Reconfiguration of the immune system network during food limitation in the caterpillar Manduca sexta

Resources modulate developmental shifts but not infection tolerance upon coinfection in an insect system

Energetic resources fuel immune responses and parasite growth within organisms, but it is unclear whether energy allocation is sufficient to explain changes in infection outcomes under the threat of multiple parasites. We manipulated diet in flour beetles ( Tribolium confusum ) infected with two natural parasites to investigate the role of resources in shifting metabolic and immune responses after single and co-infection. Our results suggest that gregarine parasites alter the within-host energetic environment, and by extension juvenile development time, in a diet- dependent manner. Gregarines do not affect host resistance to acute bacterial infection but do stimulate the expression of an alternative set of immune genes and promote damage to the gut, ultimately contributing to reduced survival regardless of diet. Thus, energy allocation is not sufficient to explain the immunological contribution to coinfection outcomes, emphasizing the importance of mechanistic insight for predicting the impact of coinfection across levels of biological organization.

The complex interactions between nutrition, immunity and infection in insects

Insects are the most diverse animal group on the planet. Their success is reflected by the diversity of habitats in which they live. However, these habitats have undergone great changes in recent decades; understanding how these changes affect insect health and fitness is an important challenge for insect conservation. In this Review, we focus on the research that links the nutritional environment with infection and immune status in insects. We first discuss the research from the field of nutritional immunology, and we then investigate how factors such as intracellular and extracellular symbionts, sociality and transgenerational effects may interact with the connection between nutrition and immunity. We show that the interactions between nutrition and resistance can be highly specific to insect species and/or infection type - this is almost certainly due to the diversity of insect social interactions and life cycles, and the varied environments in which insects live. Hence, these connections cannot be easily generalised across insects. We finally suggest that other environmental aspects - such as the use of agrochemicals and climatic factors - might also influence the interaction between nutrition and resistance, and highlight how research on these is essential.

From perplexing to predictive: are we ready to forecast insect disease susceptibility in a warming world?

Insects are critical to our ecosystems, but we do not fully understand their future in our warming world. Rising temperatures are affecting insect physiology in myriad ways, including changes to their immune systems and the ability to fight infection. Whether predicted changes in temperature will contribute to insect mortality or success, and the role of disease in their future survival, remains unclear. Although heat can enhance immunity by activating the integrated defense system (e.g. via the production of protective molecules such as heat-shock proteins) and accelerating enzyme activity, heat can also compromise the immune system through energetic-resource trade-offs and damage. The responses to heat are highly variable among species. The reasons for this variability are poorly known, and we are lagging in our understanding of how and why the immune system responds to changes in temperature. In this Commentary, we highlight the variation in insect immune responses to heat and the likely underlying mechanisms. We suggest that we are currently limited in our ability to predict the effects of rising temperatures on insect immunity and disease susceptibility, largely owing to incomplete information, coupled with a lack of tools for data integration. Moreover, existing data are concentrated on a relatively small number of insect Orders. We provide suggestions for a path towards making more accurate predictions, which will require studies with realistic temperature exposures and housing design, and a greater understanding of both the thermal biology of the immune system and connections between immunity and the physiological responses to heat.

Influence of Wireworm Diet on its Susceptibility to and Control With the Entomopathogenic Fungus Metarhizium brunneum (Hypocreales: Clavicipitaceae) in Laboratory and Field Settings

Entomopathogenic fungi (EPF) represent promising control agents against wireworms but success in field experiments is inconsistent. The physiological condition of the targeted insect is crucial for its ability to withstand fungal infection. In particular, nutritional status is among the most important determinants of the insects' immune defense. In this study, we investigated the effects of diet on the development of the wireworm Agriotes obscurus (L.) (Coleoptera: Elateridae) and its subsequent susceptibility to the fungal pathogen Metarhizium brunneum (Petch) (Hypocreales: Clavicipitaceae) in a pot experiment. After being reared on one of five plant diets for eight weeks, wireworms were exposed to an environment inoculated with the EPF and monitored for their susceptibility to fungal infection. We then performed a field experiment in which three plant diets (clover, radish, and a cover crop mix), selected according to the insects' performance in the laboratory experiment, were grown as a cover crop with EPF application. Plant diet influenced growth and development of larvae, but there were no strong differences in susceptibility toward fungal infection in the laboratory experiment. Damage levels in EPF-treated plots in the field varied depending on the cover crop. Damage was highest in plots planted with a mix of cover crop species, whereas damage was lowest in plots with clover or radish alone. This agrees with the laboratory results where insect performance was inferior when fed on clover or radish. Cover crop effects on wireworm damage in the subsequent cash crop may thus vary depending on the cover crop species selected.

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.

Survival, Body Condition, and Immune System of Apis mellifera liguistica Fed Avocado, Maize, and Polyfloral Pollen Diet

Nutritional stress is the major factor contributing to decline in the honey bee (Apis mellifera L.) populations given the high degree of dependence on floral resources, and due to the habitat loss. In this sense, monocultures of maize and avocado have great extensions in Mexico, but their impact on the physiology and morphology of A. mellifera is unknown. This research evaluated the effect of total protein content in monofloral (maize or avocado pollen diets) and polyfloral (using five types of pollen: Persea americana Mill., Zea mays L., Melampodium perfoliatum Cav., Drymaria villosa Cham Schltdl., and Lopezia racemosa Cav.) on their survival, body condition (controlled density, head mass, and development of hypopharyngeal glands; protein content in hemolymph), and immune response [lytic activity and activity of prophenoloxidase in the hemolymph (proPO)]. Corbicular pollen of P. americana had the highest protein content, followed by the corbicular pollen of Z. mays, M. perfoliatum, D. villosa, and L. racemosa. Polyfloral diet seems to be better for A. mellifera than the monofloral maize and avocado. Bees fed polyfloral pollen diet showed a high content of protein in the hemolymph in comparison with that fed maize or avocado pollen diets. Bees fed polyfloral and avocado pollen diet had the highest lytic activity but showed a decrease in proPO activity. In conclusion, polyfloral diets seem to be better for A. mellifera than the monofloral maize and avocado.

Changes in haemolymph parameters and insect ability to respond to immune challenge during overwintering

Overwintering is a challenging period in the life of temperate insects. A limited energy budget characteristic of this period can result in reduced investment in immune system. Here, we investigated selected physiological and immunological parameters in laboratory-reared and field-collected harlequin ladybirds (Harmonia axyridis). For laboratory-reared beetles, we focused on the effects of winter temperature regime (cold, average, or warm winter) on total haemocyte concentration aiming to investigate potential effects of ongoing climate change on immune system in overwintering insects. We recorded strong reduction in haemocyte concentration during winter; however, there were only limited effects of winter temperature regime on changes in haemocyte concentration in the course of overwintering. For field-collected beetles, we measured additional parameters, specifically: total protein concentration, antimicrobial activity against Escherichia coli, and haemocyte concentration before and after overwintering. The field experiment did not investigate effects of winter temperature, but focused on changes in inducibility of insect immune system during overwintering, that is, measured parameters were compared between naïve beetles and those challenged by Escherichia coli. Haemocyte concentration decreased during overwintering, but only in individuals challenged by Escherichia coli. Prior to overwintering, the challenged beetles had a significantly higher haemocyte concentration compared to naïve beetles, whereas no difference was observed after overwintering. A similar pattern was observed also for antimicrobial activity against Escherichia coli as challenged beetles outperformed naïve beetles before overwintering, but not after winter. In both sexes, total protein concentration increased in the course of overwintering, but females had a significantly higher total protein concentration in their hemolymph compared to males. In general, our results revealed that insect's ability to respond to an immune challenge is significantly reduced in the course of overwintering.

Pyrethroid Exposure Reduces Growth and Development of Monarch Butterfly (Lepidoptera: Nymphalidae) Caterpillars

Insecticide exposure has been identified as a contributing stressor to the decline in the North American monarch butterfly Danaus plexippus L. (Lepidoptera: Nymphalidae) population. Monarch toxicity data are currently limited and available data focuses on lethal endpoints. This study examined the 72-h toxicity of two pyrethroid insecticides, bifenthrin and β-cyfluthrin, and their effects on growth and diet consumption. The toxicity of bifenthrin to caterpillars was lower than β-cyfluthrin after 72 h. Survival was the most sensitive endpoint for bifenthrin, but diet consumption and caterpillar growth were significantly reduced at sublethal levels of β-cyfluthrin. Using AgDRIFT spray drift assessment, the aerial application of bifenthrin or β-cyfluthrin is predicted to pose the greatest risk to fifth-instar caterpillars, with lethal insecticide deposition up to 28 m for bifenthrin and up to 23 m for β-cyfluthrin from treated edges of fields. Low boom ground applications are predicted to reduce distances of lethal insecticide exposure to 2 m from the treated field edge for bifenthrin and β-cyfluthrin. Growth and survival of fifth-instar monarch caterpillars developing within the margins of a treated field may be significantly impacted following foliar applications of bifenthrin or β-cyfluthrin. These findings provide evidence that pyrethroid insecticides commonly used for soybean pest control are a potential risk to monarch caterpillars in agricultural landscapes.

Vertically Transmitted Gut Bacteria and Nutrition Influence the Immunity and Fitness of Bactrocera dorsalis Larvae

Symbiotic bacterial communities that colonize the digestive tract of tephritid fruit flies interact with nutrient intake to improve the flies' fitness and immunity. Some bacterial species consistently inhabit the tephritid guts and are transmitted to the next generation vertically. These species contribute significantly to some aspects of their host's physiology. In the current study, we examined the role of four vertically transmitted bacteria (Citrobacter, Enterobacter, Klebsiella, and Providencia) on the fitness parameters and immunity of Bactrocera dorsalis larvae that were fed a nutritionally manipulated diet. For this purpose, eggs were collected from axenic, gnotobiotic, and symbiotic adult flies, and larvae were reared on four types of diets in which carbohydrate and/or protein contents were reduced and then compared with larvae raised on a control diet. The diet and bacterial interactions significantly affected the fitness and immunity of B. dorsalis. Larvae of axenic flies grew slower and displayed weaker immune-based responses (PO activity, antibacterial activity, survival) than larvae of gnotobiotic and symbiotic flies. Overall, larvae reared on the low-protein diet grew slower than those reared on the control or low-carbohydrate diets. Survival, PO activity, and antibacterial activity were significantly lower in the hemolymph of larvae reared on low-protein diets. Our results also revealed that the levels of hemolymph protein, glucose, trehalose, and triglyceride in larvae from axenic flies were significantly lower than those in larvae of the symbiotic group after they fed on most of the tested diets. These results strongly infer that diet and vertically transmitted bacteria are both essential contributors to the fitness and immunity of B. dorsalis.

Friend or foe? Effects of host immune activation on the gut microbiome in the caterpillar Manduca sexta

For many animals, the gut microbiome plays an essential role in immunity and digestion. However, certain animals, such as the caterpillar Manduca sexta, do not have a resident gut microbiome. Although these animals do have bacteria that pass through their gut from their natural environment, the absence of such bacteria does not reduce growth or survival. We hypothesized that M. sexta would sterilize their gut as a protective measure against secondary infection when faced with a gut infection or exposure to heat-killed bacteria in the blood (haemolymph). However, we found that gut sterilization did not occur during either type of immune challenge, i.e. bacterial numbers did not decrease. By examining the pattern of immune-related gene expression, gut pH, live bacterial counts and mass change (as a measure of sickness behaviour), we found evidence for physiological trade-offs between regulating the microbiome and defending against systemic infections. Caterpillars exposed to both gut pathogens and a systemic immune challenge had higher numbers of bacteria in their gut than caterpillars exposed to a single challenge. Following a multivariate analysis of variance, we found that the response patterns following an oral challenge, systemic challenge or dual challenge were unique. Our results suggest that the immune response for each challenge resulted in a different configuration of the immunophysiological network. We hypothesize that these different configurations represent different resolutions of physiological trade-offs based on the immune responses needed to best protect the animal against the present immune challenges.

Selection of Beauveria bassiana (Hypocreales: Cordycipitaceae) strains to control Xyleborus affinis (Curculionidae: Scolytinae) females

BACKGROUND

Xyleborus affinis Eichhoff (Coleoptera: Curculionidae) is an ambrosia beetle reported to affect avocado trees (Persea americana Mill.). The use of the entomopathogenic fungus (EPF) Beauveria bassiana (Bals.-Criv.) Vuill. for ambrosia beetle control represents an alternative to insecticides.

METHODS

This study was designed in two stages to select B. bassiana strains with potential to control X. affinis females. In the first stage, 19 B. bassiana Mexican strains from EPF collection, isolated from Coleoptera (CHE-CNRCB, http://www.gob.mx/senasica/documentos/coleccion-de-hongos-entomopatogenos), were tested. Analyses included radial growth rate, conidial yield, spore germination, and germ tube length. Results were analysed by Principal Component Analysis (PCA) to identify clusters within favourable growth phenotypes. For the second stage, 10 selected strains were re-analysed for virulence-related metabolic characteristic, including cell wall-bound cuticle-degrading enzymes-Pr1-like proteases and β-N-acetyl glucosaminidases (NAGase) chitinases, conidial hydrophobicity and monopolar germination parameters. A second PCA analysis was run for those virulence parameters analysed, and upon results strains CHE-CNRCB 44, 171, 431 and 485 were selected and tested against X. affinis females. Females were treated with a 1 × 108 conidia mL-1 suspension (recommended rate), using a Potter Tower.

RESULTS

All strains showed insecticidal activity, inducing up to 58% mortality; about 30% dead beetles developed aerial mycelia (CHE-CNRCB 485) and the fastest mortality rate was t0 = 1.95 (CHE-CNRCB 44).

CONCLUSION

Since all selected strains showed virulence against X. affinis females, results indicated the possibility of selecting B. bassiana strains based on multiple metabolic attributes, as a preliminary test to perform bioassays against order-related target insects.

Animals have a Plan B: how insects deal with the dual challenge of predators and pathogens

When animals are faced with a life-threatening challenge, they mount an organism-wide response (i.e. Plan A). For example, both the stress response (i.e. fight-or-flight) and the immune response recruit molecular resources from other body tissues, and induce physiological changes that optimize the body for defense. However, pathogens and predators often co-occur. Animals that can optimize responses for a dual challenge, i.e. simultaneous predator and pathogen attacks, will have a selective advantage. Responses to a combined predator and pathogen attack have not been well studied, but this paper summarizes the existing literature in insects. The response to dual challenges (i.e. Plan B) results in a suite of physiological changes that are different from either the stress response or the immune response, and is not a simple summation of the two. It is also not a straight-forward trade-off of one response against the other. The response to a dual challenge (i.e. Plan B) appears to resolve physiological trade-offs between the stress and immune responses, and reconfigures both responses to provide the best overall defense. However, the dual response appears to be more costly than either response occurring singly, resulting in greater damage from oxidative stress, reduced growth rate, and increased mortality.

The role of fasting on spine regeneration and bacteremia in the purple sea urchin Strongylocentrotus purpuratus

Fasting has been shown to increase longevity and alter immune function in a variety of animals, but little is understood about how reduced caloric intake may impact regeneration and infections in animals that must regularly repair and regenerate tissue in marine environments that contain high levels of bacteria. We examined the possibility that fasting could enhance spine regeneration and reduce bacteremia in the purple sea urchin Strongylocentrotus purpuratus. A small number of spines were removed from urchins and rates of spine regrowth and levels of culturable bacteria from the coelomic fluid were measured for 21 days in fed and fasted urchins. Fasted urchins had higher rates of spine regrowth and lower levels of colony-forming units (CFU) per milliliter of coeolomic fluid. The predominant bacteria in the coelomic fluid was isolated and identified by DNA sequence-based methods as Vibrio cyclitrophicus. After 21 days, fasted and fed urchins were injected with V. cyclitrophicus. Two hours after injection, fed urchins had about 25% more culturable bacteria remaining in their coelomic fluid compared to fasted urchins. We found no evidence that fasting altered coelomic fluid cell number or righting response, indicators of physiologic and behavioral stress in urchins. Our results demonstrate that V. cyclitrophicus is present in purple urchin coelomic fluid, that fasting can increase spine regeneration and that fasted urchins have much lower levels of culturable bacteria in their coelomic fluid than fed urchins. Overall, our data suggests that fasting may ultimately reduce bacteremia and infection in injured or damaged urchins.

From phenoloxidase to fecundity: food availability does not influence the costs of oxidative challenge in a wing-dimorphic cricket

Stressed animals often struggle to maintain optimal investment into a number of fitness-related traits, which can result in some traits being more adversely affected than others. Variation in stress-related costs may also depend on the environment-costs can be facultative and only occur when resources are limited, or they may be obligate and occur regardless of resource availability. Dynamics of oxidative stress may be important in life-history evolution given their role in a range of biological processes-from reproduction to immunity to locomotion. Thus, we examined how resource (food) availability influences the costs of oxidative challenge to fitness-related traits spanning several levels of biological organization. We manipulated food availability and oxidative status in females of the wing-dimorphic sand field cricket (Gryllus firmus) during early adulthood. We then determined investment into several traits: reproduction (ovary mass), soma (body mass and flight musculature), and immune function (total phenoloxidase activity). Oxidative challenge (paraquat exposure) obligated costs to somatic tissue and a parameter of immune function regardless of food availability, but it did not affect reproduction. We show that the costs of oxidative challenge are trait-specific, but we did not detect a facultative (food-dependent) cost of oxidative challenge to any trait measured. Although the dynamics of oxidative stress are complex, our study is an important step toward a more complete understanding of the roles that resource availability and redox systems play in mediating life histories.

Listening to your gut: immune challenge to the gut sensitizes body wall nociception in the caterpillar Manduca sexta

Immune-nociceptor connections are found in animals across phyla. Local inflammation and/or damage results in increased nociceptive sensitivity of the affected area. However, in mammals, immune responses far from peripheral nociceptors, such as immune responses in the gut, produce a general increase in peripheral nociceptive sensitivity. This phenomenon has not, to our knowledge, been found in other animal groups. We found that consuming heat-killed pathogens reduced the tactile force needed to induce a defensive strike in the caterpillar Manduca sexta. This increase in the nociceptive sensitivity of the body wall is probably part of the reconfiguration of behaviour and physiology that occurs during an immune response (e.g. sickness behaviour). This increase may help enhance anti-predator behaviour as molecular resources are shifted towards the immune system. This article is part of the Theo Murphy meeting issue 'Evolution of mechanisms and behaviour important for pain'.

Sugar intake interacts with temperature to influence reproduction and immunity in adult Culex pipiens mosquitoes

Disease transmission by insect vectors will depend on integrated physiological responses to interacting environmental variables. We explored how interactions between temperature and sucrose concentration affected immunity and fecundity, two variables that contribute to vectorial capacity, in Culex pipiens Linnaeus, 1758 mosquitoes. We provided female C. pipiens with either 2% or 20% sucrose and exposed them to low (22 °C), moderate (25 °C), or high (30 °C) temperatures for 8 days. We then measured the strength of the melanization response in one subpopulation of females and the number of eggs laid as a measure of fecundity in another subpopulation. Temperature interacted with diet to weaken immunity under 2% sucrose at 22 and 25 °C. This effect disappeared at 30 °C, suggesting that high temperatures allowed mosquitoes to compensate for the effects of decreased sucrose. Conversely, increasing temperature increased egg production on a diet of 20% sucrose, but heat exposure on a diet of 2% sucrose decreased fecundity. Overall, we suggest that heat exposure requires investment in thermal protection, which may prompt reconfiguration of the immune system and (or) decreased investment in reproduction. Thus, our understanding of the effects of climate change rest on which physiological system we measure and under which combinations of stressors.

Sexual dimorphism in immunity across animals: a meta-analysis

In animals, sex differences in immunity are proposed to shape variation in infection prevalence and intensity among individuals in a population, with females typically expected to exhibit superior immunity due to life-history trade-offs. We performed a systematic meta-analysis to investigate the magnitude and direction of sex differences in immunity and to identify factors that shape sex-biased immunocompetence. In addition to considering taxonomic and methodological effects as moderators, we assessed age-related effects, which are predicted to occur if sex differences in immunity are due to sex-specific resource allocation trade-offs with reproduction. In a meta-analysis of 584 effects from 124 studies, we found that females exhibit a significantly stronger immune response than do males, but the effect size is relatively small, and became non-significant after controlling for phylogeny. Female-biased immunity was more pronounced in adult than immature animals. More recently published studies did not report significantly smaller effect sizes. Among taxonomic and methodological subsets of the data, some of the largest effect sizes were in insects, further supporting previous suggestions that testosterone is not the only potential driver of sex differences in immunity. Our findings challenge the notion of pervasive biases towards female-biased immunity and the role of testosterone in driving these differences.

Mitochondrial Dysfunction and Infection Generate Immunity-Fecundity Tradeoffs in Drosophila

Physiological responses to short-term environmental stressors, such as infection, can have long-term consequences for fitness, particularly if the responses are inappropriate or nutrient resources are limited. Genetic variation affecting energy acquisition, storage, and usage can limit cellular energy availability and may influence resource-allocation tradeoffs even when environmental nutrients are plentiful. Here, we utilized Drosophila mitochondrial-nuclear genotypes to test whether disrupted mitochondrial function interferes with nutrient-sensing pathways, and whether this disruption has consequences for tradeoffs between immunity and fecundity. We found that an energetically-compromised genotype was relatively resistant to rapamycin-a drug that targets nutrient-sensing pathways and mimics resource limitation. Dietary resource limitation decreased survival of energetically-compromised flies. Furthermore, survival of infection with a natural pathogen was decreased in this genotype, and females of this genotype experienced immunity-fecundity tradeoffs that were not evident in genotypic controls with normal energy metabolism. Together, these results suggest that this genotype may have little excess energetic capacity and fewer cellular nutrients, even when environmental nutrients are not limiting. Genetic variation in energy metabolism may therefore act to limit the resources available for allocation to life-history traits in ways that generate tradeoffs even when environmental resources are not limiting.

Immune responses to bacterial and fungal infections in the silkworm, Bombyx mori

The silkworm Bombyx mori, an economically important insect that is usually reared indoors, is susceptible to various pathogens, including bacteria, fungi, viruses, and microsporidia. As with other insects, the silkworm lacks an adaptive immune system and relies solely on innate immunity to defend itself against infection. Compared to other intensively studied insects, such as the fruit fly and tobacco hornworm, the principal immune pathways in the silkworm remain unclear. In this article, we review the literature concerning silkworm immune responses to bacteria and fungi and present our perspectives on future research into silkworm immunity.

Eating when ill is risky: immune defense impairs food detoxification in the caterpillar Manduca sexta

Mounting an immune response consumes resources, which should lead to increased feeding. However, activating the immune system reduces feeding (i.e. illness-induced anorexia) in both vertebrates and invertebrates, suggesting that it may be beneficial. We suggest that illness-induced anorexia may be an adaptive response to conflicts between immune defense and food detoxification. We found that activating an immune response in the caterpillar Manduca sexta increased its susceptibility to the toxin permethrin. Conversely, a sublethal dose of permethrin reduced resistance to the bacterium Serratia marcescens, demonstrating a negative interaction between detoxification and immune defense. Immune system activation and toxin challenge each depleted the amount of glutathione in the hemolymph. Increasing glutathione concentration in the hemolymph increased survival for both toxin- and immune+toxin-challenged groups. The results of this rescue experiment suggest that decreased glutathione availability, such as occurs during an immune response, impairs detoxification. We also found that the expression of some detoxification genes were not upregulated during a combined immune-toxin challenge, although they were when animals received a toxin challenge alone. These results suggest that immune defense reduces food detoxification capacity. Illness-induced anorexia may protect animals by decreasing exposure to food toxins when detoxification is impaired.

Microbiome symbionts and diet diversity incur costs on the immune system of insect larvae

Communities of symbiotic microorganisms that colonize the gastrointestinal tract play an important role in food digestion and protection against opportunistic microbes. Diet diversity increases the number of symbionts in the intestines, a benefit that is considered to impose no cost for the host organism. However, less is known about the possible immunological investments that hosts have to make in order to control the infections caused by symbiont populations that increase because of diet diversity. Using taxonomical composition analysis of the 16S rRNA V3 region, we show that enterococci are the dominating group of bacteria in the midgut of the larvae of the greater wax moth (Galleria mellonella). We found that the number of colony-forming units of enterococci and expressions of certain immunity-related antimicrobial peptide (AMP) genes such as Gallerimycin, Gloverin, 6-tox, Cecropin-D and Galiomicin increased in response to a more diverse diet, which in turn decreased the encapsulation response of the larvae. Treatment with antibiotics significantly lowered the expression of all AMP genes. Diet and antibiotic treatment interaction did not affect the expression of Gloverin and Galiomicin AMP genes, but significantly influenced the expression of Gallerimycin, 6-tox and Cecropin-D Taken together, our results suggest that diet diversity influences microbiome diversity and AMP gene expression, ultimately affecting an organism's capacity to mount an immune response. Elevated basal levels of immunity-related genes (Gloverin and Galiomicin) might act as a prophylactic against opportunistic infections and as a mechanism that controls the gut symbionts. This would indicate that a diverse diet imposes higher immunity costs on organisms.

Insect Immunity Varies Idiosyncratically During Overwintering

Overwintering insects face multiple stressors, including pathogen and parasite pressures that shift with seasons. However, we know little of how the insect immune system fluctuates with season, particularly in the overwintering period. To understand how immune activity changes across autumn, winter, and spring, we tracked immune activity of three temperate insects that overwinter as larvae: a weevil (Curculio sp., Coleoptera), gallfly (Eurosta solidaginis, Diptera), and larvae of the lepidopteran Pyrrharctia isabella. We measured baseline circulating hemocyte numbers, phenoloxidase activity, and humoral antimicrobial activity, as well as survival of fungal infection and melanization response at 12°C and 25°C to capture any potential plasticity in thermal performance. In Curculio sp. and E. solidaginis, hemocyte concentrations remained unchanged across seasons and antimicrobial activity against Gram-positive bacteria was lowest in autumn; however, Curculio sp. were less likely to survive fungal infection in autumn, whereas E. solidaginis were less likely to survive infection during the winter. Furthermore, hemocyte concentrations and antimicrobial activity decreased in P. isabella overwintering beneath snow cover. Overall, seasonal changes in activity were largely species dependent, thus it may be difficult to create generalizable predictions about the effects of a changing climate on seasonal immune activity in insects. However, we suggest that the relationship between the response to multiple stressors (e.g., cold and pathogens) drives changes in immune activity, and that understanding the physiology underlying these relationships will inform our predictions of the effects of environmental change on insect overwintering success.

The stress response and immune system share, borrow, and reconfigure their physiological network elements: Evidence from the insects

The classic biomedical view is that stress hormone effects on the immune system are largely pathological, especially if the stress is chronic. However, more recent interpretations have focused on the potential adaptive function of these effects. This paper examines stress response-immune system interactions from a physiological network perspective, using insects because of their simpler physiology. For example, stress hormones can reduce disease resistance, yet activating an immune response results in the release of stress hormones in both vertebrates and invertebrates. From a network perspective, this phenomenon is consistent with the 'sharing' of the energy-releasing ability of stress hormones by both the stress response and the immune system. Stress-induced immunosuppression is consistent with the stress response 'borrowing' molecular components from the immune system to increase the capacity of stress-relevant physiological processes (i.e. a trade off). The insect stress hormones octopamine and adipokinetic hormone can also 'reconfigure' the immune system to help compensate for the loss of some of the immune system's molecular resources (e.g. apolipophorin III). This view helps explain seemingly maladaptive interactions between the stress response and immune system. The adaptiveness of stress hormone effects on individual immune components may be apparent only from the perspective of the whole organism. These broad principles will apply to both vertebrates and invertebrates.

Stress responses sculpt the insect immune system, optimizing defense in an ever-changing world

A whole organism, network approach can help explain the adaptive purpose of stress-induced changes in immune function. In insects, mediators of the stress response (e.g. stress hormones) divert molecular resources away from immune function and towards tissues necessary for fight-or-flight behaviours. For example, molecules such as lipid transport proteins are involved in both the stress and immune responses, leading to a reduction in disease resistance when these proteins are shifted towards being part of the stress response system. Stress responses also alter immune system strategies (i.e. reconfiguration) to compensate for resource losses that occur during fight-or flight events. In addition, stress responses optimize immune function for different physiological conditions. In insects, the stress response induces a pro-inflammatory state that probably enhances early immune responses.

Predator exposure-induced immunosuppression: trade-off, immune redistribution or immune reconfiguration?

Although predator exposure increases the risk of wound infections, it typically induces immunosuppression. A number of non-mutually exclusive hypotheses have been put forward to explain this immunosuppression, including: trade-offs between the immune system and other systems required for anti-predator behaviour, redistribution of immune resources towards mechanisms needed to defend against wound infections, and reconfiguration of the immune system to optimize defence under the physiological state of fight-or-flight readiness. We tested the ability of each hypothesis to explain the effects of chronic predator stress on the immune system of the caterpillar Manduca sexta Predator exposure induced defensive behaviours, reduced mass gain, increased development time and increased the concentration of the stress neurohormone octopamine. It had no significant effect on haemocyte number, melanization rate, phenoloxidase activity, lysozyme-like activity or nodule production. Predator stress reduced haemolymph glutathione concentrations. It also increased constitutive expression of the antimicrobial peptide attacin-1 but reduced attacin-1 expression in response to an immune challenge. These results best fit the immune reconfiguration hypothesis, although the other hypotheses are also consistent with some results. Interpreting stress-related changes in immune function may require an examination at the level of the whole organism.

The parasitic wasp Cotesia congregata uses multiple mechanisms to control host (Manduca sexta) behaviour

Some parasites alter the behaviour of their hosts. The larvae of the parasitic wasp Cotesia congregata develop within the body of the caterpillar Manduca sexta During the initial phase of wasp development, the host's behaviour remains unchanged. However, once the wasps begin to scrape their way out of the caterpillar, the caterpillar host stops feeding and moving spontaneously. We found that the caterpillar also temporarily lost sensation around the exit hole created by each emerging wasp. However, the caterpillars regained responsiveness to nociception in those areas within 1 day. The temporary reduction in skin sensitivity is probably important for wasp survival because it prevents the caterpillar from attacking the emerging wasp larvae with a defensive strike. We also found that expression of plasmatocyte spreading peptide (PSP) and spätzle genes increased in the fat body of the host during wasp emergence. This result supports the hypothesis that the exiting wasps induce a cytokine storm in their host. Injections of PSP suppressed feeding, suggesting that an augmented immune response may play a role in the suppression of host feeding. Injection of wasp larvae culture media into non-parasitized caterpillars reduced feeding, suggesting that substances secreted by the wasp larvae may help alter host behaviour.

Social stress increases the susceptibility to infection in the ant Harpegnathos saltator

Aggressive interactions between members of a social group represent an important source of social stress with all its negative follow-ups. We used the ponerine ant Harpegnathos saltator to study the effects of frequent aggressive interactions on the resistance to different stressors. In these ants, removal or death of reproducing animals results in a period of social instability within the colony that is characterized by frequent ritualized aggressive interactions leading to the establishment of a new dominance structure. Animals are more susceptible to infections during this period, whereas their resistance against other stressors remained unchanged. This is associated with a shift from glutathione-S-transferase activities towards glutathione peroxidase activities, which increases the antioxidative capacity at the expense of their immune competence.