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.     

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

相似文献   

The price of protection:a defensive endosymbiont impairs nymph growth in the bird cherry-oat aphid,Rhopalosiphum padi

Bacterial endosymbionts have enabled aphids to adapt to a range of stressors,but their effects in many aphid species remain to be established.The bird cherry-oat aphid,Rhopalosiphum padi(Linnaeus),is an important pest of cereals worldwide and has been reported to form symbiotic associations with Serratia symbiotica and Sitobion miscanthi L-type symbiont endobacteria,although the resulting aphid phenotype has not been described.This study presents the first report of R.padi infection with the facultative bacterial endosymbiont Hamiltonella defensa.Individuals of R.padi were sampled from populations in Eastern Scotland,UK,and shown to represent seven R.padi genotypes based on the size of polymorphic microsatellite markers;two of these genotypes harbored H.defensa.In parasitism assays,survival of H.defensa-infected nymphs following attack by the parasitoid wasp Aphidius colemani(Viereck)was 5 fold higher than for uninfected nymphs.Aphid genotype was a major determinant of aphid performance on two Hordeum species,a modern cultivar of barley H.vulgare and a wild relative H.spontaneum,although aphids infected with H.defensa showed 16%lower nymph mass gain on the partially resistant wild relative compared with uninfected individuals.These findings suggest that deploying resistance traits in barley will favor the fittest R.padi genotypes,but symbiontinfected individuals will be favored when parasitoids are abundant,although these aphids will not achieve optimal performance on a poor quality host plant.     

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.     

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.     

Effects of elevated CO2 and plant genotype on interactions among cotton,aphids and parasitoids

Abstract Effects of CO₂ level (ambient vs. elevated) on the interactions among three cotton (Gossypium hirsutum) genotypes, the cotton aphid (Aphis gossypii Glover), and its hymenoptera parasitoid (Lysiphlebia japonica Ashrnead) were quantified. It was hypothesized that aphid‐parasitoid interactions in crop systems may be altered by elevated CO₂, and that the degree of change is influenced by plant genotype. The cotton genotypes had high (M9101), medium (HZ401) and low (ZMS13) gossypol contents, and the response to elevated CO₂ was genotype‐specific. Elevated CO₂ increased the ratio of total non‐structural carbohydrates to nitrogen (TNC: N) in the high‐gossypol genotype and the medium‐gossypol genotype. For all three genotypes, elevated CO₂ had no effect on concentrations of gossypol and condensed tannins. A. gossypii fitness declined when aphids were reared on the high‐gossypol genotype versus the low‐gossypol genotype under elevated CO₂. Furthermore, elevated CO₂ decreased the developmental time of L. japonica associated with the high‐gossypol genotype and the low‐gossypol genotype, but did not affect parasitism or emergence rates. Our study suggests that the abundance of A. gossypii on cotton will not be directly affected by increases in atmospheric CO₂. We speculate that A. gossypii may diminish in pest status in elevated CO₂ and high‐gossypol genotype environments because of reduced fitness to the high‐gossypol genotype and shorter developmental time of L. japonica.     

Host plant genotype determines bottom‐up effects in an aphid‐parasitoid‐predator system

Plant genotypes are known to affect performance of insect herbivores and the community structure of both herbivores and higher trophic levels. Still, only a limited number of studies demonstrate differences in the performance of predators and parasitoids because of plant genotypic effects and most of these focus on gall formers. We designed a greenhouse experiment to investigate the effects of host plant genotype on fitness components in a grass‐aphid‐carnivore system. We used clones of quackgrass [Elytrigia repens (L.) Desv. ex Nevski (Poaceae)], the aphid Rhopalosiphum padi (L.) (Hemiptera: Aphididae), the parasitoid wasp Aphidius colemani (Viereck) (Hymenoptera: Braconidae), and the predatory lacewing Chrysoperla carnea (Stephens) (Neuroptera: Chrysopidae). The number of aphid offspring differed considerably among plant genotypes. These differences were only in part because of differences in the production of biomass among host genotypes. Therefore, genotypes may differ in their nutritional value for phytophages. The number of aphids attacked by the parasitoid also differed among genotypes and aphid numbers only partly accounted for this effect. Moreover, pupal development time of female parasitoids was affected by plant genotype. We found no differences in mortality, body size, or sex ratio of hatching wasps between genotypes of quackgrass. Development time of the larvae and larval weight of the predatory lacewings differed among genotypes, but not weight of pupae and adults. Generally, the proportion of the total variance explained by the plant genotype was smaller for parasitoids and predators than for aphids. Overall, our experiments indicated that the plant genotype affects tri‐trophic interactions, but also that the strength of these effects decreases along the food chain.     

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.     

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.     

Ample genetic variation but no evidence for genotype specificity in an all‐parthenogenetic host–parasitoid interaction

Antagonistic coevolution between hosts and parasites can result in negative frequency‐dependent selection and may thus be an important mechanism maintaining genetic variation in populations. Negative frequency‐dependence emerges readily if interactions between hosts and parasites are genotype‐specific such that no host genotype is most resistant to all parasite genotypes, and no parasite genotype is most infective on all hosts. Although there is increasing evidence for genotype specificity in interactions between hosts and pathogens or microparasites, the picture is less clear for insect host–parasitoid interactions. Here, we addressed this question in the black bean aphid (Aphis fabae) and its most important parasitoid Lysiphlebus fabarum. Because both antagonists are capable of parthenogenetic reproduction, this system allows for powerful tests of genotype × genotype interactions. Our test consisted of exposing multiple host clones to different parthenogenetic lines of parasitoids in all combinations, and this experiment was repeated with animals from four different sites. All aphids were free of endosymbiotic bacteria known to increase resistance to parasitoids. We observed ample genetic variation for host resistance and parasitoid infectivity, but there was no significant host clone × parasitoid line interaction, and this result was consistent across the four sites. Thus, there is no evidence for genotype specificity in the interaction between A. fabae and L. fabarum, suggesting that the observed variation is based on rather general mechanisms of defence and attack.     

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.     

Biochemical and physiological mechanisms underlying effects of Cucumber mosaic virus on host‐plant traits that mediate transmission by aphid vectors

The transmission of insect‐vectored diseases entails complex interactions among pathogens, hosts and vectors. Chemistry plays a key role in these interactions; yet, little work has addressed the chemical ecology of insect‐vectored diseases, especially in plant pathosystems. Recently, we documented effects of Cucumber mosaic virus (CMV) on the phenotype of its host (Cucurbita pepo) that influence plant‐aphid interactions and appear conducive to the non‐persistent transmission of this virus. CMV reduces host‐plant quality for aphids, causing rapid vector dispersal. Nevertheless, aphids are attracted to the elevated volatile emissions of CMV‐infected plants. Here, we show that CMV infection (1) disrupts levels of carbohydrates and amino acids in leaf tissue (where aphids initially probe plants and acquire virions) and in the phloem (where long‐term feeding occurs) in ways that reduce plant quality for aphids; (2) causes constitutive up‐regulation of salicylic acid; (3) alters herbivore‐induced jasmonic acid biosynthesis as well as the sensitivity of downstream defences to jasmonic acid; and (4) elevates ethylene emissions and free fatty acid precursors of volatiles. These findings are consistent with previously documented patterns of aphid performance and behaviour and provide a foundation for further exploration of the genetic mechanisms responsible for these effects and the evolutionary processes that shape them.     

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.     

Aphid genotypes vary in their response to the presence of fungal endosymbionts in host plants

Genetic variation for fitness‐relevant traits may be maintained in natural populations by fitness differences that depend on environmental conditions. For herbivores, plant quality and variation in chemical plant defences can maintain genetic variation in performance. Apart from plant secondary compounds, symbiosis between plants and endosymbiotic fungi (endophytes) can produce herbivore‐toxic compounds. We show that there is significant variation among aphid genotypes in response to endophytes by comparing life‐history traits of 37 clones of the bird cherry‐oat aphid Rhopalosiphum padi feeding on endophyte‐free and endophyte‐infected tall fescue Lolium arundinaceum. Clonal variation for life‐history traits was large, and most clones performed better on endophyte‐free plants. However, the clones differed in the relative performance across the two environments, resulting in significant genotype × environment interactions for all reproductive traits. These findings suggest that natural variation in prevalence of endophyte infection can contribute to the maintenance of genetic diversity in aphid populations.     

Response of quaking aspen genotypes to enriched CO2: foliar chemistry and tussock moth performance

Abstract

• 1 Genetic variation in the phytochemical responses of plants to CO₂ enrichment is likely to alter trophic dynamics, and to shift intraspecific selection pressures on plant populations. We evaluated the independent and interactive effects of atmospheric CO₂ and quaking aspen (Populus tremuloides Michx.) genotype on chemical composition of foliage and performance of the whitemarked tussock moth (Orgyia leucostigma J. E. Sm.).
• 2 This research was conducted at the Aspen FACE (Free Air CO₂ Enrichment) site in northern Wisconsin, U.S.A. Leaf samples were collected periodically from each of three genetically variable aspen genotypes growing under ambient and elevated CO₂, and analysed for levels of primary and secondary metabolites. Tussock moth larvae were reared in situ on experimental trees, and development times and pupal masses were recorded.
• 3 Foliar chemical composition varied among aspen genotypes and in response to CO₂ enrichment. However, chemical responses of trees to elevated CO₂ were generally consistent across genotypes.
• 4 Larval development times varied among host genotypes and increased slightly for insects on high‐CO₂ plants. Enriched CO₂ tended to reduce insect pupal masses, particularly for females on one of the three aspen genotypes.
• 5 CO₂ × genotype interactions observed for plant chemistry and insect performance in this study with a small number of genotypes are probably too few, and too weak, to shift selection pressures in aspen populations. These results differ, however, from earlier work in which more substantial CO₂ × genotype interactions were observed for plant chemistry.

     

The effect of fertilizer‐N on the inter‐specific competition among three wheat aphids under elevated CO2

Increasing atmospheric carbon dioxide (ab. CO₂) and fertilizer‐nitrogen (ab. N) applications may have marked direct effects on the plant growth of agricultural crops, and in turn affect the higher trophic level of insect herbivores. In this study, the effects of elevated CO₂ (i.e., 650 µl/L vs. ambient 400 µl/L) and fertilizer‐N (0, 50, 100, 200 kg/ha) on the population abundances and the inter‐specific competition among three co‐occurring species of wheat aphids, Sitobion avenae, Rhopalosiphum padi and Schizaphis graminum, were studied. The grain weight per ear and the 1,000‐grain weight were generally increased when grown under elevated CO₂ and showed a significant effect at the 100 kg/ha (grain weight per ear) and 0, 50 and 100 kg/ha (1,000‐grain weight) N. These two yield indexes increased with increasing fertilizer‐N levels within reasonable limits and reached a maximum at 100 kg/ha. Elevated CO₂ combined with fertilizer‐N levels formed complex indirect effects on the three wheat aphids through the wheat crops they fed on. Elevated CO₂ significantly decreased the niche overlap index (ab. NOI) between S. avenae and R. padi under 0 and 100 kg/ha and that between R. padi and S. graminum under 0 kg/ha, while significantly increased the three NOIs under 50 kg/ha and that between R. padi and S. graminum under 100 and 200 kg/ha. S. avenae and R. padi had the larger population and stronger competition in low‐N condition (0 and 50 kg/ha), which was harmful to wheat yield and quality when combined with its own poor nutrition. Overall, the 100 kg/ha N level was the best option based on the aphid population, competition and wheat yields. Therefore, the balanced relationship formed among fertilizers, plants and insects under 100 kg/ha N was vital for the interactive ecosystem.     

Gypsy moth feeding in the canopy of a CO2-enriched mature forest

Rising atmospheric carbon dioxide (CO₂) concentration is expected to change plant tissue quality with important implications for plant–insect interactions. Taking advantage of canopy access by a crane and long‐term CO₂ enrichment (530 μ mol mol^?1) of a natural old‐growth forest (web‐free air carbon dioxide enrichment), we studied the responses of a generalist insect herbivore feeding in the canopy of tall trees. We found that relative growth rates (RGR) of gypsy moth (Lymantria dispar) were reduced by 30% in larvae fed on high CO₂‐exposed Quercus petraea, but increased by 29% when fed on high CO₂‐grown Carpinus betulus compared with control trees at ambient CO₂ (370 μ mol mol^?1). In Fagus sylvatica, there was a nonsignificant trend for reduced RGR under elevated CO₂. Tree species‐specific changes in starch to nitrogen ratio, water, and the concentrations of proteins, condensed and hydrolyzable tannins in response to elevated CO₂ were identified to correlate with altered RGR of gypsy moth larvae. Our data suggest that rising atmospheric CO₂ will have strong species‐specific effects on leaf chemical composition of canopy trees in natural forests leading to contrasting responses of herbivores such as those reported here. A future change in host tree preference seems likely with far‐ranging consequences for forest community dynamics.     

Host plant and competitor identity matter in genotype × genotype × environment interactions between vetch and pea aphids

1. Selection does not only operate in a genotype (G) × environment (E) context, but can also be modulated by the activities of organisms interacting with their environment (G × G × E). 2. The influences of aphid clonal identity and host plant (Vicia faba) intraspecific genetic variation on the performance of five genotypes of pea aphid (Acyrthosiphon pisum) were investigated – with and without interaction with a competing heterospecific clone of vetch aphid (Megoura viciae) – across three cultivars of V. faba. 3. Pea aphid performance in the presence of a competing vetch aphid clone (G × G × E) compared with the absence of competition (G × E) revealed strong context‐dependent, genotype‐specific shifts in performance, influenced by plant cultivar, competitor presence and their interaction. 4. The performance of vetch aphid in competition with each pea aphid clone was also compared. Here, competitor's genotype and abundance underlay a remarkably varied response by vetch aphid across interactions. 5. The study shows that aphid genotypes exhibit a varying degree of risk spreading, contingent on competitor identity and the patterns of aggregation across three plant cultivars. Owing to feedback loops between species activities and selective forces acting on them, our findings suggest that there are context‐dependent responses by competitors that are shaped via the interplay of the co‐occurring species and their biotic environment. 6. This work highlights the complexity of species interactions and the importance of investigating reciprocity between competition and intraspecific genetic variation. A better understanding of the eco‐evolutionary interactions between phloem‐feeding insects and their host plants can potentially be used to enhance crop protection and pest control.     

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.