Effects of host plant drought stress on the performance of the bird cherry-oat aphid, Rhopalosiphum padi (L.): a mechanistic analysis

Abstract.  1. The growth (increase in height and leaf number) of four grass species was reduced by a −0.5 MPa drought stress, but the performance of an associated herbivore, Rhopalosiphum padi (L.), was not affected consistently. The intrinsic rate of increase of R. padi was reduced by drought stress on three grass species, including Dactylis glomerata (L.), but was unaffected on Arrhenatherum elatius (L.). Therefore, there is no general relationship in the effect of plant drought on an insect herbivore, even among closely related host plant species.
2. Drought stress increased the quality of plant phloem sap, as indicated by increased sieve element osmotic pressure and essential amino acid concentrations. Thus, diet quality could not account for the reduced performance of R. padi under drought stress. The concentration of essential amino acids in the phloem of well-watered A. elatius was, however, lower than that of well-watered D. glomerata , correlating with the decreased performance of aphids on well-watered A. elatius .
3. There were no differences in aphid feeding duration between watering treatments or plant species but sap ingestion rates were reduced significantly under drought stress.
4. Using the measure of dietary amino acid concentrations and the estimate of sap ingestion, the essential amino acid flux through aphids was calculated. Compared with the flux through aphids feeding on well-watered D. glomerata , there was a reduction in aphids feeding on drought-stressed D. glomerata and drought-stressed A. elatius due to lower sap ingestion rates. The flux through aphids on well-watered A. elatius was also reduced due to low phloem essential amino acid concentrations. Thus, the performance of an aphid is correlated with the availability and accessibility of essential amino acids.     

Performance of pea aphid (Acyrthosiphon pisum) clones on host plants and synthetic diets mimicking the same plants phloem amino acid composition

The performances of three clones of pea aphids, with different host affiliations, were evaluated on four host plants species and on four artificial diets. The amino acid compositions of the diets mimicked those of the phloem sap of the respective host plants. The total concentration of amino acids was the same in all the diets. The pea aphid clones performance were significantly affected by amino acid composition of the diets in different ways, implying physiological and/or behavioural differences among coexisting pea aphid clones in response to amino acids in artificial diets. The observed differences in performance on diets between clones were not related to host plant affiliations. Thus, even if the variation in amino acid composition in phloem sap among the host plants affects the pea aphid clones when tested on artificial diets, this variation has no observable effect on pea aphid performance on natural host plants.     

How nutritionally imbalanced is phloem sap for aphids?

Aphids harbour intracellular symbionts (Buchnera) that provide their host with amino acids present in low amounts in their diet, phloem sap. To find out the extent to which aphids depend on their symbionts for synthesis of individual essential amino acids, we have evaluated how well phloem sap amino acid composition matches the aphids' needs. Amino acid compositions of the ingested phloem sap were compared to amino acids in aphid body proteins and also to available information about optimal diet composition for other plant feeding insects. Phloem sap data from severed stylets of two aphid species, Rhopalosiphum padi (L.) (Homoptera: Aphididae) feeding on wheat, and Uroleucon sonchi (L.) (Homoptera: Aphididae) feeding on Sonchus oleraceus (L.), together with published information on phloem sap compositions from other plant species were used.Phloem sap has in general only around 20% essential amino acids, with a range from 15–48%. Aphid body proteins and optimal diets for two other plant feeding insects have around 50%. The phloem sap of early flowering S. oleraceus ingested by U. sonchi contained 48%, which seems to be exceptional. Aphids feeding on different plants appear to be very differently dependent on their symbionts for their overall essential amino acid synthesis, due to the large variation in proportion of essential amino acids in phloem sap from different plants.The profile of the essential amino acids in phloem sap from different plant species corresponds rather well to profiles of both aphid body proteins and optimal diets determined for plant feeding insects. However, methionine and leucine in phloem sap are in general low in these comparisons, suggesting a higher dependence on the symbiont for synthesis of these amino acids. Concentrations of several essential amino acids in phloem from different plant species seem to vary together, suggesting that levels of symbiont provisioning of different amino acids are adjusted in parallel.     

The role of intracellular symbiotic bacteria in the amino acid nutrition of embryos from the black bean aphid,Aphis fabae

Aphids are well‐known for their symbiotic relationship with intracellular bacteria of the genus Buchnera (γ‐Proteobacteria). The symbiosis has a nutritional basis in that the bacteria supplement the aphid diet of phloem sap through the provision of essential amino acids. To date, few studies have considered the spatial complexity of the association, particularly the delineation of the symbiosis into embryo and maternal compartments. Here, we generate aposymbiotic (bacteria‐free) embryos of the black bean aphid, Aphis fabae (Scopoli) (Hemiptera: Aphididae), as our experimental model and demonstrate that embryos reared in culture media require an external supply of essential amino acids. Analysis of individual amino acid deletions from the culture medium indicate that the key individual amino acids for embryo growth are phenylalanine and valine derived from the maternal tissues, and tryptophan derived from Buchnera. These results are discussed in relation to our current limited understanding of nutrient supply to aphid embryos.     

Accumulation,selection and covariation of amino acids in sieve tube sap of tansy (Tanacetum vulgare) and castor bean (Ricinus communis): evidence for the function of a basic amino acid transporter and the absence of a γ‐amino butyric acid transporter

Sieve tube sap was obtained from Tanacetum by aphid stylectomy and from Ricinus after apical bud decapitation. The amino acids in sieve tube sap were analyzed and compared with those from leaves. Arginine and lysine accumulated in the sieve tube sap of Tanacetum more than 10‐fold compared to the leaf extracts and they were, together with asparagine and serine, preferably selected into the sieve tube sap, whereas glycine, methionine/tryptophan and γ‐amino butyric acid were partially or completely excluded. The two basic amino acids also showed a close covariation in sieve tube sap. The acidic amino acids also grouped together, but antagonistic to the other amino acids. The accumulation ratios between sieve tube sap and leaf extracts were smaller in Ricinus than in Tanacetum. Arginine, histidine, lysine and glutamine were enriched and preferentially loaded into the phloem, together with isoleucine and valine. In contrast, glycine and methionine/tryptophan were partially and γ‐amino butyric acid almost completely excluded from sieve tube sap. The covariation analysis grouped arginine together with several neutral amino acids. The acidic amino acids were loaded under competition with neutral amino acids. It is concluded from comparison with the substrate specificities of already characterized plant amino acid transporters, that an AtCAT1‐like transporter functions in phloem loading of basic amino acids, whereas a transporter like AtGAT1 is absent in phloem. Although Tanacetum and Ricinus have different minor vein architecture, their phloem loading specificities for amino acids are relatively similar.     

Seasonal changes in the free amino acids of oat and barley phloem sap in relation to plant growth stage and growth of Rhopalosiphum padi

The concentrations and composition of free amino acids in phloem sap from two cultivars of oats and barley, both susceptible to the aphid Rhopalosiphum padi, were determined by means of high performance liquid chromatography. Sap was collected from excised aphid stylets at three developmental stages (seedlings, tillering plants and plants undergoing stem elongation) from plants given or not given fertiliser and grown outdoors. In connection, the growth of individual R. padi nymphs was estimated at the same phenological stages on plants grown in the greenhouse. The content of free amino acids was consistently higher in seedlings than in plants at the early tillering stage. Only in seedlings did the addition of fertiliser increase amino acid levels. Barley phloem sap contained more free amino acids than that of oats when fertiliser was added and at later developmental stages. Phloem sap of oats and barley showed similar patterns in their composition of free amino acids at the seedling stage, but as the plants grew older the patterns became increasingly different. Plants given fertiliser had higher amounts of dicarboxylic amino acids (glutamic and aspartic acid) than unfertilised plants. The concentrations of γ-amino butyric acid, glycine, histidine, and methionine were very low in all treatments. The relative growth rates of R. padi nymphs were low when amino acid content was low and vice versa. The results are discussed in relation to host plant suitability and plant resistance mechanisms.     

How symbiotic bacteria influence plant utilisation by the polyphagous aphid, Aphis fabae

To explore the effect of rearing-plant species on the contribution of the symbiotic bacterium, Buchnera, to aphid performance, larvae of Aphis fabae that contained the bacteria (symbiotic aphids) and larvae experimentally deprived of the bacteria (aposymbiotic aphids) were reared on 16 plant species. Mortality of aphids was low on most plant species. The relative growth rate (RGR) of the larvae varied with plant species, and was generally depressed by elimination of the bacteria; the mean values of RGR varied between 0 and 0.29 μg μg⁻¹ day⁻¹ for symbiotic aphids and 0 and 0.17 μg μg⁻¹ day⁻¹ for aposymbiotic aphids. The extent to which RGR was depressed by aposymbiosis varied significantly between plant species, suggesting that aphid host plant may influence the contribution of the bacteria to plant utilisation. It is proposed that the bacteria may be particularly important on plants with phloem sap of high amino acid content of low quality, i.e. low concentrations of essential amino acids. Received: 18 August 1996 / Accepted: 13 January 1997     

Amino acid budgets in three aphid species using the same host plant

Abstract. Nutrient provisioning in aphids depends both on the composition of ingested phloem sap and on the biosynthetic capabilities of the aphid and its intracellular symbionts. Amino acid budgets for three aphid species, Rhopalosiphum padi (L.), Schizaphis graminum (Rondani) and Diuraphis noxia (Mordvilko), were compared on a single host plant species, wheat Triticum aestivum L. Ingestion of amino acids from phloem, elimination of amino acids in honeydew, and the content of amino acids in aphids tissue were measured. From these values, ingestion rates were estimated and compared to honeydew and to estimated composition of aphid proteins. Ingestion rate was lowest in D. noxia due to low growth rate and low honeydew production; intermediate in S. graminum due to higher growth rate and intermediate honeydew production; and highest in R. padi , which had the highest rates for both variables. Both D. noxia and S. graminum induced increases in the amino acid content of ingested phloem. These changes in phloem content, combined with differences in ingestion rates, resulted in large differences among aphids in estimated rates of ingestion of individual amino acids. In honeydew, most essential amino acids were found in low amounts compared with the amounts ingested, especially for methionine and lysine. A few amino acids (arginine, cystine, histidine and tryptophan) were more abundant in honeydew of some aphids, suggesting oversupply. Aphid species differed in the composition of free amino acids in tissue but showed very similar composition in protein, implying similar requirements among the aphids. In R. padi and D. noxia , most essential amino acids were ingested in amounts insufficient for growth, implying dependence on symbiont provisioning. In S. graminum , most amino acids were ingested in amounts apparently sufficient for growth.     

Patterns in aphid honeydew production parallel diurnal shifts in phloem sap composition

Variation in phloem sap composition is important in determining aphid performance and is known to occur at both diurnal timescales and in response to plant age. For field grown potato plants, Solanum tuberosum L. (Solanaceae), we determined diurnal variation in components of phloem sap, measured by ethylene diamine tetra‐acetate exudation, and tested for impacts of plant age. The effects of plant age and diurnal cycles on honeydew production by Macrosiphum euphorbiae (Thomas) and Myzus persicae (Sulzer) (both Hemiptera: Aphididae) were also quantified. Both the ratio of sucrose to amino acids and the composition of amino acids in phloem sap varied significantly with time of day. Dietary essential amino acids contributed a smaller proportion of amino acids in the phloem sap of older plants and during early phases of the diurnal cycle. The only significant effect on aphid honeydew production was of the diurnal cycle for Ma. euphorbiae, although increased honeydew production during the day when compared with the production at night, was consistent across the two species. In contrast with studies carried out at seasonal scales, we found limited evidence for variation in phloem sap composition in response to plant age, consistent with our results for honeydew production. These data highlight the need for improved understanding of how seasonal and diurnal physiology of plants influence performance in phloem sap feeding insects.     

How aphids decide what is good for them: experiments to test aphid feeding behaviour on Tanacetum vulgare (L.) using different nitrogen regimes

Leaf-chewing herbivores select food with a protein/carbohydrate ratio of 0.8–1.5, whereas phloem sap, which aphids feed on, has a ratio of ~0.1. Enhanced N fertilization increases the amino acid concentration in phloem sap and elevates the N/C ratio. The study examines: (1) whether aphids select between plants of different N nutrition, (2) whether feeding time correlates with the amino acid composition of phloem sap, and (3) at which stage of probing aphids identify the quality of the plant. Uroleucon tanaceti (Mordvilko) and Macrosiphoniella tanacetaria (Kaltenbach), specialist aphids feeding on tansy (Tanacetum vulgare L.), were reared on this host plant grown essentially hydroponically (in Vermiculite) in the greenhouse on 1, 3, 6, or 12 mM NH₄NO₃. One and 3 mM NH₄NO₃ corresponds to the situation found in natural tansy stands. Aphid stylet penetration was monitored by electrical penetration graphs whilst phloem sap was sampled by stylectomy. Both aphid species settled 2–3 times more frequently on plants fertilized with 6 or 12 mM NH₄NO₃. The phloem sap of these plants contained up to threefold higher amino acid concentrations, without a change in the proportion of essential amino acids. No time differences were observed before stylet penetration of plant tissue. After the first symplast contact, most aphids penetrated further, except M. tanacetaria on low-N plants, where 50% withdrew the stylet after the first probing. The duration of phloem feeding was 2–3 times longer in N-rich plants and the time spent in individual sieve tubes was up to tenfold longer. Aphids identified the nutritional quality of the host plant mainly by the amino acid concentration of phloem sap, not by leaf surface cues nor the proportion of essential amino acids. However, U. tanaceti infestation increased the percentage of methionine plus tryptophan in phloem tenfold, thus manipulating the plants nutritional quality, and causing premature leaf senescence.     

Barley yellow dwarf virus, wheat, and Sitobion avenae: a case of trilateral interactions

We analysed interactions in the system of two Barley Yellow Dwarf Virus (BYDV) strains (MAV and PAV), and wheat (cv. Tinos) as host plant for the virus, and the cereal aphid Sitobion avenae (F.) as vector, in particular whether or not infection by the virus might alter host plant suitability in favour of vector development. By measuring the amino acid and sugar content in the phloem sap of infected and non‐infected wheat plants we found a significant reduction in the concentration of the total amount of amino acids on BYDV‐infected plants. Qualitative and quantitative analysis of honeydew and honeydew excretion indicated a lower efficiency of phloem sap utilisation by S. avenae on infected plants. In addition, S. avenae excreted less honeydew on infected plants. Both BYDV strains significantly affected aphid development by a reduction in the intrinsic rate of natural increase. Hence, infection by the virus reduced the host suitability in terms of aphid population growth potential on BYDV‐infected plants. However, more alate morphs developed on virus‐infected plants. These findings are discussed in relation to the population dynamics of S. avenae, and, as a consequence, the spread of BYDV.     

Transport of organic solutes in Phloem and xylem of a nodulated legume

Collections of xylem exudate of root stumps or detached nodules, and of phloem bleeding sap from stems, petioles, and fruits were made from variously aged plants of Lupinus albus L. relying on nodules for their N supply. Sucrose was the major organic solute of phloem, asparagine, glutamine, serine, aspartic acid, valine, lysine, isoleucine, and leucine, the principal N solutes of both xylem and phloem. Xylem sap exhibited higher relative proportions of asparagine, glutamine and aspartic acid than phloem sap, but lower proportions of other amino acids. Phloem sap of petioles was less concentrated in asparagine and glutamine but richer in sucrose than was phloem sap of stem and fruit, suggesting that sucrose was unloaded from phloem and amides added to phloem as translocate passed through stems to sinks of the plant. Evidence was obtained of loading of histidine, lysine, threonine, serine, leucine and valine onto phloem of stems but the amounts involved were small compared with amides. Analyses of petiole phloem sap from different age groups of leaves indicated ontogenetic changes and effects of position on a shoot on relative rates of export of sucrose and N solutes. Diurnal fluctuations were demonstrated in relative rates of loading of sucrose and N solutes onto phloem of leaves. Daily variations in the ability of stem tissue to load N onto phloem streams were of lesser amplitude than, or out of phase with fluctuations in translocation of N from leaves. Data were related to recent information on C and N transport in the species.     

Nutritional enhancement of host plants by aphids - a comparison of three aphid species on grasses

Three aphid species were compared with respect to ability of enhancing the nutritional quality of their host plants. Rhopalosiphum padi, which does not induce macroscopic changes in its host plants, was compared with Schizaphis graminum and Diuraphis noxia, both of which induce distinctive types of chlorotic lesions. Phloem sap samples were collected from severed stylets of feeding aphids and from exudates of cut leaves of plants uninfested or infested with each aphid species. Samples were analyzed for concentrations of individual amino acids.Compared to R. padi, S. graminum ingested phloem sap with a two-fold higher concentration of amino acids and a much higher proportion of essential amino acids. Similar differences between these two aphid species were observed on both wheat and barley. For each aphid species, the absolute concentrations of amino acids and the relative proportions of essential amino acids were similar between the two host plants. Effects of D. noxia were similar to those of S. graminum, though less dramatic. Exudates from leaves infested with each aphid species showed relative concentrations of individual amino acids that were similar to those in the corresponding stylet exudates. Based on comparison of stylet exudates and cut leaf exudates from infested and uninfested plants, R. padi seems to have little effect on amino acid composition of phloem. Changes in the phloem induced by both S. graminum and D. noxia appear to be systemic, affecting at least the whole leaf they are feeding on. The changes observed for D. noxia and for S. graminum are likely to be nutritionally advantageous for the aphids and are expected to affect the aphids' dependence on nutritional supplementation by intracellular symbionts (Buchnera).     

Aphid-host plant interactions: does aphid honeydew exactly reflect the host plant amino acid composition?

Plants provide aphids with unbalanced and low concentrations of amino acids. Likely, intracellular symbionts improve the aphid nutrition by participating to the synthesis of essential amino acids. To compare the aphid amino acid uptakes from the host plant and the aphids amino acid excretion into the honeydew, host plant exudates (phloem + xylem) from infested and uninfested Vicia faba L. plants were compared to the honeydew produced by two aphid species (Acyrthosiphon pisum Harris and Megoura viciae Buckton) feeding on V. faba. Our results show that an aphid infestation modifies the amino acid composition of the infested broad bean plant since the global concentration of amino acids significantly increased in the host plant in response to aphid infestations. Specifically, the concentrations of the two amino acids glutamine and asparagine were strongly enhanced. The amino acid profiles from honeydews were similar for the two aphid species, but the concentrations found in the honeydews were generally lower than those measured in the exudates of infested plants (aphids uptakes). This work also highlights that aphids take large amounts of amino acids from the host plant, especially glutamine and asparagine, which are converted into glutamic and aspartic acids but also into other essential amino acids. The amino acid profiles differed between the host plant exudates and the aphid excretion product. Finally, this study highlights that the pea aphid, a “specialist” for the V. faba host plant, induced more important modifications into the host plant amino acid composition than the “generalist” aphid M. viciae.     

Host and symbiont genetic determinants of nutritional phenotype in a natural population of the pea aphid

A defining feature of the nutritional ecology of plant sap‐feeding insects is that the dietary deficit of essential amino acids (EAAs) in plant sap is supplemented by EAA‐provisioning microbial symbionts in the insect. Here, we demonstrated substantial variation in the nutritional phenotype of 208 genotypes of the pea aphid Acyrthosiphon pisum collected from a natural population. Specifically, the genotypes varied in performance (larval growth rates) on four test diets lacking the EAAs arginine, histidine and methionine or aromatic EAAs (phenylalanine and tryptophan), relative to the diet containing all EAAs. These data indicate that EAA supply from the symbiotic bacteria Buchnera can meet total aphid nutritional demand for only a subset of the EAA/aphid genotype combinations. We then correlated single nucleotide polymorphisms (SNPs) identified in the aphid and Buchnera genomes by reduced genome sequencing against aphid performance for each EAA deletion diet. This yielded significant associations between performance on the histidine‐free diet and Buchnera SNPs, including metabolism genes predicted to influence histidine biosynthesis. Aphid genetic correlates of performance were obtained for all four deletion diets, with associations on the arginine‐free diet and aromatic‐free diets dominated by genes functioning in the regulation of metabolic and cellular processes. The specific aphid genes associated with performance on different EAA deletion diets are largely nonoverlapping, indicating some independence in the regulatory circuits determining aphid phenotype for the different EAAs. This study demonstrates how variation in the phenotype of associations collected from natural populations can be applied to elucidate the genetic basis of ecologically important traits in systems intractable to traditional forward/reverse genetic techniques.     

Honeydew amino acids in relation to sugars and their role in the establishment of ant-attendance hierarchy in eight species of aphids feeding on tansy (Tanacetum vulgare)

Abstract. The ratio of the concentration of honeydew total amino acids to total sugars in the honeydew of eight species of aphids, all feeding on tansy, Tanacetum vulgare (L.), was determined and correlated with honeydew production and ant‐attendance. The honeydew of the five ant‐attended aphid species [Metopeurum fuscoviride (Stroyan), Trama troglodytes (v. Hayd), Aphis vandergooti (Börner), Brachycardus cardui (L.), Aphis fabae (Scopoli)] was rich in total amino acids, ranging from 12.9 to 20.8 nmol µL^?1 compared with the unattended aphid Macrosiphoniella tanacetaria (Kalt.) with only 3 nmol µL^?1. Asparagine, glutamine, glutamic acid and serine (all nonessential amino acids) were the predominant amino acids in the honeydew of all species. The total concentration of amino acids in the phloem sap of tansy was much higher (78.7 nmol µL^?1) then in the honeydew samples, and the predominant amino acids were glutamate (34.3%) and threonine (17.7%). A somewhat unexpected result was the finding that those aphid species with the highest total amino acid concentration in the honeydew always had the highest concentration of sugars. The lowest amino acid–sugar combined value was 104–28.8 nmol µL^?1 in the non ant‐attended species M. tanacetaria, and the highest value was an average of 270–89.9 nmol µL^?1 for the three most intensely attended aphid species M. fuscoviride, A. vandergooti and T. troglodytes. There is no evidence that any single amino acid or group of amino acids in the honeydew acted as an attractant for ant‐attendance in these eight aphid species. The richness of the honeydew (rate of secretion × total concentration of sugars), along with the presence of the attractant sugar melezitose, comprised the critical factors determining the extent of ant‐attendance of the aphids feeding on T. vulgare. The high total amino acid concentration in sugar‐rich honeydews can be explained by the high flow‐through of nutrients in aphids that are particularly well attended by ants.     

The nutritional quality of phloem sap utilized by natural aphid populations

Abstract.

• 1 The amino acid content of phloem exudates from leaves and of aphid honeydew were adopted as indices of the nutritional quality of phloem sap for aphids. Four plant species and associated leaf-dwelling aphids were investigated: the sycamore Acer pseudoplanatus and sycamore aphid Drepanosiphum platanoides; Prunus domestica (victoria plum) and the mealy plum aphid Hyalopterus pruni; and the spindle tree Euonymus europaeus and broad-bean Vicia faba, both hosts of the black bean aphid Aphis fabae.
• 2 The concentration of amino acids in the phloem exudates varied with: (a) plant species (greater in the herb Vicia than in the tree species), (b) season (greater in the autumn than summer for Acer and Euonymus), and (c) position (greater in flush leaves than mature leaves of Prunus).
• 3 For Acer and Prunus and their aphids, the concentration of amino acids in phloem exudates was significantly correlated with the amino acid content of the aphid honeydew.
• 4 The amino acids in all exudates and honeydew were dominated by non-essential amino acids (glutamic acid, glutamine, asparagine or serine, varying with season and between plant species). The sole major discrepancy between the amino acid profiles of exudates and honeydew was the production of asparagine-rich honeydew by aphids feeding on leaves, whose exudates were dominated by glutamic acid; this applied to both H.pruni on mature Prunus leaves and Drepanosiphum platanoides on summer-leaves of Acer.
• 5 It is suggested that EDTA-exudation may be a useful technique to study nutritional correlates of aphid life cycles, e.g. the time of migration between primary and secondary plant hosts.

     

Phloem-sap feeding by animals: problems and solutions

The incidence of phloem sap feeding by animals appears paradoxical. Although phloem sap is nutrient-rich compared with many other plant products and generally lacking in toxins and feeding deterrents, it is consumed as the dominant or sole diet by a very restricted range of animals, exclusively insects of the order Hemiptera. These insects display two sets of adaptations. First, linked to the high ratio of non-essential:essential amino acids in phloem sap, these insects contain symbiotic micro-organisms which provide them with essential amino acids. For example, bacteria of the genus Buchnera contribute up to 90% of the essential amino acids required by the pea aphid Acyrthosiphon pisum feeding on Vicia faba. Second, the insect tolerance of the very high sugar content and osmotic pressure of phloem sap is promoted by their possession in the gut of sucrase-transglucosidase activity, which transforms excess ingested sugar into long-chain oligosaccharides voided via honeydew. Various other animals consume phloem sap by proxy, through feeding on the honeydew of phloem-feeding hemipterans. Honeydew is physiologically less extreme than phloem sap, with a higher essential:non-essential amino acid ratio and lower osmotic pressure. Even so, ant species strongly dependent on honeydew as food may benefit from nutrients derived from their symbiotic bacteria Blochmannia.     

Differential utilization of ash phloem by emerald ash borer larvae: ash species and larval stage effects

• 1 Two experiments were performed to determine the extent to which ash species (black, green and white) and larval developmental stage (second, third and fourth instar) affect the efficiency of phloem amino acid utilization by emerald ash borer (EAB) Agrilus planipennis Fairmaire (Coleoptera: Buprestidae) larvae.
• 2 EAB larvae generally utilized green ash amino acids more efficiently than those of the other two species. For example, the concentrations of only six (two essential) and seven (two essential) amino acids were lower in frass from EAB that fed upon black and white ash than in the corresponding phloem, respectively. By contrast, concentrations of 16 (eight essential) amino acids were lower in the frass from EAB that fed upon green ash than in the phloem. In addition, in green ash, the frass : phloem ratios of 13 amino acids were lower than their counterparts in black and white ash.
• 3 The concentrations of non‐essential amino acids glycine and hydroxylproline were greater in frass than in phloem when EAB fed on black ash, although not when EAB fed on green or white ash.
• 4 The concentration of total phenolics (a group of putative defensive compounds to EAB, expressed as antioxidant activity of acetone extraction) was high in EAB frass but even higher in the phloem samples when the data were pooled across ash species and EAB larval stages. This suggests EAB larvae may eliminate phenolics through a combination of direct excretion and enzymatic conversion of phenolics to nonphenolics before excretion. Because the ratio of frass total phenolics to phloem total phenolics in white ash was lower than the ratios in black and green ash, the ability to destroy phenolics or convert them to nonphenolics was greater when EAB larvae fed on white ash.
• 5 Fourth‐instar EAB extracted phloem amino acids, including threonine, more efficiently than third‐instar EAB. The different larval developmental stages of EAB did not differ in their apparent ability to destroy phenolics or convert them to nonphenolics.

     

Aphid galls accumulate high concentrations of amino acids: a support for the nutrition hypothesis for gall formation

The nutrition hypothesis for the adaptive significance of insect gall formation postulates that galls accumulate higher concentrations of nutritive compounds than uninfested plant tissue, resulting in a high performance of the gall former. This hypothesis has been supported by some taxa of gall insects, but not by taxa such as cynipid wasps. Aphid galls are expected to require higher levels of nitrogen than other insects’ galls with a single inhabitant, because aphid galls are required to sustain a number of aphids reproducing parthenogenetically over two generations. The present study tested this hypothesis by evaluating aphid performance and amino acid concentration in phloem sap, using the aphid Rhopalosiphum insertum (Walker) (Homoptera: Aphididae), which establishes colonies on leaves of Sorbus commixta Hedlund or in galls of the aphid Sorbaphis chaetosiphon Shaposhnikov (Homoptera: Aphididae). We prepared the gall and non‐gall treatments on trees of S. commixta, in which R. insertum fundatrices were reared and allowed to reproduce. In S. chaetosiphon galls, R. insertum colonies propagated more rapidly, and the second generation grew larger and more fecund than on ungalled leaves. The amount of amino acids exuding from cut galled leaves was fivefold that in ungalled leaves; however, there was no significant difference in the amino acid composition between galled and ungalled leaves. In the intact leaves, total amino acid concentration in the phloem sap declined rapidly from late April to late May; however, the galls retained this high amino acid concentration in developing leaves for 1 month. These results indicate that the improved performance in R. insertum is ascribed to the increased concentration of amino acids in galled leaves. We suggest that S. chaetosiphon galls function to promote the breakdown of leaf protein, leading to an increased performance of gall‐inhabiting aphids.