changes both qualitatively (presence of two viral‐encoded proteins) and quantitatively (abundance of several caterpillar‐resident enzymes, including elicitors such as glucose oxidase). In turn, plant phenotypic changes triggered by the altered composition of caterpillar oral secretions affect the performance of herbivores. Ichnovirus manipulation of plant responses to herbivory leads to benefits for their parasitoid partners in terms of reduced developmental time within the parasitized caterpillar. Interestingly, plant‐mediated ichnovirus‐induced effects also enhance the performances of unparasitized herbivores which in natural conditions may feed alongside parasitized ones. We discuss these findings in the context of ecological costs imposed to the plant by the viral symbiont of the parasitoid. Our results provide intriguing novel findings about the role played by carnivore‐associated symbionts on plant‐insect‐parasitoid systems and underline the importance of placing mutualistic associations in an ecological perspective.

A chromosome-level genome assembly of the parasitoid wasp, Cotesia glomerata (Hymenoptera: Braconidae)

Hymenopterans make up about 20% of all animal species, but most are poorly known and lack high-quality genomic resources. One group of important, yet under-studied hymenopterans, are parasitoid wasps in the family Braconidae. Among this under-studied group are braconid wasps in the genus Cotesia; a clade of ~1,000 species routinely used in studies of physiology, ecology, biological control, and genetics. However, our ability to understand these organisms has been hindered by a lack of genomic resources. We helped bridge this gap by generating a high-quality genome assembly for the parasitoid wasp, Cotesia glomerata (Braconidae; Microgastrinae). We generated this assembly using multiple sequencing technologies, including Oxford Nanopore, whole-genome shotgun sequencing, and 3-D chromatin contact information (Hi-C). Our assembly is one of the most contiguous, complete, and publicly available hymenopteran genomes, represented by 3,355 scaffolds with a scaffold N50 of ~28Mb and a BUSCO score of ~99%. Given the genome sizes found in closely related species, our genome assembly was ~50% larger than expected, which was apparently induced by runaway amplification of three types of repetitive elements: simple repeats, Long Terminal Repeats (LTRs), and Long Interspersed Nuclear Elements (LINEs). This assembly is another step forward for genomics across this hyper-diverse, yet understudied, order of insects. The assembled genomic data and metadata files are publicly available via Figshare (https://doi.org/10.6084/m9.figshare.13010549).

Microbiota, pathogens, and parasites as mediators of tritrophic interactions between insect herbivores, plants, and pollinators

Insect-associated microbes, including pathogens, parasites, and symbionts, influence the interactions of herbivorous insects and pollinators with their host plants. Moreover, herbivory-induced changes in plant resource allocation and defensive chemistry can influence pollinator behavior. This suggests that the outcomes of interactions between herbivores, their microbes and host plants could have implications for pollinators. As epizootic diseases occur at high population densities, pathogen and parasite-mediated effects on plants could have landscape-level impacts on foraging pollinators. The goal of this minireview is to highlight the potential for an herbivore's multitrophic interactions to trigger plant-mediated effects on the immunity and health of pollinators. We highlight the importance of plant quality and gut microbiomes in bee health, and how caterpillars as model herbivores interact with pathogens, parasites, and symbionts to affect plant quality, which forms the centerpiece of multitrophic interactions between herbivores and pollinators. We also discuss the impacts of other herbivore-associated factors, such as agricultural inputs aimed at decreasing herbivorous pests, on pollinator microbiomes.

Influence of parasitoid-associated viral symbionts on plant-insect interactions and biological control

Insect parasitoids have evolved symbiotic interactions with several viruses and thousands of parasitoid species have established mutualistic associations with polydnaviruses (PDVs). While PDVs have often been described as virulence factors allowing development of immature parasitoids inside their herbivore hosts, there is increasing awareness that PDVs can affect plant-insect interactions. We review recent literature showing that PDVs alter not only host physiology, but also feeding patterns and composition of herbivore's oral secretions. In turn PDV-induced changes in herbivore phenotype affect plant responses to herbivory with consequences ranging from differential expression of plant defense-related genes to wider ecological effects across multiple trophic levels. In this opinion paper we also highlight important missing gaps to fully understand the role of PDVs and other parasitoid-associated viral symbionts in a plant-insect interaction perspective. Because PDVs negatively impact performance and survival of herbivore pests, we conclude arguing that PDV genomes offer potential opportunities for biological control.

Metacommunity theory for transmission of heritable symbionts within insect communities

Microbial organisms are ubiquitous in nature and often form communities closely associated with their host, referred to as the microbiome. The microbiome has strong influence on species interactions, but microbiome studies rarely take interactions between hosts into account, and network interaction studies rarely consider microbiomes. Here, we propose to use metacommunity theory as a framework to unify research on microbiomes and host communities by considering host insects and their microbes as discretely defined "communities of communities" linked by dispersal (transmission) through biotic interactions. We provide an overview of the effects of heritable symbiotic bacteria on their insect hosts and how those effects subsequently influence host interactions, thereby altering the host community. We suggest multiple scenarios for integrating the microbiome into metacommunity ecology and demonstrate ways in which to employ and parameterize models of symbiont transmission to quantitatively assess metacommunity processes in host-associated microbial systems. Successfully incorporating microbiota into community-level studies is a crucial step for understanding the importance of the microbiome to host species and their interactions.

Microbial Symbionts of Parasitoids

Parasitoids depend on other insects for the development of their offspring. Their eggs are laid in or on a host insect that is consumed during juvenile development. Parasitoids harbor a diversity of microbial symbionts including viruses, bacteria, and fungi. In contrast to symbionts of herbivorous and hematophagous insects, parasitoid symbionts do not provide nutrients. Instead, they are involved in parasitoid reproduction, suppression of host immune responses, and manipulation of the behavior of herbivorous hosts. Moreover, recent research has shown that parasitoid symbionts such as polydnaviruses may also influence plant-mediated interactions among members of plant-associated communities at different trophic levels, such as herbivores, parasitoids, and hyperparasitoids. This implies that these symbionts have a much more extended phenotype than previously thought. This review focuses on the effects of parasitoid symbionts on direct and indirect species interactions and the consequences for community ecology.

Bacterial Semiochemicals and Transkingdom Interactions with Insects and Plants

A peculiar feature of all living beings is their capability to communicate. With the discovery of the quorum sensing phenomenon in bioluminescent bacteria in the late 1960s, it became clear that intraspecies and interspecies communications and social behaviors also occur in simple microorganisms such as bacteria. However, at that time, it was difficult to imagine how such small organisms-invisible to the naked eye-could influence the behavior and wellbeing of the larger, more complex and visible organisms they colonize. Now that we know this information, the challenge is to identify the myriad of bacterial chemical signals and communication networks that regulate the life of what can be defined, in a whole, as a meta-organism. In this review, we described the transkingdom crosstalk between bacteria, insects, and plants from an ecological perspective, providing some paradigmatic examples. Second, we reviewed what is known about the genetic and biochemical bases of the bacterial chemical communication with other organisms and how explore the semiochemical potential of a bacterium can be explored. Finally, we illustrated how bacterial semiochemicals managing the transkingdom communication may be exploited from a biotechnological point of view.

Parasitic Wasp Mediates Plant Perception of Insect Herbivores

Microplitis croceipes is a solitary parasitoid that specializes on noctuid larvae of Helicoverpa zea and Heliothis virescens. Both the parasitoid and its hosts are naturally distributed across a large part of North America. When parasitoids deposit their eggs into hosts, venom and polydnaviruses (PDVs) are also injected into the caterpillars, which can suppress host immune responses, thus allowing parasitoid larvae to develop. In addition, PDVs can regulate host oral cues, such as glucose oxidase (GOX). The purpose of this study was to determine if parasitized caterpillars differentially induce plant defenses compared to non-parasitized caterpillars using two different caterpillar host/plant systems. Heliothis virescens caterpillars parasitized by M. croceipes had significantly lower salivary GOX activity than non-parasitized caterpillars, resulting in lower levels of tomato defense responses, which benefited parasitoid performance by increasing the growth rate of parasitized caterpillars. In tobacco plants, parasitized Helicoverpa zea caterpillars had lower GOX activity but induced higher plant defense responses. The higher tobacco defense responses negatively affected parasitoid performance by reducing the growth rate of parasitized caterpillars, causing longer developmental periods, and reduced cocoon mass and survival of parasitoids. These studies demonstrate a species-specific effect in different plant-insect systems. Based on these results, plant perception of insect herbivores can be affected by parasitoids and lead to positive or negative consequences to higher trophic levels depending upon the particular host-plant system.

The microbiome of the Melitaea cinxia butterfly shows marked variation but is only little explained by the traits of the butterfly or its host plant

Understanding of the ecological factors that shape intraspecific variation of insect microbiota in natural populations is relatively poor. In Lepidopteran caterpillars, microbiota is assumed to be mainly composed of transient bacterial symbionts acquired from the host plant. We sampled Glanville fritillary (Melitaea cinxia) caterpillars from natural populations to describe their gut microbiome and to identify potential ecological factors that determine its structure. Our results demonstrate high variability of microbiota composition even among caterpillars that shared the same host plant individual and most likely the same genetic background. We observed that the caterpillars harboured microbial classes that varied among individuals and alternated between two distinct communities (one composed of mainly Enterobacteriaceae and another with more variable microbiota community). Even though the general structure of the microbiota was not attributed to the measured ecological factors, we found that phylogenetically similar microbiota showed corresponding responses to the sex and the parasitoid infection of the caterpillar and to those of the host plant's microbial and chemical composition. Our results indicate high among-individual variability in the microbiota of the M. cinxia caterpillar and contradict previous findings that the host plant is the major driver of the microbiota communities of insect herbivores.

Pathogen-Mediated Tritrophic Interactions: Baculovirus-Challenged Caterpillars Induce Higher Plant Defenses than Healthy Caterpillars

Although the tritrophic interactions of plants, insect herbivores and their natural enemies have been intensely studied for several decades, the roles of entomopathogens in their indirect modulation of plant-insect relationships is still unclear. Here, we employed a sublethal dose of a baculovirus with a relatively broad host range (AcMNPV) to explore if feeding by baculovirus-challenged Helicoverpa zea caterpillars induces direct defenses in the tomato plant. We examined induction of plant defenses following feeding by H. zea, including tomato plants fed on by healthy caterpillars, AcMNPV-challenged caterpillars, or undamaged controls, and subsequently compared the transcript levels of defense related proteins (i.e., trypsin proteinase inhibitors, peroxidase and polyphenol oxidase) and other defense genes (i.e., proteinase inhibitor II and cysteine proteinase inhibitor) from these plants, in addition to comparing caterpillar relative growth rates. As a result, AcMNPV-challenged caterpillars induced the highest plant anti-herbivore defenses. We examined several elicitors and effectors in the secretions of these caterpillars (i.e., glucose oxidase, phospholipase C, and ATPase hydrolysis), which surprisingly did not differ between treatments. Hence, we suggest that the greater induction of plant defenses by the virus-challenged caterpillars may be due to differences in the amount of these secretions deposited during feeding or to some other unknown factor(s).

Cascading effects of defensive endosymbionts

Defensive endosymbionts are now understood to be widespread among insects, targeting many different threats, including predators, parasites and disease. The effects on natural enemies can be significant, resulting in dramatic changes in the outcome of interactions between insects and their attackers. Evidence is now emerging from laboratory and field work that defensive symbionts can have important effects on the surrounding insect community, as well as on vulnerable enemy species; for example, by reducing prey available for the trophic level above the enemy. However, there is a need for more experimental work across a greater taxonomic range of species in order to understand the different ways in which defensive symbionts influence insect communities.

Hyperparasitoids exploit herbivore-induced plant volatiles during host location to assess host quality and non-host identity

Although consumers often rely on chemical information to optimize their foraging strategies, it is poorly understood how top carnivores above the third trophic level find resources in heterogeneous environments. Hyperparasitoids are a common group of organisms in the fourth trophic level that lay their eggs in or on the body of other parasitoid hosts. Such top carnivores use herbivore-induced plant volatiles (HIPVs) to find caterpillars containing parasitoid host larvae. Hyperparasitoids forage in complex environments where hosts of different quality may be present alongside non-host parasitoid species, each of which can develop in multiple herbivore species. Because both the identity of the herbivore species and its parasitization status can affect the composition of HIPV emission, hyperparasitoids encounter considerable variation in HIPVs during host location. Here, we combined laboratory and field experiments to investigate the role of HIPVs in host selection of hyperparasitoids that search for hosts in a multi-parasitoid multi-herbivore context. In a wild Brassica oleracea-based food web, the hyperparasitoid Lysibia nana preferred HIPVs emitted in response to caterpillars parasitized by the gregarious host Cotesia glomerata over the non-host Hyposoter ebeninus. However, no plant-mediated discrimination occurred between the solitary host C. rubecula and the non-host H. ebeninus. Under both laboratory and field conditions, hyperparasitoid responses were not affected by the herbivore species (Pieris brassicae or P. rapae) in which the three primary parasitoid species developed. Our study shows that HIPVs are an important source of information within multitrophic interaction networks allowing hyperparasitoids to find their preferred hosts in heterogeneous environments.

Symbiotic polydnavirus and venom reveal parasitoid to its hyperparasitoids

Symbiotic relationships may provide organisms with key innovations that aid in the establishment of new niches. For example, during oviposition, some species of parasitoid wasps, whose larvae develop inside the bodies of other insects, inject polydnaviruses into their hosts. These symbiotic viruses disrupt host immune responses, allowing the parasitoid's progeny to survive. Here we show that symbiotic polydnaviruses also have a downside to the parasitoid's progeny by initiating a multitrophic chain of interactions that reveals the parasitoid larvae to their enemies. These enemies are hyperparasitoids that use the parasitoid progeny as host for their own offspring. We found that the virus and venom injected by the parasitoid during oviposition, but not the parasitoid progeny itself, affected hyperparasitoid attraction toward plant volatiles induced by feeding of parasitized caterpillars. We identified activity of virus-related genes in the caterpillar salivary gland. Moreover, the virus affected the activity of elicitors of salivary origin that induce plant responses to caterpillar feeding. The changes in caterpillar saliva were critical in inducing plant volatiles that are used by hyperparasitoids to locate parasitized caterpillars. Our results show that symbiotic organisms may be key drivers of multitrophic ecological interactions. We anticipate that this phenomenon is widespread in nature, because of the abundance of symbiotic microorganisms across trophic levels in ecological communities. Their role should be more prominently integrated in community ecology to understand organization of natural and managed ecosystems, as well as adaptations of individual organisms that are part of these communities.