Impact of hot events at different developmental stages of a moth: the closer to adult stage, the less reproductive output

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.

Evidence for transient deleterious thermal acclimation in field recapture rates of an invasive tropical species, Bactrocera dorsalis (Diptera: Tephritidae)

Knowing how environmental conditions affect performance traits in pest insects is important to improve pest management strategies. It can be informative for monitoring, but also for control programs where insects are mass-reared, and field-released. Here, we investigated how adult thermal acclimation in sterile Bactrocera dorsalis affects dispersal and recapture rates in the field using a mark-release-recapture method. We also considered how current abiotic factors may affect recapture rates and interact with thermal history. We found that acclimation at 20 or 30 °C for 4 d prior to release reduced the number of recaptures in comparison with the 25 °C control group, but with no differences between groups in the willingness to disperse upon release. However, the deleterious effects of acclimation were only detectable in the first week following release, whereafter only the recent abiotic conditions explained recapture rates. In addition, we found that recent field conditions contributed more than thermal history to explain patterns of recaptures. The two most important variables affecting the number of recaptures were the maximum temperature and the average relative humidity experienced in the 24 h preceding trapping. Our results add to the handful of studies that have considered the effect of thermal acclimation on insect field performance, but notably lend support to the deleterious acclimation hypothesis among the various hypotheses that have been proposed. Finally, this study shows that there are specific abiotic conditions (cold/hot and dry) in which recaptures will be reduced, which may therefore bias estimates of wild population size.

Effects of short-term high temperature at different life stages on reproductive fitness in Mythimna separata (Lepidoptera: Noctuidae)

Extreme heat events commonly occur under climate warming. All life stages of insects may experience the occurrence of extremely high temperatures. However, the effects of short-term extreme heat events on life-history traits remain unclear in most migratory pests. Here, we investigated the biological effects of short-term heat exposure (35 °C for 4 h) at different life stages on Mythimna separata Walker (Lepidoptera: Noctuidae), a typical migratory pest. We found that the reproductive sensitivity of pupae and adults was higher than that of 3rd-instar larvae. Increasing the frequency of heat exposure decreased the reproductive performance of M. separata at all life stages. Parental short-term heat exposures could cause transgenerational damage to offspring survival and reproductive fitness when the exposure frequency reached 3 times. Our results suggest that short-term exposure to extreme temperatures could impact reproductive fitness across different life stages in M. separata. This should be taken into consideration in the population prediction of migratory pests under climate change.

Physiological and behavioral basis of diamondback moth Plutella xylostella migration and its association with heat stress

BACKGROUND

Migration is a strategy that shifts insects to more favorable habitats in response to deteriorating local environmental conditions. The ecological factors that govern insect migration are poorly understood for many species. Plutella xylostella causes great losses in Brassica vegetable and oilseed crops, and undergoes mass migration. However, the physiological and behavioral basis for distinguishing migratory individuals and the factors driving its migration remain unclear.

RESULTS

Daily light trap catches conducted from April to July in a field population of P. xylostella in central China revealed a sharp decline in abundance from late-May. Analysis of ovarian development levels showed that the proportion of sexually immature females gradually increased, while the mating rate decreased, indicating that generations occurring in May mainly resulted from local breeding and that emigration began in late-May. Physiological and behavioral analyses revealed that emigrant populations had a higher take-off proportion, stronger flight capacity and greater energy reserves of triglyceride compared to residents. Furthermore, a gradual increase in temperature from 24 °C to >30 °C during larval development resulted in a significant delay in oogenesis and increased take-off propensity of adults compared with the control treatment reared at a constant temperature of 24 °C.

CONCLUSION

Our results provide the physiological and behavioral factors that underpin mass migration in P. xylostella, and demonstrate that exposure to increased temperature increases their migration propensity at the cost of reproductive output. This study sheds light on understanding the factors that influence population dynamics, migratory propensity and reproductive tradeoffs in migratory insects. © 2023 Society of Chemical Industry.

The Pupa Stage Is the Most Sensitive to Hypoxia in Drosophila melanogaster

Hypoxia not only plays a critical role in multiple disease conditions; it also influences the growth and development of cells, tissues and organs. To identify novel hypoxia-related mechanisms involved in cell and tissue growth, studying a precise hypoxia-sensitive time window can be an effective approach. Drosophila melanogaster has been a useful model organism for studying a variety of conditions, and we focused in this study on the life cycle stages of Drosophila to investigate their hypoxia sensitivity. When normoxia-grown flies were treated with 4% O2 at the pupa stage for 3, 2 and 1 day/s, the eclosion rates were 6.1%, 66.7% and 96.4%, respectively, and, when 4% O2 was kept for the whole pupa stage, this regimen was lethal. Surprisingly, when our hypoxia-adapted flies who normally live in 4% O2 were treated with 4% O2 at the pupa stage, no fly eclosed. Within the pupa stage, the pupae at 2 and 3 days after pupae formation (APF), when treated for 2 days, demonstrated 12.5 ± 8.5% and 23.6 ± 1.6% eclosion, respectively, but this was completely lethal when treated for 3 days. We conclude that pupae, at 2 days APF and for a duration of a minimum of 2 days, were the most sensitive to hypoxia. Our data from our hypoxia-adapted flies clearly indicate that epigenetic factors play a critical role in pupa-stage hypoxia sensitivity.

Heat-induced female biased sex ratio during development is not mitigated after prolonged thermal selection

BACKGROUND

The negative impacts of climate change on biodiversity are consistently increasing. Developmental stages are particularly sensitive in many ectotherms. Moreover, sex-specific differences in how organisms cope with thermal stress can produce biased sex ratios upon emergence, with potentially major impacts on population persistence. This is an issue that needs investigation, particularly testing whether thermal selection can alleviate sex ratio distortions in the long-term is a critical but neglected issue. Here, we report an experiment analyzing the sex ratio patterns at different developmental temperatures in Drosophila subobscura populations subjected to long-term experimental evolution (~ 30 generations) under a warming environment.

RESULTS

We show that exposure to high developmental temperatures consistently promotes sex ratio imbalance upon emergence, with a higher number of female than male offspring. Furthermore, we found that thermal selection resulting from evolution in a warming environment did not alleviate such sex ratio distortions generated by heat stress.

CONCLUSIONS

We demonstrate that heat stress during development can lead to clear sex ratio deviations upon emergence likely because of differential survival between sexes. In face of these findings, it is likely that sex ratio deviations of this sort occur in natural populations when facing environmental perturbation. The inability of many insects to avoid thermal shifts during their (more) sessile developmental stages makes this finding particularly troublesome for population subsistence in face of climate warming events.

Bees exposed to climate change are more sensitive to pesticides

Bee populations are exposed to multiple stressors, including land-use change, biological invasions, climate change, and pesticide exposure, that may interact synergistically. We analyze the combined effects of climate warming and sublethal insecticide exposure in the solitary bee Osmia cornuta. Previous Osmia studies show that warm wintering temperatures cause body weight loss, lipid consumption, and fat body depletion. Because the fat body plays a key role in xenobiotic detoxification, we expected that bees exposed to climate warming scenarios would be more sensitive to pesticides. We exposed O. cornuta females to three wintering treatments: current scenario (2007-2012 temperatures), near-future (2021-2050 projected temperatures), and distant-future (2051-2080). Upon emergence in spring, bees were orally exposed to three sublethal doses of an insecticide (Closer, a.i. sulfoxaflor; 0, 4.55 and 11.64 ng a.i./bee). We measured the combined effects of wintering and insecticide exposure on phototactic response, syrup consumption, and longevity. Wintering treatment by itself did not affect winter mortality, but body weight loss increased with increasing wintering temperatures. Similarly, wintering treatment by itself hardly influenced phototactic response or syrup consumption. However, bees wintered at the warmest temperatures had shorter longevity, a strong fecundity predictor in Osmia. Insecticide exposure, especially at the high dose, impaired the ability of bees to respond to light, and resulted in reduced syrup consumption and longevity. The combination of the warmest winter and the high insecticide dose resulted in a 70% longevity decrease. Smaller bees, resulting from smaller pollen-nectar provisions, had shorter longevity suggesting nutritional stress may further compromise fecundity in O. cornuta. Our results show a synergistic interaction between two major drivers of bee declines, and indicate that bees will become more sensitive to pesticides under the current global warming scenario. Our findings have important implications for pesticide regulation and underscore the need to consider multiple stressors to understand bee declines.

Seasonal energetics: are insects constrained by energy during dormancy?

In seasonal environments, many animals, including insects, enter dormancy, where they are limited to a fixed energy budget. The inability to replenish energetic stores during these periods suggests insects should be constrained by pre-dormancy energy stores. Over the last century, the community of researchers working on survival during dormancy has operated under the strong assumption that energy limitation is a key fitness trait driving the evolution of seasonal strategies. That is, energy use has to be minimized during dormancy because insects otherwise run out of energy and die during dormancy, or are left with too little energy to complete development, reproductive maturation or other costly post-dormancy processes such as dispersal or nest building. But if energy is so strongly constrained during dormancy, how can some insects - even within the same species and population - be dormant in very warm environments or show prolonged dormancy for many successive years? In this Commentary, we discuss major assumptions regarding dormancy energetics and outline cases where insects appear to align with our assumptions and where they do not. We then highlight several research directions that could help link organismal energy use with landscape-level changes. Overall, the optimal energetic strategy during dormancy might not be to simply minimize metabolic rate, but instead to maintain a level that matches the demands of the specific life-history strategy. Given the influence of temperature on energy use rates of insects in winter, understanding dormancy energetic strategies is critical in order to determine the potential impacts of climate change on insects in seasonal environments.

Effects of heat tolerance on the gut microbiota of Sarcophaga peregrina (Diptera: Sarcophagidae) and impacts on the life history traits

BACKGROUND

Heat tolerance is a distinct abiotic factor affecting the distribution and abundance of insects. Gut microbiota can contribute to host fitness, thereby increasing resistance to abiotic stress conditions. In this study, Sarcophaga peregrina is closely associated with human life in ecological habits and shows remarkable adaptability to daily and seasonal temperature fluctuations. To date, the role of gut microbiota in S. peregrina response to heat stress and its influence on the host phenotypic variability remain poorly studied.

METHODS

We exposed S. peregrina to heat stress at 40 °C for 3 h every day throughout the developmental stages from newly hatched larva to adult, after which gut DNA was extracted from third-instar larvae, early pupal stage, late pupal stage, and newly emerged adults, respectively. Then, 16S rRNA microbial community analyses were performed.

RESULTS

Firstly, we analyzed whether heat stress could have an impact on the life history traits of S. peregrina and showed that the growth rate of larvae was higher and the developmental time was significantly shorter after heat stress. We then proposed the role of the gut microbiota in the heat tolerance of S. peregrina, which indicated that the bacterial abundance and community structure changed significantly after heat tolerance. In particular, the relative abundance of Wohlfahrtiimonas and Ignatzschineria was higher in the third-instar larval larvae; the former increased and the latter decreased significantly after heat stress. To further explore the effect of disturbing the microbial community on thermotolerant phenotype, newly hatched larvae were fed with amikacin under heat stress, which indicated that the larval length and the whole developmental cycle was significantly shorter.

CONCLUSION

This study indicated that Wohlfahrtiimonas and Ignatzschineria should play an important role in the post-feeding stage under heat stress, but further study is still needed. In general, heat tolerance can affect the gut microbial community structure, which in turn affects the fitness of the host.

Weathering the hunt: The role of barometric pressure in predator insects' foraging behaviour

Abiotic factors strongly influence ecological interactions and the spatial distribution of organisms. Despite the essential role of barometric pressure, its influence on insect behaviour remains poorly understood, particularly in predators. The effect of barometric pressure variation can significantly impact biological control programs involving entomophagous insects, as they must efficiently allocate time and energy to search for prey in challenging environments. We investigated how predatory insects from different taxonomic groups (Coleoptera, Dermaptera and Neuroptera) adapt their foraging behaviour in response to variations in barometric pressure (low, medium and high). We also examined the response of different life stages to changes in pressure regimes during foraging activities. Our results showed that the searching time of Doru luteipes (Dermaptera: Forficulidae) was faster in a favourable high-pressure regime, whereas Chrysoperla externa (Neuroptera: Chrysopidae) and Eriopis connexa (Coleoptera: Coccinellidae) had similar searching times under varying pressure regimes. Although no differences in prey feeding time were observed among the studied species, the consumption rate was influenced by low barometric pressure leading to a decrease in the number of preyed eggs. Moreover, we provide novel insights into how hemimetabolous (D. luteipes) and holometabolous (E. connexa) species at different life stages respond to barometric pressure. Doru luteipes nymphs and adults had similar consumption rates across all pressure regimes tested, whereas E. connexa larvae consumed fewer eggs under low barometric pressure, but adults were unaffected. This highlights the importance of investigating how abiotic factors affect insects foraging efficiency and predator-prey interactions. Such studies are especially relevant in the current context of climate change, as even subtle changes in abiotic factors can have strong effects on insect behaviour. Barometric pressure is a key meteorological variable that serve as a warning signal for insects to seek shelter and avoid exposure to weather events that could potentially increase their mortality. Understanding the effects of barometric pressure on predatory insects' behaviour can help us develop more effective pest management strategies and promote the resilience of agroecosystems. We provide new insights into the complex relationship between barometric pressure and predator-prey interactions.

Effects of short-term extreme temperature treatment on the development and reproductive capacity of Encarsia formosa

Encarsia formosa is a natural enemy of the invasive pest Bemisia tabaci and is known to be a dominant parasitic. The frequency and magnitude of climate extremes, particularly temperature extremes, have increased, which has put insect populations at risk. However, the effects of temperature extremes on E. formosa are not well understood. To examine the impact of short-term extreme temperature exposure on the development and reproduction of E. formosa, eggs, larvae, pupae, and adults were exposed to high/low temperature treatments (HLT₂₅, HLT₅₀, LLT₂₅, and LLT₅₀). Our findings indicate that the pupal stage of E. formosa exhibited the strongest tolerance to both heat and cold, while adults exhibited a weaker tolerance. The shortest egg-to-adult development period of 12.65 days was observed in E. formosa exposed to HLT₅₀ treatment during the egg-larval stage. The parasitism peak of the adult stage was delayed by 1-6 days after exposure to extreme temperatures during the egg-larval stage. Conversely, the parasitism peak was advanced by 1-3 days after exposure to extreme temperatures during the pupal and adult stages. The eclosion rate, total parasitism, eclosion rate of the F₁ generation, and adult longevity of the F₁ generation were lower in the treatment groups than in the control groups. The F₁ generation's development period was prolonged to 15.49 and 15.19 days after exposure to HLT₂₅ and HLT₅₀ treatments, respectively, during the egg-larval stage. The F₁ generation's development period was shortened to 13.33 days after exposure to LLT₅₀ treatment during the pupal stage. Male individuals appeared in the F₁ generation after exposure to HLT₅₀ treatment during the pupal stage, with females accounting for only 56.38%. Our results demonstrate that short-term exposure to extreme temperatures has detrimental effects on the growth and reproduction of E. formosa. In field biocontrol against E. formosa, the release of E. formosa should be avoided as much as possible when the ambient temperature is higher than 35°C or lower than 0°C. During extreme temperature conditions, timely supplementation and release of E. formosa population, along with ventilation and cooling in greenhouse facilities during summer, are necessary for better pest control efficacy.

Still standing: The heat protection delivered by a facultative symbiont to its aphid host is resilient to repeated thermal stress

Insects have evolved diverse strategies to resist extreme high temperatures (EHT). The adaptive value of such strategies has to be evaluated when organisms experience multiple EHT events during their lifetime, as predicted in a changing climate. This is particularly the case for associations with facultative microbial partners involved in insect heat tolerance, the resilience of which to repeated heat stress has never been studied. We compared two artificial lines of the pea aphid (Acyrthosiphon pisum) differing by the absence or presence of the heat-protective facultative bacterium Serratia symbiotica. We exposed insect nymphs to a varying number of EHT events (between 0 and 3), and recorded fitness parameters. Except survival traits, fitness estimates were affected by the interaction between aphid infection status (absence/presence of S. symbiotica) and thermal treatment (number of heat shocks applied). Costs of bacterial infection were detected in the absence of thermal stress: symbiont-hosting aphids incurred longer development, decreased fecundity and body size. However, symbiotic infection turned neutral, and even beneficial for some traits (development and body size), as the number of heat shocks increased, and compared to the aposymbiotic strain. Conversely, symbiotic infection mediated aphid response to heat shock(s): fitness decreased only in the uninfected group. These findings suggest that (i) the facultative symbiont may alternatively act as a pathogen, commensal or mutualist depending on thermal environment, and (ii) the heat protection it delivered to its host persists under frequent EHT. We discuss eco-evolutionary implications and the role of potentially confounding factors (stage-specific effects, genetic polymorphism displayed by the obligate symbiont).

Effects of periodically repeated high-temperature exposure on the immediate and subsequent fitness of different developmental stages of Propylaea japonica

BACKGROUND

Repeated extreme high temperature occurs frequently in summer. Propylaea japonica is a predominant predator in South-East Asia and has been considered as a successful natural enemy to control aphids. However, how repeated extreme high temperature affects the fitness of P. japonica remains unclear. This study evaluated the immediate and subsequent fitness of P. japonica when egg, larva, pupa, and adult were exposed to repeated high temperatures (39, 41, or 43 °C for 3 h exposure duration per day) during several days.

RESULTS

The effect of repeated high temperatures on P. japonica fitness was stage-specific: the egg stage was the most sensitive, the larval and pupal stages were moderately resistant and the adult stage was the most resistant to heat. Repeated high temperatures extended the immature developmental time and decreased the sex ratio of eggs treated with these temperatures, compared to control eggs. A temperature of 39 °C had no significant effect on the pre-oviposition period, oviposition period, fecundity (except stress pupa), or longevity compared with the control, but negative carry-over effects above 39 °C on subsequent stages were found.

CONCLUSION

Repeated high temperature for consecutive days not only had a significant effect on the immediate survival and developmental time, but also had deleterious effects on the subsequent development and performance of P. japonica. The present study provides valuable information for understanding and utilizing P. japonica to control aphids in challenging environments. © 2022 Society of Chemical Industry.

Characterization of Cold and Heat Tolerance of Bactrocera tau (Walker)

Simple Summary

Insects are often stressed by adverse factors in their natural environment. Temperature is a crucial driver of insect activity, adaptability, and distribution, and therefore, it greatly impacts the invasive success of alien pests. Bactrocera tau (Walker) is an invasive polyphagous herbivore of vegetables and fruits, now a pest of global importance. This study provides useful information about B. tau’s cold- and heat tolerance to extremely low and high temperatures. Its different life stages (i.e., egg, larvae, pupae, and adult) had high survival rates under adverse temperatures spanning −5 to 0 °C and 39 to 42 °C. These findings suggest that B. tau possesses a wide temperature threshold range for survival, which likely contributes to its better establishment and expansion in new regions. Meanwhile, fitted curves were used to quantify B. tau’s tolerance potential as a function of both stress intensity (heat or cold) and exposure duration. The information generated in this study will contribute to our understanding of thermal tolerance in B. tau and could also provide insights for devising phytosanitary control approaches.

Abstract

Bactrocera tau (Walker) (Diptera: Tephritidae) is a serious, economically important invasive pest that has spread and been established in many regions worldwide. Temperature is a crucial abiotic factor governing insect activity, fitness, and geographical distribution. Yet, surprisingly, the tolerance of B. tau to extreme cold and heat stress remains unclear. Here, we measured the supercooling point (SCP) of different life stages of B. tau. Further, several life stages of B. tau (egg, 1st, 2nd, and 3rd instar larvae, 1-day-old pupae, and 3-day-old adult) were subjected to six low temperatures (−9, −7, −5, −3, −1, and 0 °C) and six high temperatures (39, 40, 41, 42, 43, and 44 °C) for various durations (0.5, 1.0, 2.0, and 4.0 h), and three-way survival–time–temperature relationships were investigated. We found that the SCPs differed significantly among different life stages of B. tau, being the lowest for SCP of eggs, at −25.82 ± 0.51 °C. There was no significant effect of sex on the mean SCPs of B. tau adults, except for 45- to 50-day-old flies. In addition, an interaction effect was uncovered between tested temperatures and exposure duration upon B. tau mortality at different life stages. Eggs exhibited the strongest cold tolerance, yet the weakest heat tolerance. The 3rd instar larvae were the most heat- and cold tolerant among larval stages, followed by the 2nd and 1st instar larvae. The upper limit of the chill injury zone (ULCIZ) for 3-day-old adult and 1-day-old pupae was −2.51 °C and −2.50 °C, respectively, while their corresponding lower limit of thermal injury zone (LLTIZ) was 39.39 °C and 38.29 °C. This paper presents valuable data to provide an integrated knowledge for understanding the cold and heat tolerance potential of B. tau and ensure the proper implementation of post-harvest phytosanitary protocols for this pest’s disinfestation.

Developmental timing of extreme temperature events (heat waves) disrupts host-parasitoid interactions

When thermal tolerances differ between interacting species, extreme temperature events (heat waves) will alter the ecological outcomes. The parasitoid wasp Cotesia congregata suffers high mortality when reared throughout development at temperatures that are nonstressful for its host, Manduca sexta. However, the effects of short-term heat stress during parasitoid development are unknown in this host-parasitoid system.Here, we investigate how duration of exposure, daily maximum temperature, and the developmental timing of heat waves impact the performance of C. congregata and its host¸ M. sexta. We find that the developmental timing of short-term heat waves strongly determines parasitoid and host outcomes.Heat waves during parasitoid embryonic development resulted in complete wasp mortality and the production of giant, long-lived hosts. Heat waves during the 1st-instar had little effect on wasp success, whereas heat waves during the parasitoid's nutritionally and hormonally critical 2nd instar greatly reduced wasp emergence and eclosion. The temperature and duration of heat waves experienced early in development determined what proportion of hosts had complete parasitoid mortality and abnormal phenotypes.Our results suggest that the timing of extreme temperature events will be crucial to determining the ecological impacts on this host-parasitoid system. Discrepancies in thermal tolerance between interacting species and across development will have important ramifications on ecosystem responses to climate change.

Plastic responses of survival and fertility following heat stress in pupal and adult Drosophila virilis

The impact of rising global temperatures on survival and reproduction is putting many species at risk of extinction. In particular, it has recently been shown that thermal effects on reproduction, especially limits to male fertility, can underpin species distributions in insects. However, the physiological factors influencing fertility at high temperatures are poorly understood. Key factors that affect somatic thermal tolerance such as hardening, the ability to phenotypically increase thermal tolerance after a mild heat shock, and the differential impact of temperature on different life stages are largely unexplored for thermal fertility tolerance. Here, we examine the impact of high temperatures on male fertility in the cosmopolitan fruit fly Drosophila virilis. We first determined whether temperature stress at either the pupal or adult life history stage impacts fertility. We then tested the capacity for heat-hardening to mitigate heat-induced sterility. We found that thermal stress reduces fertility in different ways in pupae and adults. Pupal heat stress delays sexual maturity, whereas males heated as adults can reproduce initially following heat stress, but become sterile within seven days. We also found evidence that while heat-hardening in D. virilis can improve high temperature survival, there is no significant protective impact of this same hardening treatment on fertility. These results suggest that males may be unable to prevent the costs of high temperature stress on fertility through heat-hardening, which limits a species' ability to quickly and effectively reduce fertility loss in the face of short-term high temperature events.

Fluctuating heat stress during development exposes reproductive costs and putative benefits

Temperature and thermal variability are increasing worldwide, with well-known survival consequences. However, effects on other potentially more thermally sensitive reproductive traits are less understood, especially when considering thermal variation. Studying the consequences of male reproduction in the context of climate warming and ability to adapt is becoming increasingly relevant. Our goals were to test how exposure to different average temperatures that either fluctuated or remained constant impacts different male reproductive performance traits and to assess adaptive potential to future heat stress. We took advantage of a set of Drosophila melanogaster isogenic lines of different genotypes, exposing them to four different thermal conditions. These conditions represented a benign and a stressful mean temperature, applied either constantly or fluctuating around the mean and experienced during development when heat stress avoidance is hindered because of restricted mobility. We measured subsequent male reproductive performance for mating success, fertility, number of offspring produced and offspring sex ratio, and calculated the influence of thermal stress on estimated heritability and evolvability of these reproductive traits. Both costs and benefits to different thermal conditions on reproductive performance were found, with some responses varying between genotypes. Mating success improved under fluctuating benign temperature conditions and declined as temperature stress increased regardless of genotype. Fertility and productivity were severely reduced at fluctuating mean high temperature for all genotypes, but some genotypes were unaffected at constant high mean temperature. These more thermally robust genotypes showed a slight increase in productivity under the fluctuating benign condition compared to constant high temperature, despite both thermal conditions sharing the same temperature for 6 hr daily. Increasing thermal stress resulted in higher heritability and evolvability. Overall, the effects of temperature on reproductive performance depended on the trait and genotype; performance of some traits slightly increased when high temperatures were experienced for short periods but decreased substantially even when experiencing a benign temperature for a portion of each day. While thermal stress increased genetic variation that could provide adaptive potential against climate warming, this is unlikely to compensate for the overall severe negative effect on reproductive performance as mean temperature and variance increase.

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.

Effects of brief exposure to high temperatures on the survival and fecundity of Athetis lepigone (Lepidoptera: Noctuidae)

In Huang-Huai-Hai Summer Corn Region of China, brief periods of high summer temperatures have been reported with increasing frequency in recent years. Athetis lepigone is a cosmopolitan insect which causes severe damage on summer corn seedlings. To understand how high summer temperatures may affect the population dynamics of A. lepigone, we exposed different developmental stages (1, 2 and 4-day old eggs; 1, 6, 12 and 18-day old larvae; 1, 3 and 6-day old pupae; and 1 and 2-day old female and male adults) to 41 °C for periods of various length (0.5, 1, 2, 4 and 6 h): The rearing temperature (constant 26 °C) was used as control. After heat treatment, all individuals were transferred to a 26 °C climate chamber for further development. The effects on immediate survival, maturation success to adulthood, and female fecundity were studied. Eggs, young larvae, late pupae and newly emerged adults had relatively higher immediate survival rates than the other experimental groups. Heat treatment at the egg and larval stages had no impact on development to adulthood and on female fecundity, while it significantly reduced the survival rate of larvae but not of eggs. Brief exposure to high temperature during the early pupal stage and as adults depressed female fecundity whereas exposure during the late pupal stage had no effect.

Within- and Trans-Generational Life History Responses to Diurnal Temperature Amplitudes of the Pupal Stage in the Diamondback Moth

Diurnal temperature fluctuations in nature can have a significant effect on many ectodermic traits. However, studies on the effects of diurnal temperature fluctuations on organisms, especially the effects on specific life stages, are still limited. We examined the immediate effects of the same average temperature (25°C) and different temperature amplitudes (±4, ±6, ±8, ±10, ±12°C) on the development and survival of Plutella xylostella (Lepidoptera: Plutellidae). We also assessed carry-over effects on adult longevity, reproduction, development, and survival of offspring across generations. The effect of moderate temperature amplitudes was similar to that of constant temperature. Wide temperature amplitudes inhibited the development of pupae, reduced total reproduction, lowered intrinsic rates of population growth, and slowed the development and survival of eggs on the first day, but the proportion of females ovipositing on the first three days increased. Insects coped with the adverse effects of wide temperature amplitudes by laying eggs as soon as possible. Our results confirmed that a logistic model based on daily average temperature cannot predict development rates under wide temperature amplitudes. These findings highlight the effect of environmental temperature fluctuations at the pupal stage on the development and oviposition patterns of P. xylostella and should be fully considered when predicting field occurrence.

Demography of Mythimna separata (Lepidoptera: Noctuidae) at outdoor fluctuating temperatures

The oriental armyworm Mythimna separata (Walker) (Lepidoptera: Noctuidae) is a major migratory pest of cereal crops in East Asia, South Asia and Australia. To comprehensively understand the ecological tolerance of M. separata, we collected life table data of individuals from four consecutive generations reared under outdoor natural fluctuating temperatures from 15 April to 17 October 2018 in Yangling, Shaanxi, China. The results showed that the immature stage in early summer and summer were shorter than in spring and autumn. High mortality in late larval instar and pupal stages was observed in the summer generation. The adult pre-oviposition period in autumn was longer than the other seasons. The population in the earlier two seasons had heavier pupae and higher fecundity than the population in the latter two seasons. The intrinsic rate of increase and the finite rate of increase was the highest in early summer (r = 0.1292 day-1, λ = 1.1391 day-1), followed by spring (r = 0.1102 day-1, λ = 1.1165 day-1), and was the lowest in summer (r = 0.0281 day-1, λ = 1.0293 day-1). The results of this study would be useful to predict the population dynamics of M. separata and deepen our standing of the adaptiveness of this migratory pest in natural fluctuating ambient environments.

Water immersion tolerance by larval instars of stable fly, Stomoxys calcitrans, L1758 (Diptera: Muscidae) impairs the fitness performance of their subsequent stages

BACKGROUND

In holometabolous insects, environmental factors experienced in pre-imaginal life stages affect the life-history traits within that stage and can also influence subsequent life stages. Here, I assessed tolerance to water immersion by the larval instars of the stable fly, Stomoxys calcitrans L. (Diptera: Muscidae) and its impact on the life-history traits of their subsequent life stages.

RESULTS

After submerging the three larval instars of S. calcitrans in distilled water, I found that the first instar larvae remained active for longer as compared to the second and third instar larvae. Also, the first instar larvae took a longer period to recover from the stress-induced immobility when removed from the water and returned to ambient temperature. When I followed the development of individuals of each larval instar that survived from water immersion, I found that their developmental time, weight, pupation percentage, adult emergence percentage and adult weight were negatively affected by this stressor. However, the weight of S. calcitrans adults developed from immersed first larval instar individuals was not affected by water immersion whereas their counterparts developed from immersed second and third larval instars had lower body weight. This suggests that in S. calcitrans, water immersion stress at the earlier stage is less detrimental than that experienced at late stages.

CONCLUSION

This study provides a comparative overview of the fitness consequences associated with water immersion stress during S. calcitrans larval ontogeny. The results prove that the fitness shift induced by water immersion in S. calcitrans is stage-specific. My results illustrate the importance of considering each larval instar when assessing the impact of environmental factors on holometabolous insect performance as these may be decoupled by metamorphosis.

How to stand the heat? Post-stress nutrition and developmental stage determine insect response to a heat wave

Organisms are increasingly confronted with intense and long-lasting heat waves. In insects, the effects of heat waves on individual performance can vary in magnitude both within (e.g. from one larval instar to another) and between life stages. However, the reasons underlying these stage-dependent effects are not fully understood. There are several lines of evidence suggesting that individual ability to withstand a heat stress depends on mechanisms based on nutrition and supporting energetically physiological stress responses. Hence, we tested the hypothesis that the efficiency of these food-based buffering mechanisms may vary between different larval instars of a phytophagous insect. Using larvae of the moth Lobesia botrana, we examined the importance of post-stress food quality in insect response to a non-lethal heat wave at two distinct larval instars. Three major conclusions were drawn from this work. First, heat waves induced an overall decline in larval performance (delayed development, depressed immunity). Second, food quality primarily mediated the insect's ability to respond to the heat stress: the reduction in performance following heat wave application was mostly restricted to individuals with access to low-quality food after the heat stress. Third, larval instars differed in their susceptibility to this combination of thermal and food stressors, but conclusions about the instar being the most vulnerable differed in a trait-specific manner. In a global warming context, this study may shed additional light on the combination of direct and indirect (through alteration of plant nutritional value) effects of rising temperatures on the ecology and the evolution of phytophagous insects.

Responses of Manduca sexta larvae to heat waves

Climate change is increasing the frequency of heat waves and other extreme weather events experienced by organisms. How does the number and developmental timing of heat waves affect survival, growth and development of insects? Do heat waves early in development alter performance later in development? We addressed these questions using experimental heat waves with larvae of the tobacco hornworm, Manduca sexta. The experiments used diurnally fluctuating temperature treatments differing in the number (0-3) and developmental timing (early, middle and/or late in larval development) of heat waves, in which a single heat wave involved three consecutive days with a daily maximum temperature of 42°C. Survival to pupation declined with increasing number of heat waves. Multiple (but not single) heat waves significantly reduced development time and pupal mass; the best models for the data indicated that both the number and developmental timing of heat waves affected performance. In addition, heat waves earlier in development significantly reduced growth and development rates later in larval development. Our results illustrate how the frequency and developmental timing of sublethal heat waves can have important consequences for life history traits in insects.

Fertility and mortality impacts of thermal stress from experimental heatwaves on different life stages and their recovery in a model insect

With climate change creating a more volatile atmosphere, heatwaves that create thermal stress for living systems will become stronger and more frequent. Using the flour beetle Tribolium castaneum, we measure the impacts of thermal stress from experimental heatwaves in the laboratory on reproduction and survival across different insect life stages, and the extent and pace of any recovery. We exposed larvae, pupae, juvenile and mature adult male beetles to 5-day periods of heat stress where temperatures were maintained at either 40°C or 42°C, a few degrees above the 35°C optimum for this species' population productivity, and then measured survival and reproduction compared with controls at 30°C. Mortality due to thermal stress was greatest among juvenile life stages. Male reproductive function was specifically damaged by high temperatures, especially if experienced through pupal or immature life stages when complete sterility was shown at reproductive maturity; larval exposure did not damage adult male fertility. High temperatures impaired testis development and the production of viable sperm, with damage being strongest when experienced during pupal or juvenile adult stages. Despite this disruption, males recovered from heat stress and, depending on the stage of exposure, testis size, sperm production and fertility returned to normal 15-28 days after exposure. Our experiments reveal how thermal stress from heatwave conditions could impact on insect survival and reproduction across different life stages, and the potential and timescales of recovery.

Survive a Warming Climate: Insect Responses to Extreme High Temperatures

Global change includes a substantial increase in the frequency and intensity of extreme high temperatures (EHTs), which influence insects at almost all levels. The number of studies showing the ecological importance of EHTs has risen in recent years, but the knowledge is rather dispersed in the contemporary literature. In this article, we review the biological and ecological effects of EHTs actually experienced in the field, i.e., when coupled to fluctuating thermal regimes. First, we characterize EHTs in the field. Then, we summarize the impacts of EHTs on insects at various levels and the processes allowing insects to buffer EHTs. Finally, we argue that the mechanisms leading to positive or negative impacts of EHTs on insects can only be resolved from integrative approaches considering natural thermal regimes. Thermal extremes, perhaps more than the gradual increase in mean temperature, drive insect responses to climate change, with crucial impacts on pest management and biodiversity conservation.

Growth, stress, and acclimation responses to fluctuating temperatures in field and domesticated populations of Manduca sexta

Diurnal fluctuations in temperature are ubiquitous in terrestrial environments, and insects and other ectotherms have evolved to tolerate or acclimate to such fluctuations. Few studies have examined whether ectotherms acclimate to diurnal temperature fluctuations, or how natural and domesticated populations differ in their responses to diurnal fluctuations. We examine how diurnally fluctuating temperatures during development affect growth, acclimation, and stress responses for two populations of Manduca sexta: a field population that typically experiences wide variation in mean and fluctuations in temperature, and a laboratory population that has been domesticated in nearly constant temperatures for more than 300 generations. Laboratory experiments showed that diurnal fluctuations throughout larval development reduced pupal mass for the laboratory but not the field population. The differing effects of diurnal fluctuations were greatest at higher mean temperature (30°C): Here diurnal fluctuations reduced pupal mass and increased pupal development time for the laboratory population, but had little effect for the field population. We also evaluated how mean and fluctuations in temperature during early larval development affected growth rate during the final larval instar as a function of test temperature. At an intermediate (25°C) mean temperature, both the laboratory and field population showed a positive acclimation response to diurnal fluctuations, in which subsequent growth rate was significantly higher at most test temperatures. In contrast at higher mean temperature (30°C), diurnal fluctuations significantly reduced subsequent growth rate at most test temperatures for the laboratory population, but not for the field population. These results suggest that during domestication in constant temperatures, the laboratory population has lost the capacity to tolerate or acclimate to high and fluctuating temperatures. Population differences in acclimation capacity in response to temperature fluctuations have not been previously demonstrated, but they may be important for understanding the evolution of reaction norms and performance curves.

High developmental temperature leads to low reproduction despite adult temperature

Phenotypic plasticity can help organisms cope with changing thermal conditions and it may depend on which life-stage the thermal stress is imposed: for instance, exposure to stressful temperatures during development can trigger a positive plastic response in adults. Here, we analyze the thermal plastic response of laboratory populations of Drosophila subobscura, derived from two contrasting latitudes of the European cline. We measured reproductive performance through fecundity characters, after the experimental populations were exposed to five thermal treatments, with different combinations of developmental and adult temperatures (14 °C, 18 °C, or 26 °C). Our questions were whether (1) adult performance changes with exposure to higher (or lower) temperatures during development; (2) flies raised at lower temperatures outperform those developed at higher ones, supporting the "colder is better" hypothesis; (3) there is a cumulative effect on adult performance of exposing both juveniles and adults to higher (or lower) temperatures; (4) there is evidence for biogeographical effects on adult performance. Our main findings were that (1) higher developmental temperatures led to low reproductive performance regardless of adult temperature, while at lower temperatures reduced performance only occurred when colder conditions were persistent across juvenile and adult stages; (2) flies raised at lower temperatures did not always outperform those developed at other temperatures; (3) there were no harmful cumulative effects after exposing both juveniles and adults to higher temperatures; (4) both latitudinal populations showed similar thermal plasticity patterns. The negative effect of high developmental temperature on reproductive performance, regardless of adult temperature, highlights the developmental stage as very critical and most vulnerable to climate change and associated heat waves.

Ontogenetic variation in thermal sensitivity shapes insect ecological responses to climate change

Insects have distinct life stages that can differ in their responses to environmental factors. We discuss empirical evidence and theoretical models for ontogenetic variation in thermal sensitivity and performance curves (TPCs). Data on lower thermal limits for development (T₀) demonstrate variation between stages within a species that is of comparable magnitude to variation among species; we illustrate the consequences of such ontogenetic variation for developmental responses to changing temperature. Ontogenetic variation in optimal temperatures and upper thermal limits has been reported in some systems, but current data are too limited to identify general patterns. The shapes of TPCs for different fitness components such as juvenile survival, adult fecundity, and generation time differ in characteristic ways, with important consequences for understanding fitness in varying thermal environments. We highlight a theoretical framework for incorporating ontogenetic variation into process-based models of population responses to seasonal variation and climate change.

Comprehensive analysis of differentially expressed proteins in the male and female Bombyx mori larval instars exposed to thermal stress

Gender sensitivity to ambient heat, despite well known in insect species, how it manifests during young and late larval instars of Bombyx mori is still unclear. To uncover this cryptic feature, different instars male and female larvae were subjected to varied thermal stress separately and sex-stage specific expression of proteins was investigated. Interestingly, heat shock proteins (HSPs) 90 and 70 were expressed differently in all the instars and also between male and female larvae as confirmed by immunoblot assay. Besides up- and downregulation of few HSPs and other normal proteins, discreet expression of protein was noticed in the two-dimensional gel electrophoresis of male than female larvae which were identified as HSP70 by mass spectrometry. Furthermore, quantitative polymerase chain reaction results show 3.98- and 4.86-fold higher levels of Bmhsp70 and Bmhsp90 transcripts in male and female larvae, respectively, as a response to 40°C heat shock (HS) treatment. Conversely, in spite of the massive production of HSPs due to HS at 45°C, all the larvae were found dead, which is a strong proof of concept for autophagy. Comparatively, female larvae HS at 40°C succeed to spin cocoons with increased weight and silk contents than non-HS larvae. Comprehensively, in the present study, we have noticed a strong correlation for the first time that Bmhsp90 and Bmhsp70 genes expressions due to thermal stress are not only sex specific but also explicit preferential and coordinated action on survivability and biosynthetic potential of the silkworm, B. mori larvae during different instars.

Effect of short-term high-temperatures on the growth, development and reproduction in the fruit fly, Bactrocera tau (Diptera: Tephritidae)

Bactrocera tau (Walker) (Diptera: Tephritidae) is an economically important invasive pest, that is capable of seriously reducing the quality and yield of vegetables and fruits, it was first recorded from Fujian province in 1849 and later introduced to Yunnan province in 1912 as a result in trade fruits and vegetables of China. In recent years, with the onset of global climate change and the accompanying increase in the greenhouse effect, elevated climatic temperatures have become one of the main environmental factors affecting growth and reproduction in insects, and the optimal developmental temperature of B. tau was found to be from 25 °C to 31 °C, the growth, development and reproduction of B. tau are normal under the optimal temperature conditions. In order to determine the repercussions that elevated temperature have on B. tau, we assessed the effects that short-term (12 h) high-temperature exposures (34 °C, 36 °C, 38 °C, 40 °C, 42 °C, 44 °C, 46 °C, and 48 °C) had on the growth, development and reproduction of B. tau at different developmental stages of the fly. The results showed that the survival rate of B. tau gradually decreased in all stages following exposure to short-term high-temperatures. The pupal stage was the least sensitive to increased temperatures. The pupae withstood the highest lethal temperature, having an LT₅₀ of 42.060 °C, followed by female adults (40.447 °C), male adults (40.013 °C), and larvae (36.740 °C). The egg stage, which was the most susceptible to heat increases, had the lowest LT₅₀ (38.310 °C). No significant effects were observed in the developmental stages of B. tau at temperatures from 24 °C to 38 °C. The development duration was significantly prolonged at 40 °C (P < 0.05) in the eggs (2.830d), larvae (7.330d), and pupae (8.170d) (P < 0.05). B. tau was unable to survive at temperatures above 42 °C. The pre-oviposition of female adults was extended, the average egg number per female showed a downward trend, the longevity of adults gradually shortened, and the ratio of female to male offspring increased as temperature increments were increased. In summary, short-term high-temperatures over 42 °C were not suitable for successful development of B. tau, while short-term high-temperatures over 40 °C were not suitable for successful reproduction in B. tau.

Effects of fluctuating thermal regimes on cold survival and life history traits of the spotted wing Drosophila (Drosophila suzukii)

Drosophila suzukii is an invasive pest causing severe damages to a large panel of cultivated crops. To facilitate its biocontrol with strategies such as sterile or incompatible insect techniques, D. suzukii must be mass-produced and then stored and transported under low temperature. Prolonged cold exposure induces chill injuries that can be mitigated if the cold period is interrupted with short warming intervals, referred to as fluctuating thermal regimes (FTR). In this study, we tested how to optimally use FTR to extend the shelf life of D. suzukii under cold storage. Several FTR parameters were assessed: temperature (15, 20, 25 °C), duration (0.5, 1, 2, 3 h), and frequency (every 12, 24, 36, 48 h) of warming intervals, in two wild-type lines and in two developmental stages (pupae and adults). Generally, FTR improved cold storage tolerance with respect to constant low temperatures (CLT). Cold mortality was lower when recovery temperature was 20 °C or higher, when duration was 2 h per day or longer, and when warming interruptions occurred frequently (every 12 or 24 h). Applying an optimized FTR protocol to adults greatly reduced cold mortality over long-term storage (up to 130 d). Consequences of FTR on fitness-related traits were also investigated. For adults, poststorage survival was unaffected by FTR, as was the case for female fecundity and male mating capacity. On the other hand, when cold storage occurred at pupal stage, poststorage survival and male mating capacity were altered under CLT, but not under FTR. After storage of pupae, female fecundity was lower under FTR compared to CLT, suggesting an energy trade-off between repair of chill damages and egg production. This study provides detailed information on the application and optimization of an FTR-based protocol for cold storage of D. suzukii that could be useful for the biocontrol of this pest.

Population Numbers and Physiological Response of an Invasive and Native Thrip Species Following Repeated Exposure to Imidacloprid

Frankliniella occidentalis and F. intonsa are devastating pest insects that target Rosa rugosa, Chrysanthemum morifolium, and Phaseolus vulgaris, which are important economical horticultural plants in China. Meanwhile, R. rugosa and C. morifolium are important cash plants in Kunming, South China. We focus on the population performance of these two thrips species on these three host plants with or without repeated exposure to imidacloprid in Kunming. In the field, the population numbers of F. occidentalis developed faster and were larger on these three sampled host plants, especially under imidacloprid exposure, compared with F. intonsa. The activity of the detoxifying enzymes (CarE, AchE, and MFO) and the antioxidant enzymes (CAT and POD) in both thrips species were significantly enhanced under imidacloprid exposure, whereas the activities of SOD in both thrips were significantly decreased on these three host plants, compared with the control. Overall, enzyme activity of F. occidentalis showed a greater increase than that observed in F. intonsa in most cases, which could be exploited in further studies on thrips resistance management.

Heat sensitivity of eggs attributes to the reduction in Agasicles hygrophila population

Agasicles hygrophila has been introduced worldwide as a control agent for the invasive weed Alternanthera philoxeroides. However, global warming has potential impact on its controlling efficacy. The aim of this research was to explore the primary factors responsible for the greatly reduced A. hygrophila population in hot summers. To imitate the temperature conditions in summers, different developmental stages of A. hygrophila were treated with high temperatures from 32.5 °C to 45 °C for 1-5 h. Based on the survival rate, the heat tolerance of each developmental stage was ranked from lowest to highest as follows: egg, 1st, 2nd, 3rd instar larva, adult and pupa. Eggs showed the lowest heat tolerance with 37.5 °C as the critical temperature affecting larval hatching. Heat treatment of the A. hygrophila eggs at 37.5 °C for 1 h decreased the hatch rate to 24%. Our results indicated that when compared with the control at 25 °C, 1 h treatment at 37.5 °C prolonged the duration of the egg stage, shortened the duration of oviposition and total longevity, and changed the reproductive pattern of A. hygrophila. The net reproductive rate, intrinsic rate and finite rate were all significantly reduced. The results suggest that low heat tolerance of the eggs was the major factor responsible for the reduction of A. hygrophila populations, and the key temperature was 37.5 °C. Therefore, appropriate measures should be taken to protect eggs in order to maintain the efficacy of A. hygrophila in the biological control of A. philoxeroides in hot summers.

Comparative Transcriptome Analysis of the Heat Stress Response in Monochamus alternatus Hope (Coleoptera: Cerambycidae)

Temperature is a critical factor of insect population abundance and distribution. Monochamus alternatus Hope (Coleoptera: Cerambycidae) is a significant concern since it is transmitted vector of the pinewood nematode posing enormous economic and environmental losses. This pest shows tolerance to heat stress, especially extremely high temperatures. Exposing for 6, 12, 24, 48, or 96 h, the 50% median lethal temperatures (Ltem₅₀) for fourth-instar larvae were 47.5, 45.5, 43.9, 43.4, and 42.3°C, respectively. A total of 63,360 unigenes were obtained from complementary DNA libraries of M. alternatus fourth-instar larvae (kept at 25°C and exposed to 40°C for 3 h) and annotated with six databases. Five hundred sixty-one genes were significantly upregulated, and 245 genes were downregulated after heat stress. The Gene Ontology enrichment analysis showed that most different expression genes are categorized into “protein folding” and “unfold protein binding” terms. In addition, “Longevity regulating pathway-multiple species,” “Antigen processing and presentation” as well as “MAPK signaling pathway” were significantly enriched Kyoto Encyclopedia of Genes and Genomes pathways. Further analysis of different expression genes showed that metabolism processes were suppressed, while ubiquitin proteolytic system, heat shock proteins, immune response, superoxide dismutase, cytochrome P450s, and aldehyde dehydrogenase were induced after heat shock. The stress signaling transduction pathways such as MAPK, Hippo, and JAK-STAT might be central convergence points in M. alternatus heat tolerance mechanism. The expression levels from quantitative real-time PCR of 13 randomly selected genes were consistent with the transcriptome results. These results showed that M. alternatus possessed strong heat tolerance and genes related to protein activity, immune response, and signal transduction composed of a complicated heat tolerance mechanism of M. alternatus. This research provided new insights into the mechanisms of thermal tolerance in other insects and aided in exploring the function of heat resistance-related genes.

The Effect of Oxygen Limitation on a Xylophagous Insect's Heat Tolerance Is Influenced by Life-Stage Through Variation in Aerobic Scope and Respiratory Anatomy

Temperature has a profound impact on insect fitness and performance via metabolic, enzymatic or chemical reaction rate effects. However, oxygen availability can interact with these thermal responses in complex and often poorly understood ways, especially in hypoxia-adapted species. Here we test the hypothesis that thermal limits are reduced under low oxygen availability - such as might happen when key life-stages reside within plants - but also extend this test to attempt to explain that the magnitude of the effect of hypoxia depends on variation in key respiration-related parameters such as aerobic scope and respiratory morphology. Using two life-stages of a xylophagous cerambycid beetle, Cacosceles (Zelogenes) newmannii we assessed oxygen-limitation effects on metabolic performance and thermal limits. We complement these physiological assessments with high-resolution 3D (micro-computed tomography scan) morphometry in both life-stages. Results showed that although larvae and adults have similar critical thermal maxima (CTmax) under normoxia, hypoxia reduces metabolic rate in adults to a greater extent than it does in larvae, thus reducing aerobic scope in the former far more markedly. In separate experiments, we also show that adults defend a tracheal oxygen (critical) setpoint more consistently than do larvae, indicated by switching between discontinuous gas exchange cycles (DGC) and continuous respiratory patterns under experimentally manipulated oxygen levels. These effects can be explained by the fact that the volume of respiratory anatomy is positively correlated with body mass in adults but is apparently size-invariant in larvae. Thus, the two life-stages of C. newmannii display key differences in respiratory structure and function that can explain the magnitude of the effect of hypoxia on upper thermal limits.

Physiological Metabolic Responses of Ophraella communa to High Temperature Stress

Considering the predicted rising temperatures under current climate change and heat wave scenarios, organisms are expected to suffer more intense and frequent thermal stress. Induced heat is accumulated by organisms and can cause a variety of physiological stress responses. Ophraella communa is an effective biological control agent of common ragweed, Ambrosia artemisiifolia, but the responses of this biocontrol agent to heat stress have not been fully elucidated and, therefore, its potential responses to climate change are uncertain. We investigated the physiological metabolism of subsequent O. communa adults after: (1) different developmental stages (egg, larval, pupal, and adult) were exposed to thermal stress for 3 h each day for 3, 5, 5, and 5 days, respectively (by stage); and (2) individuals were exposed to thermal stress throughout the egg-to-adult period for 3 h each day. The high temperatures of 40, 42, and 44°C were used to induce thermal stress. A control group was reared at 28 ± 2°C. The results showed that short- or long-term exposure to daily phasic high temperatures significantly decreased water and lipid contents and significantly increased glycogen and glycerol contents in all adults (i.e., after exposure of different stages or throughout the egg-to-adult period). However, the total sugar content significantly increased in adults after the eggs and larvae were exposed to brief short-term thermal stress. Compared to the control, the total sugar content was also significantly higher in the adults and pupae exposed to 44°C. Total sugar content in females increased significantly in response to long-term phasic thermal stress at 40°C. However, sugar content of males exposed to 44°C decreased significantly. After long-term phasic thermal stress, water and glycogen contents in males were significantly higher than in females; however, females had higher total sugar and lipid contents. Therefore, our study provides a basic understanding of the metabolic responses of O. communa to thermal stress and offers insights into its potential as a natural biocontrol agent against A. artemisiifolia during the summer season and under predicted climate change scenarios.

Comparative studies of critical physiological limits and vulnerability to environmental extremes in small ectotherms: How much environmental control is needed?

Researchers and practitioners are increasingly using comparative assessments of critical thermal and physiological limits to assess the relative vulnerability of ectothermic species to extreme thermal and aridity conditions occurring under climate change. In most assessments of vulnerability, critical limits are compared across taxa exposed to different environmental and developmental conditions. However, many aspects of vulnerability should ideally be compared when species are exposed to the same environmental conditions, allowing a partitioning of sources of variation such as used in quantitative genetics. This is particularly important when assessing the importance of different types of plasticity to critical limits, using phylogenetic analyses to test for evolutionary constraints, isolating genetic variants that contribute to limits, characterizing evolutionary interactions among traits limiting adaptive responses, and when assessing the role of cross generation effects. However, vulnerability assessments based on critical thermal/physiological limits also need to take place within a context that is relevant to field conditions, which is not easily provided under controlled environmental conditions where behavior, microhabitat, stress exposure rates and other factors will differ from field conditions. There are ways of reconciling these requirements, such as by taking organisms from controlled environments and then testing their performance under field conditions (or vice versa). While comparisons under controlled environments are challenging for many taxa, assessments of critical thermal limits and vulnerability will always be incomplete unless environmental effects within and across generations are considered, and where the ecological relevance of assays measuring critical limits can be established.

Effects of Host Plants Reared under Elevated CO2 Concentrations on the Foraging Behavior of Different Stages of Corn Leaf Aphids Rhopalosiphum maidis

Climate change is a major environmental concern and is directly related to the increasing concentrations of greenhouse gases. The increase in concentrations of atmospheric carbon dioxide (CO2), not only affects plant growth and development, but also affects the emission of plant organic volatile compounds (VOCs). Changes in the plant odor profile may affect the plant-insect interactions, especially the behavior of herbivorous insects. In this study, we compared the foraging behavior of corn leaf aphid (Rhopalosiphum maidis) on barley (Hordeum vulgare L.) seedlings grown under contrasted CO2 concentrations. During the dual choice bioassays, the winged and wingless aphids were more attracted by the VOCs of barley seedlings cultivated under ambient CO2 concentrations (aCO2; 450 ppm) than barley seedlings cultivated under elevated CO2 concentrations (eCO2; 800 ppm), nymphs were not attracted by the VOCs of eCO2 barley seedlings. Then, volatile compositions from 14-d-old aCO2 and eCO2 barley seedlings were investigated by GC-MS. While 16 VOCs were identified from aCO2 barley seedlings, only 9 VOCs were found from eCO2 barley seedlings. At last, we discussed the potential role of these chemicals observed during choice bioassays. Our findings lay foundation for functional response of corn leaf aphid under climate change through host plant modifications.

Comparative Transcriptome Analysis of Megacopta cribraria (Hemiptera: Plataspidae) in Response to High-Temperature Stress

Kudzu bug, Megacopta cribraria (Fabricius), is mainly distributed in southern China and has been considered an invasive species in the southeastern United States. Megacopta cribraria is a soybean pest with high-temperature resistance, but the molecular mechanisms underlying its thermal adaptation are largely unknown. Here, we performed comparative transcriptome analysis to unravel the molecular response of M. cribraria toward high-temperature stress. Following RNA-seq, we identified 93,959 assembled unigenes, 14,073 of which were annotated in M. cribraria transcriptome libraries. In addition, 127 differentially expressed unigenes (DEGs) were detected, 88 of them were significantly upregulated, whereas the remaining 39 genes were significantly downregulated. Functional classification revealed that the pathways of metabolic process, cellular processes, and single-organism processes were considered to be significantly enriched. In the COG classification, DEGs were mainly localized into O: post-translational modification, protein turnover, chaperone. Moreover, protein processing in endoplasmic reticulum and linoleic acid metabolism were significantly enriched among the 38 KEGG pathways. Further gene annotation analysis indicated that nine heat shock protein-related genes were significantly upregulated. Finally, five HSP DEGs were selected for real-time quantitative polymerase chain reaction validation and demonstrated a similar upregulation trend with RNA-seq expression profiles. Taken altogether, these findings provide new insights into the molecular mechanisms of thermal adaptation in M. cribraria.

Sporadic short temperature events cannot be neglected in predicting impacts of climate change on small insects

Climate warming is characterized by increase in extreme heat events (EHEs). EHEs and mild temperature periods alternate with each other and form complex climate scenarios. Among these scenarios, low-frequency and short-duration extreme heat events during long mild periods (sporadic short EHEs) and low-frequency and short-duration mild periods during long extreme heat events (sporadic short mild periods) commonly occur in nature. The biological effects of these two types of temperature events have not been thoroughly elucidated to date. To clarify the biological effects of these temperature events on organisms, we selected the English grain aphid, a globally important cereal pest, as our model system. We exposed aphids to simulated 24-h diurnal fluctuating temperatures, inserted these events during the wheat growing season and then investigated development, adult longevity, fecundity, survival, and demographic parameters. We found that sporadic short mild periods during a long EHE could improve their life history traits. Increasing the duration of mild periods from 1 day to 2 days did not significantly change their demographic performance. Sporadic short EHEs during a long mild period did not significantly affect vital rates, while increasing the duration of EHEs from 1 day to 2 days worsened the aphids' performance. We found that short mild episodes in the hot season may benefit small insects to buffer long duration heatwaves. We discussed how sporadic short mild periods during a long EHE supplied aphids a chance to recover from heat stress. Thus, we suggest that sporadic temperature events should be considered in population prediction of small insects under climate change and should be integrated into pest management.

Comparative assessment of the thermal tolerance of spotted stemborer, Chilo partellus (Lepidoptera: Crambidae) and its larval parasitoid, Cotesia sesamiae (Hymenoptera: Braconidae)

Under stressful thermal environments, insects adjust their behavior and physiology to maintain key life-history activities and improve survival. For interacting species, mutual or antagonistic, thermal stress may affect the participants in differing ways, which may then affect the outcome of the ecological relationship. In agroecosystems, this may be the fate of relationships between insect pests and their antagonistic parasitoids under acute and chronic thermal variability. Against this background, we investigated the thermal tolerance of different developmental stages of Chilo partellus Swinhoe (Lepidoptera: Crambidae) and its larval parasitoid, Cotesia sesamiae Cameron (Hymenoptera: Braconidae) using both dynamic and static protocols. When exposed for 2 h to a static temperature, lower lethal temperatures ranged from -9 to 6 °C, -14 to -2 °C, and -1 to 4 °C while upper lethal temperatures ranged from 37 to 48 °C, 41 to 49 °C, and 36 to 39 °C for C. partellus eggs, larvae, and C. sesamiae adults, respectively. Faster heating rates improved critical thermal maxima (CT_max ) in C. partellus larvae and adult C. partellus and C. sesamiae. Lower cooling rates improved critical thermal minima (CT_min ) in C. partellus and C. sesamiae adults while compromising CT_min in C. partellus larvae. The mean supercooling points (SCPs) for C. partellus larvae, pupae, and adults were -11.82 ± 1.78, -10.43 ± 1.73 and -15.75 ± 2.47, respectively. Heat knock-down time (HKDT) and chill-coma recovery time (CCRT) varied significantly between C. partellus larvae and adults. Larvae had higher HKDT than adults, while the latter recovered significantly faster following chill-coma. Current results suggest developmental stage differences in C. partellus thermal tolerance (with respect to lethal temperatures and critical thermal limits) and a compromised temperature tolerance of parasitoid C. sesamiae relative to its host, suggesting potential asynchrony between host-parasitoid population phenology and consequently biocontrol efficacy under global change. These results have broad implications to biological pest management insect-natural enemy interactions under rapidly changing thermal environments.

Effect of short-term high-temperature exposure on the life history parameters of Ophraella communa

Extreme heat in summer is frequent in parts of China, and this likely affects the fitness of the beetle Ophraella communa, a biological control agent of invasive common ragweed. Here, we assessed the life history parameters of O. communa when its different developmental stages were exposed to high temperatures (40, 42 and 44 °C, with 28 °C as a control) for 3 h each day for 3, 5, 5, and 5 days, respectively (by stage). The larval stage was the most sensitive stage, with the lowest survival rate under heat stress. Egg and pupal survival significantly decreased only at 44 °C, and these two stages showed relative heat tolerance, while the adult stage was the most tolerant stage, with the highest survival rates. High temperatures showed positive effects on the female proportion, but there was no stage-specific response. Treated adults showed the highest fecundity under heat stress and a similar adult lifespan to that in the control. High temperatures decreased the F₁ egg hatching rate, but the differences among stages were not significant. Negative carry-over effects of heat stress on subsequent stages and progenies’ survival were also observed. Overall, heat effects depend on the temperature and life stage, and the adult stage was the most tolerant stage. Ophraella communa possesses a degree of heat tolerance that allows it to survive on hot days in summer.

Antioxidant Responses of Ragweed Leaf Beetle Ophraella communa (Coleoptera: Chrysomelidae) Exposed to Thermal Stress

Ophraellacommuna LeSage is an effective biological control agent of common ragweed, Ambrosia artemisiifolia L., which competes with crops and causes allergic rhinitis and asthma. However, thermal stress negatively affects the developmental fitness and body size of this beetle. High temperatures cause a variety of physiological stress responses in insects, which can cause oxidative damage. We investigated the total protein content and activity of antioxidant enzymes including superoxide dismutase (SOD), catalase (CAT), and peroxidases (PODs) in O. communa adults when its different developmental stages were exposed to high temperatures (40, 42, and 44°C) for 3 h each day for 3, 5, 5, and 5 days, respectively (by stage), and a whole generation to high temperatures (40, 42, and 44°C) for 3 h each day. A control group was reared at 28 ± 2°C. Under short-term daily phasic high-temperature stress, total protein contents were close to the control as a whole; overall, SOD activities increased significantly, CAT activities were closer to or even higher than the control, POD activities increased at 40°C, decreased at 42 or 44°C; stage-specific response was also observed. Under long-term daily phasic high-temperature stress, total protein content increased significantly at 44°C, SOD activities increased at higher temperatures, decreased at 44°C; CAT activities of females increased at ≤42°C, and decreased at 44°C, CAT activities of males decreased significantly; POD activities of females increased at 40°C, decreased at ≥42°C, POD activities of males decreased at 44°C; and antioxidant enzymes activities in females were significantly higher than those in males. Antioxidative enzymes protect O. communa from oxidative damage caused by thermal stress.

Are adult life history traits in oriental fruit moth affected by a mild pupal heat stress?

Thermal stress at one life stage can affect fitness at a later stage in ectotherms with complex life cycles. Most relevant studies have focused on extreme stress levels, but here we also show substantial fitness effects in a moth when pupae are exposed to a relatively mild and sublethal heat stress. We consider the impact of a 35°C heat stress of 2h in three geographically separate populations of the oriental fruit moth (OFM, Grapholita molesta) from northern, middle and southern China. Heat stress negatively affected fecundity but increased adult heat resistance and adult longevity. Fitness effects were mostly consistent across populations but there were also some population differences. In the Shenyang population from northern China, there was a hormetic effect of heat on female longevity not evident in the other populations. Adults from all populations had higher LT₅₀s due to heat stress after pupal exposure to the sublethal stress. These results highlight that the pupal stage is a particularly sensitive window for development and they have implications for seasonal adaptation in uncertain environments as well as changes in pest dynamics under climate warming.

Carried over: Heat stress in the egg stage reduces subsequent performance in a butterfly

Increasing heat stress caused by anthropogenic climate change may pose a substantial challenge to biodiversity due to associated detrimental effects on survival and reproduction. Therefore, heat tolerance has recently received substantial attention, but its variation throughout ontogeny and effects carried over from one developmental stage to another remained largely neglected. To explore to what extent stress experienced early in life affects later life stages, we here investigate effects of heat stress experienced in the egg stage throughout ontogeny in the tropical butterfly Bicyclus anynana. We found that detrimental effects of heat stress in the egg stage were detectable in hatchlings, larvae and even resulting adults, as evidenced by decreased survival, growth, and body mass. This study shows that even in holometabalous insects with discrete life stages effects of stress experienced early in life are carried over to later stages, substantially reducing subsequent fitness. We argue that such effects need to be considered when trying to forecast species responses to climate change.