Within and between population variation in disease resistance in cyclic populations of western tent caterpillars: a test of the disease defence hypothesis

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

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

Warmer temperatures reduce the transmission of a virus in a gregarious forest insect

Understanding how climate warming will influence species interactions is a key question in ecology and predicting changes in the prevalence of disease outbreaks is particularly challenging. Ectotherms are likely to be more influenced by climatic changes as temperature governs their growth, feeding, development, and behavior. We test the hypothesis that pathogen transmission and host mortality will increase at warmer temperatures using a cyclic forest insect, the western tent caterpillar (WTC), Malacosoma californicum pluviale, and its baculovirus. The virus causes population declines at peak host density. WTC are gregarious and clustering is predicted to increase the risk of within family infection; however, how temperature influences this has not been examined. We investigated the impact of temperature on different components of the transmission process in order to pinpoint the possible mechanisms involved. In the laboratory, leaf consumption increased linearly with rising temperature between 15 and 30°C. Insects died more rapidly from virus infection as temperature increased, but this did not translate into differences in the production of viral transmission stages. To examine the influence of temperature on virus transmission, we created a temperature difference between two greenhouses containing potted red alder trees, Alnus rubra. The cooler greenhouse (mean 19.5°C) was roughly similar to ambient temperatures in the field, while the warmer greenhouse was 10°C higher (mean 29°C). As predicted, both larval movement and feeding were higher at the warmer temperature, while the likelihood of the preinfected, inoculum larvae dying on the tents was twice as high in the cooler greenhouse. This resulted in increased virus mortality and a higher transmission parameter under cooler conditions. Therefore, we suggest that, contrary to our prediction, the reduced movement of infected larvae at colder temperatures increased the risk of infection in these gregarious insects and had a greater impact on virus transmission than the increased activity of the susceptible larvae in warmer conditions. Long-term population data from the field, however, show no relationship between temperature and infection levels, suggesting that local changes in virus transmission might not scale up to population infection levels.

Parasite dynamics in North American monarchs predicted by host density and seasonal migratory culling

Insect-pathogen dynamics can show seasonal and inter-annual variation that covaries with fluctuations in insect abundance and climate. Long-term analyses are especially needed to track parasite dynamics in migratory insects, in part because their vast habitat ranges and high mobility might dampen local effects of density and climate on infection prevalence. Monarch butterflies (Danaus plexippus) are commonly infected with the protozoan Ophryocystis elektroscirrha (OE). Because this parasite lowers monarch survival and flight performance, and because migratory monarchs have experienced declines in recent decades, it is important to understand patterns and drivers of infection. 3. Here we compiled data on OE infection spanning 50 years, from wild monarchs sampled in the USA, Canada, and Mexico during summer breeding, fall migrating, and overwintering periods. We examined eastern versus western North American monarchs separately, to ask how abundance estimates, resource availability, climate, and breeding season length impact infection trends. We further assessed the intensity of migratory culling, which occurs when infected individuals are removed from the population during migration. 4. Average infection prevalence was four times higher in western compared to eastern subpopulations. In eastern North America, the proportion of infected monarchs increased three-fold since the mid-2000s. In the western region, the proportion of infected monarchs declined sharply from 2000-2015, and increased thereafter. For both eastern and western subpopulations, years with greater summer adult abundance predicted greater infection prevalence, indicating that transmission increases with host breeding density. Environmental variables (temperature and NDVI) were not associated with changes in infected adults. We found evidence for migratory culling of infected butterflies, based on declines in parasitism during fall migration. We estimated that tens of millions fewer monarchs reach overwintering sites in Mexico as a result of OE, highlighting the need to consider the parasite as a potential threat to the monarch population. 5. Increases in infection among eastern North American monarchs post-2002 suggest that changes to the host's ecology or environment have intensified parasite transmission. Further work is needed to examine the degree to which human practices, such as mass caterpillar rearing and the widespread planting of exotic milkweed, have contributed to this trend.

Confirmation of Oryctes rhinoceros nudivirus infections in G-haplotype coconut rhinoceros beetles (Oryctes rhinoceros) from Palauan PCR-positive populations

Coconut rhinoceros beetle (CRB), Oryctes rhinoceros, is a pest of palm trees in the Pacific. Recently, a remarkable degree of palm damage reported in Guam, Hawaii, Papua New Guinea and Solomon Islands has been associated with a particular haplotype (clade I), known as "CRB-G". In the Palau Archipelago, both CRB-G and another haplotype (clade IV) belonging to the CRB-S cluster coexist in the field. In this study, more than 75% of pheromone trap-captured adults of both haplotypes were Oryctes rhinoceros nudivirus (OrNV)-positive by PCR. No significant difference in OrNV prevalence between the haplotypes was detected. In PCR-positive CRB-G tissue specimens from Palau, viral particles were observed by electron microscopy. Hemocoel injection of CRB larvae with crude virus homogenates from these tissues resulted in viral infection and mortality. OrNV isolated from Palauan-sourced CRB was designated as OrNV-Palau1. Both OrNV-Palau1 and OrNV-X2B, a CRB biological control isolate released in the Pacific, were propagated using the FRI-AnCu-35 cell line for production of inoculum. However, the OrNV-Palau1 isolate exhibited lower viral production levels and longer larval survival times compared to OrNV-X2B in O. rhinoceros larvae. Full genome sequences of the OrNV-Palau1 and -X2B isolates were determined and found to be closely related to each other. Altogether these results suggest CRB adults in Palau are infected with a less virulent virus, which may affect the nature and extent of OrNV-induced pathology in Palauan populations of CRB.

The paradoxical rarity of a fruit fly fungus attacking a broad range of hosts

Understanding the factors that determine the realized and potential distribution of a species requires knowledge of abiotic, physiological, limitations as well as ecological interactions. Fungi of the order Laboulbeniales specialize on arthropods and are typically thought to be highly specialized on a single species or closely related group of species. Because infections are almost exclusively transmitted through direct contact between the hosts, the host ecology, to a large extent, determines the distribution and occurrence of the fungus. We examined ~20,000 fruit flies (Diptera: Dacinae) collected in Malaysia, Sulawesi, Australia, and the Solomon Islands between 2017 and 2019 for fungal infections and found 197 infected flies across eight different Bactrocera species. Morphology and 1,363 bps of small subunit (18S) DNA sequences both support that the infections are from a single polyphagous fungal species Stigmatomyces dacinus-a known ectoparasite of these fruit flies. This leads to the question: why is S. dacinus rare, when its hosts are widespread and abundant? In addition, the hosts are all Bactrocera, a genus with ~480 species, but 37 Bactrocera species found sympatric with the hosts were never infected. Host-selection does not appear to be phylogenetically correlated. These results suggest a hidden complexity in how different, but closely related, host species vary in their susceptibility, which somehow limits the abundance and dispersal capability of the fungus.

Separate seasons of infection and reproduction can lead to multi-year population cycles

Many host-pathogen systems are characterized by a temporal order of disease transmission and host reproduction. For example, this can be due to pathogens infecting certain life cycle stages of insect hosts; transmission occurring during the aggregation of migratory birds; or plant diseases spreading between planting seasons. We develop a simple discrete-time epidemic model with density-dependent transmission and disease affecting host fecundity and survival. The model shows sustained multi-annual cycles in host population abundance and disease prevalence, both in the presence and absence of density dependence in host reproduction, for large horizontal transmissibility, imperfect vertical transmission, high virulence, and high reproductive capability. The multi-annual cycles emerge as invariant curves in a Neimark-Sacker bifurcation. They are caused by a carry-over effect, because the reproductive fitness of an individual can be reduced by virulent effects due to infection in an earlier season. As the infection process is density-dependent but shows an effect only in a later season, this produces delayed density dependence typical for second-order oscillations. The temporal separation between the infection and reproduction season is crucial in driving the cycles; if these processes occur simultaneously as in differential equation models, there are no sustained oscillations. Our model highlights the destabilizing effects of inter-seasonal feedbacks and is one of the simplest epidemic models that can generate population cycles.

Chrysodeixis chalcites nucleopolyhedrovirus (ChchNPV): Natural occurrence and efficacy as a biological insecticide on young banana plants in greenhouse and open-field conditions on the Canary Islands

Chrysodeixis chalcites, an important pest of banana crops on the Canary Islands, is usually controlled by chemical insecticides. The present study aimed to evaluate the efficacy of the most prevalent isolate of the Chrysodeixis chalcites nucleopolyhedrovirus (ChchNPV, Baculoviridae) as a biological insecticide. Overall the prevalence of ChchNPV infection in C. chalcites populations was 2.3% (103 infected larvae out of 4,438 sampled), but varied from 0–4.8% on Tenerife and was usually low (0–2%) on the other islands. On Tenerife, infected larvae were present at 11 out of 17 plantations sampled. The prevalence of infection in larvae on bananas grown under greenhouse structures was significantly higher (3%) than in open-field sites (1.4%). The ChchNPV-TF1 isolate was the most abundant and widespread of four genetic variants of the virus. Application of 1.0x10⁹ viral occlusion bodies (OBs)/l of ChchNPV-TF1 significantly reduced C. chalcites foliar damage in young banana plants as did commonly used pesticides, both in greenhouse and open-field sites. The insecticidal efficacy of ChchNPV-TF1 was similar to that of indoxacarb and a Bacillus thuringiensis (Bt)-based insecticide in one year of trials and similar to Bt in the following year of trails in greenhouse and field crops. However, larvae collected at different time intervals following virus treatments and reared in the laboratory experienced 2–7 fold more mortality than insects from conventional insecticide treatments. This suggests that the acquisition of lethal dose occurred over an extended period (up to 7 days) compared to a brief peak in larvae on plants treated with conventional insecticides. These results should prove useful for the registration of a ChchNPV-based insecticide for integrated management of this pest in banana crops on the Canary Islands.

Covert Infection of Insects by Baculoviruses

Baculoviruses (Baculoviridae) are occluded DNA viruses that are lethal pathogens of the larval stages of some lepidopterans, mosquitoes, and sawflies (phytophagous Hymenoptera). These viruses have been developed as biological insecticides for control of insect pests and as expression vectors in biotechnological applications. Natural and laboratory populations frequently harbor covert infections by baculoviruses, often at a prevalence exceeding 50%. Covert infection can comprise either non-productive latency or sublethal infection involving low level production of virus progeny. Latency in cell culture systems involves the expression of a small subset of viral genes. In contrast, covert infection in lepidopterans is associated with differential infection of cell types, modulation of virus gene expression and avoidance of immune system clearance. The molecular basis for covert infection may reside in the regulation of host-virus interactions through the action of microRNAs (miRNA). Initial findings suggest that insect nudiviruses and vertebrate herpesviruses may provide useful analogous models for exploring the mechanisms of covert infection by baculoviruses. These pathogens adopt mixed-mode transmission strategies that depend on the relative fitness gains that accrue through vertical and horizontal transmission. This facilitates virus persistence when opportunities for horizontal transmission are limited and ensures virus dispersal in migratory host species. However, when host survival is threatened by environmental or physiological stressors, latent or persistent infections can be activated to produce lethal disease, followed by horizontal transmission. Covert infection has also been implicated in population level effects on host-pathogen dynamics due to the reduced reproductive capacity of infected females. We conclude that covert infections provide many opportunities to examine the complexity of insect-virus pathosystems at the organismal level and to explore the evolutionary and ecological relationships of these pathogens with major crop and forest pests.

Sources of variance in immunological traits: evidence of congruent latitudinal trends across species

Among-population differences in immunological traits allow assessment of both evolutionary and plastic changes in organisms' resistance to pathogens. Such knowledge also provides information necessary to predict responses of such traits to environmental changes. Studies on latitudinal trends in insect immunity have so far yielded contradictory results, suggesting that multispecies approaches with highly standardised experimental conditions are needed. Here, we studied among-population differences of two parameters reflecting constitutive immunity-phenoloxidase (PO) and lytic activity, using common-garden design on three distantly related moth species represented by populations ranging from northern Finland to Georgia (Caucasus). The larvae were reared at different temperatures and on different host plants under a crossed factors experimental design. Haemolymph samples for measurement of immune status were taken from the larvae strictly synchronously. Clear among-population differences could be shown only for PO activity in one species (elevated activity in the northern populations). There was some indication that the cases of total absence of lytic activity were more common in southern populations. The effects of temperature, host and sex on the immunological traits studied remained highly species specific. Some evidence was found that lytic activity may be involved in mediating trade-offs between immunity and larval growth performance. In contrast, PO activity rarely covaried with fitness-related traits, and neither were the values of PO and lytic activity correlated with each other. The relatively inconsistent nature of the detected patterns suggests that studies on geographic differences in immunological traits should involve multiple species, and rely on several immunological indices if general trends are a point of interest.

Eco-Evolutionary Theory and Insect Outbreaks

Eco-evolutionary theory argues that population cycles in consumer-resource interactions are partly driven by natural selection, such that changes in densities and changes in trait values are mutually reinforcing. Evidence that the theory explains cycles in nature, however, is almost nonexistent. Experimental tests of model assumptions are logistically impractical for most organisms, while for others, evidence that population cycles occur in nature is lacking. For insect baculoviruses in contrast, tests of model assumptions are straightforward, and there is strong evidence that baculoviruses help drive population cycles in many insects, including the gypsy moth that we study here. We therefore used field experiments with the gypsy moth baculovirus to test two key assumptions of eco-evolutionary models of host-pathogen population cycles: that reduced host infection risk is heritable and that it is costly. Our experiments confirm both assumptions, and inserting parameters estimated from our data into eco-evolutionary insect-outbreak models gives cycles closely resembling gypsy moth outbreak cycles in North America, whereas standard models predict unrealistic stable equilibria. Our work shows that eco-evolutionary models are useful for explaining outbreaks of forest insect defoliators, while widespread observations of intense selection on defoliators in nature and of heritable and costly resistance in defoliators in the lab together suggest that eco-evolutionary dynamics may play a general role in defoliator outbreaks.

Ecology and evolution of pathogens in natural populations of Lepidoptera

Pathogens are ubiquitous in insect populations and yet few studies examine their dynamics and impacts on host populations. We discuss four lepidopteran systems and explore their contributions to disease ecology and evolution. More specifically, we elucidate the role of pathogens in insect population dynamics. For three species, western tent caterpillars, African armyworm and introduced populations of gypsy moth, infection by nucleopolyhedrovirus (NPV) clearly regulates host populations or reduces their outbreaks. Transmission of NPV is largely horizontal although low levels of vertical transmission occur, and high levels of covert infection in some cases suggest that the virus can persist in a nonsymptomatic form. The prevalence of a mostly vertically transmitted protozoan parasite, Ophryocystis elektroscirrha, in monarch butterflies is intimately related to their migratory behaviour that culls highly infected individuals. Virulence and transmission are positively related among genotypes of this parasite. These systems clearly demonstrate that the interactions between insects and pathogens are highly context dependent. Not only is the outcome a consequence of changes in density and genetic diversity: environmental factors, particularly diet, can have strong impacts on virulence, transmission and host resistance or tolerance. What maintains the high level of host and pathogen diversity in these systems, however, remains a question.

Recurrent Domestication by Lepidoptera of Genes from Their Parasites Mediated by Bracoviruses

Bracoviruses are symbiotic viruses associated with tens of thousands of species of parasitic wasps that develop within the body of lepidopteran hosts and that collectively parasitize caterpillars of virtually every lepidopteran species. Viral particles are produced in the wasp ovaries and injected into host larvae with the wasp eggs. Once in the host body, the viral DNA circles enclosed in the particles integrate into lepidopteran host cell DNA. Here we show that bracovirus DNA sequences have been inserted repeatedly into lepidopteran genomes, indicating this viral DNA can also enter germline cells. The original mode of Horizontal Gene Transfer (HGT) unveiled here is based on the integrative properties of an endogenous virus that has evolved as a gene transfer agent within parasitic wasp genomes for ≈100 million years. Among the bracovirus genes thus transferred, a phylogenetic analysis indicated that those encoding C-type-lectins most likely originated from the wasp gene set, showing that a bracovirus-mediated gene flux exists between the 2 insect orders Hymenoptera and Lepidoptera. Furthermore, the acquisition of bracovirus sequences that can be expressed by Lepidoptera has resulted in the domestication of several genes that could result in adaptive advantages for the host. Indeed, functional analyses suggest that two of the acquired genes could have a protective role against a common pathogen in the field, baculovirus. From these results, we hypothesize that bracovirus-mediated HGT has played an important role in the evolutionary arms race between Lepidoptera and their pathogens.

Eukaryotes are generally thought to evolve mainly through the modification of existing genetic information. However, evidence of horizontal gene transfer (HGT) in eukaryotes-the accidental acquisition of a novel gene from another species, allowing acquisition of novel traits—is now recognized as an important factor in their evolution. We show here that in several lineages, lepidopteran genomes have acquired genes from a bracovirus that is symbiotically used by parasitic wasps to inhibit caterpillar host immune defences. Integration of parts of the viral genome into host caterpillar DNA strongly suggests that integration can sporadically occur in the germline, leading to the production of lepidopteran lineages that harbor bracovirus sequences. Moreover, some of the transferred bracovirus genes reported here originate from the wasp genome, demonstrating that a gene flux exists between the two insect orders Hymenoptera and Lepidoptera that diverged ≈300 MYA. As bracovirus gene organisation has evolved to allow expression in Lepidoptera, these transferred genes can be readily domesticated. Additionally, we present functional analyses suggesting that some of the acquired genes confer to caterpillars a protection toward baculovirus, a very common pathogen in the field. This phenomenon may have implications for understanding how caterpillars acquire resistance against baculoviruses used in biological control.

Effects of pathogen exposure on life-history variation in the gypsy moth (Lymantria dispar)

Investment in host defences against pathogens may lead to trade-offs with host fecundity. When such trade-offs arise from genetic correlations, rates of phenotypic change by natural selection may be affected. However, genetic correlations between host survival and fecundity are rarely quantified. To understand trade-offs between immune responses to baculovirus exposure and fecundity in the gypsy moth (Lymantria dispar), we estimated genetic correlations between survival probability and traits related to fecundity, such as pupal weight. In addition, we tested whether different virus isolates have different effects on male and female pupal weight. To estimate genetic correlations, we exposed individuals of known relatedness to a single baculovirus isolate. To then evaluate the effect of virus isolate on pupal weight, we exposed a single gypsy moth strain to 16 baculovirus isolates. We found a negative genetic correlation between survival and pupal weight. In addition, virus exposure caused late-pupating females to be identical in weight to males, whereas unexposed females were 2-3 times as large as unexposed males. Finally, we found that female pupal weight is a quadratic function of host mortality across virus isolates, which is likely due to trade-offs and compensatory growth processes acting at high and low mortality levels, respectively. Overall, our results suggest that fecundity costs may strongly affect the response to selection for disease resistance. In nature, baculoviruses contribute to the regulation of gypsy moth outbreaks, as pathogens often do in forest-defoliating insects. We therefore argue that trade-offs between host life-history traits may help explain outbreak dynamics.

Genetic similarity of island populations of tent caterpillars during successive outbreaks

Cyclic or fluctuating populations experience regular periods of low population density. Genetic bottlenecks during these periods could give rise to temporal or spatial genetic differentiation of populations. High levels of movement among increasing populations, however, could ameliorate any differences and could also synchronize the dynamics of geographically separated populations. We use microsatellite markers to investigate the genetic differentiation of four island and one mainland population of western tent caterpillars, Malacosoma californicum pluviale, in two periods of peak or pre-peak density separated by 8 years. Populations showed high levels of genetic variation and little genetic differentiation either temporally between peaks or spatially among sites. Mitochondrial haplotypes were also shared between one island population and one mainland population in the two years studied. An isolation-by-distance analysis showed the FST values of the two geographically closest populations to have the highest level of differentiation in both years. We conclude that high levels of dispersal among populations maintain both synchrony of population dynamics and override potential genetic differentiation that might occur during population troughs. As far we are aware, this is the first time that genetic similarity between temporally separated population outbreaks in insects has been investigated. A review of genetic data for both vertebrate and invertebrate species of cyclic animals shows that a lack of spatial genetic differentiation is typical, and may result from high levels of dispersal associated with fluctuating dynamics.

Evolution and the microbial control of insects

Insect pathogens can be utilized in a variety of pest management approaches, from inundative release to augmentation and classical biological control, and microevolution and the consideration of evolutionary principles can potentially influence the success of all these strategies. Considerable diversity exists in natural entomopathogen populations and this diversity can be either beneficial or detrimental for pest suppression, depending on the pathogen and its mode of competition, and this should be considered in the selection of isolates for biological control. Target hosts can exhibit considerable variation in their susceptibility to entomopathogens, and cases of field-evolved resistance have been documented for Bacillus thuringiensis and baculoviruses. Strong selection, limited pathogen diversity, reduced gene flow, and host plant chemistry are linked to cases of resistance and should be considered when developing resistance management strategies. Pre- and post-release monitoring of microbial control programs have received little attention; however, to date there have been no reports of host-range evolution or long-term negative effects on nontarget hosts. Comparative analyses of pathogen population structure, virulence, and host resistance over time are required to elucidate the evolutionary dynamics of microbial control systems.

Multiple mating and family structure of the western tent caterpillar, Malacosoma californicum pluviale: impact on disease resistance

BACKGROUND

Levels of genetic diversity can strongly influence the dynamics and evolutionary changes of natural populations. Survival and disease resistance have been linked to levels of genetic diversity in eusocial insects, yet these relationships remain untested in gregarious insects where disease transmission can be high and selection for resistance is likely to be strong.

METHODOLOGY/PRINCIPAL FINDINGS

Here we use 8 microsatellite loci to examine genetic variation in 12 families of western tent caterpillars, Malacosoma californicum pluviale from four different island populations to determine the relationship of genetic variability to survival and disease resistance. In addition these genetic markers were used to elucidate the population structure of western tent caterpillars. Multiple paternity was revealed by microsatellite markers, with the number of sires estimated to range from one to three per family (mean ± SE  = 1.92±0.23). Observed heterozygosity (H(O)) of families was not associated to the resistance of families to a nucleopolyhedrovirus (NPV) (r = 0.161, F(1,12)  = 0.271, P = 0.614), a major cause of mortality in high-density populations, but was positively associated with larval survival (r = 0.635, F(1,10)  = 5.412, P = 0.048). Genetic differentiation among the families was high (F(ST) = 0.269, P<0.0001), and families from the same island were as differentiated as were families from other islands.

CONCLUSION/SIGNIFICANCE

We have been able to describe and characterize 8 microsatellite loci, which demonstrate patterns of variation within and between families of western tent caterpillars. We have discovered an association between larval survival and family-level heterozygosity that may be relevant to the population dynamics of this cyclic forest lepidopteran, and this will be the topic of future work.

The effect of food limitation on immunity factors and disease resistance in the western tent caterpillar

Epizootics of nucleopolyhedrovirus characterize declines of cyclic populations of western tent caterpillars, Malacosoma pluviale californicum. In field populations, infection can be apparently lacking in one generation and high in the next. This may suggest an increase in the susceptibility to infection of larvae at peak density or the triggering of a vertically transmitted virus. Here, we test the hypothesis that reduced food availability, as may occur during population outbreaks of tent caterpillars, influences the immunocompetence of larvae and increases their susceptibility to viral infection. We compared immunity factors, hemolymph phenoloxidase and hemocyte numbers, and the susceptibility to nucleopolyhedroviral infection of fifth instar larvae that were fully or partially fed as fourth instars. To determine if maternal or transgenerational influences occurred, we also determined the susceptibility of the offspring of the treated parents to viral infection. Food limitation significantly reduced larval survival, development rate, larval and pupal mass, moth fecundity and levels of hemolymph phenoloxidase, but not the numbers of hemocytes. Neither the food-reduced larvae nor their offspring were more susceptible to viral infection and were possibly even less susceptible at intermediate viral doses. Food reduction did not activate latent or covert viral infection of larvae as might be expected as a response to stress. We conclude that reducing the food intake of fourth instar larvae to an extent that had measurable and realistic impacts on their life history characteristics was not translated into increased susceptibility to viral infection.

Within- and among-population variation in chytridiomycosis-induced mortality in the toad Alytes obstetricans

Background

Chytridiomycosis is a fungal disease linked to local and global extinctions of amphibians. Susceptibility to chytridiomycosis varies greatly between amphibian species, but little is known about between- and within-population variability. However, this kind of variability is the basis for the evolution of tolerance and resistance evolution to disease.

Methodology/Principal Findings

In a common garden experiment, we measured mortality after metamorphosis of Alytes obstetricans naturally infected with Batrachochytrium dendrobatidis. Mortality rates differed significantly among populations and ranged from 27 to 90%. Within populations, mortality strongly depended on mass at and time through metamorphosis.

Conclusions/Significance

Although we cannot rule out that the differences observed resulted from differences in skin microbiota, different pathogen strains or environmental effects experienced by the host or the pathogen prior to the start of the experiment, we argue that genetic differences between populations are a likely source of at least part of this variation. To our knowledge, this is the first study showing differences in survival between and within populations under constant laboratory conditions. Assuming that some of this intraspecific variation has a genetic basis, this may suggest that there is the potential for the evolution of resistance or tolerance, which might allow population persistence.

Inter-annual variation in prevalence and intensity of mite parasitism relates to appearance and expression of damselfly resistance

BACKGROUND

Insects can resist parasites using the costly process of melanotic encapsulation. This form of physiological resistance has been studied under laboratory conditions, but the abiotic and biotic factors affecting resistance in natural insect populations are not well understood. Mite parasitism of damselflies was studied in a temperate damselfly population over seven seasons to determine if melanotic encapsulation of mite feeding tubes was related to degree of parasitism, host sex, host size, emergence timing, duration of the emergence period, and average daily air temperature.

RESULTS

Although parasite prevalence in newly emerged damselflies was > 77% each year, hosts did not resist mites in the early years of study. Resistance began the year that there was a dramatic increase in the number of mites on newly emerged damselflies. Resistance continued to be correlated with mite prevalence and intensity throughout the seven-year study. However, the percentage of hosts resisting only ranged from 0-13% among years and resistance was not sex-biased and was not correlated with host size. Resistance also was not correlated with air temperature or with timing or duration of damselfly emergence.

CONCLUSIONS

Resistance in host damselflies was weakly and variably expressed over the study period. Factors such as temperature, which have been identified in laboratory studies as contributing to resistance by similar hosts, can be irrelevant in natural populations. This lack of temperature effect may be due to the narrow range in temperatures observed at host emergence among years. Degree of mite parasitism predicted both the appearance and continued expression of resistance among parasitized damselflies.

Disease spread, susceptibility and infection intensity: vicious circles?

Epidemiological models and studies of disease ecology typically ignore the role of host condition and immunocompetence when trying to explain the distribution and dynamics of infections and their impact on host dynamics. Recent research, however, indicates that host susceptibility should be considered carefully if we are to understand the mechanism by which parasite dynamics influence host dynamics and vice versa. Studies in insects, fish, amphibians and rodents show that infection occurrence and intensity are more probable and more severe in individuals with an underlying poor condition. Moreover, infection itself results in further deterioration of the host and a 'vicious circle' is created. We argue that this potential synergy between host susceptibility and infection should be more widely acknowledged in disease ecology research.

Enhancing the multiplication of nucleopolyhedrovirus in vitro by manipulation of the pH

Insect nucleopolyhedroviruses (NPVs) are studied widely as agents for biological control, as expression vectors for the production of heterologous proteins, and as transduction vectors for gene therapy applications. Most of these applications rely on the existence of cell lines that allow in vitro multiplication of the virus. The influence of pH in the medium culture on the multiplication of SeMNPV, HearSNPV and AcMNPV in different cell culture lines was investigated. The study showed a strong influence of the medium pH on the virus multiplication with the best results at pH 6.5, about half pH unit above the pH of insect culture media used most commonly. Additional experiments using a recombinant AcMNPV, expressing the green fluorescent protein, suggested that the enhanced virus multiplication at pH 6.5 is due mainly to a facilitated entry of the budded virions into the cells.