Host plant‐related variation in thermal tolerance of Eldana saccharina

Understanding tolerance of thermal extremes by pest insects is essential for developing integrated management strategies, as tolerance traits can provide insights into constraints on activity and survival. A major question in thermal biology is whether thermal limits vary systematically with microclimate variation, or whether other biotic or abiotic factors can influence these limits in a predictable manner. Here, we report the results of experiments determining thermal limits to activity and survival at extreme temperatures in the stalk borer Eldana saccharina Walker (Lepidoptera: Pyralidae), collected from either Saccharum spp. hybrids (sugarcane) (Poaceae) or Cyperus papyrus L. (Cyperaceae) and then reared under standard conditions in the laboratory for 1–2 generations. Chill‐coma temperature (CT_min), critical thermal maximum (CT_max), lower lethal temperatures (LLT), and freezing temperature between E. saccharina collected from the two host plants were compared. CT_min and CT_max of E. saccharina moths collected from sugarcane were significantly lower than those from C. papyrus (CT_min = 2.8 ± 0.4 vs. 3.9 ± 0.4 °C; CT_max = 44.6 ± 0.1 vs. 44.9 ± 0.2 °C). By contrast, LLT of moths and freezing temperatures of pupae did not vary with host plant [LLT for 50% (LT₅₀) of the moth population, when collected from sugarcane: ?3.2 ± 0.5 °C, from C. papyrus: ?3.9 ± 0.8 °C]. Freezing temperatures of pupae collected from C. papyrus were ?18.0 ± 1.0 °C and of those from sugarcane ?17.5 ± 1.8 °C. The E. saccharina which experienced the lowest minimum temperature (in C. papyrus) did not have the lowest CT_min, although the highest estimate of CT_max was found in E. saccharina collected from C. papyrus and this was also the microsite which reported the highest maximum temperatures. These results therefore suggest that host plant may strongly mediate lower critical thermal limits, but not necessarily LLT or freezing temperatures. These results have significant implications for ongoing pest management and thermal biology of these and other insects.     

The thermal physiology of Stenopelmus rufinasus and Neohydronomus affinis (Coleoptera: Curculionidae), two biological control agents for the invasive alien aquatic weeds,Azolla filiculoides and Pistia stratiotes in South Africa

Water lettuce, Pistia stratiotes, and red water fern, Azolla filiculoides, are floating aquatic macrophytes that have become problematic in South Africa. Two weevils, Neohydronomus affinis and Stenopelmus rufinasus, are successful biological control agents of these two species in South Africa. The aim of this study was to investigate the thermal requirements of these two species to explain their establishment patterns in the field. Laboratory results showed that both weevils are widely tolerant to cold and warm temperatures. The critical thermal minima (CT_min) of N. affinis was determined to be 5.58?±?0.31°C and the critical thermal maxima (CT_max) was 44.52?±?0.27°C, while the CT_min of S. rufinasus was 5.38?±?0.33°C and the CT_max?44.0?±?0.17°C. In addition, the lower lethal temperatures were ?9.85?±?0.06°C for N. affinis and ?6.85?±?0.13°C for S. rufinasus, and the upper lethal temperatures were 42.7?±?0.85°C for N. affinis and 41.9?±?2.52°C S. rufinasus. Using the reduced major axis regression method, the development for N. affinis was described using the formula y?=?12.976x?+?435.24, while the development of S. rufinasus was described by y?=?13.6x?+?222.45. These results showed that S. rufinasus develops twice as fast as N. affinis. Using these formulae and temperature data obtained from the South African Weather Service, N. affinis was predicted to complete between 4 and 9 generations per year in South Africa, while S. rufinasus was predicted to complete between 5 and 14 generations per year around the country. These results suggest that both species should not be limited by cold winter, nor warm summer temperatures, and should establish throughout the ranges of the weeds in South Africa.     

Low‐temperature physiology of climatically distinct south African populations of the biological control agent Neochetina eichhorniae

1. Neochetina eichhorniae is the most widely established biocontrol agent on water hyacinth populations around South Africa. However, some N. eichhorniae populations have failed to adequately control their host population, specifically those exposed to cold conditions.
2. The aim of this study was to determine whether two climatically distinct populations of N. eichhorniae in South Africa differ in their low‐temperature physiology, which tests whether local‐climate adaptation has occurred.
3. We estimated weevil CT_min, LLT₅₀, SCP, and SCP mortality using standard approaches. Contrary to expectation based on climatic thermal profiles at the two sites, weevils from the warm locality ((mean ± SE) CT_min = 5.0 °C ± 0.2, LLT₅₀ = ?11.3 °C ± 0.03, SCP = ?15.8 °C ± 0.6) were able to maintain activity and tolerate colder temperatures than the weevils from the colder site (CT_min = 6.0 °C ± 0.5, LLT₅₀ = ?10.1 °C ± 0.1, SCP = ?12.9 °C ± 0.8).
4. These contradictory outcomes are likely explained by the poor nutrient quality of the plants at the cold site, driving low‐temperature performance variation that overrode any macroclimate variation among sites. The cold site weevils may also have adapted to survive wide‐temperature variability, rather than perform well under very cold conditions. In contrast, the mass‐reared population of insects from the warm site has likely adapted to the consistent conditions that they experience over many years in confinement.

     

Within‐generation variation of critical thermal limits in adult Mediterranean and Natal fruit flies Ceratitis capitata and Ceratitis rosa: thermal history affects short‐term responses to temperature

Insect thermal tolerance shows a range of responses to thermal history depending on the duration and severity of exposure. However, few studies have investigated these effects under relatively modest temperature variation or the interactions between short‐ and longer‐term exposures. In the present study, using a full‐factorial design, 1 week‐long acclimation responses of critical thermal minimum (CT_min) and critical thermal maximum (CT_max) to temperatures of 20, 25 and 30 °C are investigated, as well as their interactions with short‐term (2 h) sub‐lethal temperature exposures to these same conditions (20, 25 and 30 °C), in two fruit fly species Ceratitis capitata (Wiedemann) and Ceratitis rosa Karsch from South Africa. Flies generally improve heat tolerance with high temperature acclimation and resist low temperatures better after acclimation to cooler conditions. However, in several cases, significant interaction effects are evident for CT_max and CT_min between short‐ and long‐term temperature treatments. Furthermore, to better comprehend the flies' responses to natural microclimate conditions, the effects of variation in heating and cooling rates on CT_max and CT_min are explored. Slower heating rates result in higher CT_max, whereas slower cooling rates elicit lower CT_min, although more variation is detected in CT_min than in CT_max (approximately 1.2 versus 0.5 °C). Critical thermal limits estimated under conditions that most closely approximate natural diurnal temperature fluctuations (rate: 0.06 °C min^?1) indicate a CT_max of approximately 42 °C and a CT_min of approximately 6 °C for these species in the wild, although some variation between these species has been found previously in CT_max. In conclusion, the results suggest critical thermal limits of adult fruit flies are moderated by temperature variation at both short and long time scales and may comprise both reversible and irreversible components.     

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.     

Fluctuating environments impact thermal tolerance in an invasive insect species Bactrocera dorsalis (Diptera: Tephritidae)

The incidence and severity of environmental stressors associated with global climate change are increasing and insects frequently face variability in temperature and moisture regimes at variable spatio-temporal scales. Coincidental with this, is increased thermal and hydric stress on insects as warming increases vapour pressure deficit (VPD), the drying power of the air. While the effects of mean temperatures on fitness are widely documented, fluctuations in both temperature and relative humidity (RH) are largely unexplored. Here, we investigated the effects of dynamic temperature and RH fluctuations (around the mean [28°C; 65% RH]) on low and high thermal tolerance of laboratory-reared adult invasive Bactrocera dorsalis (Hendel) (Diptera: Tephritidae), measured as critical thermal minima (CT_min), critical thermal maxima (CT_max), chill coma recovery time (CCRT) and heat knockdown time (HKDT). Our results show that increased environmental amplitude significantly influenced low and high temperature responses and varied across traits tested. The highest amplitude (δ12°C; 28% RH) compromised CT_min, CCRT and HKDT traits while enhancing CT_max. Similarly, acclimation to δ3°C; 7% RH compromised both low (CT_min and CCRT) and high (CT_max and HKDT) fitness traits. Variations in fitness reported here indicate significant roles of combined thermal and moisture fluctuations on B. dorsalis fitness suggesting caveats that are worthy considering when predicting species responses to climate change. These results are significant for B. dorsalis population phenology, management, quantifying vulnerability to climate variability and may help modelling future biogeographical patterns.     

Individual variation in metabolism and thermal tolerance in juvenile qingbo (Spinibarbus sinensis)

Studies of individual variation in the physiological performance of animals and their relationship with metabolism may provide insight into how selection influences diversity in phenotypic traits. Thus, the aims of the present study were to investigate variation in thermal tolerance and its relationship with individual metabolism in juvenile qingbo (Spinibarbus sinensis). To fulfill our goal, we first measured the resting metabolic rate (RMR), maximum metabolic rate (MMR), metabolic scope (MS, MMR–RMR) and excess post-exercise oxygen consumption (EPOC) of 40 fish at 25 °C. We then measured the critical thermal minimum (CT_min), lethal thermal minimum (LT_min), critical thermal maximum (CT_max), and lethal thermal maximum (LT_max) of 20 fish. Both MMR and MS were positively correlated with the metabolic recovery rate (MRR) (p = 0.001), indicating that high aerobic metabolic performance individuals possessed an advantage for the recovery of anaerobic metabolism. However, the negative correlation between EPOC and MRR (p = 0.017) indicated a slow recovery of the metabolism of high anaerobic metabolic capacity individuals. The RMR was positively correlated with CT_min and LT_min, whereas all of the metabolic rate parameters (RMR, MMR, and MS) were negatively correlated with CT_max and LT_max (p < 0.05), indicating that high aerobic metabolic performance individuals have a weakened thermal tolerance. These results suggested that there is a trade-off between aerobic metabolic performance and thermal tolerance.     

Rain shadow effects predict population differences in thermal tolerance of leaf-cutting ant workers (Atta cephalotes)

Tests of hypotheses for the evolution of thermal physiology often rely on mean temperatures, but mounting evidence suggests geographic variation in temperature extremes is also an important predictor of species’ thermal tolerances. Although the tropics are less thermally variable than higher latitude regions, rain shadows on the leeward sides of mountains can experience greater diel and seasonal variation in temperature than windward sites. Rain shadows provide opportunities to test predictions about the relationships of extreme temperatures with thermal physiology while controlling for latitude. We tested the hypothesis that populations of leaf-cutting ants (Atta cephalotes) in leeward, montane, and windward sites in Costa Rica would differ in upper thermal tolerances (CT_max) of workers. As predicted from rain shadow effects via extreme high temperatures, the leeward rain shadow site yielded the highest mean CT_max (rain shadow site 42.1 ± 0.3°C, Montane site 38.2 ± 0.5°C, and windward site 38.2 ± 0.3°C). This suggests that high-temperature extremes in tropical rain shadow forests can select for higher thermal tolerances. CT_max increased with worker body size within sites, but CT_max increased with body size more gradually at the two lowland sites, as predicted if local high temperatures selected more strongly on the most thermally vulnerable society members (small workers). This suggests that warmer lowland climates selected for colonies with less variation in heat tolerance than cooler high elevation climates.     

Constant and fluctuating temperature acclimations have similar effects on phenotypic plasticity in springtails

Much interest exists in the extent to which constant versus fluctuating temperatures affect thermal performance traits and their phenotypic plasticity. Theory suggests that effects should vary with temperature, being especially pronounced at more extreme low (because of thermal respite) and high (because of Jensen's inequality) temperatures. Here we tested this idea by examining the effects of constant temperatures (10 to 30 °C in 5 °C increments) and fluctuating temperatures (means equal to the constant temperatures, but with fluctuations of ±5 °C) temperatures on the adult (F2) phenotypic plasticity of three thermal performance traits – critical thermal minimum (CT_min), critical thermal maximum (CT_max), and upper lethal temperature (ULT₅₀) in ten species of springtails (Collembola) from three families (Isotomidae 7 spp.; Entomobryidae 2 spp.; Onychiuridae 1 sp.). The lowest mean CT_min value recorded here was -3.56 ± 1.0 °C for Paristoma notabilis and the highest mean CT_max was 43.1 ± 0.8 °C for Hemisotoma thermophila. The Acclimation Response Ratio for CT_min was on average 0.12 °C/°C (range: 0.04 to 0.21 °C/°C), but was much lower for CT_max (mean: 0.017 °C/°C, range: -0.015 to 0.047 °C/°C) and lower also for ULT₅₀ (mean: 0.05 °C/°C, range: -0.007 to 0.14 °C/°C). Fluctuating versus constant temperatures typically had little effect on adult phenotypic plasticity, with effect sizes either no different from zero, or inconsistent in the direction of difference. Previous work assessing adult phenotypic plasticity of these thermal performance traits across a range of constant temperatures can thus be applied to a broader range of circumstances in springtails.     

Thermal biology of the alien ground beetle <Emphasis Type="Italic">Merizodus soledadinus</Emphasis> introduced to the Kerguelen Islands

Thermal tolerance is one of the major determinants of successful establishment and spread of invasive aliens. Merizodus soledadinus (Coleoptera, Carabidae) was accidentally introduced to Kerguelen from the Falkland Islands in 1913. On Kerguelen, the climate is cooler than the Falklands Islands but has been getting warmer since the 1990s, in synchrony with the rapid expansion of M. soledadinus. We aimed to investigate the thermal sensitivity in adults of M. soledadinus and hypothesised that climate warming has assisted the colonisation process of M. soledadinus. We examined (1) survival of constant low temperatures and at fluctuating thermal regimes, (2) the critical thermal limits (CT_min and CT_max) of acclimated individuals (4, 8 and 16°C), (3) the metabolic rates of acclimated adults at temperatures from 0 to 16°C. The FTRs moderately increased the duration of survival compared to constant cold exposure. M. soledadinus exhibited an activity window ranged from −5.5 ± 0.3 to 38 ± 0.5°C. The Q ₁₀ after acclimation to temperatures ranging from 0 to 16°C was 2.49. Our work shows that this species is only moderately cold tolerant with little thermal plasticity. The CT_min of M. soledadinus are close to the low temperatures experienced in winter on Kerguelen Islands, but the CT_max are well above summer conditions, suggesting that this species has abundant scope to deal with current climate change.     

Effects of acclimation on the thermal tolerance of the brown planthopper Nilaparvata lugens (Stål)

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Predicting the distribution of Eccritotarsus catarinensis, a natural enemy released on water hyacinth in South Africa

Water hyacinth [Eichhornia crassipes (Mart.) Solms (Pontederiaceae)] is the most damaging aquatic weed in South Africa, where five arthropod biological control agents have been released against it. The most recent introduction of Eccritotarsus catarinensis (Carvalho) (Heteroptera: Miridae) has failed to establish permanent populations at a number of sites in South Africa where water hyacinth is a problem. Cold winter temperatures at these sites are assumed to be the reason for these establishment failures. This assumption was tested by investigating the thermal physiology of the mirid, then incorporating these data into various predictive distribution models. Degree‐day models predict 3–14 generations per year at different localities in South Africa, and five generations at a Johannesburg site where the mirid failed to overwinter. The inability to develop sufficiently rapidly during winter months may hinder overwintering of this insect, which was predicted to develop through only one generation during the winter months of April to August in Johannesburg. A CLIMEX model also showed that cold stress limits the mirid's ability to overwinter in the interior of the country, while determination of the lower lethal limit (–3.5 °C) and critical thermal minimum (1.2 ± 1.17 °C) also indicated that extreme temperatures will limit establishment at certain sites. It is concluded that E. catarinensis is limited in its distribution in South Africa by low winter temperatures.     

The role of nest surface temperatures and the brain in influencing ant metabolic rates

Thermal limits of insects can be influenced by recent thermal history: here we used thermolimit respirometry to determine metabolic rate responses and thermal limits of the dominant meat ant, Iridomyrmex purpureus. Firstly, we tested the hypothesis that nest surface temperatures have a pervasive influence on thermal limits. Metabolic rates and activity of freshly field collected individuals were measured continuously while ramping temperatures from 44 °C to 62 °C at 0.25 °C/minute. At all the stages of thermolimit respirometry, metabolic rates were independent of nest surface temperatures, and CT_max did not differ between ants collected from nest with different surface temperatures. Secondly, we tested the effect of brain control on upper thermal limits of meat ants via ant decapitation experiments (‘headedness’). Decapitated ants exhibited similar upper critical temperature (CT_max) results to living ants (Decapitated 50.3±1.2 °C: Living 50.1±1.8 °C). Throughout the temperature ramping process, ‘headedness’ had a significant effect on metabolic rate in total (Decapitated V̇CO₂ 140±30 µl CO₂ mg⁻¹ min⁻¹: Living V̇CO₂ 250±50 CO₂ mg⁻¹ min⁻¹), as well as at temperatures below and above CT_max. At high temperatures (>44 °C) pre- CT_max the relationships between I. purpureus CT_max values and mass specific metabolic rates for living ants exhibited a negative slope whilst decapitated ants exhibited a positive slope. The decapitated ants also had a significantly higher Q₁₀:25–35 °C when compared to living ants (1.91±0.43 vs. 1.29±0.35). Our findings suggest that physiological responses of ants may be able to cope with increasing surface temperatures, as shown by metabolic rates across the thermolimit continuum, making them physiologically resilient to a rapidly changing climate. We also demonstrate that the brain plays a role in respiration, but critical thermal limits are independent of respiration levels.     

Thermal adaptation generates a diversity of thermal limits in a rainforest ant community

The Thermal Adaptation Hypothesis posits that the warmer, aseasonal tropics generates populations with higher and narrower thermal limits. It has largely been tested among populations across latitudes. However, considerable thermal heterogeneity exists within ecosystems: across 31 trees in a Panama rainforest, surfaces exposed to sun were 8 °C warmer and varied more in temperature than surfaces in the litter below. Tiny ectotherms are confined to surfaces and are variously submerged in these superheated boundary layer environments. We quantified the surface CT_min and CT_maxs (surface temperatures at which individuals grew torpid and lost motor control, respectively) of 88 ant species from this forest; they ranged in average mass from 0.01 to 57 mg. Larger ants had broader thermal tolerances. Then, for 26 of these species we again tested body CT_maxs using a thermal dry bath to eliminate boundary layer effects: body size correlations observed previously disappeared. In both experiments, consistent with Thermal Adaptation, CT_maxs of canopy ants averaged 3.5–5 °C higher than populations that nested in the shade of the understory. We impaled thermocouples in taxidermy mounts to further quantify the factors shaping operative temperatures for four ant species representing the top third (1–30 mg) of the size distribution. Extrapolations suggest the smallest 2/3rds of species reach thermal equilibrium in <10s. Moreover, the large ants that walk above the convective superheated surface air also showed more net heating by solar radiation, with operative temperatures up to 4 °C higher than surrounding air. The thermal environments of this Panama rainforest generate a range of CT_max subsuming 74% of those previously recorded for ant populations worldwide. The Thermal Adaptation Hypothesis can be a powerful tool in predicting diversity of thermal limits within communities. Boundary layer temperatures are likely key to predicting the future of Earth's tiny terrestrial ectotherm populations.     

Thermal plasticity and microevolution enhance establishment success and persistence of a water hyacinth biological control agent

Aspects of the thermal physiology of the water hyacinth biological control agent Eccritotarsus catarinensis Carvalho (Hemiptera: Miridae) have been extensively investigated over the past 20 years to understand and improve post‐release establishment in the field. Thermal physiology studies predicted that the agent would not establish at a number of cold sites in South Africa, where it has nonetheless subsequently established and thrived. Recently, studies have begun to incorporate the plastic nature of insect thermal physiology into models of agent establishment. This study determined whether season and locality influenced the thermal physiology of two field populations of E. catarinensis, one collected from the hottest site where the agent has established in South Africa, and one from the coldest site. The thermal physiology of E. catarinensis was significantly influenced by season and site, demonstrating a degree of phenotypic plasticity, and that some post‐release local adaptation to climatic conditions has occurred through microevolution. We then determined whether cold acclimation under laboratory conditions was possible. Successfully cold‐acclimated E. catarinensis had a significantly lower critical thermal minimum (CT_min) compared to the field cold‐acclimated population. This suggests that cold acclimation of agents could be conducted in the laboratory before future releases to improve their cold tolerance, thereby increasing their chance of establishment at cold sites and allowing further adaptation to colder climates to occur in the field. Although the thermal tolerance of E. catarinensis is limited by local adaptations to climatic conditions in the native range, the plastic nature of the insect's thermal physiology has allowed it to survive in the very different climatic conditions of the introduced range, and there has been some adaptive change to the insect's thermal tolerance since establishment. This study highlights the importance of plasticity and microevolutionary processes in the success of biological control agents under the novel climatic conditions in the introduced range.     

Divergence of thermal physiological traits in terrestrial breeding frogs along a tropical elevational gradient

Critical thermal limits are thought to be correlated with the elevational distribution of species living in tropical montane regions, but with upper limits being relatively invariant compared to lower limits. To test this hypothesis, we examined the variation of thermal physiological traits in a group of terrestrial breeding frogs (Craugastoridae) distributed along a tropical elevational gradient. We measured the critical thermal maximum (CT_max; n = 22 species) and critical thermal minimum (CT_min; n = 14 species) of frogs captured between the Amazon floodplain (250 m asl) and the high Andes (3,800 m asl). After inferring a multilocus species tree, we conducted a phylogenetically informed test of whether body size, body mass, and elevation contributed to the observed variation in CT_max and CT_min along the gradient. We also tested whether CT_max and CT_min exhibit different rates of change given that critical thermal limits (and their plasticity) may have evolved differently in response to different temperature constraints along the gradient. Variation of critical thermal traits was significantly correlated with species’ elevational midpoint, their maximum and minimum elevations, as well as the maximum air temperature and the maximum operative temperature as measured across this gradient. Both thermal limits showed substantial variation, but CT_min exhibited relatively faster rates of change than CT_max, as observed in other taxa. Nonetheless, our findings call for caution in assuming inflexibility of upper thermal limits and underscore the value of collecting additional empirical data on species’ thermal physiology across elevational gradients.     

Acclimation responses to short‐term temperature treatments during early life stages causes long lasting changes in spontaneous activity of adult Drosophila melanogaster

Ecotherms adjust their physiology to environmental temperatures. Long‐term exposures to heat or cold typically induce acclimation responses that generate directional, but reversible shifts in thermal tolerance and performance. However, less is known about how short exposure in different life stages will affect the adult phenotype. In the present study, we compared the effects of long‐term temperature exposure to 15, 19 and 31 °C with that of brief (16 h) exposure periods at the same temperatures in Drosophila melanogaster eggs, larvae, pupae, or adults, respectively. The acclimation responses are evaluated using activity measurements at 11, 15, 19, 27, 31 and 33 °C and by measuring upper and lower thermal limits (CT_max and CT_min) in 5‐day‐old adult males. As expected, long‐term cold exposure reduces relative CT_min, whereas long‐term heat exposure increases relative CT_max. By contrast, we find little effect on thermal limits when using short‐term exposures at different life stages. Long‐term exposures to 31 and 15 °C both suppressed activity relative to the 19 °C control, suggesting that development at high and low temperatures may lead to reduced activity later in life. Short‐term cold exposure early in development reduces activity in the adult stage, whereas the effects of short‐term heat exposure on behaviour are dependent on life stage and test temperature. Together, our results highlight how the thermal sensitivity of the trait measured determines the ability to detect acclimation responses.     

Ageing and thermal performance in the sub-Antarctic wingless fly Anatalanta aptera (Diptera: Sphaeroceridae): older is better

Senescence is a progressive biological process expressed in behavioural, morphological, physiological, biochemical and cellular age-related changes. Age-associated alterations in activity are regularly found in insects when examining whole-organism senescence over the adult lifespan. In addition, overall stress resistance usually decreases with senescence. In the present study, we measured the critical thermal minimum (CT_min) and the subsequent recovery period over the lifespan of the sub-Antarctic wingless fly, Anatalanta aptera. Experiments were conducted on males and females in seven age groups: newly emerged, 1.5-, 5-, 7-, 13-, 15- and 18-month-old adults. Surprisingly, CT_min decreased significantly with ageing in A. aptera, from −3.8 ± 0.5°C just after the emergence to −5.6 ± 0.7°C in the 18-month-old flies. The subsequent recovery period remained similar between the seven groups tested. Our unexpected results contradict the previous data collected in other insects. We have demonstrated for the first time that ageing may improve rather than impair locomotor activity during unfavourable thermal conditions. It raises questions and challenges the literature dealing with ageing. These fascinating results also question the underpinning mechanisms involved in the improvement of the thermal performance with ageing in A. aptera.     

Water deprivation drives intraspecific variability in lizard heat tolerance

Quantifying intraspecific variation in heat tolerance is critical to understand how species respond to climate change. In a previous study, we recorded variability in critical thermal maxima (CT_max) by 3 °C among populations of small Iberian lizard species, which could substantially influence predictions of climate-driven activity restriction. Here, we undertake experiments to examine whether we could reproduce similar levels of heat-tolerance variability in response to water deficit. We hypothesized that deprivation of drinking water should increase variability in CT_max between populations more than deprivation of food under the theoretical expectation that the variation of the more limiting resource must trigger stronger variation in physiological performance. We measured CT_max after manipulating availability of live prey and drinking water in two populations of an arid and a mesic lizard species from the Iberian Peninsula. We quantified a mean CT_max across all studied lizards of 44.2 °C ± 0.2 SE for the arid species and 41.7 °C ± 0.3 SE for the mesic species. Using multimodel inference, we found that water deprivation (combined with food supply) caused population differences in CT_max by 3 to 4 °C which were two to three times wider than population differences due to food deprivation (combined with water supply) or to food and water provision. To highlight the need for more thermo-hydroregulatory research, we examined bias in research effort towards thermal versus hydric environmental effects on heat tolerance through a systematic literature review. We show that environmental temperature has been used five times more frequently than precipitation in ecological studies of heat tolerance of terrestrial species. Studies linking thermal tolerance of ectotherms to the interplay of air temperature and water availability are needed in the face of projected increases in aridity and drought in the 21st century, because the balance of body temperature and water resources are functionally interlinked.     

Hot hypoxic flies: Whole-organism interactions between hypoxic and thermal stressors in Drosophila melanogaster

The upper critical thermal maximum (CT_max) of metazoans varies over a wide range, and its determinative factors, such as oxygen limitation, remain controversial. Induction of thermoprotective mechanisms after challenge by sublethal heat stress has been well documented in many organisms, including the model fly Drosophila melanogaster. Interestingly, however, other challenges—notably a period of anoxia—induce post-exposure thermoprotective effects in some organisms such as locusts and houseflies. Here I show, using thermolimit respirometry, that acute hypoxia during thermal stress significantly reduced the CT_max of D. melanogaster, but only below an oxygen partial pressure of about 10 kPa (39.0±0.4 SE °C at 9.3 kPa vs. 36.0±0.2 SE °C at 3.5 kPa). Likewise, the scope for voluntary motor activity declined sharply below 10 kPa and was essentially eliminated at 2.3 kPa. Respiratory water loss increased highly significantly below about 10 kPa. The post-CT_max release of a large quantity of CO₂ is shown to be independent of loss of spiracular control, but dependent at least in part on oxygen availability. The results are broadly in accord with Pörtner's oxygen limitation hypothesis, but suggest that acute oxygen limitation only becomes an important factor at partial pressures less than half of typical atmospheric levels.