Behavioral thermoregulation in the migratory locust: a therapy to overcome fungal infection

Cross-talk between immunity and behavior: insights from entomopathogenic fungi and their insect hosts

Insects are one of the most successful animals in nature, and entomopathogenic fungi play a significant role in the natural epizootic control of insect populations in many ecosystems. The interaction between insects and entomopathogenic fungi has continuously coevolved over hundreds of millions of years. Many components of the insect innate immune responses against fungal infection are conserved across phyla. Additionally, behavioral responses, which include avoidance, grooming, and/or modulation of body temperature, have been recognized as important mechanisms for opposing fungal pathogens. In an effort to investigate possible cross-talk and mediating mechanisms between these fundamental biological processes, recent studies have integrated and/or explored immune and behavioral responses. Current information indicates that during discrete stages of fungal infection, several insect behavioral and immune responses are altered simultaneously, suggesting important connections between the two systems. This review synthesizes recent advances in our understanding of the physiological and molecular aspects influencing cross-talk between behavioral and innate immune antifungal reactions, including chemical perception and olfactory pathways.

Trade-Offs among Immune Mechanisms: Bacterial-Challenged Spodoptera frugiperda Larvae Reduce Nodulation Reactions during Behavioral Fever

Insect innate immunity is composed of cellular and humoral reactions, the former acting via circulating hemocytes and the latter via immune signaling that lead to the production of antimicrobial peptides and phenol oxidase-driven melanization. Cellular immunity involves direct interactions between circulating hemocytes and invaders; it includes internalization and killing microbes (phagocytosis) and formation of bacterial-laden microaggregates which coalesce into nodules that are melanized and attached to body walls or organs. Nodulation can entail investing millions of hemocytes which must be replaced. We hypothesized that biologically costly hemocyte-based immunity is traded off for behavioral fevers in infected larvae of fall armyworms, Spodoptera frugiperda, that were allowed to fever. We tested our hypothesis by infecting larvae with the Gram-negative bacterium, Serratia marcescens, placing them in thermal gradients (TGs) and recording their selected body temperatures. While control larvae selected about 30 °C, the experimental larvae selected up 41 °C. We found that 4 h fevers, but not 2, 6 or 24 h fevers, led to increased larval survival. Co-injections of S. marcescens with the prostaglandin (PG) biosynthesis inhibitor indomethacin (INDO) blocked the fevers, which was reversed after co-injections of SM+INDO+Arachidonic acid, a precursor to PG biosynthesis, confirming that PGs mediate fever reactions. These and other experimental outcomes support our hypothesis that costly hemocyte-based immunity is traded off for behavioral fevers in infected larvae under appropriate conditions.

Recent advances in insect thermoregulation

Ambient temperature (Ta) is a critical abiotic factor for insects that cannot maintain a constant body temperature (Tb). Interestingly, Ta varies during the day, between seasons and habitats; insects must constantly cope with these variations to avoid reaching the deleterious effects of thermal stress. To minimize these risks, insects have evolved a set of physiological and behavioral thermoregulatory processes as well as molecular responses that allow them to survive and perform under various thermal conditions. These strategies range from actively seeking an adequate environment, to cooling down through the evaporation of body fluids and synthesizing heat shock proteins to prevent damage at the cellular level after heat exposure. In contrast, endothermy may allow an insect to fight parasitic infections, fly within a large range of Ta and facilitate nest defense. Since May (1979), Casey (1988) and Heinrich (1993) reviewed the literature on insect thermoregulation, hundreds of scientific articles have been published on the subject and new insights in several insect groups have emerged. In particular, technical advancements have provided a better understanding of the mechanisms underlying thermoregulatory processes. This present Review aims to provide an overview of these findings with a focus on various insect groups, including blood-feeding arthropods, as well as to explore the impact of thermoregulation and heat exposure on insect immunity and pathogen development. Finally, it provides insights into current knowledge gaps in the field and discusses insect thermoregulation in the context of climate change.

Environment-host-parasite interactions in mass-reared insects

The mass production of insects is rapidly expanding globally, supporting multiple industrial needs. However, parasite infections in insect mass-production systems can lower productivity and can lead to devastating losses. High rearing densities and artificial environmental conditions in mass-rearing facilities affect the insect hosts as well as their parasites. Environmental conditions such as temperature, gases, light, vibration, and ionizing radiation can affect productivity in insect mass-production facilities by altering insect development and susceptibility to parasites. This review explores the recent literature on environment-host-parasite interactions with a specific focus on mass-reared insect species. Understanding these complex interactions offers opportunities to optimise environmental conditions for the prevention of infectious diseases in mass-reared insects.

Host–Pathogen Interactions between Metarhizium spp. and Locusts

The progress in research on the interactions between Metarhizium spp. and locusts has improved our understanding of the interactions between fungal infection and host immunity. A general network of immune responses has been constructed, and the pathways regulating fungal pathogenicity have also been explored in depth. However, there have been no systematic surveys of interaction between Metarhizium spp. and locusts. The pathogenesis of Metarhizium comprises conidial attachment, germination, appressorial formation, and colonization in the body cavity of the host locusts. Meanwhile, the locust resists fungal infection through humoral and cellular immunity. Here, we summarize the crucial pathways that regulate the pathogenesis of Metarhizium and host immune defense. Conidial hydrophobicity is mainly affected by the contents of hydrophobins and chitin. Appressorial formation is regulated by the pathways of MAPKs, cAMP/PKA, and Ca²⁺/calmodulin. Lipid droplets degradation and secreted enzymes contributed to fungal penetration. The humoral response of locust is coordinated by the Toll pathway and the ecdysone. The regulatory mechanism of hemocyte differentiation and migration is elusive. In addition, behavioral fever and density-dependent population immunity have an impact on the resistance of hosts against fungal infection. This review depicts a prospect to help us understand host–pathogen interactions and provides a foundation for the engineering of entomopathogenic fungi and the discovery of insecticidal targets to control insect pests.

Not Only a Formulation: The Effects of Pickering Emulsion on the Entomopathogenic Action of Metarhizium brunneum

Growing global population and environmental concerns necessitate the transition from chemical to eco-friendly pest management. Entomopathogenic fungi (EPF) are rising candidates for this task due to their ease of growing, broad host range and unique disease process, allowing EPF to infect hosts directly through its cuticle. However, EPF’s requirement for high humidity negates their integration into conventional agriculture. To mitigate this problem, we formulated Metarhizium brunneum conidia in an oil-in-water Pickering emulsion. Conidia in aqueous and emulsion formulations were sprayed on Ricinus communis leaves, and Spodoptera littoralis larvae were introduced under low or high humidity. The following were examined: conidial dispersion on leaf, larval mortality, conidial acquisition by larvae, effects on larval growth and feeding, and dynamic of disease progression. Emulsion was found to disperse conidia more efficiently and caused two-fold more adhesion of conidia to host cuticle. Mortality from conidia in emulsion was significantly higher than other treatments reaching 86.5% under high humidity. Emulsion was also found to significantly reduce larval growth and feeding, while conferring faster fungal growth in-host. Results suggest that a Pickering emulsion is able to improve physical interactions between the conidia and their surroundings, while weakening the host through a plethora of mechanisms, increasing the chance of an acute infection.

Mechanisms underlying host persistence following amphibian disease emergence determine appropriate management strategies

Emerging infectious diseases have caused many species declines, changes in communities and even extinctions. There are also many species that persist following devastating declines due to disease. The broad mechanisms that enable host persistence following declines include evolution of resistance or tolerance, changes in immunity and behaviour, compensatory recruitment, pathogen attenuation, environmental refugia, density-dependent transmission and changes in community composition. Here we examine the case of chytridiomycosis, the most important wildlife disease of the past century. We review the full breadth of mechanisms allowing host persistence, and synthesise research on host, pathogen, environmental and community factors driving persistence following chytridiomycosis-related declines and overview the current evidence and the information required to support each mechanism. We found that for most species the mechanisms facilitating persistence have not been identified. We illustrate how the mechanisms that drive long-term host population dynamics determine the most effective conservation management strategies. Therefore, understanding mechanisms of host persistence is important because many species continue to be threatened by disease, some of which will require intervention. The conceptual framework we describe is broadly applicable to other novel disease systems.

Interplay between Fungal Infection and Bacterial Associates in the Wax Moth Galleria mellonella under Different Temperature Conditions

Various insect bacterial associates are involved in pathogeneses caused by entomopathogenic fungi. The outcome of infection (fungal growth or decomposition) may depend on environmental factors such as temperature. The aim of this study was to analyze the bacterial communities and immune response of Galleria mellonella larvae injected with Cordyceps militaris and incubated at 15 °C and 25 °C. We examined changes in the bacterial CFUs, bacterial communities (Illumina MiSeq 16S rRNA gene sequencing) and expression of immune, apoptosis, ROS and stress-related genes (qPCR) in larval tissues in response to fungal infection at the mentioned temperatures. Increased survival of larvae after C. militaris injection was observed at 25 °C, although more frequent episodes of spontaneous bacteriosis were observed at this temperature compared to 15 °C. We revealed an increase in the abundance of enterococci and enterobacteria in the midgut and hemolymph in response to infection at 25 °C, which was not observed at 15 °C. Antifungal peptide genes showed the highest expression at 25 °C, while antibacterial peptides and inhibitor of apoptosis genes were strongly expressed at 15 °C. Cultivable bacteria significantly suppressed the growth of C. militaris. We suggest that fungi such as C. militaris may need low temperatures to avoid competition with host bacterial associates.

Spatial and temporal transcriptomic analyses reveal locust initiation of immune responses to Metarhizium acridum at the pre-penetration stage

Insect hemocyte and fat body tissues play critical functional roles in insect immunity. Little, however, is known concerning the dynamic responses of these tissues to fungal infection. Here, we report on a time course of locust hemocyte and fat body transcriptomic responses to infection by the acridid specific fungal pathogen, Metarhizium acridum. Fat body responses were more pronounced at all infection stages as compared to hemocytes. Immune and other related genes were induced far earlier than previously considered including at pre-penetration stages. Differential expression in hemocyte and fat body tissues persisted throughout the course of infection up until host death. Our data indicate selective pressure on the host to recognize the infection as early as possible in order to limit its spread. Overall, fat body and hemocyte tissues launch a robust multi-tiered response to combat the fungal pathogen, with our data providing potential host targets for exploitation in pest control.

Thermoregulatory response of Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae) to infection by Beauveria bassiana, and its effect on survivorship and reproductive success

Behavioral thermoregulation is a defensive strategy employed by some insects to counter infections by parasites and pathogens. Most reported examples of this type of thermoregulatory response involve behavioral fevering. However depending upon the life history of a host-insect and that of the parasite or pathogen, the host may respond by cold-seeking behavior. Thermoregulation is not only ecologically important; it may affect the success of parasites and pathogens as biological control agents. We examined if Frankliniella occidentalis (Pergande) thermoregulates in response to infection by Beauveria bassiana, a fungal pathogen commonly used for biological control. Fungal-infected thrips preferentially moved to cooler areas (~12 °C) while healthy thrips sought out warmer temperatures (~24 °C). This cold-seeking behavior suppressed the growth of B. bassiana in infected thrips, and significantly improved survivorship of infected thrips. At 24 °C, males only survived up to 10 d after infection and females up to 20 d after infection, which was substantially poorer survivorship than that of corresponding healthy individuals. However, individuals of both sexes survived up to 48 d after infection at 12 °C, which was a much less severe reduction in survivorship compared with the effect of B. bassiana infection at 24 °C. The proportion of females among progeny from infected thrips at 12 °C was higher than at 24 °C. Therefore, cold-seeking behavior is beneficial to F. occidentalis when infected by B. bassiana, and its effects should be considered in the use of B. bassiana in biological control programs.

To cool or not to cool? Intestinal coccidians disrupt the behavioral hypothermia of lizards in response to tick infestation

It is generally accepted that parasites exert negative effects on their hosts and that natural selection favors specific host responses that mitigate this impact. It is also known that some components of the host immune system often co-evolve with parasite antigens resulting in a host-parasite arms race. In addition to immunological components of the anti-parasitic response, host behavioral responses are also important in this arms race and natural selection may favor avoidance strategies that preclude contact with parasites, or shifts in the host's thermoregulatory strategy to combat active infections (e.g., behavioral fever). Ticks are widespread parasites with direct and indirect costs on their vertebrate hosts. Their saliva provokes hemolysis in the blood of their hosts and can transmit a plethora of tick-borne pathogens. We enquired whether tick infestation by Ixodes pacificus can provoke a thermoregulatory response in Sceloporus occidentalis. For this, we compared the thermoregulatory behavior of tick-infested lizards against tick-infested lizards co-infected with two different species of coccidians (Lankesterella occidentalis and Acroeimeria sceloporis). After this, lizards were kept in individual terraria with a basking spot and fed ad libitum. We found that tick-infested lizards sought cooler temperatures in proportion to their tick load, and this response was independent of the co-infection status by L. occidentalis. This was consistent in April and June (when tick loads were significantly lower) and suggests a conservative strategy to save energy which might have been selected to overcome tick infestations during phenological peaks of this parasite. However, this behavior was not observed in lizards co-infected with A. sceloporis, suggesting that co-infection with this intestinal parasite prompt lizards to be active. Cost of tick infestation was confirmed because housed lizards lost weight at a constant ratio to initial tick load, independently of other infections. The broader implications of these findings are discussed in the context of climate change.

Behavioural fever reduces ranaviral infection in toads

1. Host behaviour is known to influence disease dynamics. Additionally, hosts often change their behaviours in response to pathogen detection to resist and avoid disease. The capacity of wildlife populations to respond to pathogens using behavioural plasticity is critical for reducing the impacts of disease outbreaks. However, there is limited information regarding the ability of ectothermic vertebrates to resist diseases via behavioural plasticity. 2. Here, we experimentally examine the effect of host behaviour on ranaviral infections, which affect at least 175 species of ectothermic vertebrates. We placed metamorphic (temporal block 1) or adult (block 2) Southern toads (Anaxyrus terrestris) in thermal gradients, tested their temperature preferences before and after oral inoculation by measuring individual-level body temperature over time, and measured ranaviral loads of viral-exposed individuals. 3. We found significant individual-level variation in temperature preference and evidence for behavioural fever in both metamorph and adult A. terrestris during the first two days after exposure. Additionally, we found that individual-level change in temperature preference was negatively correlated with ranaviral load and a better predictor of load than average temperature preference or maximum temperature reached by an individual. In other words, an increase in baseline temperature preference was more important than simply reaching an absolute temperature. 4. These results suggest that behavioural fever is an effective mechanism for resisting ranaviral infections.

Efficacy of Two Entomopathogenic Fungi, Metarhizium brunneum, Strain F52 Alone and Combined with Paranosema locustae against the Migratory Grasshopper, Melanoplus sanguinipes, under Laboratory and Greenhouse Conditions

Grasshopper outbreaks cause significant damage to crops and grasslands in US. Chemical control is widely used to suppress these pests but it reduces environmental quality. Biological control of insect pests is an alternative way to reduce the use of chemical insecticides. In this context, two entomopathogenic fungi, Metarhizium brunneum strain F52 and Paranosema locustae were evaluated as control agents for the pest migratory grasshopper Melanoplus sanguinipes under laboratory and greenhouse conditions. Third-instar grasshoppers, reared in the laboratory, were exposed up to fourteen days to wheat bran treated with different concentrations of each of the fungi alone or the two pathogens combined. In the greenhouse, nymphs were placed individually in cages where they were able to increase their body temperatures by basking in the sun in an attempt to inhibit the fungal infection, and treated with each pathogen alone or in combination. Mortality was recorded daily and presence of fungal outgrowth in cadavers was confirmed by recording fungal mycosis for two weeks’ post-treatment (PT). For combination treatment, the nature of the pathogen interaction (synergistic, additive, or antagonistic effects) was also determined. In laboratory conditions, all treatments except P. locustae alone resulted in grasshopper mortality. The application of the pathogen combinations caused 75% and 77%, mortality for lower and higher concentrations, respectively than each of the pathogens alone. We infer a synergistic effect occurred between the two agents. In greenhouse conditions, the highest mortalities were recorded in combination fungal treatments with a M. brunneum dose (60% mortality) and with a combination of the two pathogens in which M. brunneum was applied at high rate (50%) two weeks after application. This latter combination also exhibited a synergistic effect. Exposure to the P. locustae treatment did not lead to mortality until day 14 PT. We infer that these pathogens are promising for developing a biopesticide formulation for rangeland pest grasshopper management.

Sublethal effects of mixed fungal infections on the Moroccan locust, Dociostaurus maroccanus

The effects of single and mixed infections with Beauveria bassiana (EABb 90/2-Dm) and Metarhizium acridum (IMI 330189) strains on survival, feeding and reproduction of thermoregulating Dociostaurus maroccanus were evaluated. Adult locusts (2-3 days post fledging) were treated with low dosages of both fungal pathogens alone and in mixture (total dosage for single treatments and combinations = 1 × 10² and 1 × 10³ spores per insect). M. acridum IMI 330189 was more virulent than B. bassiana EABb 90/2-Dm at both dosages. In the mixed infections, in which half of the infective units of the more virulent pathogen were replaced by the less virulent pathogen, the analysis of the cumulative insect mortality after 30 days suggested additive interaction in the lethal effects between the two strains. All fungal treatments, except EABb 90/2-Dm at 1 × 10² spores per insect showed reduction in per capita feeding, as indicated by fecal production per insect per day when insects were maintained at 27 ± 2 °C (32-51% of reduction compared with the control); but only IMI 330189 caused significant reduction in per capita feeding (50%) when those insects were allowed to thermoregulate. Both strains and their mixtures caused a significant reduction of locust fecundity, with a 21-53% reduction in the number of egg-pods per female, and 30-65% reduction in the number of fertile eggs per female. In both sublethal effects (feeding and fecundity) a potential antagonistic interaction between the fungal strains was detected. Locust fecundity (egg-pods per female) and per capita feeding were positively correlated (r = 0.783). Implications of these findings on the potential use of both strains to control D. maroccanus populations are discussed.

Relationship of baseline and maximum glucocorticoid concentrations to migration propensity: a field test with wild subadult brown trout (Salmo trutta)

There is considerable variation in glucocorticoid (GC) baseline status and stress responses of individuals, yet the cause and consequence of this variation remains ambiguous. Attempts to relate GC levels to fitness and life-history trade-offs have yielded variable results. In this study, we evaluated whether baseline and poststressor GC hormone concentrations predicted migration strategy (i.e., resident or migrant) and successful seaward migration in a partially migrating population of juvenile brown trout (Salmo trutta Linnaeus, 1758). Baseline (N = 99) or poststressor (N = 102) plasma cortisol concentrations were obtained from brown trout and they were tagged with passive integrated transponder (PIT) and released in a natural Danish stream. Subsequently, fish were tracked with PIT reader systems and the stream was resampled for resident individuals. GC levels were not found to be associated with recapture of resident individuals or migration propensity to our first tracking station (S1), but increased baseline (and not poststressor) GC levels were associated with increased passage from S1 to our second tracking station, which anecdotally was an area of high predation or challenge. Our study found no evidence to suggest that cortisol regulates the migration life history in juvenile brown trout, but intermediate increases in baseline GC (and not poststressor GC) levels may favor migration performance.

Behavioral thermoregulation in Locusta migratoria manilensis (Orthoptera: Acrididae) in response to the entomopathogenic fungus, Beauveria bassiana

Insects such as locusts and grasshoppers can reduce the effectiveness of pathogens and parasites by adopting different defense strategies. We investigated the behavioral thermopreference of Locusta migratoria manilensis (Meyen) (Orthoptera: Acrididae) induced by the fungus Beauveria bassiana, and the impact this behavior had on the fungal mycosis under laboratory conditions. By basking in higher temperature locations, infected nymphs elevated their thoracic temperature to 30-32.6 °C, which is higher than the optimum temperature (25°C) for B. bassiana conidial germination and hyphal development. A minimum thermoregulation period of 3 h/day increased survival of infected locusts by 43.34%. The therapeutic effect decreased when thermoregulation was delayed after initial infection. The fungus grew and overcame the locusts as soon as the thermoregulation was interrupted, indicating that thermoregulation helped the insects to cope with infection but did not completely rid them of the fungus. A significant enhancement in the number of haemocytes was observed in infected thermoregulating locusts, reaching levels that were even higher than those observed in the controls. In contrast, haemocyte concentration was severely reduced in infected insects that did not thermoregulate. In infected non-thermoregulating locusts, the reduction in haemocyte number was accompanied by an increase in fungal blastospore concentration that was obvious in the haemolymph by day four. In contrast, no circulating blastospores were found in the haemolymph of infected thermoregulating locusts three days post-inoculation. We also examined the phagocytic activity of infected insects in vivo by using fluorescein isothiocyanate (FITC)-labelled silica beads. The proportion of beads that was engulfed by haemocytes in infected, thermoregulating insects was similar to that in the controls throughout the experiment, whereas the rate of phagocytosis in infected, non-thermoregulating insects progressively decreased after infection. These findings demonstrated that behavioural thermoregulation can adversely affect B. bassiana mycosis in infected L. migratoria manilensis, thereby limiting the development of lethal entomopathogenic fungi in locusts. This is apparently accomplished through an increase in the levels of haemocytes, leading to greater phagocytic activity under certain environmental conditions.

Dose-dependent behavioural fever responses in desert locusts challenged with the entomopathogenic fungus Metarhizium acridum

Behavioural fever is a common response to immune challenge in ectotherms and confers survival benefits. However, costs accrue rapidly as body temperature rises. Thus, the magnitude of adaptive fever responses might reflect the balance of costs and benefits. We investigated behavioural fever in desert locusts, Schistocerca gregaria, infected with the entomopathogenic fungus Metarhizium acridum. We first tracked the time course of behavioural fever in infected locusts, demonstrating that body temperatures rose on the day following inoculation (day 1), and reached peak intensity on the day after that (day 2). Subsequently, the magnitude of fever responses varied during a day, and locusts tended to exhibit high-intensity fever responses in the mornings when basking was first possible. We speculate that this may have resulted from increased fungal load caused by unimpeded growth overnight when locusts could not fever. We next inoculated locusts with different M. acridum doses ranging from 0 to ca. 75,000 conidia. The magnitude of their behavioural fever responses on day 2 post-inoculation was positively related to fungal dose. Thus, we demonstrate dose-dependency in the behavioural fever responses of desert locusts and suggest that this may reflect the adaptive deployment of behavioural fever to minimize costs relative to benefits.

Fungal infection dynamics in response to temperature in the lepidopteran insect Galleria mellonella

This study examines how the dynamics of fungus-insect interactions can be modulated by temperature. The wax moth, Galleria mellonella, is a well-studied and important model insect whose larvae in the wild develop optimally at around 34 °C in beehives. However, surprisingly little research on wax moths has been conducted at relevant temperatures. In this study, the entomopathogenic fungus Metarhizium robertsii inflicted rapid and substantial mortality on wax moth larvae maintained at a constant temperature of 24 °C, but at 34 °C a 10 fold higher dose was required to achieve an equivalent mortality. The cooler temperature favored fungal pathogenicity, with condial adhesion to the cuticle, germination and hemocoel invasion all significantly enhanced at 24 °C, compared with 34 °C. The wax moth larvae immune responses altered with the temperature, and with the infective dose of the fungus. Enzyme-based immune defenses (lysozyme and phenoloxidase) exhibited enhanced activity at the warmer temperature. A dramatic upregulation in the basal expression of galiomicin and gallerimycin was triggered by cooling, and this was augmented in the presence of the fungus. Profiling of the predominant insect epicuticular fatty acids revealed a 4-7 fold increase in palmetic, oleic and linoleic acids in larvae maintained at 24 °C compared with those at 34 °C, but these failed to exert fungistatic effects on topically applied fungus. This study demonstrates the importance of choosing environmental conditions relevant to the habitat of the insect host when determining the dynamics and outcome of insect/fungus interactions, and has particular significance for the application of entomopathogens as biocontrol agents.

Impact of rearing temperature on encapsulation and the accumulation of transcripts putatively involved in capsule formation in a parasitized lepidopteran host

Encapsulation and melanisation are innate immune reactions of insects against foreign intruders such as parasitoids. In an earlier study, we observed that immature life stages of the endoparasitoid Tranosema rostrale (Hymenoptera: Ichneumonidae) parasitizing Choristoneura fumiferana (Lepidoptera: Tortricidae) larvae experienced higher mortality due to encapsulation and melanisation when reared at high (30 °C) than at lower (10 °C, 20 °C) temperatures. Downregulation of T. rostrale polydnavirus genes in parasitized hosts and upregulation of two genes involved in the spruce budworm's melanisation process were identified as likely contributors to parasitoid mortality at high temperature. However, levels of transcripts of genes involved in the spruce budworm's cellular encapsulation process were not measured inasmuch as candidate genes, in the spruce budworm, had not yet been identified. In addition, our assessment of temperature-dependent encapsulation and melanisation of foreign objects in spruce budworm larvae was only partial. To fill these knowledge gaps, we injected Sephadex™ beads into unparasitized spruce budworm larvae and assessed their encapsulation/melanisation after the insects had been held at three different temperatures (10, 20, and 30 °C), and we identified spruce budworm genes putatively involved in the encapsulation process and quantified their transcripts at the same three temperatures, using a qPCR approach. As expected, both encapsulation and melanisation of Sephadex™ beads increased as a function of temperature. At the molecular level, three of the five genes examined (Integrin β1, Hopscotch, Stat92E) clearly displayed temperature-dependent upregulation. The results of this study further support the hypothesis that a temperature-dependent increase in the encapsulation response of C. fumiferana against T. rostrale is due to the combined effects of reduced expression of polydnavirus genes and enhanced expression of host immune genes.

Species of the Metarhizium anisopliae complex with diverse ecological niches display different susceptibilities to antifungal agents

Species of the Metarhizium anisopliae complex are globally ubiquitous soil-inhabiting and predominantly insect-pathogenic fungi. The Metarhizium genus contains species ranging from specialists, such as Metarhizium acridum that only infects acridids, to generalists, such as M. anisopliae, Metarhizium brunneum, and Metarhizium robertsii that infect a broad range of insects and can also colonize plant roots. There is little information available about the susceptibility of Metarhizium species to clinical and non-clinical antifungal agents. We determined the susceptibility of 16 isolates comprising four Metarhizium species with different ecological niches to seven clinical (amphotericin B, ciclopirox olamine, fluconazole, griseofulvin, itraconazole, tebinafine, and voriconazole) and one non-clinical (benomyl) antifungal agents. All isolates of the specialist M. acridum were clearly more susceptible to most antifungals than the isolates of the generalists M. anisopliae sensu lato, M. brunneum, and M. robertsii. All isolates of M. anisopliae, M. brunneum, and M. robertsii were resistant to fluconazole and some were also resistant to amphotericin B. The marked differences in susceptibility between the specialist M. acridum and the generalist Metarhizium species suggest that this characteristic is associated with their different ecological niches, and may assist in devising rational antifungal treatments for the rare cases of mycoses caused by Metarhizium species.

Behavioural effects of temperature on ectothermic animals: unifying thermal physiology and behavioural plasticity

Temperature imposes significant constraints on ectothermic animals, and these organisms have evolved numerous adaptations to respond to these constraints. While the impacts of temperature on the physiology of ectotherms have been extensively studied, there are currently no frameworks available that outline the multiple and often simultaneous pathways by which temperature can affect behaviour. Drawing from the literature on insects, we propose a unified framework that should apply to all ectothermic animals, generalizing temperature's behavioural effects into: (1) kinetic effects, resulting from temperature's bottom-up constraining influence on metabolism and neurophysiology over a range of timescales (from short to long term), and (2) integrated effects, where the top-down integration of thermal information intentionally initiates or modifies a behaviour (behavioural thermoregulation, thermal orientation, thermosensory behavioural adjustments). We discuss the difficulty in distinguishing adaptive behavioural changes from constraints when observing animals' behavioural responses to temperature. We then propose two complementary approaches to distinguish adaptations from constraints, and categorize behaviours according to our framework: (i) 'kinetic null modelling' of temperature's effects on behaviour; and (ii) behavioural ecology experiments using temperature-insensitive mutants. Our framework should help to guide future research on the complex relationship between temperature and behaviour in ectothermic animals.

Sickness behaviour in the cricket Gryllus texensis: Comparison with animals across phyla

Immune activation alters behaviour (i.e. sickness behaviour) in animals across phyla and is thought to aid recovery from infection. Hypotheses regarding the adaptive function of different sickness behaviours (e.g. decreased movement and appetite) include the energy conservation and predator avoidance hypotheses. These hypotheses were originally developed for mammals (e.g. Hart, 1988), however similar sickness behaviours are also observed in insects (e.g., crickets). We predicted that immune-challenged crickets (Gryllus texensis) would reduce feeding, grooming, and locomotion as well as increase shelter use, consistent with the energy conservation and predator avoidance hypotheses. We found evidence of illness-induced anorexia in adult and juvenile crickets, consistent with previous research (Adamo et al., 2010), but contrary to expectations, we found an increase in grooming, and no evidence that crickets decreased locomotion or increased shelter use in response to immune challenge. Therefore, our results do not support the energy conservation or predator avoidance hypotheses. The difference in sickness behaviour between insects and mammals is probably due, in part, to the lack of physiological fever in insects. We hypothesize that the lack of physiological fever reduces the need for energy conservation, decreasing the benefits of some sickness behaviours such as increased shelter use. These results, taken together with others in the literature, suggest that ectotherms and endotherms may differ significantly in the selective forces leading to the evolution of most sickness behaviours.

Drosophila melanogaster does not exhibit a behavioural fever response when infected with Drosophila C virus

Behavioural fever is a widely conserved response to infection. The host increases body temperature (T b) by altering their preferred temperature (T p), generating a fever and delaying or avoiding pathogen-induced mortality. This response is not ubiquitous in insects, however, although few studies have investigated this response to viral infection. Here, we examined the change in T p of Drosophila in response to virus infection using a thermal gradient. No difference in T p was observed. We suggest that the lack of behavioural fever could be due to the increased energy cost of maintaining a higher T b whilst the immune response is active. To the best of our knowledge, this is the first study to assay for changes in T p of infected Drosophila.

Impact Of Environmental Variation On Host Performance Differs With Pathogen Identity: Implications For Host-Pathogen Interactions In A Changing Climate

Specialist and generalist pathogens may exert different costs on their hosts; thereby altering the way hosts cope with environmental variation. We examined how pathogen-challenge alters the environmental conditions that maximize host performance by simultaneously varying temperature and nutrition (protein to carbohydrate ratio; P:C) after exposure to two baculoviruses; one that is specific to the cabbage looper, Trichoplusia ni (TnSNPV) and another that has a broad host range (AcMNPV). Virus-challenged larvae performed better on more protein-biased diets, primarily due to higher survival, whereas unchallenged larvae performed best on a balanced diet. The environmental conditions that maximized host performance differed with virus identity because TnSNPV-challenge inflicted fitness costs (reduced pupal weight and prolonged development) whereas AcMNPV-challenge did not. The performance of TnSNPV-challenged larvae rose with increasing P:C across all temperatures, whereas temperature modulated the optimal P:C in AcMNPV-challenged larvae (slightly protein-biased at 16 °C to increasingly higher P:C as temperature increased). Increasing temperature reduced pupal size, but only at more balanced P:C ratios, indicating that nutrition moderates the temperature-size rule. Our findings highlight the complex environmental interactions that can alter host performance after exposure to pathogens, which could impact the role of entomopathogens as regulators of insect populations in a changing climate.

Heat-induced post-stress growth delay: a biological trait of many Metarhizium isolates reducing biocontrol efficacy?

The habitats of many pest insects have fluctuating climatic conditions. To function effectively, the pathogens of these pests must be capable of infecting and developing disease at a wide range of temperatures. The current study examines ten Metarhizium spp. isolates as to their ability to recover normal metabolic activity after exposure to high temperature for several hours daily; and whether such recovery, with at least some isolates, requires a temporary repair ("retooling") period. Fungal colonies were exposed to 40°C for 4h or 8h followed by 20h or 16h at 28°C, respectively, for three consecutive days. Growth rates during treatments were compared to control plates (constant 28°C) and to plates with growth stoppage by cold treatment (4h or 8h at 5°C per day). All ten isolates survived 3days of cycled heat treatment and resumed normal growth afterward; some isolates however, were considerably more negatively affected by heat-cycling than others. In fact, some isolates underwent greatly reduced growth not only during 8h heating, but also some hours after cessation of heat treatment. This phenomenon is labeled in the current study as "post-stress growth delay" (PSGD). In contrast, all isolates stopped growing during 8h cold treatments, but immediately recommenced growing on return to 28°C. The delay in recommencing growth of some isolates after heat treatment amplifies the effect of this stress. In addition to the studies on the effects of heat cycling on fungal cultures, the effects of imposing such temperature cycling on fungal infection of insects was documented in the laboratory. Three Metarhizium isolates were bioassayed using Galleria mellonella larvae. Treated insects were placed at daily temperature regimes matching those used for the in vitro fungus rate-of-growth study, and insect mortality recorded daily. For all three isolates the levels of insect mortality at the highest-heat dose (40°C at 8h daily) significantly reduced infection. Fluctuating temperatures are likely to be a factor in most pest-insect habitats; therefore, the presence and level of PSGD of each isolate should be a primary consideration in selecting field-appropriate fungal isolates.

Thermoregulatory behavior and fungal infection of Anoplophora glabripennis (Coleoptera: Cerambycidae)

Asian longhorned beetles, Anoplophora glabripennis (Motschulsky), are invasive wood borers that are native to China and Korea but have been introduced to North America and Europe. These beetles have great potential to negatively impact economic and environmental interests in hardwood and urban forests if they become established. The entomopathogenic fungus Metarhizium brunneum Petch (previously Metarhizium anisopliae (Metschnikoff) Sorokin) is under development for control of A. glabripennis. Some insect species eliminate pathogens or delay disease progression through thermoregulation. Because Asian longhorned beetles had been observed occupying sunlit areas of the tree canopy, we hypothesized that behavioral fevering could be used to delay mortality of fungal-infected beetles. M. brunneum cultures incubated at 34°C for 5 h/d grew significantly slower compared with cultures incubated at lower temperatures. Holding M. brunneum-infected A. glabripennis at 34°C for 5 h/d significantly delayed mortality by 2 d compared with infected beetles held at ≤31°C. Adult A. glabripennis did not exhibit behavioral fever when infected. Uninfected males, when provided with food, and both uninfected males and females when deprived of food, slightly increased their preferred temperature over time. When held at 15°C before being placed into temperature gradients, uninfected beetles did not increase their temperatures above ambient. Results demonstrate that M. brunneum-infected A. glabripennis do not exhibit behaviors necessary to elevate their body temperatures enough to combat M. brunneum infections through thermoregulation.

Effect of temperature and humidity on pathogenicity of native Beauveria bassiana isolate against Musca domestica L

Beauveria bassiana HQ917687 virulence to housefly larvae and adult was assessed at different relative humidity, RH (50, 75, 90, and 100 %) and temperature (15, 20, 25, 30, 35, 40, 45 °C) conditions at the fungal dose of 10(8) conidia/ml. Depending on the temperature and RH regime tested, difference in mortality rates of housefly adult and larvae were detected. During assay on adult housefly, 100 % mortality was achieved at RH, 90 and 100 % while the temperature of 30 °C showed maximum mortality at all the tested humidity conditions. Lethal time, LT50 was 2.9 days at 100 % RH. Larval mortality at different humidity conditions varied between 30 and 74 %, with maximum mortality at 100 % RH and 30 °C. Optimum temperature for B. bassiana virulence to housefly larvae was also found to be 30 °C. The interaction between temperature and RH revealed significant effect of RH at moderate temperature range (20-35 °C), while such an interaction was not observed at extreme temperatures. The results obtained in this study have useful implications in understanding the pathogen behavior under actual field conditions. This in turn may help devising suitable entomopathogen release schedules for maximum fungal infection.

Internal receptors in insect appendages project directly into a special brain neuropile

Background

The great majority of afferent neurons of insect legs project into their segmental ganglion. Intersegmental projections are rare and are only formed by sense organs associated with the basal joints of the legs. Such intersegmental projections never ascend as far as the brain and they form extensive ramifications within thoracic ganglia. A few afferents of chordotonal organs of the subcoxal joints ascend as far as the suboesophageal ganglion.

Results

We describe novel afferent neurons in distal segments of locust legs that project directly into the brain without forming ramifications in other ganglia. In the brain, the fibres terminate with characteristic terminals in a small neuropile previously named the superficial ventral inferior protocerebrum. The somata of these neurons are located in the tibiae and tarsi of all legs and they are located within branches of peripheral nerves, or closely associated with such branches. They are not associated with any accessory structures such as tendons or connective tissue strands as typical for insect internal mechanoreceptors such as chordotonal organs or stretch receptors. Morphologically they show great similarity to certain insect infrared receptors.

We could not observe projections into the superficial ventral inferior protocerebrum after staining mandibular or labial nerves, but we confirm previous studies that showed projections into the same brain neuropile after staining maxillary and antennal nerves, indicating that most likely similar neurons are present in these appendages also.

Conclusion

Because of their location deep within the lumen of appendages the function of these neurons as infrared receptors is unlikely. Their projection pattern and other morphological features indicate that the neurons convey information about an internal physiological parameter directly into a special brain neuropile. We discuss their possible function as thermoreceptors.

Behavioral Immunity in Insects

Parasites can dramatically reduce the fitness of their hosts, and natural selection should favor defense mechanisms that can protect hosts against disease. Much work has focused on understanding genetic and physiological immunity against parasites, but hosts can also use behaviors to avoid infection, reduce parasite growth or alleviate disease symptoms. It is increasingly recognized that such behaviors are common in insects, providing strong protection against parasites and parasitoids. We review the current evidence for behavioral immunity in insects, present a framework for investigating such behavior, and emphasize that behavioral immunity may act through indirect rather than direct fitness benefits. We also discuss the implications for host-parasite co-evolution, local adaptation, and the evolution of non-behavioral physiological immune systems. Finally, we argue that the study of behavioral immunity in insects has much to offer for investigations in vertebrates, in which this topic has traditionally been studied.

The inhibitory effect of the fungal toxin, destruxin A, on behavioural fever in the desert locust

During an infection locusts behaviourally fever by seeking out higher environmental temperatures. This behaviour places the pathogen at sub-optimal growth temperatures while improving the efficiency of the immune system, thereby prolonging the lifespan of the host. It is therefore in the interest of the pathogen to either adapt to fever-like temperatures or to evolve mechanisms to interfere with, or inhibit fever. We investigated the behavioural fever response of desert locusts to two fungal pathogens. A prolonged fever was observed in locusts infected with Metarhizium acridum. However, fever was comparatively short-lived during infection with Metarhizium robertsii. In both cases restriction of thermoregulation reduced lifespan. Destruxin A (dtx A) produced by M. robertsii, but not M. acridum has previously been associated with the inhibition of the insect immune system. Injection of dtx A during infection with the fever-causing M. acridum inhibited fever and was particularly effective when administered early on in infection. Furthermore, locusts injected with dtx A were more susceptible to M. acridum infection. Therefore engineering M. acridum isolates currently used for locust biocontrol, to express dtx A may improve efficiency of control by interfering with fever.

Thermal behaviour of Anopheles stephensi in response to infection with malaria and fungal entomopathogens

Background

Temperature is a critical determinant of the development of malaria parasites in mosquitoes, and hence the geographic distribution of malaria risk, but little is known about the thermal preferences of Anopheles. A number of other insects modify their thermal behaviour in response to infection. These alterations can be beneficial for the insect or for the infectious agent. Given current interest in developing fungal biopesticides for control of mosquitoes, Anopheles stephensi were examined to test whether mosquitoes showed thermally-mediated behaviour in response to infection with fungal entomopathogens and the rodent malaria, Plasmodium yoelii.

Methods

Over two experiments, groups of An. stephensi were infected with one of three entomopathogenic fungi, and/or P. yoelii. Infected and uninfected mosquitoes were released on to a thermal gradient (14 – 38°C) for "snapshot" assessments of thermal preference during the first five days post-infection. Mosquito survival was monitored for eight days and, where appropriate, oocyst prevalence and intensity was assessed.

Results and conclusion

Both infected and uninfected An. stephensi showed a non-random distribution on the gradient, indicating some capacity to behaviourally thermoregulate. However, chosen resting temperatures were not altered by any of the infections. There is thus no evidence that thermally-mediated behaviours play a role in determining malaria prevalence or that they will influence the performance of fungal biopesticides against adult Anopheles.

Intraspecific tolerance of Metarhizium anisopliae conidia to the upper thermal limits of summer with a description of a quantitative assay system

Conidial tolerance to the upper thermal limits of summer is important for fungal biocontrol agents, whose conidia are formulated into mycoinsecticides for field application. To develop an efficient assay system, aerial conidia of eight Metarhizium anisopliae, four M. anisopliae var. anisopliae, and six M. anisopliae var. acridum isolates with different host and geographic origins were wet-stressed for <or=180 min at 48 degrees C or incubated for 14 d colony growths at 10-35 degrees C. The survival ratios (relative to unstressed conidia) of each isolate, examined at 15-min intervals, fit a logistic equation (r2>or=0.975), yielding median lethal times (LT50s) of 14.3-150.3 min for the 18 isolates stressed at 48 degrees C. Seven grasshopper isolates from Africa had a mean LT50 of 110 (73-150) min, but could not grow at 10 or 15 degrees C. The mean LT50 of five non-grasshopper isolates capable of growing at 10-35 degrees C was 16 (10-26) min only. Three isolates with typically low (type I), medium (type II), and high (type III) levels of tolerance to 48 degrees C were further assayed for <or=4-d tolerance of their conidia to the wet stress at 38, 40, 42, or 45 degrees C. The resultant LT50s decreased to 20, 53 and 167 min at 48 degrees C from 507, 1612, and 8256 min at 38 degrees C for types I, II and III, respectively. For the distinguished types, the logarithms of the LT50s were significantly correlated to the temperatures of 38-48 degrees C with an inverse linearity (r2>or=0.88). The method developed to assay quantitatively fungal thermotolerance would be useful for screening of fungal candidates for improved pest control in summer.

Physiology of temperature regulation: comparative aspects

Few environmental factors have a larger influence on animal energetics than temperature, a fact that makes thermoregulation a very important process for survival. In general, endothermic species, i.e., mammals and birds, maintain a constant body temperature (Tb) in fluctuating environmental temperatures using autonomic and behavioural mechanisms. Most of the knowledge on thermoregulatory physiology has emerged from studies using mammalian species, particularly rats. However, studies with all vertebrate groups are essential for a more complete understanding of the mechanisms involved in the regulation of Tb. Ectothermic vertebrates-fish, amphibians and reptiles-thermoregulate essentially by behavioural mechanisms. With few exceptions, both endotherms and ectotherms develop fever (a regulated increase in Tb) in response to exogenous pyrogens, and regulated hypothermia (anapyrexia) in response to hypoxia. This review focuses on the mechanisms, particularly neuromediators and regions in the central nervous system, involved in thermoregulation in vertebrates, in conditions of euthermia, fever and anapyrexia.

Bizarre interactions and endgames: entomopathogenic fungi and their arthropod hosts

Invertebrate pathogens and their hosts are taxonomically diverse. Despite this, there is one unifying concept relevant to all such parasitic associations: Both pathogen and host adapt to maximize their own reproductive output and ultimate fitness. The strategies adopted by pathogens and hosts to achieve this goal are almost as diverse as the organisms themselves, but studies examining such relationships have traditionally concentrated only on aspects of host physiology. Here we review examples of host-altered behavior and consider these within a broad ecological and evolutionary context. Research on pathogen-induced and host-mediated behavioral changes demonstrates the range of altered behaviors exhibited by invertebrates including behaviorally induced fever, elevation seeking, reduced or increased activity, reduced response to semiochemicals, and changes in reproductive behavior. These interactions are sometimes quite bizarre, intricate, and of great scientific interest.