Effects of elevated atmospheric CO<Subscript>2</Subscript> on the nutritional ecology of C<Subscript>3</Subscript> and C<Subscript>4</Subscript> grass-feeding caterpillars

It is plausible that the nutritional quality of C₃ plants will decline more under elevated atmospheric CO₂ than will the nutritional quality of C₄ plants, causing herbivorous insects to increase their feeding on C₃ plants relative to C₄ plants. We tested this hypothesis with a C₃ and C₄ grass and two caterpillar species with different diet breadths. Lolium multiflorum (C₃) and Bouteloua curtipendula (C₄) were grown in outdoor open top chambers at ambient (370 ppm) or elevated (740 ppm) CO₂. Bioassays compared the performance and digestive efficiencies of Pseudaletia unipuncta (a grass-specialist noctuid) and Spodoptera frugiperda (a generalist noctuid). As expected, the nutritional quality of L. multiflorum changed to a greater extent than did that of B. curtipendula when grown in elevated CO₂; levels of protein (considered growth limiting) declined in the C₃ grass, while levels of carbohydrates (sugar, starch and fructan) increased. However, neither insect species increased its feeding rate on the C₃ grass to compensate for its lower nutritional quality when grown in an elevated CO₂ atmosphere. Consumption rates of P. unipuncta and S. frugiperda were higher on the C₃ grass than the C₄ grass, the opposite of the result expected for a compensatory response to the lower nutritional quality of the C₄ grass. Although our results do not support the hypothesis that grass-specialist insects compensate for lower nutritional quality by increasing their consumption rates more than do generalist insects, the performance of the specialist was greater than that of the generalist on each grass species and at both CO₂ levels. Mechanisms other than compensatory feeding, such as increased nutrient assimilation efficiency, appear to determine the relative performance of these herbivores. Our results also provide further evidence against the hypothesis that C₄ grasses would be avoided by insect herbivores because a large fraction of their nutrients is unavailable to herbivores. Instead, our results are consistent with the hypothesis that C₄ grasses are poorer host plants primarily because of their lower nutrient levels, higher fiber levels, and greater toughness.     

Growth and reproduction of the alpine grasshopper <Emphasis Type="Italic">Miramella alpina</Emphasis> feeding on CO<Subscript>2</Subscript>-enriched dwarf shrubs at treeline

The consequences for plant-insect interactions of atmospheric changes in alpine ecosystems are not well understood. Here, we tested the effects of elevated CO₂ on leaf quality in two dwarf shrub species (Vaccinium myrtillus and V. uliginosum) and the response of the alpine grasshopper (Miramella alpina) feeding on these plants in a field experiment at the alpine treeline (2,180 m a.s.l.) in Davos, Switzerland. Relative growth rates (RGR) of M. alpina nymphs were lower when they were feeding on V. myrtillus compared to V. uliginosum, and were affected by elevated CO₂ depending on plant species and nymph developmental stage. Changes in RGR correlated with CO₂-induced changes in leaf water, nitrogen, and starch concentrations. Elevated CO₂ resulted in reduced female adult weight irrespective of plant species, and prolonged development time on V. uliginosum only, but there were no significant differences in nymphal mortality. Newly molted adults of M. alpina produced lighter eggs and less secretion (serving as egg protection) under elevated CO₂. When grasshoppers had a choice among four different plant species grown either under ambient or elevated CO₂, V. myrtillus and V. uliginosum consumption increased under elevated CO₂ in females while it decreased in males compared to ambient CO₂-grown leaves. Our findings suggest that rising atmospheric CO₂ distinctly affects leaf chemistry in two important dwarf shrub species at the alpine treeline, leading to changes in feeding behavior, growth, and reproduction of the most important insect herbivore in this system. Changes in plant-grasshopper interactions might have significant long-term impacts on herbivore pressure, community dynamics and ecosystem stability in the alpine treeline ecotone.     

A meta-analysis of plant physiological and growth responses to temperature and elevated CO<Subscript>2</Subscript>

Atmospheric carbon dioxide (CO₂) and global mean temperature are expected to be significantly higher by the end of the 21st century. Elevated CO₂ (eCO₂) and higher temperature each affect plant physiology and growth, but their interactive effects have not been reviewed statistically with respect to higher chronic mean temperatures and abrupt heat stress. In this meta-analysis, we examined the effect of CO₂ on the physiology and growth of plants subjected to different temperature treatments. The CO₂ treatments were categorized into ambient (<400 ppm) or elevated (>560 ppm) levels, while temperature treatments were categorized into ambient temperature (AT), elevated temperature (ET; AT + 1.4–6°C), or heat stress (HS; AT + >8°C). Plant species were grouped according to photosynthetic pathways (C₃, C₄), functional types (legumes, non-legumes), growth forms (herbaceous, woody), and economic purposes (crop, non-crop). eCO₂ enhanced net photosynthesis at AT, ET, and HS in C₃ species (especially at the HS level), but in C₄ species, it had no effect at AT, a positive effect at ET, and a negative effect at HS. The positive effect of eCO₂ on net photosynthesis was greater for legumes than for non-legumes at HS, for non-crops than crops at ET, and for woody than herbaceous species at ET and HS. Total (W _T) and above- (W _AG) and below-ground (W _BG) biomass were increased by eCO₂ for most species groups at all temperatures, except for C₄ species and W _BG of legumes at HS. Hence, eCO₂ × heat effects on growth were often not explained by effects on net photosynthesis. Overall, the results show that eCO₂ effects on plant physiology and growth vary under different temperature regimes, among functional groups and photosynthetic pathways, and among response variables. These findings have important implications for biomass accumulation and ecosystem functioning in the future when the CO₂ level is higher and climate extremes, such as heat waves, become more frequent.     

Increased chemical resistance explains low herbivore colonization of introduced seaweed

The success of introduced species is often attributed to release from co-evolved enemies in the new range and a subsequent decreased allocation to defense (EICA), but these hypotheses have rarely been evaluated for systems with low host-specificity of enemies. Here, we compare herbivore utilization of the brown seaweed, Fucus evanescens, and its coexisting competitors both in its native and new ranges, to test certain predictions derived from these hypotheses in a system dominated by generalist herbivores. While F. evanescens was shown to be a preferred host in its native range, invading populations supported a less diverse herbivore fauna and it was less preferred in laboratory choice experiments with important herbivores, when compared to co-occurring seaweeds. These results are consistent with the enemy release hypothesis, despite the fact that the herbivore communities in both regions were mainly composed of generalist species. However, in contrast to the prediction of EICA, analysis of anti-grazing compounds indicated a higher allocation to defense in introduced compared to native F. evanescens. The results suggest that the invader is subjected to less intense enemy control in the new range, but that this is due to an increased allocation to defense rather than release from specialized herbivores. This indicates that increased resistance to herbivory might be an important strategy for invasion success in systems dominated by generalist herbivores.     

Interacting effects of elevated CO2, nutrient availability and plant species on a generalist invertebrate herbivore

By affecting plant growth and phytochemistry elevated CO₂ may have indirect effects on the performance of herbivores. These effects show considerable variability across studies and may depend on nutrient availability, the carbon/nutrient‐balance in plant tissues and the secondary metabolism of plants. We studied the responses to elevated CO₂ and different nutrient availability of 12 herbaceous plant species differing in their investment into secondary compounds. Caterpillars of the generalist herbivore Spodoptera littoralis were reared on the leaves produced and their consumption and growth rates analysed. Elevated CO₂ resulted in a similar increase of biomass in all plant species, whereas the positive effect of fertilization varied among plant species. Specific leaf weight was influenced by elevated CO₂, but the effect depended on nutrient level and identity of plant species. Elevated CO₂ increased the C/N ratio of the leaves of most species. Caterpillars consumed more leaf material when plants were grown under elevated CO₂ and low nutrients. This indicates compensatory feeding due to lower tissue quality. However, the effects of elevated CO₂, nutrient availability and plant species identity on leaf consumption interacted. Both the effects of CO₂ and nutrient availability on the relative growth rate of the herbivore depended on the plant species. The feeding rate of S. littoralis on plant species that do not produce nitrogen‐containing secondary compounds (NCSC) was higher under low nutrient availability. In contrast, in plants producing NCSC nutrient availability had no effect on the feeding rate. This suggests that compensatory feeding in response to low nutrient contents may not be possible if plants produce NCSC. We conclude that elevated CO₂ causes species‐specific changes in the quality of plant tissues and consequently in changes in the preferences of herbivores for plant species. This could result in changes in plant community composition.     

Choice,performance and heritability of performance of specialist and generalist insect herbivores towards cacalol and seneciphylline,two allelochemicals of Adenostyles alpina (Asteraceae)

We compared the effects of a sesquiterpene (ST, cacalol) and a pyrrolizidine alkaloid (PA, seneciphylline), both occurring in Adenostyles alliariae, on food choice and performance of specialist and generalist insect herbivores which are all known to feed or live on A. alliariae. In choice experiments we investigated whether the compounds were preferred, deterrent or had no effect. All specialist species Aglaostigma discolor (Hymenoptera, Tenthredinidae), Oreina cacaliae (Coleoptera, Chrysomelidae) and O. speciosissima avoided feeding when confronted with the combination of compounds. Only larvae of A. discolor avoided the single ST treatment as well. Larvae of the generalist species Callimorpha dominula (Lepidoptera, Arctiidae), Cylindrotoma distinctissima (Diptera, Tipulidae) and Miramella alpina (Caelifera, Acrididae) generally avoided feeding from PA, ST and PAST treatments. The only exception were caterpillars of C. dominula which were indiscriminate towards PA when naive, and preferred to feed on the PA treatment when they had experienced the compound before. Performance, measured as the growth of larvae on the different treatments in a no choice situation over a period of 10–17 days, was not different between treatments in the specialist leaf beetles O. cacaliae and O. speciosissima. Their avoidance of the combination treatment in the choice experiments had no obvious effect on growth when forced to feed from the treatment. In the generalist C. dominula only the high concentration combination treatment (PAST) reduced growth of the larvae due to decreased consumption. In C. distinctissima we found reduced growth in all treatments except one (PA3%). Poor growth performance in C. distinctissima was due to postingestive physiological effects of all treatments and additionally to consumption reduction in high‐dose ST treatments. Genetic variability (broad sense heritability) of growth performance metabolism varied in accordance with the specialization degree of the species. O. cacaliae, the most specialized species, had no significant heritability; O. speciosissima, the less specialized specialist, had a heritability of 0.46; C. dominula, the PA adapted generalist species, had a heritability of 0.64; C. distinctissima, the generalist with no apparent adaptations, had a heritability of 0.84.     

Drought changes plant chemistry and causes contrasting responses in lepidopteran herbivores

Drought events are predicted to increase due to climate change, yet consequences for plant–insect interactions are only partially understood. Drought‐mediated interactions between herbivores and their host plants are affected by a combination of factors, including characteristics of the affected plant, its associated herbivore and of the prevailing drought. Studying the effect of these factors in combination may provide important insight into plant and herbivore responses to drought. We studied drought effects on plant resistance to two leaf‐chewing herbivores by considering differing growth conditions, plant chemistry and insect responses in concert. We exposed Alliaria petiolata plants from several wild populations to different intensities of intermittent drought stress and quantified drought‐mediated changes in plant chemistry. Simultaneously, we assessed behavior (feeding preference) and performance of two lepidopteran herbivores: Pieris brassicae, a specialist, and Spodoptera littoralis, a generalist. Drought led to lowest concentrations of secondary defense compounds in severely stressed plants, without affecting total nitrogen content. Additionally, drought evoked opposite patterns in feeding preferences (plant palatability) between the herbivore species. Pieris brassicae consumed most of well‐watered plants, while S. littoralis preferred severely drought‐stressed plants. Hence, feeding preferences of S. littoralis reflected changes in plant secondary chemistry. Contrary to their feeding preference, P. brassicae performed better on drought‐stressed than on well‐watered plants, with faster development and higher attained pupal mass (plant suitability). Spodoptera littoralis showed retarded development in all treatments. In conclusion, drought caused plant secondary defense compounds to decrease consistently across all studied plant populations, which evoked contrasting feeding preferences of two herbivore species of the same feeding guild. These results suggest herbivore specificity as a possible explanation for herbivore responses to drought and emphasize the importance of herbivore characteristics such as feeding specialization in understanding and predicting consequences of future drought events.     

CO<Subscript>2</Subscript> enrichment increases element concentrations in grass mixtures by changing species abundances

Atmospheric carbon dioxide (CO₂) enrichment may increase plant growth more than the uptake of chemical elements from soil. Increased CO₂ also may alter element levels in biomass from multi-species vegetation by changing plant species abundances. We measured concentrations of ten elements in aboveground tissues of three C₄ grasses that had been exposed for 2–3 growing seasons to a continuous gradient in CO₂ from 250 to 500 μmol mol⁻¹. The grasses, Bouteloua curtipendula, Schizachyrium scoparium, and Sorghastrum nutans, are competitive dominants in assemblages of tallgrass prairie vegetation growing on each of three soil types along a field CO₂ gradient in central Texas, USA. Our objective was to determine whether CO₂ influences element concentrations in grass mixtures by changing concentrations in individual species or shifting species abundances. Increased CO₂ had little effect on element concentrations in grasses compared to differences observed among grass species and soils. Increasing CO₂ from the pre-Industrial to elevated levels reduced the phosphorus concentration in grasses grown on a clay and sandy loam soil. Concentrations of most other elements did not respond to CO₂ treatment. Cover of the mid-grass Bouteloua declined at higher CO₂ levels as cover of the taller grass Sorghastrum increased. Concentrations of several elements were lower in Bouteloua than Sorghastrum; hence, this exchange of species at higher CO₂ increased element concentrations in grass assemblages. Potential consequences include an improvement in the nutritional quality of plants for herbivores. Results highlight the underappreciated impact that CO₂ enrichment may have on ecosystem functioning by changing plant composition.     

CO2‐mediated changes of plant traits and their effects on herbivores are determined by leaf age

1. Concentration of atmospheric CO₂ is predicted to double during the 21st century. However, quantitative effects of increased CO₂ levels on natural herbivore–plant interactions are still little understood. 2. In this study, we assess whether increased CO₂ quantitatively affects multiple defensive and nutritive traits in different leaf stages of cyanogenic wildtype lima bean plants (Phaseolus lunatus), and whether plant responses influence performance and choice behaviour of a natural insect herbivore, the Mexican bean beetle (Epilachna varivestis). 3. We cultivated lima bean plants in climate chambers at ambient, 500, 700, and 1000 ppm CO₂ and analysed cyanogenic precursor concentration (nitrogen‐based defence), total phenolics (carbon‐based defence), leaf mass per area (LMA; physical defence), and soluble proteins (nutritive parameter) of three defined leaf age groups. 4. In young leaves, cyanide concentration was the only parameter that quantitatively decreased in response to CO₂ treatments. In intermediate and mature leaves, cyanide and protein concentrations decreased while total phenolics and LMA increased. 5. Depending on leaf stage, CO₂‐mediated changes in leaf traits significantly affected larval performance and choice behaviour of adult beetles. We observed a complete shift from highest herbivore damage in mature leaves under natural CO₂ to highest damage of young leaves under elevated CO_2. Our study shows that leaf stage is an essential factor when considering CO₂‐mediated changes of plant defences against herbivores. Since in the long run preferred consumption of young leaves can strongly affect plant fitness, variable effects of elevated CO₂ on different leaf stages should receive highlighted attention in future research.     

Within‐plant distribution of 1,4‐benzoxazin‐3‐ones contributes to herbivore niche differentiation in maize

Plant defences vary in space and time, which may translate into specific herbivore‐foraging patterns and feeding niche differentiation. To date, little is known about the effect of secondary metabolite patterning on within‐plant herbivore foraging. We investigated how variation in the major maize secondary metabolites, 1,4‐benzoxazin‐3‐one derivatives (BXDs), affects the foraging behaviour of two leaf‐chewing herbivores. BXD levels varied substantially within plants. Older leaves had higher levels of constitutive BXDs while younger leaves were consistently more inducible. These differences were observed independently of plant age, even though the concentrations of most BXDs declined markedly in older plants. Larvae of the well‐adapted maize pest Spodoptera frugiperda preferred and grew better on young inducible leaves irrespective of plant age, while larvae of the generalist Spodoptera littoralis preferred and tended to grow better on old leaves. In BXD‐free mutants, the differences in herbivore weight gain between old and young leaves were absent for both species, and leaf preferences of S. frugiperda were attenuated. In contrast, S. littoralis foraging patterns were not affected. In summary, our study shows that plant secondary metabolites differentially affect performance and foraging of adapted and non‐adapted herbivores and thereby likely contribute to feeding niche differentiation.     

Spatial distributions of multiple plant species affect herbivore foraging selectivity

Spatial distribution of food resources is an important factor determining herbivore foraging. Previous studies have demonstrated that clumped distribution of preferred species increases its consumption by herbivores in single‐ or two‐species systems. However, the potential impact of distribution pattern of less preferred species on foraging was ignored. In natural grasslands with high species diversity and complexity, the spatial distribution of preferred species impacts on herbivore foraging may be strongly correlated with the distribution of less preferred species. Our aims were to determine the effect of distribution of both preferred and other plant species on herbivore foraging under conditions close to a native, multi‐species foraging environment, and conceptualize the relationships between spatial distribution of food resources and herbivore consumption. We hypothesized that random distribution of non‐preferred species reduces herbivore consumption of preferred species because the dispersion of less preferred species likely disturbs herbivore foraging. We conducted an experiment using three species with five combinations of clumped and random distribution patterns. Three species Lathyrus quinquenervius, Phragmites australis and Leymus chinensis, were of high, intermediate and low preferences by sheep, respectively. Results showed that distribution of low preferred species, but not that of high preferred one, affected the consumption of preferred species. Sheep obtained higher consumption of high preferred species when low preferred species followed a clumped distribution than a random distribution. Distance between aggregations of high and low preferred species did not affect sheep foraging. It was concluded that the effects of spatial distribution of preferred species on its consumption are dependent on herbivore foraging strategy, and sheep can consume more preferred species when there is a consistent spatial pattern between preferred species and the entire food resource, and that the random dispersion of low preferred species in grassland may reduce herbivore consumption of high preferred species, thus minimizing selective grazing.     

An ecogenomic analysis of herbivore‐induced plant volatiles in Brassica juncea

Upon herbivore feeding, plants emit complex bouquets of induced volatiles that may repel insect herbivores as well as attract parasitoids or predators. Due to differences in the temporal dynamics of individual components, the composition of the herbivore‐induced plant volatile (HIPV) blend changes with time. Consequently, the response of insects associated with plants is not constant either. Using Brassica juncea as the model plant and generalist Spodoptera spp. larvae as the inducing herbivore, we investigated herbivore and parasitoid preference as well as the molecular mechanisms behind the temporal dynamics in HIPV emissions at 24, 48 and 72 h after damage. In choice tests, Spodoptera litura moth preferred undamaged plants, whereas its parasitoid Cotesia marginiventris favoured plants induced for 48 h. In contrast, the specialist Plutella xylostella and its parasitoid C. vestalis preferred plants induced for 72 h. These preferences matched the dynamic changes in HIPV blends over time. Gene expression analysis suggested that the induced response after Spodoptera feeding is mainly controlled by the jasmonic acid pathway in both damaged and systemic leaves. Several genes involved in sulphide and green leaf volatile synthesis were clearly up‐regulated. This study thus shows that HIPV blends vary considerably over a short period of time, and these changes are actively regulated at the gene expression level. Moreover, temporal changes in HIPVs elicit differential preferences of herbivores and their natural enemies. We argue that the temporal dynamics of HIPVs may play a key role in shaping the response of insects associated with plants.     

Simultaneous inbreeding modifies inbreeding depression in a plant–herbivore interaction

Because inbreeding is common in natural populations of plants and their herbivores, herbivore‐induced selection on plants, and vice versa, may be significantly modified by inbreeding and inbreeding depression. In a feeding assay with inbred and outbred lines of both the perennial herb, Vincetoxicum hirundinaria, and its specialist herbivore, Abrostola asclepiadis, we discovered that plant inbreeding increased inbreeding depression in herbivore performance in some populations. The effect of inbreeding on plant resistance varied among plant and herbivore populations. The among‐population variation is likely to be driven by variation in plant secondary compounds across populations. In addition, inbreeding depression in plant resistance was substantial when herbivores were outbred, but diminished when herbivores were inbred. These findings demonstrate that in plant–herbivore interactions expression of inbreeding depression can depend on the level of inbreeding of the interacting species. Furthermore, our results suggest that when herbivores are inbred, herbivore‐induced selection against self‐fertilisation in plants may diminish.     

CO2 and O3 effects on host plant preferences of the forest tent caterpillar (Malacosoma disstria)

Elevated levels of CO₂ and O₃ affect plant growth and phytochemistry, which in turn can alter physiological performance of associated herbivores. Little is known, however, about how generalist insect herbivores respond behaviorally to CO₂‐ and O₃‐mediated changes in their host plants. This research examined the effects of elevated CO₂ and O₃ levels on host plant preferences and consumption of forest tent caterpillar (FTC, Malacosoma disstria Hbn.) larvae. Dual choice feeding assays were performed with foliage from birch (Betula papyrifera Marsh.) and aspen (Populus tremuloides Michx., genotypes 216 and 259). Trees were grown at the Aspen Free Air CO₂ Enrichment (FACE) facility near Rhinelander, WI, USA, and had been exposed to ambient or elevated concentrations of CO₂ and/or O₃. Levels of nutritional and secondary compounds were quantified through phytochemical analyses. The results showed that elevated O₃ levels increased FTC larval preferences for birch compared with aspen, whereas elevated CO₂ levels had the opposite effect. In assays with the two aspen genotypes, addition of both CO₂ and O₃ caused a shift in feeding preferences from genotype 259 to genotype 216. Consumption was unaffected by experimental treatments in assays comparing aspen and birch, but were increased for larvae given high O₃ foliage in the aspen genotype assays. Elevated levels of CO₂ and O₃ altered tree phytochemistry, but did not explain shifts in feeding preferences. The results demonstrate that increased levels of CO₂ and O₃ can alter insect host plant preferences both between and within tree species. Also, consequences of altered host quality (e.g., compensatory consumption) may be buffered by partial host shifts in situations when alternative plant species are available. Environmentally induced changes in host plant preferences may have the potential to alter the distribution of herbivory across plant genotypes and species, as well as competitive interactions among them.     

Behavioral archives link the chemistry and clonal structure of trembling aspen to the food choice of North American porcupine

Understanding the links among plant genotype, plant chemistry, and food selection by vertebrate herbivores is critical to assess the role of herbivores in the evolution of plant secondary chemistry. Some specialized vertebrate herbivores have been shown to select plants differentially according to plant genotype, but examples from generalists, which constitute the vast majority of vertebrate herbivores, are few, especially in natural conditions. We examined the relationship between the North American porcupine (Erethizon dorsatum), a generalist mammalian herbivore, and clonal trembling aspen (Populus tremuloides), a preferred food source of porcupines. We determined preference for certain aspen trees through visual examination of porcupine climbing scars left on tree bark, and through a controlled feeding experiment. We used genetic and biochemical analyses to link the behavioral archives (climbing scars) left by porcupines on aspen trunks to the clonal structure and chemical composition of trees. We show that two phenolic glycosides (tremulacin and salicortin), which are under a high degree of genetic control and thus vary in concentration across clones, are the chemical variables that most influence (deter) feeding choices by porcupines. Using behavioral archives left by a wild herbivore on a natural stand of plants thus allowed us to demonstrate that a generalist vertebrate herbivore can choose plants according to their clonal structure and genetically based chemical composition. Our results contribute to extending previous findings obtained with generalist herbivores studied in controlled conditions, and with specialist herbivores studied in the field. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Variability in the behavioural responses of three generalist herbivores to the most abundant coumarin in Daphne laureola leaves

We evaluated the responses of three generalist herbivores (Lepidoptera: Noctuidae) to the most abundant coumarin in young leaves of spurge-laurel, Daphne laureola L. (Thymelaeaceae), a perennial shrub consistently fed upon by several noctuid species. Pseudenargia ulicis Staud and Noctua janthe Borkhausen are natural herbivores of the species in southeastern Spain, whereas Spodoptera littoralis Boisduval is a model species frequently used to address the antifeedant role of secondary compounds. Discrimination between control and coumarin-coated D. laureola leaves was investigated by short-term choice experiments. We found species-specific behavioural responses to the treatment: P. ulicis preferred the control leaves, whereas S. littoralis did not discriminate between leaf treatments, although it clearly avoided coumarin when incorporated into an artificial diet, and N. janthe preferred the coumarin-coated leaves. Furthermore, N. janthe larvae reduced consumption when the proportion of treated leaves ingested increased and consumption of S. littoralis larvae was also reduced in coumarin-containing diet under no-choice conditions. Our results highlight that different herbivore species feeding simultaneously on the same host plant respond differently to a single chemical defence compound, likely constraining a directional response of the plant to selection.     

Elevated CO<Subscript>2</Subscript> reduces sap flux in mature deciduous forest trees

We enriched in CO₂ the canopy of 14 broad-leaved trees in a species-rich, ca. 30-m-tall forest in NW Switzerland to test whether elevated CO₂ reduces water use in mature forest trees. Measurements of sap flux density (J_S) were made prior to CO₂ enrichment (summer 2000) and throughout the first whole growing season of CO₂ exposure (2001) using the constant heat-flow technique. The short-term responses of sap flux to brief (1.5–3 h) interruptions of CO₂ enrichment were also examined. There were no significant a priori differences in morphological and physiological traits between trees which were later exposed to elevated CO₂ (n=14) and trees later used as controls (n=19). Over the entire growing season, CO₂ enrichment resulted in an average 10.7% reduction in mean daily J_S across all species compared to control trees. Responses were most pronounced in Carpinus, Acer, Prunus and Tilia, smaller in Quercus and close to zero in Fagus trees. The J_S of treated trees significantly increased by 7% upon transient exposure to ambient CO₂ concentrations at noon. Hence, responses of the different species were, in the short term, similar in magnitude to those observed over the whole season (though opposite because of the reversed treatment). The reductions in mean J_S of CO₂-enriched trees were high (22%) under conditions of low evaporative demand (vapour pressure deficit, VPD <5 hPa) and small (2%) when mean daily VPD was greater than 10 hPa. During a relatively dry period, the effect of elevated CO₂ on J_S even appeared to be reversed. These results suggest that daily water savings by CO₂-enriched trees may have accumulated to a significantly improved water status by the time when control trees were short of soil moisture. Our data indicate that the magnitude of CO₂ effects on stand transpiration will depend on rainfall regimes and the relative abundance of the different species, being more pronounced under humid conditions and in stands dominated by species such as Carpinus and negligible in mono-specific Fagus forests.     

Plant genotype and induced responses affect resistance to herbivores on evening primrose (Oenothera biennis)

Abstract. 1. Although both genotype and induced responses affect a plant's resistance to herbivores, little is known about their relative and interactive effects. This study examined how plant genotype of a native plant (Oenothera biennis) and induced plant responses to herbivory affect resistance to, and interactions among, several herbivores. 2. In a field experiment, genetic and environmental variation among habitats led to variation in the amount of early season damage and plant quality. The pattern of variation in early season infestation by spittlebugs (Philaenus spumarius, a piercing–sucking herbivore) negatively correlated with oviposition preference by a later feeding specialist weevil (Tyloderma foveolatum, a leaf‐chewer). 3. To determine if plant genotype and induced responses to herbivory might be responsible for these field patterns, we performed no‐choice and choice bioassays using four genotypes of O. biennis that varied in resistance. Plants were induced by either spittlebugs or weevils and assays measured the responses of the same specialist weevil as well as a generalist caterpillar (Spodoptera exigua). 4. Resistance to adult weevils was largely unaffected by plant genotype, while they experienced induced resistance following damage by conspecific weevils in no‐choice assays. Caterpillars were more strongly affected by plant genotype than induced responses in both no‐choice and choice assays, but they also fed less and experienced higher mortality on plants previously damaged by weevils. In contrast to the pattern suggested by the field experiment, spittlebugs did consistently induce resistance against either weevils or caterpillars in the bioassay experiment. 5. These results support recent findings that show herbivore species can compete via induced plant responses. Additionally, a quantitative review of the literature demonstrates that plant genotype tends to be more important than interspecific competition among herbivores (plant‐mediated or otherwise) in affecting herbivore preference and performance.     

Strong effects of hydrologic environment and weak effects of elevated CO<Subscript>2</Subscript> on the invasive weed <Emphasis Type="Italic">Alternanthera philoxeroides</Emphasis> and the biocontrol beetle <Emphasis Type="Italic">Agasicles hygrophila</Emphasis>

Global change, such as elevated CO₂, may alter interactions between invasive plants and biocontrol agents, impacting biocontrol efficacy. Here, we conducted four experiments in Texas, USA to test how elevated CO₂ influences an invasive plant (Alternanthera philoxeroides) and its interactions with an introduced biocontrol beetle (Agasicles hygrophila) in terrestrial (well-watered) and flooded environments. We grew plants for 9 months in ambient or elevated CO₂ (800 ppm) chambers in continuously flooded or well-watered conditions. In no-choice trials, flooding increased leaf toughness and decreased beetle consumption but beetles only oviposited on ambient CO₂ leaves. In choice trials, beetles preferred to feed and oviposit on terrestrial plants but were also less likely to damage elevated CO₂ leaves. Caged beetle populations were larger in terrestrial conditions than aquatic conditions for a second set of plants grown in the chambers. With a third set of plants grown in the ambient or elevated CO₂ chambers, damage for plants placed in the field (aquatic setting) was higher for plants grown in terrestrial conditions vs. flooded conditions at ambient CO₂. Our results suggest that elevated CO₂ will have minor effects on the efficacy of this biocontrol agent by decreasing oviposition and number of leaves damaged, and hydrologic environment may affect invasive plant performance by altering herbivore oviposition and feeding preferences. A broader understanding of the effects of global change on biocontrol will help prevent and manage future spread of invasive plants.     

Will chemical defenses become more effective against specialist herbivores under elevated CO2?

Elevated atmospheric CO₂ is known to affect plant–insect herbivore interactions. Elevated CO₂ causes leaf nitrogen to decrease, the ostensible cause of herbivore compensatory feeding. CO₂ may also affect herbivore consumption by altering chemical defenses via changes in plant hormones. We considered the effects of elevated CO₂, in conjunction with soil fertility and damage (simulated herbivory), on glucosinolate concentrations of mustard (Brassica nigra) and collard (B. oleracea var. acephala) and the effects of leaf nitrogen and glucosinolate groups on specialist Pieris rapae consumption. Elevated CO₂ affected B. oleracea but not B. nigra glucosinolates; responses to soil fertility and damage were also species‐specific. Soil fertility and damage also affected B. oleracea glucosinolates differently under elevated CO₂. Glucosinolates did not affect P. rapae consumption at either CO₂ concentration in B. nigra, but had CO₂‐specific effects on consumption in B. oleracea. At ambient CO₂, leaf nitrogen had strong effects on glucosinolate concentrations and P. rapae consumption but only gluconasturtiin was a feeding stimulant. At elevated CO₂, direct effects of leaf nitrogen were weaker, but glucosinolates had stronger effects on consumption. Gluconasturtiin and aliphatic glucosinolates were feeding stimulants and indole glucosinolates were feeding deterrents. These results do not support the compensatory feeding hypothesis as the sole driver of changes in P. rapae consumption under elevated CO₂. Support for hormone‐mediated CO₂ response (HMCR) was mixed; it explained few treatment effects on constitutive or induced glucosinolates, but did explain patterns in SEMs. Further, the novel feeding deterrent effect of indole glucosinolates under elevated CO₂ in B. oleracae underscores the importance of defensive chemistry in CO₂ response. We speculate that P. rapae indole glucosinolate detoxification mechanisms may have been overwhelmed under elevated CO₂ forcing slowed consumption. Specialists may have to contend with hosts with poorer nutritional quality and more effective chemical defenses under elevated CO₂.