Temperature and population density: interactional effects of environmental factors on phenotypic plasticity, immune defenses, and disease resistance in an insect pest

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.

Defence response of Galleria mellonella larvae to oral and intrahemocelic infection with Pseudomonasentomophila

We report differences in the course of infection of G. mellonella larvae with P. entomophila via intrahemocelic and oral routes. Survival curves, larval morphology, histology, and induction of defence response were investigated. Larvae injected with 10 and 50 cells of P. entomophila activated a dose-dependent immune response, which was manifested by induction of immune-related genes and dose-dependent defence activity in larval hemolymph. In contrast, after the oral application of the pathogen, antimicrobial activity was detected in whole hemolymph of larvae infected with the 103 but not 105 dose in spite of the induction of immune response manifested as immune-relevant gene expression and defence activity of electrophoretically separated low-molecular hemolymph components. Among known proteins induced after the P. entomophila infection, we identified proline-rich peptide 1 and 2, cecropin D-like peptide, galiomycin, lysozyme, anionic peptide 1, defensin-like peptide, and a 27 kDa hemolymph protein. The expression of the lysozyme gene and the amount of protein in the hemolymph were correlated with inactivity of hemolymph in insects orally infected with a higher dose of P. entomophila, pointing to its role in the host-pathogen interaction.

Mutualisms in a warming world

Predicting the impacts of global warming on mutualisms poses a significant challenge given the functional and life history differences that usually exist among interacting species. However, this is a critical endeavour since virtually all species on Earth depend on other species for survival and/or reproduction. The field of thermal ecology can provide physiological and mechanistic insights, as well as quantitative tools, for addressing this challenge. Here, we develop a conceptual and quantitative framework that connects thermal physiology to species' traits, species' traits to interacting mutualists' traits and interacting traits to the mutualism. We first identify the functioning of reciprocal mutualism-relevant traits in diverse systems as the key temperature-dependent mechanisms driving the interaction. We then develop metrics that measure the thermal performance of interacting mutualists' traits and that approximate the thermal performance of the mutualism itself. This integrated approach allows us to additionally examine how warming might interact with resource/nutrient availability and affect mutualistic species' associations across space and time. We offer this framework as a synthesis of convergent and critical issues in mutualism science in a changing world, and as a baseline to which other ecological complexities and scales might be added.

Impact of the Diet on the Mortality and on Gene Expression of the Antimicrobial Peptide Tenecin 3 in Tenebrio molitor Larvae Infected by Beauveria bassiana

Large-scale insect rearing can be subjected to microbial infections, leading to serious economic losses. When possible, the use of antibiotics is to be avoided for insects intended as feed or food and new strategies to preserve the health of the farmed insects are required. The effectiveness of insect immune system depends on several factors, including the nutritional composition of the diet. The possibility to modulate immune responses through the diet is currently a topic of great interest from an application point of view. We evaluated the effect of two different diets on the survival rate and gene expression of the antimicrobial peptide Tenecin 3 in uninfected and Beauveria bassiana-infected Tenebrio molitor L. larvae. A wheat bran diet, mixed 50% with brewers' spent grains, could positively influence the expression of Tenecin 3 gene when uninfected T. molitor is allowed to develop on such a substrate from early larval stages. Even if, in our trial, the diet with added brewers' spent grains could not reduce the mortality of the larvae infected with B. bassiana, higher transcriptional levels of the antifungal peptide were observed in insects fed this diet, depending on the timing of diet administration.

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

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

Realism in Immune Ecology Studies: Artificial Diet Enhances a Caterpillar's Immune Defense but Does Not Mask the Effects of a Plastic Immune Strategy

The immune system is considered a functional trait in life-history theory and its modulation is predicted to be costly and highly dependent on the host's nutrition. Therefore, the nutritional status of an individual has a great impact on an animal's immune ecology. Herbivorous insects are commonly used as model organisms in eco-immunology studies and the use of an artificial diet is the predominant rearing procedure to test them. However, this diet differs from what herbivores experience in nature and it is unclear to what degree this distinction might impact on the relevance of these studies for the real world. Here, we compared plant-based vs. artificial diet in a set of three experiments to investigate the interaction of both diets with a plastic immune strategy known as Density-Dependent Prophylaxis (DDP). We used as a model organism the velvetbean caterpillar Anticarsia gemmatalis, which is known to adjust its immune defense in line with the DDP hypothesis. Our main results showed that larvae fed with artificial diet had 20.5% more hemocytes circulating in the hemolymph and died 20% more slowly when infected with an obligate (viral) pathogen. Crucially, however, we did not find any indication of fitness costs related to DDP. The use of artificial diet did not interact with that of DDP except in the case of host survival after infection, where the DDP effect was only observable in this diet. Our findings suggest the use of an artificial diet does not mask resource allocation conflicts between immune investment and fitness related traits, but to some extent it might lead to an overestimation of immune parameters and host survival time after infection. We believe that this is the first study to compare an artificial diet and a host plant covering all these aspects: immune parameters, life-history traits, and host survival after infection. Here we provide evidence that, besides the quantitative effects in immune parameters and host survival time, the use of artificial diet interacts only marginally with a density-dependent immune response. This provides support for the use of artificial diets in eco-immunology studies with insects.

Immune Response in Crayfish Is Species-Specific and Exhibits Changes along Invasion Range of a Successful Invader

Immunity is an important component of invasion success since it enables invaders' adaptation to conditions of the novel environment as they expand their range. Immune response of invaders may vary along the invasion range due to encountered parasites/microbial communities, conditions of the local environment, and ecological processes that arise during the range expansion. Here, we analyzed changes in the immune response along the invasion range of one of the most successful aquatic invaders, the signal crayfish, in the recently invaded Korana River, Croatia. We used several standard immune parameters (encapsulation response, hemocyte count, phenoloxidaze activity, and total prophenoloxidaze) to: i) compare immune response of the signal crayfish along its invasion range, and between species (comparison with co-occurring native narrow-clawed crayfish), and ii) analyze effects of specific predictors (water temperature, crayfish abundance, and body condition) on crayfish immune response changes. Immune response displayed species-specificity, differed significantly along the signal crayfish invasion range, and was mostly affected by water temperature and population abundance. Specific immune parameters showed density-dependent variation corresponding to increased investment in them during range expansion. Obtained results offer baseline insights for elucidating the role of immunocompetence in the invasion success of an invertebrate freshwater invader.

Haemocyte variations in 35 species of grasshoppers and locusts

INTRODUCTION

Grasshoppers and locusts are widely distributed worldwide, causing significant losses in agriculture. The origin and functions of their haemocytes are not entirely understood.

OBJECTIVES

Insect haemocytes arbitrate cellular defence and participate in humoral defences. Due to their importance, the haemocytes of 35 species of grasshoppers and locusts from China were morphologically examined in this study. We aim to highlight a simple method for the morphological examination of insect haemocytes.

METHODS

The haemocytes were observed, counted and compared under a light microscope after Wright-Giemsa staining.

RESULTS

High complexity in form and shape were observed in the haemocytes. These include prohaemocytes, plasmatocytes, granulocytes, vermicytes, podocytes and megakaryocytes. No clear relationship was seen between the haemocyte type and their phylogenetic relationship among the three families examined. The high abundance of plasmatocytes and granulocytes suggests their importance in the immunity of grasshoppers and locusts. The minor haemocyte populations including prohaemocytes, vermicytes and podocytes may not be always present in individuals.

CONCLUSION

All examined species shared similarities in their haemocyte types. Wright-Giemsa staining is a simple and efficient method for evaluating haemocytes.

Insects and their pathogens in a changing climate

The complex nature of climate change-mediated multitrophic interaction is an underexplored area, but has the potential to dramatically shift transmission and distribution of many insects and their pathogens, placing some populations closer to the brink of extinction. However, for individual insect-pathogen interactions climate change will have complicated hard-to-anticipate impacts. Thus, both pathogen virulence and insect host immunity are intrinsically linked with generalized stress responses, and in both pathogen and host have extensive trade-offs with nutrition (e.g., host plant quality), growth and reproduction. Potentially alleviating or exasperating these impacts, some pathogens and hosts respond genetically and rapidly to environmental shifts. This review identifies many areas for future research including a particular need to identify how altered global warming interacts with other environmental changes and stressors, and how consistent these impacts are across pathogens and hosts. With that achieved we would be closer to producing an overarching framework to integrate knowledge on all environmental interplay and infectious disease events.

Wide Diurnal Temperature Amplitude and High Population Density Can Positively Affect the Life History of Sitobion avenae (Hemiptera: Aphididae)

Diurnal temperature amplitude is known to have a large influence on insect life history. Population density affects intraspecific competition and many other aspects of insect life history. However, there is limited information on the interactive effects of these factors on insects. Here, we tested the interactive effects of three diurnal temperature amplitudes (22 ± 0°C, 22 ± 6°C, and 22 ± 12°C) and three population densities on the development, survival, longevity, and fecundity of the English grain aphid Sitobion avenae (Fabricius) (Homoptera: Aphididae). At a constant temperature, increasing population density reduced the growth and survival of early-instar nymphs, increased longevity, and reduced fecundity. At a low population density, increasing temperature amplitude inhibited nymph development. However, even at a high temperature amplitude, nymph survival rate was higher than expected, and reproduction was possible because the recovery of the lower night-temperatures eliminated thermal stress. Increasing the population density reduced, and even reversed, the negative effects of the wide temperature amplitude. This may reflect synergistic interactions between population density and wide temperature amplitude as these stressors each incur energetic costs. These findings emphasize the importance of temperature amplitude and population density for improving prediction accuracy and damage assessment during pest control modeling.

Lower bioenergetic costs but similar immune responsiveness under a heat wave in urban compared to rural damselflies

There is mounting evidence that the widespread phenotypic changes in response to urbanization may reflect adaptations caused by rapid evolutionary processes driven by urban-related stressors. Compared to increased habitat fragmentation and pollution, adaptations towards another typical urban-related stressor, that is higher and longer lasting very high temperatures (heat waves), are much less studied. Notably, the sensitivities to heat waves of life-history traits and important fitness-related physiological traits such as immune responsiveness and bioenergetic variables (energy availability, energy consumption and their balance) have never been contrasted between urban and rural populations. By conducting a laboratory common-garden experiment, we compared effects of a simulated heat wave on life history (survival and growth rate), immune responsiveness and bioenergetic variables between three urban and three rural populations of the damselfly Coenagrion puella. Because energy-mediated trade-off patterns may only be detected under energetically costly manipulations, all larvae were immune-challenged by simulating ectoparasitism by water mites. As expected, the simulated heat wave caused negative effects on nearly all response variables. The immune responsiveness, on the other hand, increased under the heat wave, consistent with a trade-off pattern between immune function and growth, and this similarly between urban and rural populations. A key finding was that urban larvae suffered less from the simulated heat wave compared to the rural larvae in terms of a lower heat wave-induced depletion in energy availability. This suggests an adaptation of urban populations to better cope with the stronger and more frequent heat waves in cities. Notably, this urbanization-driven evolution in the bioenergetic variables was not apparent in the absence of a heat wave. Given that changes in energy budgets have strong fitness consequences, our findings suggest that the evolved higher ability to cope with heat waves is fundamental for the survival of urban damselfly populations.

Effects of Larval Density on Plutella xylostella Resistance to Granulosis Virus

Simple Summary

Generally, the transmission of pathogens is positively density-dependent; therefore, the risk of insects becoming infected by diseases increases with increasing population density. It has been reported that some phase-polyphenic insects from high-density conditions are more resistant (or susceptible or identical) to pathogens than those from low-density conditions. This phenomenon is termed “density-dependent prophylaxis” (DDP). The diamondback moth (DBM), Plutella xylostella, one of the most destructive insect pests affecting cruciferous crops, is non phase-polyphenic. Biological control, especially by pathogens, plays an important role in the integrated pest management program of DBM. However, whether the P. xylostella larval population exhibits DDP has not been elucidated. The resistance of DBM larvae to P. xylostella granulosis virus (Plxy GV) and their immune response to the virus under different density treatments were investigated under laboratory conditions. Our results demonstrated that P. xylostella larvae exhibited DDP within a certain limited density. This study may help to elucidate the biocontrol effect of different density populations of P. xylostella by granulosis virus and guide improvements in future management strategy.

Abstract

It has been reported that some phase-polyphenic insects from high-density conditions are more resistant to pathogens than those from low-density conditions. This phenomenon is termed “density-dependent prophylaxis” (DDP). However, whether non phase-polyphenic insects exhibit DDP has rarely been elucidated. The diamondback moth (DBM), Plutella xylostella, one of the most destructive insect pests affecting cruciferous crops, is non phase-polyphenic. In this study, the resistance of DBM larvae to P. xylostella granulosis virus (Plxy GV) and their immune response to the virus when reared at densities of 1, 2, 5, 10, 15, and 20 larvae per Petri dish were investigated under laboratory conditions. Compared with larvae reared at lower densities, larvae reared at moderate density showed a significantly higher survival rate, but the survival rate significantly decreased with further increases in rearing density. Furthermore, the phenoloxidase, lysozyme and antibacterial activity and total hemocyte count in the hemolymph of the larvae, regardless of whether they were challenged with the virus, from different larval densities corresponded to the observed differences in resistance to Plxy GV. These results demonstrated that P. xylostella larvae exhibited DDP within a certain limited density. This study may help to elucidate the biocontrol effect of different density populations of P. xylostella by granulosis virus and guide improvements in future management strategy.

Immune Defenses of a Beneficial Pest: The Mealworm Beetle, Tenebrio molitor

The mealworm beetle, Tenebrio molitor, is currently considered as a pest when infesting stored grains or grain products. However, mealworms are now being promoted as a beneficial insect because their high nutrient content makes them a viable food source and because they are capable of degrading polystyrene and plastic waste. These attributes make T. molitor attractive for mass rearing, which may promote disease transmission within the insect colonies. Disease resistance is of paramount importance for both the control and the culture of mealworms, and several biotic and abiotic environmental factors affect the success of their anti-parasitic defenses, both positively and negatively. After providing a detailed description of T. molitor’s anti-parasitic defenses, we review the main biotic and abiotic environmental factors that alter their presentation, and we discuss their implications for the purpose of controlling the development and health of this insect.

Variation in the immune state of Gammarus pulex (Crustacea, Amphipoda) according to temperature: Are extreme temperatures a stress?

Temperature is known to impact host-parasite interactions in various ways. Such effects are often regarded as the consequence of the increased metabolism of parasites with increasing temperature. However, the effect of temperature on hosts' immune system could also be a determinant. Here we assessed the influence of temperature on the immunocompetence of the crustacean amphipod Gammarus pulex. Amphipods play a key ecological role in freshwater ecosystems that can be altered by several parasites. We investigated the consequences of three weeks of acclimatization at four temperatures (from 9 °C to 17 °C) on different immunological parameters. Temperature influenced both hemocyte concentration and active phenoloxidase enzymatic activity, with lower values at intermediate temperatures, while total phenoloxidase activity was not affected. In addition, the ability of gammarids to clear a bacterial infection was at the highest at intermediate temperatures. These results suggest a dysregulation of the immune system of gammarids in response to stress induced by extreme temperature.

Daily temperature variation and extreme high temperatures drive performance and biotic interactions in a warming world

We review the major patterns on the effects of daily temperature variation (DTV) and extreme high temperatures (EXT) on performance traits and the resulting outcome of biotic interactions in insects. EXT profoundly affects the outcome of all types of biotic interactions: competitive, predator-prey, herbivore-plant, host-pathogen/parasitoid and symbiotic interactions. Studies investigating effects of DTV on biotic interactions are few but also show strong effects on competitive and host-pathogen/parasitoid interactions. EXT typically reduces predation, and is expected to reduce parasitoid success. The effects of EXT and DTV on the outcome of the other interaction types are highly variable, yet can be predicted based on comparisons of the TPCs of the interacting species, and challenges the formulation of general predictions about the change in biotic interactions in a warming world.