The impact of thermal fluctuations on reaction norms in specialist and generalist parasitic wasps

High-temperature responses of Myzus persicae and its parasitoid Aphidius gifuensis in relation to heat level, duration and developmental stage

BACKGROUND

Understanding how parasitoids respond to temperature is crucial for improving biological control strategies under the context of global warming. This study examined the suitability of Myzus persicae and its parasitoid Aphidius gifuensis to varying temperature conditions, as well as the stage-specific response of A. gifuensis to high temperatures.

RESULTS

High temperatures had a significant impact on the both M. persicae and A. gifuensis. When exposed to 36°C, M. persicae developed more slowly and produced smaller adults compared to control, regardless of the duration of exposure (2, 4 or 6 h); additionally, the survival rate of M. persicae nymphs sharply decreased under these conditions. Exposure to 36°C for 4 h negatively impacted the development of A. gifuensis. Female parasitoids exposed to 32°C developed into smaller adults, whereas males exposed to all three temperature levels were smaller compared to control group. Female parasitoids exposed to high temperatures, regardless of the specific heat level and duration, exhibited reduced longevity and decreased fecundity. None of the parasitoids exposed to 36°C for 6 h daily developed into adults. Heat treated during early developmental stages (2 and 4 days old) had a greater influence on parasitoid development, whereas heat treatment at 4 and 6 days old had a more significant impact on its fecundity.

CONCLUSION

High temperatures not only directly affected the performance of A. gifuensis, but also exerted indirect effects by influencing the quality of the host aphids M. persicae. The deleterious effects of high temperature on larvae can persist into the adult stage, affecting the longevity and reproduction of adults. These findings are important for the utilization of A. gifuensis in the control of M. persicae in warming environments. © 2024 Society of Chemical Industry.

Modelling thermal reaction norms for development and viability in Drosophila suzukii under constant, fluctuating and field conditions

Phenological models for insect pests often rely on knowledge of thermal reaction norms. These may differ in shape depending on developmental thermal conditions (e.g. constant vs. fluctuating) and other factors such as life-stages. Here, we conducted an extensive comparative study of the thermal reaction norms for development and viability in the invasive fly, Drosophila suzukii, under constant and fluctuating thermal regimes. Flies, were submitted to 15 different constant temperatures (CT) ranging from 8 to 35 °C. We compared responses under CT with patterns observed under 15 different fluctuating temperature (FT) regimes. We tested several equations for thermal performance curves and compared various models to obtain thermal limits and degree-day estimations. To validate the model's predictions, the phenology was monitored in two artificial field-like conditions and two natural conditions in outdoor cages during spring and winter. Thermal reaction norm for viability from egg to pupa was broader than that from egg to adult. FT conditions yielded a broader thermal breadth for viability than CT, with a performance extended towards the colder side, consistent with our field observations in winter. Models resulting from both CT and FT conditions made accurate predictions of degree-day as long as the temperature remained within the linear part of the developmental rate curve. Under cold artificial and natural winter conditions, a model based on FT data made more accurate predictions. Model based on CT failed to predict adult's emergence in winter. We also document the first record of development and adult emergence throughout winter in D. suzukii. Population dynamics models in D. suzukii are all based on summer phenotype and CT. Accounting for variations between seasonal phenotypes, stages, and thermal conditions (CT vs. FT) could improve the predictive power of the models.

A perspective on insect-microbe holobionts facing thermal fluctuations in a climate-change context

Temperature influences the ecology and evolution of insects and their symbionts by impacting each partner independently and their interactions, considering the holobiont as a primary unit of selection. There are sound data about the responses of these partnerships to constant temperatures and sporadic thermal stress (mostly heat shock). However, the current understanding of the thermal ecology of insect-microbe holobionts remains patchy because the complex thermal fluctuations (at different spatial and temporal scales) experienced by these organisms in nature have often been overlooked experimentally. This may drastically constrain our ability to predict the fate of mutualistic interactions under climate change, which will alter both mean temperatures and thermal variability. Here, we tackle down these issues by focusing on the effects of temperature fluctuations on the evolutionary ecology of insect-microbe holobionts. We propose potentially worth-investigating research avenues to (i) evaluate the relevance of theoretical concepts used to predict the biological impacts of temperature fluctuations when applied to holobionts; (ii) acknowledge the plastic (behavioural thermoregulation, physiological acclimation) and genetic responses (evolution) expressed by holobionts in fluctuating thermal environments; and (iii) explore the potential impacts of previously unconsidered patterns of temperature fluctuations on the outcomes and the dynamic of these insect-microbe associations.

Effects of Constant versus Fluctuating Temperatures on Fitness Indicators of the Aphid Dysaphis plantaginea and the Parasitoid Aphidius matricariae

Simple Summary

Like all organisms, insects encounter temperatures that fluctuate on different time scales: within a day, between days, or throughout the seasons. However, most studies on the impact of temperature on insect physiology, behavior, morphology, or ecology have focused on constant temperatures tested in the laboratory. In our study, we wanted to know if fluctuating temperatures during the day (7–17 °C, average 12 °C) can affect insects differently compared to a constant temperature of 12 °C. We used, as a model, the apple aphid Dysaphis plantaginea, a major threat to apple orchards worldwide, and its parasitoid Aphidius matricariae, which is used in biological control. We found that many traits—but not all—were affected. In particular, the fluctuating thermal regime reduced the development time of aphids and parasitoids, improved the rate of parasitism, and tended (albeit slightly) to increase the longevity of both species. In contrast, we did not find strong effects on morphological traits. Our results can be used to better predict how these agronomically important insects behave in orchards, how ecologically-relevant fluctuating temperatures affect host–parasitoid relationships, and ultimately what the implications are in the context of climate change and biological control.

Abstract

Testing fluctuating rather than constant temperatures is likely to produce more realistic datasets, as they are ecologically more similar to what arthropods experience in nature. In this study, we evaluated the impact of three constant thermal regimes (7, 12, and 17 °C) and one fluctuating thermal regime (7–17 °C with a mean of 12 °C) on fitness indicators in the rosy apple aphid Dysaphis plantaginea, a major pest of apple orchards, and the parasitoid Aphidius matricariae, one of its natural enemies used in mass release biological control strategies. For some—but not all—traits, the fluctuating 7–17 °C regime was beneficial to insects compared to the constant 12 °C regime. Both aphid and parasitoid development times were shortened under the fluctuating regime, and there was a clear trend towards an increased longevity under the fluctuating regime. The fecundity, mass, and size were affected by the mean temperature, but only the mass of aphids was higher at 7–17 °C than at a constant 12 °C. Parasitism rates, but not emergence rates, were higher under the fluctuating regime than under the constant 12 °C regime. Results are discussed within the framework of insect thermal ecology and Jensen’s inequality. We conclude that incorporating thermal fluctuations in ecological studies could allow for the more accurate consideration of how temperature affects host–parasitoid interactions and insect responses to temperature change over time.

Sperm Production Is Reduced after a Heatwave at the Pupal Stage in the Males of the Parasitoid Wasp Microplitisrufiventris Kok (Hymenoptera; Braconidae)

Simple Summary

Biocontrol with natural enemies of insect pests needs an optimal reproduction of beneficial insects. Most insects are sensitive to heat, and many males suffer from sperm decrease when exposed to warmth during their development. It is dramatic in hymenoptera because males are issued from the development of unfertilized oocytes and only females develop from fertilized eggs. The sex ratio of populations then results from the availability of sperm for egg laying females. Microplitisrufiventris is a parasite of the cotton worm; this moth is a major pest for cotton fields in Egypt. Because the temperature is high in Egypt, reproduction of M. rufiventris must be studied to optimize its use in the fields. We conducted experiments to measure the sperm number of males after heat periods during their development. It shows that M. rufiventris males have less sperm than controls when they were exposed to 36 °C and 40 °C short periods during their development. Moreover, those males live shorter than males that were maintained at 25 °C. In conclusion, we found, males to be sensitive to heat waves, which results in lower fertility, resulting in a lower availability of sperm for females leading to a sex ratio bias. It may lead to a decrease of the efficacy of biocontrol in cotton fields.

Abstract

Understanding reproduction is essential for controlling pests and supporting beneficial insects. Among the many factors allowing optimal reproduction, sperm availability is key to sex ratio control in hymenopteran parasitoids since males are haploid and only females come from fertilization. Microplitisrufiventris (Hymenoptera; Braconidae) is a solitary endoparasitoid of some noctuids. This insect could be used for the control of the cotton leafworm Spodopteralittoralis. Under controlled conditions, sperm quantity was measured in virgin males at 1, 5, 10, and 15 days; it increases in adult males until the fifth day. Sperm stock of control males increased from 2500 at one day to 6700 at 15 days. With the control climatic condition being 25 °C, we tested the effects of a time-limited increase of temperature that can be found in Egypt (36 and 40 °C) during one day at the early pupal stage. Emerging males had 1500 and 420 sperm at 36 and 40 °C, respectively; both lived shorter than the control. The sperm potential of males is dependent on both age and temperature during the early pupal stage. It could have dramatic consequences on the sex ratio of M. rufiventris in natural and controlled populations.

Developmental Temperature Affects Life-History Traits and Heat Tolerance in the Aphid Parasitoid Aphidius colemani

Developmental temperature plays important roles in the expression of insect traits through thermal developmental plasticity. We exposed the aphid parasitoid Aphidius colemani to different temperature regimes (10, 20, or 28 °C) throughout larval development and studied the expression of morphological and physiological traits indicator of fitness and heat tolerance in the adult. We showed that the mass decreased and the surface to volume ratio of parasitoids increased with the development temperature. Water content was not affected by rearing temperature, but parasitoids accumulated more lipids when reared at 20 °C. Egg content was not affected by developmental temperature, but adult survival was better for parasitoids reared at 20 °C. Finally, parasitoids developed at 20 °C showed the highest heat stupor threshold, whereas parasitoids developed at 28 °C showed the highest heat coma threshold (better heat tolerance CTmax1 and CTmax2, respectively), therefore only partly supporting the beneficial acclimation hypothesis. From a fundamental point of view, our study highlights the role of thermal plasticity (adaptive or not) on the expression of different life history traits in insects and the possible correlations that exist between these traits. From an applied perspective, these results are important in the context of biological control through mass release techniques of parasitoids in hot environments.

Winter warm-up frequency and the degree of temperature fluctuations affect survival outcomes of spotted-wing drosophila winter morphotypes

Among overwintering Drosophila suzukii, discrete environmental changes in temperature and photoperiod induce a suite of biochemical changes conferring cold tolerance. However, little is known regarding how temperature fluctuations, which can influence metabolic and cellular repair activity, affect survival outcomes in this species. For that reason, we designed three experiments to test the effects of intermittent warm-up periods and the degree of temperature fluctuation on winter-morphotype (WM) D. suzukii survival. We found that at 5 °C, a temperature sufficient to induce reproductive diapause, but warm enough to allow foraging, increasing warm-up frequency (warmed to 25 °C at various interval schedules) was associated with decreased survival. In contrast, when the nightly low temperature was 0 °C, daily fluctuations that warmed the environment to temperatures above freezing (5, or 15 °C) appeared beneficial and resulted in improved survival compared to flies held at 0 °C during day and night. When we next evaluated cold tolerance using a 24-hour stress test assay (-5 °C), we found that again, thermal fluctuations improved survival compared to static freezing conditions. However, we also found that WM D. suzukii exposed to freezing temperatures during acclimation were less cold tolerant, regardless of the thermal fluctuation schedule, indicating that there may be tradeoffs between adequate acclimation temperature, which is required to induce cold tolerance, and the ensuing effects of incidental chill injury. Moving forward, these data, which account for the nuanced interactions between the thermal environment and in the internal physiology of D. suzukii, may help refine seasonal populations models, which aim to forecast pest pressure based on conditions the previous winter.

Short communication: Domesticated and wild fathead minnows differ in growth and thermal tolerance

Many populations have evolved in response to laboratory environments (lack of predators, continual food availability, etc.). Another potential agent of selection in the lab is exposure to constant thermal environments. Here, we examined changes in growth, critical thermal maximum (CT_max), and food consumption under constant (25 °C) and fluctuating (22-28 °C and 19-31 °C) conditions in two populations of fathead minnows, Pimephales promelas: one that has been kept in a laboratory setting for over 120 generations (~40 years) and a corresponding wild one. We found that under thermal fluctuations, domesticated fathead minnows grew faster than their wild counterparts, but also exhibited lower thermal tolerance. Food consumption was significantly higher in the lab population under the constant and large fluctuation thermal treatments. Our results suggest that the lab population has adjusted to the stable conditions in the laboratory and that we should carefully apply lessons learned in the lab to wild populations.

Evolutionary genomics can improve prediction of species' responses to climate change

Global climate change (GCC) increasingly threatens biodiversity through the loss of species, and the transformation of entire ecosystems. Many species are challenged by the pace of GCC because they might not be able to respond fast enough to changing biotic and abiotic conditions. Species can respond either by shifting their range, or by persisting in their local habitat. If populations persist, they can tolerate climatic changes through phenotypic plasticity, or genetically adapt to changing conditions depending on their genetic variability and census population size to allow for de novo mutations. Otherwise, populations will experience demographic collapses and species may go extinct. Current approaches to predicting species responses to GCC begin to combine ecological and evolutionary information for species distribution modelling. Including an evolutionary dimension will substantially improve species distribution projections which have not accounted for key processes such as dispersal, adaptive genetic change, demography, or species interactions. However, eco-evolutionary models require new data and methods for the estimation of a species' adaptive potential, which have so far only been available for a small number of model species. To represent global biodiversity, we need to devise large-scale data collection strategies to define the ecology and evolutionary potential of a broad range of species, especially of keystone species of ecosystems. We also need standardized and replicable modelling approaches that integrate these new data to account for eco-evolutionary processes when predicting the impact of GCC on species' survival. Here, we discuss different genomic approaches that can be used to investigate and predict species responses to GCC. This can serve as guidance for researchers looking for the appropriate experimental setup for their particular system. We furthermore highlight future directions for moving forward in the field and allocating available resources more effectively, to implement mitigation measures before species go extinct and ecosystems lose important functions.

The Fitness and Economic Benefits of Rearing the Parasitoid Telenomus podisi Under Fluctuating Temperature Regime

Successful biological control requires detailed knowledge about the mass rearing conditions of the control agents in order to ensure higher quality of field-released insects. Thus, we investigated whether rearing fluctuating thermal condition would affect the fitness and costs of the parasitoid wasp Telenomus podisi Ashmead (a biocontrol agent used for controlling the Neotropical brown stink bug Euschistus heros (Fabricius)) when compared with parasitoid reared at constant temperature condition, which is commonly used in insect facilities. Parasitoids were reared under either constant (continuous exposure at 25 ± 2°C) or fluctuating temperature conditions (i.e., 30 ± 2°C during day and 20 ± 2°C at night) during four consecutive generations. Our results indicated that tested fluctuating temperature is more suitable for rearing of T. podisi as such temperature condition not only resulted in fitness benefits (e.g., shorter developmental time, longer female longevity, higher fecundity/fertility) but also reduced (approximately 23.5%) the estimated costs for producing the parasitoids. Furthermore, rearing T. podisi under fluctuating temperatures improved tolerance to low constant temperatures (i.e., 20°C) without changing the tolerance to constant high temperatures (30°C) in the fourth generation. Surprisingly, even parasitoids that developed under fluctuating thermal conditions performed better than those reared at constant temperature of 25°C. Collectively, our findings suggest that T. podisi reared under fluctuating thermal condition can tolerate better fluctuating temperatures that normally occur both during long periods of transport and in agricultural ecosystems, which will increase the quality and productivity of mass-reared T. podisi for inundative releases.

Selective Sweep at a QTL in a Randomly Fluctuating Environment

Adaptation is mediated by phenotypic traits that are often near continuous, and undergo selective pressures that may change with the environment. The dynamics of allelic frequencies at underlying quantitative trait loci (QTL) depend on their own phenotypic effects, but also possibly on other polymorphic loci affecting the same trait, and on environmental change driving phenotypic selection. Most environments include a substantial component of random noise, characterized both by its magnitude and its temporal autocorrelation, which sets the timescale of environmental predictability. I investigate the dynamics of a mutation affecting a quantitative trait in an autocorrelated stochastic environment that causes random fluctuations of an optimum phenotype. The trait under selection may also exhibit background polygenic variance caused by many polymorphic loci of small effects elsewhere in the genome. In addition, the mutation at the QTL may affect phenotypic plasticity, the phenotypic response of given genotype to its environment of development or expression. Stochastic environmental fluctuations increase the variance of the evolutionary process, with consequences for the probability of a complete sweep at the QTL. Background polygenic variation critically alters this process, by setting an upper limit to stochastic variance of population genetics at the QTL. For a plasticity QTL, stochastic fluctuations also influences the expected selection coefficient, and alleles with the same expected trajectory can have very different stochastic variances. Finally, a mutation may be favored through its effect on plasticity despite causing a systematic mismatch with optimum, which is compensated by evolution of the mean background phenotype.

Are life-history traits equally affected by global warming? A case study combining a multi-trait approach with fine-grain climate modeling

Predicting species responses to climate change requires tracking the variation in individual performance following exposure to warming conditions. One ecologically relevant approach consists of examining the thermal responses of a large number of traits, both related with population dynamics and trophic interactions (i.e. a multi-trait approach). Based on in situ climatic data and projections from climate models, we here designed two daily fluctuating thermal regimes realistically reflecting current and future conditions in Eastern France. These models detected an increase in mean temperature and in the range of daily thermal fluctuations as two local facets of global warming likely to occur in our study area by the end of this century. We then examined the responses of several fitness-related traits in caterpillars of the moth Lobesia botrana - including development, pupal mass, survival rates, energetic reserves, behavioral and immune traits expressed against parasitoids - to this experimental imitation of global warming. Increasing temperatures positively affected development (leading to a 31% reduction in the time needed to complete larval stage), survival rates (+19%), and movement speed as a surrogate for larval escape ability to natural enemies (+60%). Conversely, warming elicited detrimental effects on lipid reserves (-26%) and immunity (total phenoloxidase activity: -34%). These findings confirm that traits should differ in their sensitivity to global warming, underlying complex consequences for population dynamics and trophic interactions. Our study strengthens the importance of combining a multi-trait approach with the use of realistic fluctuating regimes to forecast the consequences of global warming for individuals, species and species assemblages.

Environmental degradation amplifies species' responses to temperature variation in a trophic interaction

Land‐use and climate change are two of the primary drivers of the current biodiversity crisis. However, we lack understanding of how single‐species and multispecies associations are affected by interactions between multiple environmental stressors.

We address this gap by examining how environmental degradation interacts with daily stochastic temperature variation to affect individual life history and population dynamics in a host–parasitoid trophic interaction, using the Indian meal moth, Plodia interpunctella, and its parasitoid wasp Venturia canescens.

We carried out a single‐generation individual life‐history experiment and a multigeneration microcosm experiment during which individuals and microcosms were maintained at a mean temperature of 26°C that was either kept constant or varied stochastically, at four levels of host resource degradation, in the presence or absence of parasitoids.

At the individual level, resource degradation increased juvenile development time and decreased adult body size in both species. Parasitoids were more sensitive to temperature variation than their hosts, with a shorter juvenile stage duration than in constant temperatures and a longer adult life span in moderately degraded environments. Resource degradation also altered the host's response to temperature variation, leading to a longer juvenile development time at high resource degradation. At the population level, moderate resource degradation amplified the effects of temperature variation on host and parasitoid populations compared with no or high resource degradation and parasitoid overall abundance was lower in fluctuating temperatures. Top‐down regulation by the parasitoid and bottom‐up regulation driven by resource degradation contributed to more than 50% of host and parasitoid population responses to temperature variation.

Our results demonstrate that environmental degradation can strongly affect how species in a trophic interaction respond to short‐term temperature fluctuations through direct and indirect trait‐mediated effects. These effects are driven by species differences in sensitivity to environmental conditions and modulate top‐down (parasitism) and bottom‐up (resource) regulation. This study highlights the need to account for differences in the sensitivity of species’ traits to environmental stressors to understand how interacting species will respond to simultaneous anthropogenic changes.

Variable temperatures across different stages have novel effects on behavioral response and population viability in a host-feeding parasitoid

Parasitoids are insects (usually wasps or flies) that lay eggs within or on other insects (their hosts). Host-feeding parasitoids lay eggs to parasitize the host and feed directly on the host for nourishment. Temperature is the most critical factor affecting insect behavioral responses. Few studies have focused on the impacts of variable temperatures across different life stages on the behaviors of host-feeding parasitoids. This study investigated the effects of temperature experienced during the preadult and adult stages on the life history traits and life table parameters of females of a host-feeding parasitoid, Eretmocerus hayati. Our results show that the temperatures experienced during the preadult and adult stages significantly change life history traits (immature development, adult longevity, host feeding and fecundity). Increasing the preadult temperature resulted in shorter development times for immature stages of the parasitoid, and decreasing the temperature during the adult stage increased reproduction and longevity. Most importantly, we found that host-feeding events changed with temperature rather than life stage. The daily host-feeding ability of the parasitoid increased with increasing temperature at all temperatures except the stress temperature (34 °C). Furthermore, switching temperatures at the immature stage and adult stage can increase the values of life table parameters, with the highest intrinsic rate of increase (r) occurring in the 30/26 °C treatment. This study provides new insight into the mass rearing of parasitic natural enemies.

Impact of differences in nutritional quality of wingless and winged aphids on parasitoid fitness

Winged aphids are described as hosts of lesser quality for parasitoids because a part of their resources is used to produce wings and associated muscles during their development. Host lipid content is particularly important for parasitoid larvae as they lack lipogenesis and therefore rely entirely on the host for this resource. The goal of this study was to determine to what extent winged and wingless aphids differ from a nutritional point of view and whether these differences impact parasitoid fitness, notably the lipid content. We analysed the energetic budget (proteins, lipids and carbohydrates) of aphids of different ages (third instars, fourth instars and adults) according to the morph (winged or wingless). We also compared fitness indicators for parasitoids emerging from winged and wingless aphids (third and fourth instars). We found that in third instars, parasitoids are able to inhibit wing development whereas this is not the case in fourth instars. Both winged instars allow the production of heavier and fattier parasitoids. The presence of wings in aphids seems to have little effect on the fitness of emerging parasitoids and did not modify female choice for oviposition. Finally, we demonstrate that Aphidius colemani, used as a biological control agent, is able to parasitize wingless as well as winged Myzus persicae, at least in the juvenile stages. If the parasitism occurs in third instars, the parasitoid will prevent the aphid from flying, which could in turn reduce virus transmission.

Fluctuating thermal environments and time-dependent effects on fruit fly egg-hatching performance

Organismal performance in a changing environment is dependent on temporal patterns and duration of exposure to thermal variability. We experimentally assessed the time-dependent effects of thermal variability (i.e., patterns of thermal exposure) on the hatching performance of Drosophila melanogaster. Flies were collected in central Chile and maintained for four generations in laboratory conditions. Fourth generation eggs were acclimated to different thermal fluctuation cycles until hatching occurred. Our results show that the frequency of extreme thermal events has a significant effect on hatching success. Eggs exposed to 24 hr cycles of thermal fluctuation had a higher proportion of eggs that hatched than those acclimated to shorter (6 and 12 hr) and longer cycles (48 hr). Furthermore, eggs subjected to frequent thermal fluctuations hatched earlier than those acclimated to less frequent thermal fluctuations. Overall, we show that, egg-to-adult viability is dependent on the pattern of thermal fluctuations experienced during ontogeny; thus, the pattern of thermal fluctuation experienced by flies has a significant and until now unappreciated impact on fitness.

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.

Adaptations to different habitats in sexual and asexual populations of parasitoid wasps: a meta-analysis

BACKGROUND

Coexistence of sexual and asexual populations remains a key question in evolutionary ecology. We address the question how an asexual and a sexual form of the parasitoid Venturia canescens can coexist in southern Europe. We test the hypothesis that both forms are adapted to different habitats within their area of distribution. Sexuals inhabit natural environments that are highly unpredictable, and where density of wasps and their hosts is low and patchily distributed. Asexuals instead are common in anthropic environments (e.g., grain stores) where host outbreaks offer periods when egg-load is the main constraint on reproductive output.

METHODS

We present a meta-analysis of known adaptations to these habitats. Differences in behavior, physiology and life-history traits between sexual and asexual wasps were standardized in term of effect size (Cohen's d value; Cohen, 1988).

RESULTS

Seeking consilience from the differences between multiple traits, we found that sexuals invest more in longevity at the expense of egg-load, are more mobile, and display higher plasticity in response to thermal variability than asexual counterparts.

DISCUSSION

Thus, each form has consistent multiple adaptations to the ecological circumstances in the contrasting environments.

Impact of constant versus fluctuating temperatures on the development and life history parameters of Tetranychus urticae (Acari: Tetranychidae)

The impact of daily temperature fluctuations on arthropod life history parameters is inadequately studied compared with the ample amount of research that has been conducted on the effects of constant temperatures. Fluctuating temperatures are likely to be more realistic, as they are ecologically more similar to what these arthropods experience in nature. Here, we compared the impact of 11 constant temperatures that ranged from 10 to 35 °C with fluctuating temperatures with the same corresponding mean temperature and an amplitude of 10 °C between high (12 h) and low (12 h) temperatures on the development and life history parameters of Tetranychus urticae under continuous light conditions. No eggs hatched at constant 10 °C, whereas 81.5% of eggs successfully completed development at fluctuating 10 °C (15/5 °C). Egg-to-female adult development was faster under fluctuating temperatures from 12.5 to 27.5 °C than under constant temperatures, whereas the opposite trend was observed at >30 °C. The lower thermal thresholds (T) were 11.63 and 8.63 °C, and thermal constants (K) were 127.81 and 150.69 degree-days for egg-to-female adults at constant and fluctuating temperatures, respectively. The numbers of oviposition days were significantly higher at fluctuating 15 °C than at the corresponding constant temperature, whereas the opposite trend was observed from 20 to 30 °C. The intrinsic rate of increase (r) was higher at fluctuating than at constant 15 °C. The net reproductive rate (R ₀) was also higher at fluctuating than at constant 15 and 35 °C, but showed an opposite trend at 20 and 25 °C. We conclude that fluctuating temperatures should be considered to accurately predict spider mite population dynamics in nature.

Cognitive adaptation in asexual and sexual wasps living in contrasted environments

Differences in learning and memory dynamics between populations are suspected to result from differences in ecological constraints such as resource distribution. The two reproductive modes (strains) of the parasitoid wasp Venturia canescens share the same geographical areas but live in contrasting habitats: arrhenotokous wasps live in the wild (generally orchards), whereas thelytokous ones live mostly in stored-products buildings (e.g. granaries). This species thus represents a relevant biological model for understanding the relationship between the ecological constraints faced by a species and its memory and learning ability. We showed that after having laid eggs in presence of both a synthetic odour and natural olfactory cues of their host, arrhenotokous wasps exhibited a change in their behavioural response towards the synthetic odour that was at least as pronounced as in thelytokous ones even though they were faster in their decision-making process. This is consistent with better learning skills in arrhenotokous wasps. The corresponding memory trace persisted in both strains for at least 51 h. We compare and discuss the learning and memory ablities of both strains as a function of their costs and benefits in their preferential habitats.

Effects of daily fluctuating temperatures on the Drosophila-Leptopilina boulardi parasitoid association

Koinobiont parasitoid insects, which maintain intimate and long-term relationships with their arthropod hosts, constitute an association of ectothermic organisms that is particularly sensitive to temperature variations. Because temperature shows pronounced natural daily fluctuations, we examined if experiments based on a constant temperature range can mask the real effects of the thermal regime on host-parasitoid interactions. The effects of two fluctuating thermal regimes on several developmental parameters of the Drosophila larval parasitoid Leptopilina boulardi were analyzed in this study. Regime 1 included a range of 16-23-16°C and regime 2 included a range of 16-21-26-21-16°C (mean temperature 20.1°C) compared to a 20.1°C constant temperature. Under an average temperature of 20.1°C, which corresponds to a cold condition of L. boulardi development, we showed that the success of parasitism is significantly higher under a fluctuating temperature regime than at constant temperature. A fluctuating regime also correlated with a reduced development time of the parasitoids. In contrast, the thermal regime did not affect the ability of Drosophila to resist parasitoid infestation. Finally, we demonstrated that daily temperature fluctuation prevented the entry into diapause for this species, which is normally observed at a constant temperature of 21°C. Overall, the results reveal that constant temperature experiments can produce misleading results, highlighting the need to study the thermal biology of organisms under fluctuating regimes that reflect natural conditions as closely as possible. This is particularly a major issue in host-parasitoid associations, which constitute a good model to understand the effect of climate warming on interacting species.

Impact of fluctuating temperatures on development of the koinobiont endoparasitoid Venturia canescens

The effect of temperature on the biology of Venturia canescens (Gravenhorst) (Hymenoptera: Ichneumonidae) is well understood under constant temperature conditions, but less so under more natural, fluctuating conditions. Herein we studied the influence of fluctuating temperatures on biological parameters of V. canescens. Parasitized fifth-instar larvae of Ephestia kuehniella Zeller (Lepidoptera: Pyralidae) were reared individually in incubators at six fluctuating temperature regimes (15-19.5°C with a mean of 17.6°C, 17.5-22.5°C with a mean of 19.8°C, 20-30°C with a mean of 22.7°C, 22.5-27.5°C with a mean of 25°C, 25.5-32.5°C with a mean of 28.3°C and 28.5-33°C with a mean of 30°C) until emergence and death of V. canescens adults. Developmental time from parasitism to adult eclosion, adult longevity and survival were recorded at each fluctuating temperature regime. In principle, developmental time decreased with an increase of the mean temperature of the fluctuating temperature regime. Upper and lower threshold temperatures for total development were estimated at 34.9 and 6.7°C, respectively. Optimum temperature for development and thermal constant were 28.6°C and 526.3 degree days, respectively. Adult longevity was also affected by fluctuating temperature, as it was significantly reduced at the highest mean temperature (7.0 days at 30°C) compared to the lowest one (29.4 days at 17.6°C). Survival was low at all tested fluctuating temperatures, apart from mean fluctuating temperature of 25°C (37%). Understanding the thermal biology of V. canescens under more natural conditions is of critical importance in applied contexts. Thus, predictions of biological responses to fluctuating temperatures may be used in population forecasting models which potentially influence decision-making in IPM programs.

Insects in fluctuating thermal environments

All climate change scenarios predict an increase in both global temperature means and the magnitude of seasonal and diel temperature variation. The nonlinear relationship between temperature and biological processes means that fluctuating temperatures lead to physiological, life history, and ecological consequences for ectothermic insects that diverge from those predicted from constant temperatures. Fluctuating temperatures that remain within permissive temperature ranges generally improve performance. By contrast, those which extend to stressful temperatures may have either positive impacts, allowing repair of damage accrued during exposure to thermal extremes, or negative impacts from cumulative damage during successive exposures. We discuss the mechanisms underlying these differing effects. Fluctuating temperatures could be used to enhance or weaken insects in applied rearing programs, and any prediction of insect performance in the field-including models of climate change or population performance-must account for the effect of fluctuating temperatures.

Experimental evolution for generalists and specialists reveals multivariate genetic constraints on thermal reaction norms

Theory predicts the emergence of generalists in variable environments and antagonistic pleiotropy to favour specialists in constant environments, but empirical data seldom support such generalist-specialist trade-offs. We selected for generalists and specialists in the dung fly Sepsis punctum (Diptera: Sepsidae) under conditions that we predicted would reveal antagonistic pleiotropy and multivariate trade-offs underlying thermal reaction norms for juvenile development. We performed replicated laboratory evolution using four treatments: adaptation at a hot (31 °C) or a cold (15 °C) temperature, or under regimes fluctuating between these temperatures, either within or between generations. After 20 generations, we assessed parental effects and genetic responses of thermal reaction norms for three correlated life-history traits: size at maturity, juvenile growth rate and juvenile survival. We find evidence for antagonistic pleiotropy for performance at hot and cold temperatures, and a temperature-mediated trade-off between juvenile survival and size at maturity, suggesting that trade-offs associated with environmental tolerance can arise via intensified evolutionary compromises between genetically correlated traits. However, despite this antagonistic pleiotropy, we found no support for the evolution of increased thermal tolerance breadth at the expense of reduced maximal performance, suggesting low genetic variance in the generalist-specialist dimension.