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.     

Thermal niche variation among individuals of the poison frog,Oophaga pumilio,in forest and converted habitats

The conversion of natural habitats to human land uses often increases local temperatures, creating novel thermal environments for species. The variable responses of ectotherms to habitat conversion, where some species decline while others persist, can partly be explained by variation among species in their thermal niches. However, few studies have examined thermal niche variation within species and across forest‐land use ecotones, information that could provide clues about the capacity of species to adapt to changing temperatures. Here, we quantify individual‐level variation in thermal traits of the tropical poison frog, Oophaga pumilio, in thermally contrasting habitats. Specifically, we examined local environmental temperatures, field body temperatures (T_b), preferred body temperatures (T_pref), critical thermal maxima (CT_max), and thermal safety margins (TSM) of individuals from warm, converted habitats and cool forests. We found that frogs from converted habitats exhibited greater mean T_b and T_pref than those from forests. In contrast, CT_max and TSM did not differ significantly between habitats. However, CT_max did increase moderately with increasing T_b, suggesting that changes in CT_max may be driven by microscale temperature exposure within habitats rather than by mean habitat conditions. Although O. pumilio exhibited moderate divergence in T_pref, CT_max appears to be less labile between habitats, possibly due to the ability of frogs in converted habitats to maintain their T_b below air temperatures that reach or exceed CT_max. Selective pressures on thermal tolerances may increase, however, with the loss of buffering microhabitats and increased frequency of extreme temperatures expected under future habitat degradation and climate warming. Abstract in Spanish is available with online material.     

Upper thermal tolerance plasticity in tropical amphibian species from contrasting habitats: Implications for warming impact prediction

Tropical ectothermic species are currently depicted as more vulnerable to increasing temperatures because of the proximity between their upper thermal limits and environmental temperatures. Yet, the acclimatory capacity of thermal limits has rarely been measured in tropical species, even though they are generally predicted to be smaller than in temperate species. We compared critical thermal maximum (CT_max) and warming tolerance (WT: the difference between CT_max and maximum temperature, T_max), as well as CT_max acclimatory capacity of toad species from the Atlantic forest (AF) and the Brazilian Caatinga (CAA), a semi-arid habitat with high temperatures. Acclimation temperatures represented the mean temperatures of AF and CAA habitats, making estimates of CT_max and WT more ecologically realistic. CAA species mean CT_max was higher compared to AF species in both acclimation treatments. Clutches within species, as well as between AF and CAA species, differed in CT_max plasticity and we discuss the potential biological meaning of these findings. We did not find a trade-off between absolute CT_max and CT_max plasticity, indicating that species can have both high CT_max and high CT_max plasticity. Although CT_max was highly correlated to T_max, CT_max plasticity was not related to T_max or T_max coefficients of variation. CAA species mean WT was lower than for AF species, but still very high for all species, diverging from other studies with tropical species. This might be partially related to over-estimation of vulnerability due to under-appreciation of realistic acclimation treatments in CT_max estimation. Thus, some tropical species might not be as vulnerable to warming as previously predicted if CT_max is considered as a shifting population parameter.     

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.     

Thermal limits to survival and activity in two life stages of false codling moth Thaumatotibia leucotreta (Lepidoptera,Tortricidae)

The present study examines life stage‐related variation in the thermal limits to activity and survival in an African pest, the false codling moth Thaumatotibia leucotreta (Lepidoptera, Tortricidae). Thermal tolerance, including the functional activity limits of critical thermal maxima and minima (CT_max and CT_min respectively), upper and lower lethal temperature, and the effect of heat and cold hardening (short‐term acute plasticity), is measured across a diverse range of low or high temperature stress conditions in both larvae and adults. We also report the sum of inducible and cognate forms of the amounts of heat shock protein 70 (HSP70) as an explanatory variable for changes in thermotolerance. The results show that the larvae have high variability in CT_max and CT_min at different ramping rates and low levels of basal (innate) thermal tolerance. By contrast, the adults show high basal tolerance and overall lower variability in CT_max and CT_min, indicating lower levels of phenotypic plasticity in thermotolerance. HSP70 responses, although variable, do not reflect these tolerance or survival patterns. Larvae survive across a broader range of temperatures, whereas adults remain active across a broader range of temperatures. Life stage‐related variation in thermal tolerance is most pronounced under the slowest (most ecologically‐relevant) ramping rate (0.06 °C min^–1) during lower critical thermal limit experiments and least pronounced during upper thermal limit experiments. Thus, the ramping rate can hinder or enhance the detection of stage‐related variation in thermal limits to activity and survival of insects.     

City limits: Heat tolerance is influenced by body size and hydration state in an urban ant community

Cities are rapidly expanding, and global warming is intensified in urban environments due to the urban heat island effect. Therefore, urban animals may be particularly susceptible to warming associated with ongoing climate change. We used a comparative and manipulative approach to test three related hypotheses about the determinants of heat tolerance or critical thermal maximum (CT_max) in urban ants—specifically, that (a) body size, (b) hydration status, and (c) chosen microenvironments influence CT_max. We further tested a fourth hypothesis that native species are particularly physiologically vulnerable in urban environments. We manipulated water access and determined CT_max for 11 species common to cities in California's Central Valley that exhibit nearly 300‐fold variation in body size. There was a moderate phylogenetic signal influencing CT_max, and inter (but not intra) specific variation in body size influenced CT_max where larger species had higher CT_max. The sensitivity of ants’ CT_max to water availability exhibited species‐specific thresholds where short‐term water limitation (8 hr) reduced CT_max and body water content in some species while longer‐term water limitation (32 hr) was required to reduce these traits in other species. However, CT_max was not related to the temperatures chosen by ants during activity. Further, we found support for our fourth hypothesis because CT_max and estimates of thermal safety margin in native species were more sensitive to water availability relative to non‐native species. In sum, we provide evidence of links between heat tolerance and water availability, which will become critically important in an increasingly warm, dry, and urbanized world that others have shown may be selecting for smaller (not larger) body size.     

Thermal tolerance in the Andean toad Rhinella spinulosa (Anura: Bufonidae) at three sites located along a latitudinal gradient in Chile

Rhinella spinulosa is one of the anuran species with the greatest presence in Chile. This species mainly inhabits mountain habitats and is distributed latitudinally along the western slope of the Andes Range. These habitats undergo great temperature fluctuations, exerting pressure on the amphibian. To identify the physiological strategies and thermal behavior of this species, we analyzed the temperature variables CT_min, CT_max, TTR, τ_heat, and τ_cool in individuals of three sites from a latitudinal gradient (22°S to 37°S). The amphibians were acclimated to 10 °C and 20 °C and fed ad libitum. The results indicate that the species has a high thermal tolerance range, with a mean of 38.14±1.34 °C, a critical thermal maxima of 34.6–41.4 °C, and a critical thermal minima of 2.6–0.8 °C, classifying the species as eurythermic. Furthermore, there were significant differences in CT_máx and TTR only in the northern site. The differences in thermal time constants between sites are due to the effects of size and body mass. For example, those from the central site had larger size and greater thermal inertia; therefore, they warmed and cooled in a slower manner.The wide thermal limits determined in R. spinulosa confirm that it is a thermo-generalist species, a characteristic that allows the species to survive in adverse microclimatic conditions. The level of plasticity in critical temperatures seems ecologically relevant and supports the acclimatization of thermal limits as an important factor for ectothermic animals to adapt to climate change.     

Testing the thermal limits of Eccritotarsus catarinensis: a case of thermal plasticity

Water hyacinth is considered the most damaging aquatic weed in South Africa. The success of biocontrol initiatives against the weed varies nation-wide, but control remains generally unattainable in higher altitude, temperate regions. Eccritotarsus catarinensis (Hemiptera: Miridae) is a biocontrol agent of water hyacinth that was first released in South Africa in 1996. By 2011, it was established at over 30 sites across the country. These include the Kubusi River, a site with a temperate climate where agent establishment and persistence was unexpected. This study compared the critical thermal limits of the Kubusi River insect population with a laboratory-reared culture to determine whether any physiological plasticity was evident that could account for its unexpected establishment. There were no significant differences in critical thermal maxima (CT_max) or minima (CT_min) between sexes, while the effect of rate of temperature change on the thermal parameters in the experiments had a significant impact in some trials. Both CT_max and CT_min differed significantly between the two populations, with the field individuals tolerating significantly lower temperatures (CT_min: ?0.3°C?±?0.063 [SE], CT_max: 42.8°C?±?0.155 [SE]) than those maintained in the laboratory (CT_min: 1.1°C?±?0.054 [SE], CT_max: 44.9°C?±?0.196 [SE]). Acclimation of each population to the environmental conditions typical of the other for a five-day period illustrated that short-term acclimation accounted for some, but not all of the variation between their lower thermal limits. This study provides evidence for the first cold-adapted strain of E. catarinensis in the field, with potential value for introduction into other colder regions where water hyacinth control is currently unattainable.     

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.     

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.     

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.     

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.     

Lower thermal tolerance in nocturnal than in diurnal ants: a challenge for nocturnal ectotherms facing global warming

1. The thermal adaptation hypothesis proposes that because thermoregulation involves a high metabolic cost, thermal limits of organisms must be locally adapted to temperatures experienced in their environments. There is evidence that tolerance to high temperatures decreases in insects inhabiting colder habitats and microclimates. However, it is not clear if thermal limits of ectotherms with contrasting temporal regimes, such as diurnal and nocturnal insects, are also adapted to temperatures associated with their circadian activities. 2. This study explores differences in heat tolerance among diurnal and nocturnal ant species in four ecosystems in Mexico: tropical montane, tropical rainforest, subtropical dry forests, and high‐elevation semi‐desert. 3. The critical thermal maximum (CT_max), i.e. the temperature at which ants lost motor control, was estimated for diurnal and nocturnal species. CT_max for 19 diurnal and 12 nocturnal ant species distributed among 45 populations was also estimated. 4. Semi‐desert and subtropical dry forest ants displayed higher tolerances to high temperatures than did ants in tropical rainforest. The lowest tolerance to high temperatures was recorded in tropical montane forest ants. In general, among all habitats, the CT_max of nocturnal ants was lower than that of diurnal ants. 5. An increase in nocturnal temperatures, combined with lower tolerance to high temperatures, may represent a substantial challenge for nocturnal ectotherms in a warming world.     

Thermal stress exposure of pupal oriental fruit fly has strong and trait-specific consequences in adult flies

Global climate change is projected to increase the incidence of heat waves, their magnitude and duration resulting in insects experiencing increasing environmental stress in both natural and managed ecosystems. While studies on insect thermal tolerance are rapidly increasing, variation across developmental or juvenile stress cross-stage effects within and across generations remain largely unexplored. Yet in holometabolous insects, heat stress at an early developmental stage may influence performance and survival during later stages. Here, we investigated the effects of pupal mild heat stress on the performance of laboratory-reared adult Bactrocera dorsalis (Hendel) (Diptera: Tephritidae) measured as longevity, critical thermal maximum (CT_max), critical thermal minima (CT_min), heat knockdown time (HKDT) and chill coma recovery time (CCRT). Pupal heat stress significantly influenced performance of B. dorsalis adults resulting in impaired longevity and heat tolerance (CT_max and HKDT) in both sexes with improved and compromised cold tolerance (CT_min and CCRT) in females and males, respectively. These findings highlight the role of juvenile stages in mediating stress responses at adult stages. For B. dorsalis, pupal heat stress largely compromised thermal tolerance implying that the species has limited potential to shift its geographic range in heat prone areas. Significant benefits in cold tolerance in females following heat stress may help in improving survival in the cold in the short-term despite restricted activity to the same traits in males. This study suggests that basal heat tolerance and not short-term compensatory thermal plasticity following heat stress may have aided the recent invasion of B. dorsalis in African landscapes.     

Superior basal and plastic thermal responses to environmental heterogeneity in invasive exotic stemborer Chilo partellus Swinhoe over indigenous Busseola fusca (Fuller) and Sesamia calamistis Hampson

Lepidopteran stemborers are the most destructive insect pests of cereal crops in sub‐Saharan Africa. In nature, these insects are often exposed to multiple environmental stressors, resulting in potent impact on their thermal tolerance. Such environmental stressors may influence their activity, survival, abundance and biogeography. In the present study, we investigate the effects of acclimation to temperature, starvation and desiccation on thermal tolerance, measured as critical thermal limits [critical thermal minima (CT_min) and maxima (CT_max)] on laboratory‐reared economic pest species Chilo partellus Swinhoe (Lepidoptera: Crambidae), Busseola fusca (Fuller) and Sesamia calamistis Hampson (Lepidoptera: Noctuidae) using established protocols. Low temperature acclimation results in improved CT_min for B. fusca and C. partellus, whereas high temperature acclimation enhances the same trait for B. fusca and S. calamistis. Similarly, high temperature and starvation pretreatment improve CT_max for C. partellus relative to S. calamistis and B. fusca. In addition, starvation and desiccation pretreatments improve CT_min for all stemborer species. Furthermore, rapid cold‐hardening (RCH) enhancs CT_min for B. fusca and C. partellus, whereas rapid heat‐hardening (RHH) improves the same trait for C. partellus. However, RCH and RHH impair CT_max for all stemborer species. These findings show differential thermal tolerances after exposure to heterogeneous environmental stress habitats. Chilo partellus, of exotic origin, shows a higher magnitude of basal thermal tolerance plasticity relative to the indigenous African species S. calamistis and B. fusca. This indicates that C. partellus may have a fitness and survival advantage under climate‐induced heterogeneous environments, and also have a greater chance for geographical range expansion and invasion success compared with the indigenous B. fusca and S. calamistis.     

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 plasticity in the invasive south American tomato pinworm Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae)

South American tomato pinworm, Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae) is a devastating invasive global insect pest of tomato, Solanum lycopersicum (Solanaceae). In nature, pests face multiple overlapping environmental stressors, which may significantly influence survival. To cope with rapidly changing environments, insects often employ a suite of mechanisms at both acute and chronic time-scales, thereby improving fitness at sub-optimal thermal environments. For T. absoluta, physiological responses to transient thermal variability remain under explored. Moreso, environmental effects and physiological responses may differ across insect life stages and this can have implications for population dynamics. Against this background, we investigated short and long term plastic responses to temperature of T. absoluta larvae (4th instar) and adults (24–48 h old) from field populations. We measured traits of temperature tolerance vis critical thermal limits [critical thermal minima (CT_min) and maxima (CT_max)], heat knockdown time (HKDT), chill coma recovery time (CCRT) and supercooling points (SCP). Our results showed that at the larval stage, Rapid Cold Hardening (RCH) significantly improved CT_min and HKDT but impaired SCP and CCRT. Heat hardening in larvae impaired CT_min, CCRT, SCP, CT_max but not HKDT. In adults, both heat and cold hardening generally impaired CT_min and CT_max, but had no effects on HKDT, SCP and CCRT. Low temperature acclimation significantly improved CT_min and HKDT while marginally compromising CCRT and CT_max, whereas high temperature acclimation had no significant effects on any traits except for HKDT in larvae. Similarly, low and high temperature acclimation had no effects on CT_min, SCPs and CT_max, while high temperature acclimation significantly compromised adult CCRT. Our results show that larvae are more thermally plastic than adults and can shift their thermal tolerance in short and long timescales. The larval plasticity reported here could be advantageous in new envirnments, suggesting an asymmetrical ecological role of larva relative to adults in facilitating T. absoluta invasion.     

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.     

Ecological differences influence the thermal sensitivity of swimming performance in two co-occurring mysid shrimp species with climate change implications

Temperature strongly affects performance in ectotherms. As ocean warming continues, performance of marine species will be impacted. Many studies have focused on how warming will impact physiology, life history, and behavior, but few studies have investigated how ecological and behavioral traits of organisms will affect their response to changing thermal environments. Here, we assessed the thermal tolerances and thermal sensitivity of swimming performance of two sympatric mysid shrimp species of the Northwest Atlantic. Neomysis americana and Heteromysis formosa overlap in habitat and many aspects of their ecological niche, but only N. americana exhibits vertical migration. In temperate coastal ecosystems, temperature stratification of the water column exposes vertical migrators to a wider range of temperatures on a daily basis. We found that N. americana had a significantly lower critical thermal minimum (CT_min) and critical thermal maximum (CT_max). However, both mysid species had a buffer of at least 4 °C between their CT_max and the 100-year projection for mean summer water temperatures of 28 °C. Swimming performance of the vertically migrating species was more sensitive to temperature variation, and this species exhibited faster burst swimming speeds. The generalist performance curve of H. formosa and specialist curve of N. americana are consistent with predictions based on the exposure of each species to temperature variation such that higher within-generation variability promotes specialization. However, these species violate the assumption of the specialist-generalist tradeoff in that the area under their performance curves is not constant. Our results highlight the importance of incorporating species-specific responses to temperature based on the ecology and behavior of organisms into climate change prediction models.     

Exposure to a fungal pathogen increases the critical thermal minimum of two frog species

The ability of an organism to tolerate seasonal temperature changes, such as extremely cold temperatures during the winter, can be influenced by their pathogens. We tested how exposure to a virulent fungal pathogen, Batrachochytrium dendrobatidis (Bd), affected the critical thermal minimum (CT_min) of two frog species, Hyla versicolor (gray treefrog) and Lithobates palustris (pickerel frog). The CT_min is the minimum thermal performance point of an organism, which we estimated via righting response trials. For both frog species, we compared the righting response of Bd‐exposed and Bd‐unexposed individuals in either a constant (15ºC) environment or with decreasing temperatures (−1°C/2.5 min) starting from 15°C. The CT_min for both species was higher for Bd‐exposed frogs than unexposed frogs, and the CT_min of H. versicolor was higher than L. palustris. We also found that Bd‐exposed frogs of both species righted themselves significantly fewer times in both decreasing and constant temperature trials. Our findings show that pathogen exposure can reduce cold tolerance and limit the thermal performance range of hosts, which may lead to increased overwintering mortality.