Pea aphid promotes amino acid metabolism both in Medicago truncatula and bacteriocytes to favor aphid population growth under elevated CO2

Rising atmospheric CO₂ levels can dilute the nitrogen (N) resource in plant tissue, which is disadvantageous to many herbivorous insects. Aphids appear to be an exception that warrants further study. The effects of elevated CO₂ (750 ppm vs. 390 ppm) were evaluated on N assimilation and transamination by two Medicago truncatula genotypes, a N‐fixing‐deficient mutant (dnf1) and its wild‐type control (Jemalong), with and without pea aphid (Acyrthosiphon pisum) infestation. Elevated CO₂ increased population abundance and feeding efficiency of aphids fed on Jemalong, but reduced those on dnf1. Without aphid infestation, elevated CO₂ increased photosynthetic rate, chlorophyll content, nodule number, biomass, and pod number for Jemalong, but only increased pod number and chlorophyll content for dnf1. Furthermore, aphid infested Jemalong plants had enhanced activities of N assimilation‐related enzymes (glutamine synthetase, Glutamate synthase) and transamination‐related enzymes (glutamate oxalate transaminase, glutamine phenylpyruvate transaminase), which presumably increased amino acid concentration in leaves and phloem sap under elevated CO₂. In contrast, aphid infested dnf1 plants had decreased activities of N assimilation‐related enzymes and transmination‐related enzymes and amino acid concentrations under elevated CO₂. Furthermore, elevated CO₂ up‐regulated expression of genes relevant to amino acid metabolism in bacteriocytes of aphids associated with Jemalong, but down‐regulated those associated with dnf1. Our results suggest that pea aphids actively elicit host responses that promote amino acid metabolism in both the host plant and in its bacteriocytes to favor the population growth of the aphid under elevated CO₂.     

Elevated CO2 Modifies N Acquisition of Medicago truncatula by Enhancing N Fixation and Reducing Nitrate Uptake from Soil

The effects of elevated CO₂ (750 ppm vs. 390 ppm) were evaluated on nitrogen (N) acquisition and assimilation by three Medicago truncatula genotypes, including two N-fixing-deficient mutants (dnf1-1 and dnf1-2) and their wild-type (Jemalong). The proportion of N acquisition from atmosphere and soil were quantified by ¹⁵N stable isotope, and N transportation and assimilation-related genes and enzymes were determined by qPCR and biochemical analysis. Elevated CO₂ decreased nitrate uptake from soil in all three plant genotypes by down-regulating nitrate reductase (NR), nitrate transporter NRT1.1 and NR activity. Jemalong plant, however, produced more nodules, up-regulated N-fixation-related genes and enhanced percentage of N derived from fixation (%Ndf) to increase foliar N concentration and N content in whole plant (Ntotal Yield) to satisfy the requirement of larger biomass under elevated CO₂. In contrast, both dnf1 mutants deficient in N fixation consequently decreased activity of glutamine synthetase/glutamate synthase (GS/GOGAT) and N concentration under elevated CO₂. Our results suggest that elevated CO₂ is likely to modify N acquisition of M. truncatula by simultaneously increasing N fixation and reducing nitrate uptake from soil. We propose that elevated CO₂ causes legumes to rely more on N fixation than on N uptake from soil to satisfy N requirements.     

Elevated CO2 shifts the focus of tobacco plant defences from cucumber mosaic virus to the green peach aphid

Elevation in CO₂ concentration broadly impacts plant physiological characteristics, which influences herbivores and biotrophic pathogens, which in turn regulate the plant defensive response. In this study, responses of tobacco plants to stress in the form of the green peach aphid, Myzus persicae (Sulzer), or cucumber mosaic virus (CMV), or both aphid and CMV combined were investigated in open‐top chambers under ambient and elevated CO₂ concentrations. We measured aboveground biomass and foliar chlorophyll, nitrogen, non‐structural carbohydrates, soluble protein, total amino acid and nicotine content in tobacco plants and also measured aphid population dynamics, body weight, honeydew production and anti‐oxidative enzyme activities in individual aphids. Plants produced more secondary metabolites for defence in both CO₂ treatments when treated with aphid and CMV combined than with either alone. Aphid density significantly increased on CMV‐infected tobacco plants (relative to uninfected plants) under ambient CO₂ but not under elevated CO₂. This suggests that plant defences against virus and aphid would be more efficient under elevated CO₂. Plant defence appears to shift from plant virus to aphid under increasing CO₂ levels, which highlights the potential influences of multiple biotic stressors on plants under elevated CO₂.     

Towards an understanding of how phloem amino acid composition shapes elevated CO2‐induced changes in aphid population dynamics

1. The performance of foliage feeders tends to decrease under elevated CO₂, but the responses of phloem‐feeding insects have been much more equivocal. As phloem tissues are less accessible than whole‐plant tissues, much less is known about how phloem composition is altered under elevated CO₂ and the mechanisms driving changes in aphid performance. 2. In this study, the plant mechanisms underlying the performance of Rhopalosiphum padi aphids on Hordeum vulgare (barley) grown under ambient (390 ppm) and elevated (700 ppm) CO₂ were examined. We used aphid stylectomy to sample pure phloem from plants in CO₂‐controlled conditions and high‐performance liquid chromatography to analyse phloem samples for amino acid concentrations. 3. Aphid abundance significantly increased by 127% under elevated CO₂. Consequently, plant biomass decreased under elevated CO₂ in trials with herbivores present, possibly due to the increased herbivore load, but increased when aphids were absent. The intrinsic rate of population increase (r_m) was significantly higher under elevated CO₂; however, there were no statistically significant effects on aphid fecundity or development time. The concentration of individual amino acids tended to increase, although these increases were statistically significant in only a few cases. A principal components analysis revealed that the relative abundance (mol %) of those amino acids considered essential for aphids tended to increase under elevated CO₂. 4. These results indicate that CO₂ may affect nutrient translocation in plants in ways that are contrary to predictions about nitrogen metabolite responses to CO₂. Such plant biochemical responses may underlie observations of improved phloem feeder performance under elevated CO₂.     

Plant stomatal closure improves aphid feeding under elevated CO2

Stomata help plants regulate CO₂ absorption and water vapor release in response to various environmental changes, and plants decrease their stomatal apertures and enhance their water status under elevated CO₂. Although the bottom‐up effect of elevated CO₂ on insect performance has been extensively studied, few reports have considered how insect fitness is altered by elevated CO₂‐induced changes in host plant water status. We tested the hypothesis that aphids induce stomatal closure and increase host water potential, which facilitates their passive feeding, and that this induction can be enhanced by elevated CO₂. Our results showed that aphid infestation triggered the abscisic acid (ABA) signaling pathway to decrease the stomatal apertures of Medicago truncatula, which consequently decreased leaf transpiration and helped maintain leaf water potential. These effects increased xylem‐feeding time and decreased hemolymph osmolarity, which thereby enhanced phloem‐feeding time and increased aphid abundance. Furthermore, elevated CO₂ up‐regulated an ABA‐independent enzyme, carbonic anhydrase, which led to further decrease in stomatal aperture for aphid‐infested plants. Thus, the effects of elevated CO₂ and aphid infestation on stomatal closure synergistically improved the water status of the host plant. The results indicate that aphid infestation enhances aphid feeding under ambient CO₂ and that this enhancement is increased under elevated CO₂.     

Phloem phytochemistry and aphid responses to elevated CO2, nitrogen fertilization and endophyte infection

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How does atmospheric elevated CO2 affect crop pests and their natural enemies? Case histories from China

Abstract Global atmospheric CO₂ concentrations have risen rapidly since the Industrial Revolution and are considered as a primary factor in climate change. The effects of elevated CO₂ on herbivore insects were found to be primarily through the CO₂‐induced changes occurring in their host plants, which then possibly affect the intensity and frequency of pest outbreaks on crops. This paper reviews several ongoing research models using primary pests of crops (cotton bollworm, whitefly, aphids) and their natural enemies (ladybeetles, parasitoids) in China to examine insect responses to elevated CO₂. It is generally indicated that elevated CO₂ prolonged the development of cotton bollworm, Helicoverpa armigera, a chewing insect, by decreasing the foliar nitrogen of host plants. In contrast, the phloem‐sucking aphid and whitefly insects had species‐specific responses to elevated CO₂ because of complex interactions that occur in the phloem sieve elements of plants. Some aphid species, such as cotton aphid, Aphis gossypii and wheat aphid, Sitobion avenae, were considered to represent the only feeding guild to respond positively to elevated CO₂ conditions. Although whitefly, Bemisia tabaci, a major vector of Tomato yellow leaf curl virus, had neutral response to elevated CO₂, the plants became less vulnerable to the virus infection under elevated CO₂. The predator and parasitoid response to elevated CO₂ were frequently idiosyncratic. These documents from Chinese scientists suggested that elevated CO₂ initially affects the crop plant and then cascades to a higher trophic level through the food chain to encompass herbivores (pests), their natural enemies, pathogens and underground nematodes, which disrupt the natural balance observed previously in agricultural ecosystems.     

How do aphids respond to elevated CO2?

The performance of herbivore insects is determined directly by the quality of host plants. Elevated CO₂ induced a decline in foliar nitrogen, which reduced the growth of chewing insects. Phloem-sucking insects (i.e. aphid), however, had species-specific responses to elevated CO₂ and were the only feeding guild to respond positively to elevated CO₂. Although many studies attempt to illuminate the interaction between aphids and plants under elevated CO₂, few studies can explain why some aphids are more successful than other chewing insects in elevated CO₂. Elevated CO₂ leads to a re-allocation of the carbon and nitrogen resources in plant tissue, which increases the thickness of the microscopic structures of leaves, reduces amino acids content of leaf phloem sap and increases the secondary metabolites. Considering the complexity of aphid–plant interactions, it is difficult and unreasonable to predict the general response of aphids to elevated CO₂ using a single plant component. Instead, it is more likely that aphids are able to overcome the disadvantages of the indirect effects of elevated CO₂ by reducing developmental times and increasing fecundity under elevated CO₂ conditions. Our results provide several clues to why some aphids are successful in elevated CO₂ conditions. We review recent studies of the effects of elevated CO₂ on aphids and discuss the effects of elevated CO₂ on aphid performance on crops using cotton and cereal aphids as examples.     

High levels of arbuscular mycorrhizal fungus colonization on Medicago truncatula reduces plant suitability as a host for pea aphids (Acyrthosiphon pisum)

This study sheds light on a poorly understood area in insect-plant-microbe interactions,focusing on aphid probing and feeding behavior on plants with varying levels of arbuscular mycorrhizal(AM)fungus root colonization.It investigates a commonly occurring interaction of three species:pea aphid Acyrthosiphon pisum,barrel medic Medicago truncatula,and the AM fungus Rhizophagus irregularis,examining whether aphid-feeding behavior changes when insects feed on plants at different levels of AM fungus colonization(42% and 84% root length colonized).Aphid probing and feeding behavior was monitored throughout 8 h of recording using the electrical penetration graph(EPG)technique,also,foliar nutrient content and plant growth were measured.Summarizing,aphids took longer to reach their 1st sustained phloem ingestion on the 84% AM plants than on the 42% AM plants or on controls.Less aphids showed phloem ingestion on the 84% AM plants relative to the 42% AM plants.Shoots of the 84% AM plants had higher percent carbon(43.7%)relative to controls(40.5%),and the 84% AM plants had reduced percent nitrogen(5.3%)relative to the 42% AM plants(6%).In conclusion,EPG and foliar nutrient data support the hypothesis that modifications in plant anatomy(e.g.,thicker leaves),and poor food quality(reduced nitrogen)in the 84% AM plants contribute to reduced aphid success in locating phloem and ultimately to differences in phloem sap ingestion.This work suggests that M.truncatula plants benefit from AM symbiosis not only because of increased nutrient uptake but also because of reduced susceptibility to aphids.     

Elevated CO2 changes the interactions between nematode and tomato genotypes differing in the JA pathway

Interactions between the root‐knot nematode Meloidogyne incognita and three isogenic tomato (Lycopersicon esculentum) genotypes were examined when plants were grown under ambient (370 ppm) and elevated (750 ppm) CO₂. We tested the hypothesis that, defence‐recessive genotypes tend to allocate ‘extra’ carbon (relative to nitrogen) to growth under elevated CO₂, whereas defence‐dominated genotypes allocate extra carbon to defence, and thereby increases the defence against nematodes. For all three genotypes, elevated CO₂ increased height, biomass, and root and leaf total non‐structural carbohydrates (TNC):N ratio, and decreased amino acids and proteins in leaves. The activity of anti‐oxidant enzymes (superoxide dismutase and catalase) was enhanced by nematode infection in defence‐recessive genotypes. Furthermore, elevated CO₂ and nematode infection did not qualitatively change the volatile organic compounds (VOC) emitted from plants. Elevated CO₂ increased the VOC emission rate only for defence‐dominated genotypes that were not infected with nematodes. Elevated CO₂ increased the number of nematode‐induced galls on defence‐dominated genotypes but not on wild‐types or defence‐recessive genotypes roots. Our results suggest that CO₂ enrichment may not only increase plant C : N ratio but can disrupt the allocation of plant resources between growth and defence in some genetically modified plants and thereby reduce their resistance to nematodes.     

Aphid response to elevated ozone and CO2

Numerous reports indicate that pollution stress caused by sulphur dioxide (SO₂), oxies of nitrogen or fluorides promote aphid growth on herbaceous and woody plants. At SO₂ exposures, the response curve of aphids is bell-shaped having the peak at 100 ppb. This curvilinear response is related to physiological stress responses of host plants exposed to pollutants. On the other hand, observations of aphid performance on ozone-exposed (O₃) or elevated carbon dioxide-exposed (CO₂) plants have given very variable results. Depending on the duration and concentration of O₃ or elevated CO₂ exposure or the age of the exposed plants, aphid growth on the same plants either decreased or increased in comparison to growth on control plants grown in filtered air. The results of these studies suggest that there is no general air pollution-induced plant stress that triggers aphid outbreaks on plants. Plants grown in elevated CO₂ usually have higher C/N ratios than plants grown in current ambient CO₂ atmosphere. A reduced proportion of nitrogen in the plant foliage decreases growth of chewing herbivorous insects, but the few studies of elevated CO₂ effects on sucking insects such as aphids have not yielded similar consistent effects. The present paper reviews recent studies of elevated CO₂ effects on aphids and discusses the effects of combined elevated O₃ and CO₂ exposures on aphid performance on woody plants using pine and birch aphids as examples.     

Response of amino acid changes in Aphis gossypii (Glover) to elevated CO2 levels

Effects of elevated CO₂ levels on the amino acid constituents of cotton aphid, Aphis gossypii (Glover), fed on transgenic Bacillus thuringiensis (Berliner) (Bt) cotton [Cryl A(c)], grown in ambient and double‐ambient CO₂ levels in closed‐dynamics CO₂ chambers, were investigated. Lower amounts of amino acids were found in cotton phloem under elevated CO₂ than under ambient CO₂ levels. However, higher amounts of free amino acids were found in A. gossypii fed on elevated CO₂‐grown cotton than those fed ambient CO₂‐grown cotton, and the contents of amino acids in honeydew were not significantly affected by elevated CO₂ levels. A larger amount of honeydew was produced by cotton aphids feeding on leaves under elevated CO₂ treatment than those feeding on leaves under ambient CO₂ treatment, which indicates that A. gossypii ingests more cotton phloem because of the higher C:N ratio of cotton phloem under elevated CO₂ levels. Moreover, the amino acid composition was similar in bodies of aphids ingesting leaves under both CO₂ treatments, except for two alkaline amino acids, lysine and arginine. This suggests that the nutritional constitution of the phloem sap was important for A. gossypii. Our data suggest that more phloem sap will be ingested by A. gossypii to satisfy its nutritional requirement and balance the break‐even point of amino acid in elevated CO₂. Larger amounts of honeydew produced by A. gossypii under elevated CO₂ will reduce the photosynthesis and result in the occurrence of some Entomophthora spp.     

Effects of elevated CO2 on five plant-aphid interactions

We investigated interactions between five species of phloem-feeding aphids (Homoptera: Aphididae) and their host plants at elevated CO₂; Acyrthosiphon pisum (Harris) on Vicia faba L., Aphis nerii Boyer de Fonscolombe on Asclepias syriaca L., Aphis oenotherae Oestlund on Oenothera biennis L., Aulacorthum solani (Kaltenbach) on Nicotiana sylvestris Speg. & Comes and Myzus persicae (Sulzer) on Solanum dulcamara L. Host plants grown at elevated CO₂ generally had greater biomass, leaf area and C:N ratios than those grown at ambient CO₂, while plants with aphids had lower biomass and leaf area than those without aphids.The responses of aphid populations to elevated CO₂ were species-specific with one species increasing (M. persicae), one decreasing (A. pisum), and the other three being unaffected. CO₂ treatment did not affect the proportion of alate individuals produced. In general, aphid abundance was not significantly related to foliar nitrogen concentration.We performed separate analyses to test whether either aphid presence or aphid abundance modified the response of host plants to elevated CO₂. In terms of aphid presence, only three of the potential 15 interactions (five aphid species x three plant traits) were significant; A. solani slightly modified the response of the plant biomass to elevated CO₂ and M. persicae affected the response of leaf area and allocation. In terms of aphid abundance, only two of the potential 15 interactions were significant with A. nerii modifying the plant response to CO₂ in terms of total leaf area and allocation.We conclude that, in contrast to other insect groups such as leaf chewers, populations of most phloem-feeders may not be negatively affected by increased CO₂ concentrations in the future. The reasons for this difference include the possibility that aphids may be able to compensate for changes in host plant quality by altering feeding behaviour or by synthesizing amino acids. In addition, there is little evidence that aphid herbivory, even at high levels, will substantially modify the response of plants to elevated CO₂.     

Host-specific aphid population responses to elevated CO2 and increased N availability

Sap-feeding insects such as aphids are the only insect herbivores that show positive responses to elevated CO₂. Recent models predict that increased nitrogen will increase aphid population size under elevated CO₂, but few experiments have tested this idea empirically. To determine whether soil nitrogen (N) availability modifies aphid responses to elevated CO₂, we tested the performance of Macrosiphum euphorbiae feeding on two host plants; a C₃ plant (Solanum dulcamara), and a C₄ plant (Amaranthus viridis). We expected aphid population size to increase on plants in elevated CO₂, with the degree of increase depending on the N availability. We found a significant CO₂× N interaction for the response of population size for M. euphorbiae feeding on S. dulcamara: aphids feeding on plants grown in ambient CO₂, low N conditions increased in response to either high N availability or elevated CO₂. No population size responses were observed for aphids infesting A. viridis. Elevated CO₂ increased plant biomass, specific leaf weight, and C : N ratios of the C₃ plant, S. dulcamara but did not affect the C₄ plant, A. viridis. Increased N fertilization significantly increased plant biomass, leaf area, and the weight : height ratio in both experiments. Elevated CO₂ decreased leaf N in S. dulcamara and had no effect on A. viridis, while higher N availability increased leaf N in A. viridis and had no effect in S. dulcamara. Aphid infestation only affected the weight : height ratio of S. dulcamara. We only observed an increase in aphid population size in response to elevated CO₂ or increased N availability for aphids feeding on S. dulcamara grown under low N conditions. There appears to be a maximum population growth rate that M. euphorbiae aphids can attain, and we suggest that this response is because of intrinsic limits on development time and fecundity.     

Aphid–hoverfly interactions under elevated CO2 concentrations: oviposition and larval development

The performance of predators of plant pests is mainly driven by their ability to find prey. Recent studies suggest that rising atmospheric carbon dioxide (CO₂) concentrations will affect the semiochemistry of plant–insect relationships, possibly altering prey‐finding behaviour. In the present study, we test the hypothesis that higher atmospheric CO₂ concentrations affect the oviposition behaviour of an aphidophagous hoverfly and alter the development of its larvae. We also test the hypothesis that volatile compounds released by the plant–aphid association are modified under elevated CO_2. Broad bean plants infested with pea aphids are grown under ambient (450 ppm) or elevated CO₂ (800 ppm) concentrations. Plants raised under each treatment are then presented to gravid hoverfly females in a dual‐choice bioassay. In addition, emerging Episyrphus balteatus larvae are directly fed with aphids reared under ambient or elevated CO₂ conditions and then measured and weighed daily until pupation. Odours emitted by the plant–aphid association are sampled. A larger number of eggs is laid on plants grown under ambient CO₂ conditions. However, no significant difference is observed between the two groups of predatory larvae grown under different CO₂ concentrations, indicating that the CO₂ concentration does not affect the quality of their aphid diet. Although plant volatiles do not differ between the ambient and elevated CO₂‐treated plants, we find that the quantity of aphid alarm pheromone is lower on the plant–aphid association raised under the elevated CO₂ condition. This suggests that an alteration of semiochemical emissions by elevated CO₂ concentrations impacts the oviposition behaviour of aphid predators.     

Ultrastructure of compatible and incompatible interactions in phloem sieve elements during the stylet penetration by cotton aphids in melon

Resistance of the melon line TGR‐1551 to the aphid Aphis gossypii is based on preventing aphids from ingesting phloem sap. In electrical penetration graphs (EPGs), this resistance has been characterized with A. gossypii showing unusually long phloem salivation periods (waveform E1) mostly followed by pathway activities (waveform C) or if followed by phloem ingestion (waveform E2), ingestion was not sustained for more than 10 min. Stylectomy with aphids on susceptible and resistant plants was performed during EPG recording while the stylet tips were phloem inserted. This was followed by dissection of the penetrated leaf section, plant tissue fixation, resin embedding, and ultrathin sectioning for transmission electron microscopic observation in order to study the resistance mechanism in the TGR. The most obvious aspect appeared to be the coagulation of phloem proteins inside the stylet canals and the punctured sieve elements. Stylets of 5 aphids per genotype were amputated during sieve element (SE) salivation (E1) and SE ingestion (E2). Cross‐sections of stylet bundles in susceptible melon plants showed that the contents of the stylet canals were totally clear and also, no coagulated phloem proteins occurred in their punctured sieve elements. In contrast, electron‐dense coagulations were found in both locations in the resistant plants. Due to calcium binding, aphid saliva has been hypothesized to play an essential role in preventing/suppressing such coagulations that cause occlusion of sieves plate and in the food canal of the aphid's stylets. Doubts about this role of E1 salivation are discussed on the basis of our results.     

Asparagine as a major factor in the N‐feedback regulation of N2 fixation in Medicago truncatula

The objective of this study was to assess whether a whole plant N‐feedback regulation impact on nitrogen fixation in Medicago truncatula would manifest itself in shifts of the composition of the amino acid flow from shoots to nodules. Detected shifts in the phloem amino acid composition were supposed to be mimicked through artificial phloem feeding and concomitant measurement of nodule activity. The amino acid composition of the phloem exudates was analyzed from plants grown under the influence of treatments (limiting P supply or application of combined nitrogen) known to reduce nodule nitrogen fixation activity. Plants in nutrient solution were supplied with sufficient (9 µM) control, limiting (1 µM) phosphorus or 3 mM NH₄NO₃ (downregulated nodule activity). Low phosphorus and the application of NH₄NO₃ reduced per plant and specific nitrogenase activity (H₂ evolution). At day 64 of growth, phloem exudates were collected from cuts of the shoot base. The amount of amino acids was strongly increased in both phloem exudates and nodules of the treatments with downregulated nodule activity. The increase in the downregulated treatments was almost exclusively the result of a higher proportion of asparagine in both phloem exudates and nodules. Leaf labeling with ¹⁵N showed that nitrogen from the leaves is retranslocated to nodules. An artificial phloem feeding with asparagine resulted in an increased concentration of asparagine in nodules and a decreased nodule activity. A possible role of asparagine in an N‐feedback regulation of nitrogen fixation in M. truncatula is discussed.     

Plant phenolics mediated bottom-up effects of elevated CO2 on Acyrthosiphon pisum and its parasitoid Aphidius avenae

Elevated concentrations of atmospheric CO2 can alter plant secondary metabolites,which play important roles in the interactions among plants,herbivorous insects and natural enemies.However,few studies have examined the cascading effects of host plant secondary metabolites on tri-trophic interactions under elevated CO2(eCO2).In this study,we determined the effects of eCO2 on the growth and foliar phenolics of Medicago truncatula and the cascading effects on two color genotypes oiAcyrthosiphon pisum(pink vs.green)and their parasitoid Aphidius avenae in the field open-top chambers.Our results showed that eCO2 increased photosynthetic rate,nodule number,yield and the total phenolic content of M.truncatula.eCO2 had contrasting effects on two genotypes of A.pisum;the green genotype demonstrated increased population abundance,fecundity,growth and feeding efficiency,while the pink genotype showed decreased fitness and these were closely associated with the foliar genstein content.Furthermore,eCO2 decreased the parasitic rate of A.avenae independent of aphid genotypes.eCO2 prolonged the emergence time and reduced the emergence rate and percentage of females when associated with the green genotype,but little difference,except for increased percentage of females,was observed in A.avenae under eCO2 when associated with the pink genotype,indicating that parasitoids can perceive and discriminate the qualities of aphid hosts.We concluded that eCO2 altered plant phenolics and thus the performance of aphids and parasitoids.Our results indicate that plant phenolics vary by different abiotic and biotic stimuli and could potentially deliver the cascading effects of eCO2 to the higher trophic levels.Our results also suggest that the green genotype is expected to perform better in future eCO2 because of decreased plant resistance after its infestation and decreased parasitic rate.     

Influence of elevated CO2 on interspecific interactions at higher trophic levels

Abstract We report the results of a study investigating the influence of elevated CO₂ on species interactions across three trophic levels: a plant (Brassica oleracea), two aphid herbivores (the generalist Myzus persicae and the specialist Brevicoryne brassicae), and two natural enemies (the coccinellid Hippodamia convergens (ladybird) and the parasitoid wasp Diaeretiella rapae). Brassica oleracea plants reared under elevated CO₂ conditions (650 ppmv vs. 350 ppmv) were larger and had decreased water and nitrogen content. Brevicoryne brassicae reared on plants grown in elevated CO₂ were larger and accumulated more fat, while there was no change in M. persicae traits. Fecundity of individual aphids appeared to be increased when reared on plants grown in elevated CO₂. However, these differences were generally lost when aphids were reared in colonies, suggesting that such changes in plant quality will have subtle effects on aphid intraspecific interactions. Nevertheless, CO₂ treatment did influence aphid distribution on plants, with significantly fewer M. persicae found on the shoots, and B. brassicae was only found on senescing leaves, when colonies were reared on plants grown in elevated CO₂. We reared B. brassicae and M. persicae in competition on plants grown at both the CO₂ concentration treatments. We found a significantly lower ratio of M. persicae: B. brassicae on plants grown under elevated CO₂ conditions, strongly suggesting that increasing CO₂ concentrations can alter the outcome of competition among insect herbivores. This was also reflected in the distribution of the aphids on the plants. While the CO₂ treatment did not influence where B. brassicae were found, fewer M. persicae were present on senescing leaves under elevated CO₂ conditions. Changes in plant quality resulting from the CO₂ treatments did not appear to alter aphid quality as prey species, as the number consumed by the ladybird H. convergens, and the number parasitised by the parasitoid wasp D. rapae, did not change. To our knowledge, this study provides the first empirical evidence that changes in host plant quality mediated by increasing levels of CO₂ can alter the outcome of interspecific competition among insect herbivores.     

Influences of elevated CO2 and pest damage on the allocation of plant defense compounds in Bt‐transgenic cotton and enzymatic activity of cotton aphid

Abstract Plant allocation to defensive compounds by elevated CO₂‐grown non‐transgenic and transgenic Bt cotton in response to infestation by cotton aphid, Aphis gossypii (Glover) in open‐top chambers under elevated CO₂ were studied. The results showed that significantly lower foliar nitrogen concentration and Bt toxin protein occurred in transgenic Bt cotton with and without cotton aphid infestation under elevated CO_2. However, significantly higher carbon/nitrogen ratio, condensed tannin and gossypol were observed in transgenic Bt cotton “GK‐12” and non‐transgenic Bt cotton ‘Simian‐3’ under elevated CO_2. The CO₂ level and cotton variety significantly influenced the foliar nitrogen, condensed tannin and gossypol concentrations in the plant leaves after feeding by A. gossypii. The interaction between CO₂ level × infestation time (24 h, 48 h and 72 h) showed a significant increase in cotton condensed tannin concentrations, while the interaction between CO₂ level × cotton variety significantly decreased the true choline esterase (TChE) concentration in the body of A. gossypi. This study exemplified the complexities of predicting how transgenic and non‐transgenic plants will allocate defensive compounds in response to herbivorous insects under differing climatic conditions. Plant defensive compound allocation patterns and aphid enzyme changes observed in this study appear to be broadly applicable across a range of plant and herbivorous insect interactions as CO₂ atmosphere rises.