The Effect of Phenoloxidase Activity on Survival Is Host Plant Dependent in Virus-Infected Caterpillars

The role of host plants in driving pathogen susceptibility in insects through chemicals, immune responses and microbiota

In this comprehensive exploration, we delve into the pivotal role of host plants in shaping the intricate interactions between herbivorous insects and their pathogens. Recent decades have seen a surge in studies that demonstrate that host plants are crucial drivers of the interactions between insects and pathogens, providing novel insights into the direct and indirect interactions that shape tri-trophic interactions. These studies have built on a wide range of pathogens, from viruses to bacteria, and from protozoans to fungi. We summarise these studies, and discuss the mechanisms of plant-mediated insect resistance to infection, ranging from the toxicity of plant chemicals to pathogens to enhancement of anti-pathogen immune responses, and modulation of the insect's microbiome. Although we provide evidence for the roles of all these mechanisms, we also point out that the majority of existing studies are phenomenological, describing patterns without addressing the underlying mechanisms. To further our understanding of these tri-trophic interactions, we therefore urge researchers to design their studies to enable them specifically to distinguish the mechanisms by which plants affect insect susceptibility to pathogens.

Dose-dependent dynamics of densovirus infection in two nymphalid butterfly species utilizing native or exotic host plants

Insects are attacked by a diverse range of microbial pathogens in the wild. In herbivorous species, larval host plants frequently play a critical role in mediating susceptibility to infection. Characterizing such plant-mediated effects on herbivore-pathogen interactions can provide insight into patterns of infection across wild populations. In this study, we investigated the effects of host plant use by two North American butterflies, Euphydryas phaeton (Nymphalidae) and Anartia jatrophae (Nymphalidae), on entomopathogen infection across a range of three doses. Both of these herbivores recently incorporated the same exotic plant, Plantago lanceolata (Plantaginaceae), into their host range and are naturally infected by the same entomopathogen, Junonia coenia densovirus (Parvoviridae), in wild populations. We performed two factorial experiments in which E. phaeton and A. jatrophae were reared on either P. lanceolata or a native host plant [Chelone glabra (Plantaginaceae) for E. phaeton; Bacopa monnieri (Plantaginaceae) for A. jatrophae] and inoculated with either a low, medium, or high dose of the virus. In E. phaeton, the outcomes of infection were highly dose-dependent, with inoculation with higher viral doses resulting in faster time to death and greater mortality. However, neither survival nor postmortem viral burdens varied depending upon the host plant that was consumed. In contrast, host plant use had a strong effect on viral burdens in A. jatrophae, with consumption of the exotic plant appearing to enhance host resistance to infection. Together, these results illustrate the variable influences of host plant use on herbivore resistance to infection, highlighting the importance of investigating plant-herbivore relationships within a tritrophic framework.

Host plant-mediation of viral transmission and its consequences for a native butterfly

Pathogens play a key role in insect population dynamics, contributing to short-term fluctuations in abundance as well as long-term demographic trends. Two key factors that influence the effects of entomopathogens on herbivorous insect populations are modes of pathogen transmission and larval host plants. In this study, we examined tritrophic interactions between a sequestering specialist lepidopteran, Euphydryas phaeton, and a viral pathogen, Junonia coenia densovirus, on its native host plant, Chelone glabra, and a novel host plant, Plantago lanceolata, to explore whether host plant mediates viral transmission, survival, and viral loads. A two-factor factorial experiment was conducted in the laboratory with natal larval clusters randomly assigned to either the native or novel host plant and crossed with either uninoculated controls or viral inoculation (20% of individuals in the cluster inoculated). Diapausing clusters were overwintered in the laboratory and checked weekly for mortality. At the end of diapause, all surviving individuals were reared to adulthood to estimate survivorship. All individuals were screened to quantify viral loads, and estimate horizontal transmission postmortem. To test for vertical transmission, adults were mated, and the progeny were screened for viral presence. Within virus-treated groups, we found evidence for both horizontal and vertical transmission. Larval clusters reared on the native host plant had slightly higher horizontal transmission. Survival probability was lower in clusters feeding on the native host plant, with inoculated groups reared on the native host plant experiencing complete mortality. Viral loads did not differ by the host plant, although viral loads decreased with increased sequestration of secondary compounds on both host plants. Our results indicate that the use of a novel host plant may confer fitness benefits in terms of survival and reduced viral transmission when larvae feeding on it are infected with this pathogen, supporting hypotheses of potential evolutionary advantages of a host range expansion in the context of tritrophic interactions.

Viral Infection Induces Changes to the Metabolome, Immune Response and Development of a Generalist Insect Herbivore

Host plant consumption and pathogen infection commonly influence insect traits related to development and immunity, which are ultimately reflected in the behavior and physiology of the insect. Herein, we explored changes in the metabolome of a generalist insect herbivore, Vanessa cardui (Lepidoptera: Nymphalidae), in response to both dietary variation and pathogen infection in order to gain insight into tritrophic interactions for insect metabolism and immunity. Caterpillars were reared on two different host plants, Plantago lanceolata (Plantaginaceae) and Taraxacum officinale (Asteraceae) and subjected to a viral infection by Junonia coenia densovirus (JcDV), along with assays to determine the insect immune response and development. Richness and diversity of plant and caterpillar metabolites were evaluated using a liquid chromatography-mass spectrometry approach and showed that viral infection induced changes to the chemical content of V. cardui hemolymph and frass dependent upon host plant consumption. Overall, the immune response as measured by phenoloxidase (PO) enzymatic activity was higher in individuals feeding on P. lanceolata compared with those feeding on T. officinale. Additionally, infection with JcDV caused suppression of PO activity, which was not host plant dependent. We conclude that viral infection combined with host plant consumption creates a unique chemical environment, particularly within the insect hemolymph. Whether and how these metabolites contribute to defense against viral infection is an open question in chemical ecology.

Use of an exotic host plant reduces viral burden in a native insect herbivore

Incorporation of exotic plants into the diets of native herbivores is a common phenomenon, influencing interactions with natural enemies and providing insight into the tritrophic costs and benefits of dietary expansion. We evaluated how use of an exotic plant, Plantago lanceolata, impacted immune performance, development and susceptibility to pathogen infection in the neotropical herbivore Anartia jatrophae (Lepidoptera: Nymphalidae). Caterpillars were reared on P. lanceolata or a native plant, Bacopa monnieri, and experimentally infected with a pathogenic virus, Junonia coenia densovirus. We found that virus-challenged herbivores exhibited higher survival rates and lower viral burdens when reared on P. lanceolata compared to B. monnieri, though immune performance and development time were largely similar on the two plants. These findings reveal that use of an exotic plant can impact the vulnerability of a native herbivore to pathogen infection, suggesting diet-mediated protection against disease as a potential mechanism facilitating the incorporation of novel resources.

Effects of Host Plants on Development and Immunity of a Generalist Insect Herbivore

Secondary plant chemistry mediates a variety of communication signals among species, playing a fundamental role in the evolutionary diversification of communities and ecosystems. Herein, we explored diet-mediated host plant effects on development and immune response of a generalist insect herbivore. Vanessa cardui (Nymphalidae) caterpillars were reared on leaves of three host plants that vary in secondary metabolites, Plantago lanceolata (Plantaginaceae), Taraxacum officinale (Asteraceae) and Tithonia diversifolia (Asteraceae). Insect development was evaluated by larval and pupal viabilities, survivorship, and development rate. Immune response was measured as phenoloxidase (PO) activity. Additionally, chemical profiles of the host plants were obtained by liquid chromatograph-mass spectrometry (LC-MS) and the discriminant metabolites were determined using a metabolomic approach. Caterpillars reared on P. lanceolata exhibited the highest larval and pupal viabilities, as well as PO activity, and P. lanceolata leaves were chemically characterized by the presence of iridoid glycosides, phenylpropanoids and flavonoids. Taraxacum officinale leaves were characterized mainly by the presence of phenylpropanoids, flavones O-glycoside and germacranolide-type sesquiterpene lactones; caterpillars reared on this host plant fully developed to the adult stage, however they exhibited lower larval and pupal viabilities compared to individuals reared on P. lanceolata. Conversely, caterpillars reared on T. diversifolia leaves, which contain phenylpropanoids, flavones and diverse furanoheliangolide-type sesquiterpene lactones, were not able to complete larval development and exhibited the lowest PO activity. These findings suggested that V. cardui have adapted to tolerate potentially toxic metabolites occurring in P. lanceolata (iridoid glycosides), however caterpillars were not able to cope with potentially detrimental metabolites occurring in T. diversifolia (furanoheliangolides). Therefore, we suggest that furanoheliangolide-type sesquiterpene lactones were responsible for the poor development and immune response observed for caterpillars reared on T. diversifolia.

Feeding on an exotic host plant enhances plasma levels of phenoloxidase by modulating feeding efficiency in a specialist insect herbivore

Background: Exotic plant species represent a novel resource for invertebrates and many herbivorous insects have incorporated exotic plants into their diet. Using a new host plant can have physiological repercussions for these herbivores that may be beneficial or detrimental. In this study, we compared how using an exotic versus native host plant affected the immune system response and feeding efficiency of a specialist lepidopteran, the common buckeye (Junonia coenia: Nymphalidae, Hübner 1822). Materials and Methods: In a lab experiment, larvae were reared on either the exotic host plant, Plantago lanceolata (Plantaginaceae), or the native host plant, Mimulus guttatus (Phrymaceae). Beginning at second instar feeding efficiency data were collected every 2 days until fifth instar when immune assays were performed. Immune assays consisted of standing phenoloxidase activity, total phenoloxidase activity, and melanization. Results: Interestingly, we found that all three immune system parameters were higher on the exotic host plant compared to the native host plant. The exotic host plant also supported higher pupal weights, faster development time, greater consumption, and more efficient approximate digestibility. In contrast, the native host plant supported higher efficiency of conversion of ingested and digested food. The relationship between immunity and feeding efficiency was more complex but showed a large positive effect of greater host plant consumption on all immune parameters, particularly for the exotic host plant. While not as strong, the efficiency of conversion of digested food tended to show a negative effect on the three immune parameters. Conclusion: Overall, the exotic host plant proved to be beneficial for this specialist insect with regard to immunity and many of the feeding efficiency parameters and continued use of this host plant is predicted for populations already using it.

Use of an exotic host plant shifts immunity, chemical defense, and viral burden in wild populations of a specialist insect herbivore

Defense against natural enemies constitutes an important driver of herbivore host range evolution in the wild. Populations of the Baltimore checkerspot butterfly, Euphydryas phaeton (Nymphalidae), have recently incorporated an exotic plant, Plantago lanceolata (Plantaginaceae), into their dietary range. To understand the tritrophic consequences of utilizing this exotic host plant, we examined immune performance, chemical defense, and interactions with a natural entomopathogen (Junonia coenia densovirus, Parvoviridae) across wild populations of this specialist herbivore. We measured three immune parameters, sequestration of defensive iridoid glycosides (IGs), and viral infection load in field-collected caterpillars using either P. lanceolata or a native plant, Chelone glabra (Plantaginaceae). We found that larvae using the exotic plant exhibited reduced immunocompetence, compositional differences in IG sequestration, and higher in situ viral burdens compared to those using the native plant. On both host plants, high IG sequestration was associated with reduced hemocyte concentration in the larval hemolymph, providing the first evidence of incompatibility between sequestered chemical defenses and the immune response (i.e., the "vulnerable host" hypothesis) from a field-based study. However, despite this negative relationship between IG sequestration and cellular immunity, caterpillars with greater sequestration harbored lower viral loads. While survival of virus-infected individuals decreased with increasing viral burden, it ultimately did not differ between the exotic and native plants. These results provide evidence that: (1) phytochemical sequestration may contribute to defense against pathogens even when immunity is compromised and (2) herbivore persistence on exotic plant species may be facilitated by sequestration and its role in defense against natural enemies.