Temperature and the effects of elemental food quality on Daphnia

1. We examined the responses of two species of Daphnia to changes in food phosphorus (P) content, with animals reared at three different water temperatures. Specifically, we measured mass‐specific growth rate (MSGR), body P content and respiration rate of Daphnia magna and Daphnia pulex acclimatised to 10, 17.5 and 25 °C and fed food carbon : phosphorus (C : P) ratios of either 150 or 500. 2. The responses of these three physiological variables to temperature–food quality interactions were species‐specific. There was a significant interactive effect of temperature and food quality on D. magna, as the greatest proportional effect of food quality on growth was observed at 10 °C and reductions in body P because of low food P content were relatively greater at 25 °C. These effects may reflect the temperature dependence of mechanisms that reduce elemental constraints associated with food quality in D. magna. By contrast, there were no interactive effects between food quality and temperature on MSGR, body P or mass‐specific respiration of D. pulex. 3. It thus appears that temperature can alter food quality effects on Daphnia but the nature of these alterations depends upon the daphniid species and its thermal adaptability. Significant temperature–food quality interactions will complicate efforts to understand zooplankton nutrition in nature and warrant future consideration.     

Testing the plant stress hypothesis: stressed plants offer better food to an insect herbivore

Temperature and nutrition are among the most important environmental factors affecting ectotherm growth. As temperature and host‐plant quality often co‐vary in nature, the interaction between the two is of potentially high ecological importance for herbivorous insects. We here use the temperate‐zone butterfly Pieris napi L. (Lepidoptera: Pieridae) to investigate interactive effects of larval rearing temperature and host‐plant quality (by manipulating water availability) on larval growth. As growth rates have been hypothesized to govern stress tolerance, we additionally assessed adult starvation resistance. Butterflies followed the ‘temperature‐size rule’, which states that body size increases at lower developmental temperatures, proximately caused by differences in growth increment, which resulted from increased consumption at the lower temperature. Larvae benefitted from feeding on stressed plants from the low‐water regime by having higher body mass, growth rate, and food conversion efficiency, thus supporting the plant stress hypothesis, which predicts that plant quality for herbivores should increase if stress is imposed on plants. Some effects of host‐plant quality on larval growth parameters were as strong as or even stronger than effects of temperature, whereas interactive effects between temperature and food quality were scarce. At the low temperature, adult starvation resistance was higher than at the higher temperature and females were more resistant than males, whereas plant water regime had no clear impact. No evidence was found for a trade‐off between growth rate and starvation resistance. This study illustrates the importance of considering effects of host‐plant quality along with variation in other environmental factors for estimating the impact of environmental changes on herbivorous species.     

The nutritional landscape of host plants for a specialist insect herbivore

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Experimental climate warming alters aspen and birch phytochemistry and performance traits for an outbreak insect herbivore

Climate change and insect outbreaks are key factors contributing to regional and global patterns of increased tree mortality. While links between these environmental stressors have been established, our understanding of the mechanisms by which elevated temperature may affect tree–insect interactions is limited. Using a forest warming mesocosm, we investigated the influence of elevated temperature on phytochemistry, tree resistance traits, and insect performance. Specifically, we examined warming effects on forest tent caterpillar (Malacosoma disstria) and host trees aspen (Populus tremuloides) and birch (Betula papyrifera). Trees were grown under one of three temperature treatments (ambient, +1.7 °C, +3.4 °C) in a multiyear open‐air warming experiment. In the third and fourth years of warming (2011, 2012), we assessed foliar nutrients and defense chemistry. Elevated temperatures altered foliar nitrogen, carbohydrates, lignin, and condensed tannins, with differences in responses between species and years. In 2012, we performed bioassays using a common environment approach to evaluate plant‐mediated indirect warming effects on larval performance. Warming resulted in decreased food conversion efficiency and increased consumption, ultimately with minimal effect on larval development and biomass. These changes suggest that insects exhibited compensatory feeding due to reduced host quality. Within the context of observed phytochemical variation, primary metabolites were stronger predictors of insect performance than secondary metabolites. Between‐year differences in phytochemical shifts corresponded with substantially different weather conditions during these two years. By sampling across years within an ecologically realistic and environmentally open setting, our study demonstrates that plant and insect responses to warming can be temporally variable and context dependent. Results indicate that elevated temperatures can alter phytochemistry, tree resistance traits, and herbivore feeding, but that annual weather variability may modulate warming effects leading to uncertain consequences for plant–insect interactions with projected climate change.     

Host–plant quality alters herbivore responses to temperature: a case study using the generalist Hyphantria cunea

The nutritional quality of host plants is an important determinant of fitness in insect herbivores. However, it remains an open question whether the ingestion of a particular plant will have the same effects on an herbivore under differing thermal conditions. We measured the performance of the generalist‐feeding caterpillars of Hyphantria cunea Drury (Lepidoptera: Arctiidae) raised on one of five natural host plants to determine their nutritional quality: Platanus occidentalis L. (Platanaceae), Sophora japonica (L.) Schott (Fabaceae), Prunus × yedoensis Matsum. (Rosaceae), Cornus kousa Hance (Cornaceae), or Betula platyphylla Sukaczev (Betulaceae). Caterpillars performed well on P. occidentalis, S. japonica, and P. × yedoensis, but poorly on C. kousa and B. platyphylla. The nutritional phenotype of caterpillars varied among host–plant groups, with the proportion of lipid‐free body mass to lipid content being higher for caterpillars raised on P. occidentalis and S. japonica (3.8–4.2:1) than for caterpillars raised on P. × yedoensis (1.6–2.1:1). A multi‐factorial experimental design was employed to investigate the interactive effects of host–plant quality and temperature on the performance of H. cunea caterpillars raised on either P. occidentalis or P. × yedoensis at three rearing temperatures (20, 25, or 30 °C). Caterpillars raised on P. occidentalis displayed a monotonic decrease in development time with increasing temperature, but the development time of those on P. × yedoensis decreased rapidly as temperature rose from 20 to 25 °C and then stayed unchanged despite further increase in temperature. The rate at which body size increased with decreasing temperature was much steeper for caterpillars raised on P. occidentalis than for those on P. × yedoensis. Collectively, these results indicate that host plant can alter the thermal reaction norms for the key life‐history traits of herbivores. This study has implications for understanding the impacts of climate change on herbivore–plant interactions.     

Warming counteracts defoliation‐induced mismatch by increasing herbivore‐plant phenological synchrony

Climate change is altering phenology; however, the magnitude of this change varies among taxa. Compared with phenological mismatch between plants and herbivores, synchronization due to climate has been less explored, despite its potential implications for trophic interactions. The earlier budburst induced by defoliation is a phenological strategy for plants against herbivores. Here, we tested whether warming can counteract defoliation‐induced mismatch by increasing herbivore‐plant phenological synchrony. We compared the larval phenology of spruce budworm and budburst in balsam fir, black spruce, and white spruce saplings subjected to defoliation in a controlled environment at temperatures of 12, 17, and 22°C. Budburst in defoliated saplings occurred 6–24 days earlier than in the controls, thus mismatching needle development from larval feeding. This mismatch decreased to only 3–7 days, however, when temperatures warmed by 5 and 10°C, leading to a resynchronization of the host with spruce budworm larvae. The increasing synchrony under warming counteracts the defoliation‐induced mismatch, disrupting trophic interactions and energy flow between forest ecosystem and insect populations. Our results suggest that the predicted warming may improve food quality and provide better growth conditions for larval development, thus promoting longer or more intense insect outbreaks in the future.     

Inclusion of host quality data improves predictions of herbivore phenology

Understanding the correspondence between ambient temperature and insect development is necessary to forecast insect phenology under novel environments. In the face of climate change, both conservation and pest control efforts require accurate phenological predictions. Here, we compare a suite of degree‐day models to assess their ability to predict the phenology of a common, oligophagous butterfly, the silver‐spotted skipper, Epargyreus clarus (Cramer) (Lepidoptera: Hesperiidae). To estimate model parameters, we used development time of eggs and larvae reared in the laboratory at six constant temperatures ranging from 8 to 38 °C and on two host plants of contrasting quality (kudzu and wisteria). We employed three approaches to determine the base temperature to calculate degree days: linear regression, modified reduced major axis regression, and application of a generic base temperature value of 10 °C, which is commonly used in the absence of laboratory data. To calculate the number of degree days required to complete a developmental stage, we used data from caterpillars feeding on high‐ and low‐quality hosts, both in the field and in the laboratory. To test model accuracy, we predicted development time of seven generations of larvae reared in the field on the same host plants across 3 years (2014–2016). To compare performance among models, we regressed predicted vs. observed development time, and found that r² values were significantly larger when accounting for host plant quality. The accuracy of development time predictions varied across the season, with estimates of the first two generations being more accurate than estimates of the third generation, when ambient temperatures dropped outside the range in which development rate and temperature have a linear relationship. Overall, we show that accounting for variation in host plant quality when calculating development time in the field is more important than the choice of the base temperature for calculating degree days.     

Interactive effects of temperature and drought on cassava growth and toxicity: implications for food security?

Cassava is an important dietary component for over 1 billion people, and its ability to yield under drought has led to it being promoted as an important crop for food security under climate change. Despite its known photosynthetic plasticity in response to temperature, little is known about how temperature affects plant toxicity or about interactions between temperature and drought, which is important because cassava tissues contain high levels of toxic cyanogenic glucosides, a major health and food safety concern. In a controlled glasshouse experiment, plants were grown at 2 daytime temperatures (23 °C and 34 °C), and either well‐watered or subject to a 1 month drought prior to harvest at 6 months. The objective was to determine the separate and interactive effects of temperature and drought on growth and toxicity. Both temperature and drought affected cassava physiology and chemistry. While temperature alone drove differences in plant height and above‐ground biomass, drought and temperature × drought interactions most affected tuber yield, as well as foliar and tuber chemistry, including C : N, nitrogen and cyanide potential (CNp; total cyanide released from cyanogenic glucosides). Conditions that most stimulated growth and yield (well‐watered × high temperature) effected a reduction in tuber toxicity, whereas drought inhibited growth and yield, and was associated with increased foliar and tuber toxicity. The magnitude of drought effects on tuber yield and toxicity were greater at high temperature; thus, increases in tuber CNp were not merely a consequence of reduced tuber biomass. Findings confirm that cassava is adaptable to forecast temperature increases, particularly in areas of adequate or increasing rainfall; however, in regions forecast for increased incidence of drought, the effects of drought on both food quality (tuber toxicity) and yield are a greater threat to future food security and indicate an increasing necessity for processing of cassava to reduce toxicity.     

Sap‐feeding insects on forest trees along latitudinal gradients in northern Europe: a climate‐driven patterns

Knowledge of the latitudinal patterns in biotic interactions, and especially in herbivory, is crucial for understanding the mechanisms that govern ecosystem functioning and for predicting their responses to climate change. We used sap‐feeding insects as a model group to test the hypotheses that the strength of plant–herbivore interactions in boreal forests decreases with latitude and that this latitudinal pattern is driven primarily by midsummer temperatures. We used a replicated sampling design and quantitatively collected and identified all sap‐feeding insects from four species of forest trees along five latitudinal gradients (750–1300 km in length, ten sites in each gradient) in northern Europe (59 to 70°N and 10 to 60°E) during 2008–2011. Similar decreases in diversity of sap‐feeding insects with latitude were observed in all gradients during all study years. The sap‐feeder load (i.e. insect biomass per unit of foliar biomass) decreased with latitude in typical summers, but increased in an exceptionally hot summer and was independent of latitude during a warm summer. Analysis of combined data from all sites and years revealed dome‐shaped relationships between the loads of sap‐feeders and midsummer temperatures, peaking at 17 °C in Picea abies, at 19.5 °C in Pinus sylvestris and Betula pubescens and at 22 °C in B. pendula. From these relationships, we predict that the losses of forest trees to sap‐feeders will increase by 0–45% of the current level in southern boreal forests and by 65–210% in subarctic forests with a 1 °C increase in summer temperatures. The observed relationships between temperatures and the loads of sap‐feeders differ between the coniferous and deciduous tree species. We conclude that climate warming will not only increase plant losses to sap‐feeding insects, especially in subarctic forests, but can also alter plant‐plant interactions, thereby affecting both the productivity and the structure of future forest ecosystems.     

Temperature responses of a plant‐insect system using a food‐web performance approach

Evaluation of the performance of a plant‐herbivore system as a whole is difficult due to the lack of fitness parameters that can be applied to both components. The individual use of traditional measures of performance (e.g., r_m, biomass) can provide useful, but incomplete information on the performance of insect herbivores and seldom incorporates plant performance. We propose the use of the net generational productivity (NGP) to evaluate the fitness of the herbivore, which can then be compared directly with the performance of the plant in biomass units, to obtain the food‐web performance ratio (φ_H/P). We compared three biotypes of the potato aphid, Macrosiphum euphorbiae Thomas (Hemiptera: Aphididae), when raised on three different host plants: potato (Solanum tuberosum L. cv. Norland) and two bell peppers (Capsicum annuum L. cv. Fascinato and cv. Crosby) (all Solanaceae) at temperatures ranging from 8 to 36 °C. The temperature profiles of the potato aphid biotypes suggest that this aphid is better suited to temperate climates, and its performance generally depends on the particular host‐plant/biotype association. Plant growth performance showed that potato has a lower thermal tolerance, but has a faster growth rate than bell peppers, especially in the range of 16–24 °C. Temperature variation in the φ_H/P ratio shows that aphids have a greater performance than plants, especially at lower temperatures, at which they can accumulate biomass up to 148 times faster. Because of the aphid's biological inability to withstand long exposures to temperatures above 28 °C, plants have a slight advantage over aphids. Nonetheless, as the performance of plants is extremely reduced at high temperatures, this advantage cannot withstand long‐term exposures to extreme temperatures. This is the first attempt to obtain a parameter capable of determining the climatic profile and performance of a food web in an inclusive yet simple manner.     

Bet hedging in a warming ocean: predictability of maternal environment shapes offspring size variation in marine sticklebacks

Bet hedging at reproduction is expected to evolve when mothers are exposed to unpredictable cues for future environmental conditions, whereas transgenerational plasticity (TGP) should be favoured when cues reliably predict the environment offspring will experience. Since climate predictions forecast an increase in both temperature and climate variability, both TGP and bet hedging are likely to become important strategies to mediate climate change effects. Here, the potential to produce variably sized offspring in both warming and unpredictable environments was tested by investigating whether stickleback (Gasterosteus aculeatus) mothers adjusted mean offspring size and within‐clutch variation in offspring size in response to experimental manipulation of maternal thermal environment and predictability (alternating between ambient and elevated water temperatures). Reproductive output traits of F1 females were influenced by both temperature and environmental predictability. Mothers that developed at ambient temperature (17 °C) produced larger, but fewer eggs than mothers that developed at elevated temperature (21 °C), implying selection for different‐sized offspring in different environments. Mothers in unpredictable environments had smaller mean egg sizes and tended to have greater within‐female egg size variability, especially at 21 °C, suggesting that mothers may have dynamically modified the variance in offspring size to spread the risk of incorrectly predicting future environmental conditions. Both TGP and diversification influenced F2 offspring body size. F2 offspring reared at 21 °C had larger mean body sizes if their mother developed at 21 °C, but this TGP benefit was not present for offspring of 17 °C mothers reared at 17 °C, indicating that maternal TGP will be highly relevant for ocean warming scenarios in this system. Offspring of variable environment mothers were smaller but more variable in size than offspring from constant environment mothers, particularly at 21 °C. In summary, stickleback mothers may have used both TGP and diversified bet‐hedging strategies to cope with the dual stress of ocean warming and environmental uncertainty.     

Temperature and nutrient availability interact to mediate growth and body stoichiometry in a detritivorous stream insect

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Nonlinear effects of temperature on body form and developmental canalization in the threespine stickleback

Theoretical models predict that nonlinear environmental effects on the phenotype also affect developmental canalization, which in turn can influence the tempo and course of organismal evolution. Here, we used an oceanic population of threespine stickleback (Gasterosteus aculeatus) to investigate temperature‐induced phenotypic plasticity of body size and shape using a paternal half‐sibling, split‐clutch experimental design and rearing offspring under three different temperature regimes (13, 17 and 21 °C). Body size and shape of 466 stickleback individuals were assessed by a set of 53 landmarks and analysed using geometric morphometric methods. At approximately 100 days, individuals differed significantly in both size and shape across the temperature groups. However, the temperature‐induced differences between 13 and 17 °C (mainly comprising relative head and eye size) deviated considerably from those between 17 and 21 °C (involving the relative size of the ectocoracoid, the operculum and the ventral process of the pelvic girdle). Body size was largest at 17 °C. For both size and shape, phenotypic variance was significantly smaller at 17 °C than at 13 and 21 °C, indicating that development is most stable at the intermediate temperature matching the conditions encountered in the wild. Higher additive genetic variance at 13 and 21 °C indicates that the plastic response to temperature had a heritable basis. Understanding nonlinear effects of temperature on development and the underlying genetics are important for modelling evolution and for predicting outcomes of global warming, which can lead not only to shifts in average morphology but also to destabilization of development.     

Stressor interactions in freshwater habitats: Effects of cold water exposure and food limitation on early‐life growth and upper thermal tolerance in white sturgeon,Acipenser transmontanus

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Factoring distribution and prevalence of Fall armyworm in southwest China

Host plant growth changing with environmental conditions can impact the distribution of herbivores. The generalist herbivore fall armyworm (FAW), Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae), is an invasive pest rapidly spreading across the world and has recently invaded southern China. We studied effects of environmental factors on the distribution of the fall armyworm and its host (maize) plant growth in the tropical mountainous area of Huize County, province of Yunnan, southwest China. Moreover, the relationships among the FAW distribution, environmental factors (altitude, temperature and humidity) and plant growth (mean kernel weight, kernel number per ear and ear weight) were analysed. The results showed that FAW predominated at altitude 1,243.3 m, temperature 21.4°C and humidity 82.1%. The host plant grew best at 1,200–1,312 m, 21.0–21.7°C and 81.2%–82.0%. Environmental factors indirectly influenced the distribution of FAW via the host plant growth. Compared with environmental factors, the host plant growth had a simplistic positive linear relationship with the density of FAW. FAW is less impacted by abiotic factors rather it determined by host plant prevalence, and thus the locations where maize plants grow best are preferred by FAW and should be the focus of insecticide applications. Understanding the distribution of FAW under various environmental conditions provides a valuable reference for Chinese maize production and food security.     

Effects of water temperature and initial weight on growth,digestion and energy budget of yellow catfish Pelteobagrus fulvidraco (Richardson, 1846)

The effects of water temperature and body weight on feeding, growth, and energy budget were inevitable in the yellow catfish Pelteobagrus fulvidraco (Richardson, 1846), an important fish cultivated in China. This study explores the interaction of water temperature and body weight on both energy utilization strategy and energy conversion efficiency to promote further healthy culture of yellow catfish. Fish with body weights of 6 g (Group S), 16 g (Group M) and 35 g (Group B) were reared in 15 circular glass steel cylinders 80 cm in diameter × 70 cm in height (180 L) at water temperatures of 21, 24, 27, 30 and 33°C (3 replicates for each temperature) for 42 days to investigate effects of water temperature and body weight on the feeding, growth, digestion and energy budget in yellow catfish. Results showed that the levels of dry matter, protein and energy in the body were significantly affected by water temperature (p < .05). Feeding, growth, feed conversion efficiency, digestion and energy allocation parameters were significantly related to both water temperature and body weight (p < .05). Yellow catfish had higher maximal food consumption (C_max), food intake rate, specific growth rate, food conversion efficiency, appear digestibility coefficient, and growth energy allocation (G) at 24–30°C, and optimal growth at a water temperature of 27°C. Two‐factor analysis of variance revealed that there was reciprocation of both water temperature and body weight on the above parameters. At the optimal temperature of 27°C, the value of energy for growth (G) was the highest, and the value of energy for feces (F) produced was the lowest. Yellow catfish with various body weights had energy budget equations of 100 A = 63.70 R + 36.30 G in Group S, 100 A = 62.54 R + 37.46 G in Group M, and 100 A = 67.47 R + 32.53 G in Group B if the equations were described as percentage of the proportion of the assimilation energy. Therefore, the optimal temperature was 27°C according to its feeding, growth and digestion.     

Temperature and water stress affect plant–pollinator interactions in Borago officinalis (Boraginaceae)

Climate change alters the abiotic constraints faced by plants, including increasing temperature and water stress. These changes may affect flower development and production of flower rewards, thus altering plant–pollinator interactions. Here, we investigated the consequences of increased temperature and water stress on plant growth, floral biology, flower‐reward production, and insect visitation of a widespread bee‐visited species, Borago officinalis. Plants were grown for 5 weeks under three temperature regimes (21, 24, and 27°C) and two watering regimes (well‐watered and water‐stressed). Plant growth was more affected by temperature rise than water stress, and the reproductive growth was affected by both stresses. Vegetative traits were stimulated at 24°C, but impaired at 27°C. Flower development was mainly affected by water stress, which decreased flower number (15 ± 2 flowers/plant in well‐watered plants vs. 8 ± 1 flowers/plant under water stress). Flowers had a reduced corolla surface under temperature rise and water stress (3.8 ± 0.5 cm² in well‐watered plants at 21°C vs. 2.2 ± 0.1 cm² in water‐stressed plants at 27°C). Both constraints reduced flower‐reward production. Nectar sugar content decreased from 3.9 ± 0.3 mg/flower in the well‐watered plants at 21°C to 1.3 ± 0.4 mg/flower in the water‐stressed plants at 27°C. Total pollen quantity was not affected, but pollen viability decreased from 79 ± 4% in the well‐watered plants at 21°C to 25 ± 9% in the water‐stressed plants at 27°C. Flowers in the well‐watered plants at 21°C received at least twice as many bumblebee visits compared with the other treatments. In conclusion, floral modifications induced by abiotic stresses related to climate change affect insect behavior and alter plant–pollinator interactions.     

Genetic factors affecting food‐plant specialization of an oligophagous seed predator

Several ecological and genetic factors affect the diet specialization of insect herbivores. The evolution of specialization may be constrained by lack of genetic variation in herbivore performance on different food‐plant species. By traditional view, trade‐offs, that is, negative genetic correlations between the performance of the herbivores on different food‐plant species favour the evolution of specialization. To investigate whether there is genetic variation or trade‐offs in herbivore performance between different food plants that may influence specialization of the oligophagous seed‐eating herbivore, Lygaeus equestris (Heteroptera), we conducted a feeding trial in laboratory using four food‐plant species. Although L. equestris is specialized on Vincetoxicum hirundinaria (Apocynaceae) to some degree, it occasionally feeds on alternative food‐plant species. We did not find significant negative genetic correlations between mortality, developmental time and adult biomass of L. equestris on the different food‐plant species. We found genetic variation in mortality and developmental time of L. equestris on some of the food plants, but not in adult biomass. Our results suggest that trade‐offs do not affect adaptation and specialization of L. equestris to current and novel food‐plant species, but the lack of genetic variation may restrict food‐plant utilization. As food‐plant specialization of herbivores may have wide‐ranging effects, for instance, on coevolving plant–herbivore interactions and speciation, it is essential to thoroughly understand the factors behind the specialization process. Our findings provide valuable information about the role of genetic factors in food‐plant specialization of this oligophagous herbivore.