Occurrence and Transmission of Facultative Endosymbionts in Aphids

The occurrence of a secondary bacterial symbiont (PASS) of pea aphid, Acyrthosiphon pisum (Harris), was detected by polymerase chain reaction (PCR) with specific nucleotide primers based on PASS 16S rDNA nucleotide sequences from over 80% (50/57) of clones of pea aphid collected from widely separated locations in California. PASS was also detected by PCR in both red and green phenotypes of rose aphid, Macrosiphum rosae (L.), but not in six other species of aphids examined, including blue alfalfa aphid (A. kondoi Shinji). The nucleotide sequences of the PCR-amplified, partial 16S rDNAs (1060 bp) from pea aphid and rose aphid were identical and 99.9% similar to the published 16S rDNA of PASS. PASS and a recently described new rickettsia of pea aphid (PAR) were transmitted by needle injection of hemolymph from positive pea aphid clones into negative clones and into blue alfalfa aphids. Both PASS and PAR were maintained in the offspring of some of the injected mother aphids via high rate of maternal transmission. Received: 18 September 1996 / Accepted: 30 September 1996     

Facultative symbionts are associated with host plant specialization in pea aphid populations

The pea aphid, Acyrthosiphon pisum, shows significant reproductive isolation and host plant specialization between populations on alfalfa and clover in New York. We examine whether specialization is seen in pea aphids in California, and whether fitness on alternative host plants is associated with the presence of bacterial symbionts. We measured the fitness of alfalfa- and clover-derived aphids on both types of plants and found no evidence for specialization when all aphid lineages were considered simultaneously. We then screened all aphids for the presence of four facultative bacterial symbionts: PAR, PASS, PABS and PAUS. Aphids with PAUS were host-plant specialized, having twice as many offspring as other aphids on clover, and dying on alfalfa. Other aphids showed no evidence of specialization. Additionally, aphids with PABS had 50% more offspring than aphids with PASS when on alfalfa. Thus, specialist and generalist aphid lineages coexist, and specialization is symbiont associated. Further work will resolve whether PAUS is directly responsible for this variation in fitness or whether PAUS is incidentally associated with host-plant specialized aphid lineages.     

High variation in host adaptation among clones of the pea aphid,Acyrthosiphon pisum on peas,Pisum sativum

The performance of one clone of the pea aphid,Acyrthosiphon pisum (Harris), was assessed on 37 different cultivars and species ofPisum L. In addition, random samples of 36 pea aphid clones collected on alfalfa and clover were tested on a selection of fivePisum sativum L. cultivars. Aphid performance was evaluated in terms of the mean relative growth rate (MRGR) during the first five days of life or other life history variables. The MRGR of the first-mentioned pea aphid clone differed little between cultivars. No significant differences in MRGR were found between wild and cultivatedPisum species or between modern and oldP. sativum cultivars. There was considerable variation in host adaptation among the 36 pea aphid clones within each sampled field. The pea aphid clones showed no consistent pattern in performance on four of the five pea cultivars i.e. there was a significant pea aphid genotype —pea genotype interaction. On one of the cultivars all clones performed well. Pea aphid clones collected from red clover generally performed relatively poorly on pea cultivars, in contrast to the pea aphid clones collected on alfalfa. There was no difference in performance between the two pea aphid colour forms tested. Possible reasons for the high variation and the observed adaptation patterns are discussed. The fact that all clones were collected in two adjacent fields indicates thatA. pisum shows high local intraspecific variability in terms of host adaptation.     

EVOLUTION OF AN APHID-PARASITOID INTERACTION: VARIATION IN RESISTANCE TO PARASITISM AMONG APHID POPULATIONS SPECIALIZED ON DIFFERENT PLANTS

The evolution of associations between herbivorous insects and their parasitoids is likely to be influenced by the relationship between the herbivore and its host plants. If populations of specialized herbivorous insects are structured by their host plants such that populations on different hosts are genetically differentiated, then the traits affecting insect-parasitoid interactions may exhibit an associated structure. The pea aphid (Acyrthosiphon pisum) is a herbivorous insect species comprised of genetically distinct groups that are specialized on different host plants (Via 1991a, 1994). Here, we examine how the genetic differentiation of pea aphid populations on different host plants affects their interaction with a parasitoid wasp, Aphidius ervi. We performed four experiments. (1) By exposing pea aphids from both alfalfa and clover to parasitoids from both crops, we demonstrate that pea aphid populations that are specialized on alfalfa are successfully parasitized less often than are populations specialized on clover. This difference in parasitism rate does not depend upon whether the wasps were collected from alfalfa or clover fields. (2) When we controlled for potential differences in aphid and parasitoid behavior between the two host plants and ensured that aphids were attacked, we found that pea aphids from alfalfa were still parasitized less often than pea aphids from clover. Thus, the difference in parasitism rates is not due to behavior of either aphids or wasps, but appears to be a physiologically based difference in resistance to parasitism. (3) Replicates of pea aphid clones reared on their own host plant and on a common host plant, fava bean, exhibited the same pattern of resistance as above. Thus, there do not appear to be nutritional or secondary chemical effects on the level of physiological resistance in the aphids due to feeding on clover or alfalfa, and therefore the difference in resistance on the two crops appears to be genetically based. (4) We assayed for genetic variation in resistance among individual pea aphid clones collected from clover fields and found no detectable genetic variation for resistance to parasitism within two populations sampled from clover. This is in contrast to Henter and Via's (1995) report of abundant genetic variation in resistance to this parasitoid within a pea aphid population on alfalfa. Low levels of genetic variation may be one factor that constrains the evolution of resistance to parasitism in the populations of pea aphids from clover, leading them to remain more susceptible than populations of the same species from alfalfa.     

Influence of Experience on the Response of Bathyplectes curculionis (Hymenoptera: Ichneumonidae), a Nonaphidophagous Parasitoid, to Aphid Odor

Bathyplectes curculionis (Thomson) is an introduced natural enemy of the alfalfa weevil in North America. The wasp requires carbohydrate foods as an adult. Adult wasps have increased longevity and fecundity when provided access to pea aphid, Acyrthosiphon pisum (Harris), honeydew in the laboratory, and adults respond positively to the presence of pea aphids in alfalfa fields. However, it is unknown how these wasps find aphid honeydew in the field. In a series of Y-tube olfactometer experiments, we evaluated the response of naïve and experienced adult female B. curculionis to odors from pea aphids, alfalfa, and pea aphids on alfalfa. Naïve adult females did not respond positively to pea aphid odor even when hungry. But adult females were able to learn aphid odor, and the mechanism of learning appears to be associative rather than by sensitization. Naïve females also showed no preference for alfalfa odor but learned alfalfa odor through sensitization. The wasps did not distinguish between alfalfa with aphids and alfalfa without aphids, even after exposure to aphids or alfalfa with aphids. However, they preferred pea aphid odor to alfalfa odor after a feeding experience in the presence of pea aphid odors. But after exposure to mixed odors of aphids and alfalfa while feeding, B. curculionis females preferred the odor of alfalfa to the odor of pea aphids. These results suggest that alfalfa odors mask or override aphid odors when aphids are associated with alfalfa (as happens naturally), thus interfering with the wasp's ability to respond to learned aphid odors. Therefore, although the wasps are capable of learning to find pea aphids and their honeydew in a simplified laboratory setting, it appears unlikely that they do so in the field.     

Facultative bacterial endosymbionts benefit pea aphids Acyrthosiphon pisum under heat stress

Abstract 1. Natural populations of pea aphids in California contain at least two facultative bacterial secondary symbionts (pea aphid secondary symbiont, PASS, or pea aphid rickettsia, PAR) in a range of frequencies throughout the state.
2. Two pea aphid clones without either of these facultative associates failed to reproduce in the first 8 days after the final moult if they had been heat-stressed for a period of about 4 h at 39 °C as 1-day-old larvae in the laboratory.
3. Aphids infected artificially with PASS, however, were able to produce up to 48% of the normal complement of offspring produced by PASS-positive aphids that had not been heat-stressed. Clones infected artificially with PAR did not have the same advantage as those with PASS after heat stress.
4. In aphids without PASS or PAR, heat stress reduced the number of bacteriocytes (in which the obligate primary symbiont, Buchnera , resides) to 7% of non-heat-stressed aphids, while aphids with only PASS retained 70% of their bacteriocytes. Bacteriocytes in aphids with PAR but not PASS were reduced to 42% of controls.
5. When larvae were heat-stressed as older instars (5 days old), a similar pattern emerged, though the effect of heat stress was less extreme. Clones containing PASS produced the most offspring, three to 14 times as many as aphids without PASS or PAR. Aphids with PAR only, or PASS and PAR together, had reduced or no advantage over aphids without facultative symbionts.
6. Aphids of all clones that had been heat-stressed as later instars gave birth to a variable number of stillborn offspring. Aphids without facultative symbionts produced the most stillborn larvae.
7. Field studies showed a higher incidence of PASS in aphids collected in California in summer compared with aphids from the same sites collected 2–4 months earlier. The difference was significant in two of three widely dispersed locations.     

Competition and dispersal in the pea aphid: clonal variation and correlations across traits

Abstract.  1. The presence of an across-species trade-off between dispersal ability and competitive ability has been proposed as a mechanism that facilitates coexistence. It is not clear if a similar trade-off exists within species. Such a trade-off would constrain the evolution of either trait and, given appropriate selection pressures, promote local adaptation in these traits.
2. This study found substantial levels of heritable variation in competitive ability of the pea aphid, Acyrthosiphon pisum Harris (Homoptera: Aphididae), measured in terms of relative survival when reared with a single clone of the vetch aphid, Megoura viciae Buckton (Homoptera: Aphididae).
3. Pea aphids can move to new patches by either flying (longer distance dispersal) or walking (local dispersal) from plant to plant. There was considerable clonal variation in dispersal ability, measured in terms of the proportion of winged offspring produced, and ability to survive away from their host plant.
4. Winged individuals showed longer off-plant survival times than wingless forms of the same pea aphid clone.
5. There was no evidence of a relationship between clonal competitive ability and either measure of dispersal ability, although the power of the test is limited by the number of pea aphid clones used in the trial.
6. However, there was a positive correlation between clonal fecundity and the proportion of winged offspring produced. Although speculative, it is suggested that clones that are more likely to either overwhelm their host plant or attract higher numbers of natural enemies as a result of having higher fecundity are more likely to produce winged morphs.     

Effects of pea aphid secondary endosymbionts on aphid resistance and development of the aphid parasitoid Aphidius ervi: a correlative study

In order to reduce parasite‐induced mortality, hosts may be involved in mutualistic interactions in which the partner contributes to resistance against the parasite. The pea aphid, Acyrthosiphon pisum Harris (Hemiptera: Aphididae), harbours secondary bacterial endosymbionts, some of which have been reported to confer resistance against aphid parasitoids. Although this resistance often results in death of the developing parasitoid larvae, some parasitoid individuals succeed in developing into adults. Whether these individuals suffer from fitness reduction compared to parasitoids developing in pea aphid clones without symbionts has not been tested so far. Using 30 pea aphid clones that differed in their endosymbiont complement, we studied the effects of these endosymbionts on aphid resistance against the parasitoid Aphidius ervi Haliday (Hymenoptera: Braconidae: Aphidiinae), host–parasitoid physiological interactions, and fitness of emerging adult parasitoids. The number of symbiont species in an aphid clone was positively correlated with a number of resistance measurements but there were also clear symbiont‐specific effects on the host–parasitoid interaction. As in previous studies, pea aphid clones infected with Hamiltonella defensa Moran et al. showed resistance against the parasitoid. In addition, pea aphid clones infected with Regiella insecticola Moran et al. and co‐infections of H. defensa–Spiroplasma, R. insecticola–Spiroplasma, and R. insecticola–H. defensa showed reduced levels of parasitism and mummification. Parasitoids emerging from symbiont‐infected aphid clones often had a longer developmental time and reduced mass. The number of teratocytes was generally lower when parasitoids oviposited in aphid clones with a symbiont complement. Interestingly, unparasitized aphids infected with Serratia symbiotica Moran et al. and R. insecticola had a higher fecundity than unparasitized aphids of uninfected pea aphid clones. We conclude that in addition to conferring resistance, pea aphid symbionts also negatively affect parasitoids that successfully hatch from aphid mummies. Because of the link between aphid resistance and the number of teratocytes, the mechanism underlying resistance by symbiont infection may involve interference with teratocyte development.     

Aphid population dynamics: does resistance to parasitism influence population size?

1. Insect population size is regulated by both intrinsic traits of organisms and extrinsic factors. The impacts of natural enemies are typically considered to be extrinsic factors, however insects have traits that affect their vulnerability to attack by natural enemies, and thus intrinsic and extrinsic factors can interact in their effects on population size. 2. Pea aphids Acyrthosiphon pisum Harris (Hemiptera: Aphididae) in New York and Maryland that are specialised on alfalfa are approximately two times more physiologically resistant to parasitism by Aphidius ervi Haliday (Hymenoptera: Braconidae) than pea aphids specialised on clover. To assess the potential influence of this genetically based difference in resistance to parasitism on pea aphid population dynamics, pea aphids, A. ervi, and other natural enemies of aphids in clover and alfalfa fields were sampled. 3. Rates of successful parasitism by A. ervi were higher and pea aphid population sizes were lower in clover, where the aphids are less resistant to parasitism. In contrast, mortality due to a fungal pathogen of pea aphids was higher in alfalfa. Generalist aphid predators did not differ significantly in density between the crops. 4. To explore whether intrinsic resistance to parasitism influences field dynamics, the relationship between resistance and successful field parasitism in 12 populations was analysed. The average level of resistance of a population strongly predicts rates of successful parasitism in the field. The ability of the parasitoid to regulate the aphid may vary among pea aphid populations of different levels of resistance.     

Temporal changes in host adaptation in the pea aphid, Acyrthosiphon pisum

Abstract.

• 1 Temporal changes in host adaptation were followed in a local population of the pea aphid, Acyrthosiphon pisum. Aphid clones were collected in one alfalfa and one clover field at three different times. In the spring, first-generation females were collected. Later, in the autumn, females belonging to the last parthenogenetic generation were collected. Lastly, sexual females were collected after mating in autumn and allowed to produce eggs which were hatched. The performance was evaluated on alfalfa and clover. The spring-collected individuals were also assessed on peas.
• 2 On the overwintering hosts clover and alfalfa, the clones performed best on the plant of origin, i.e. negative correlations in performance. Correlations between performance on the temporary summer host, pea, and that on clover/alfalfa were weak or nonsignificant.
• 3 Significant variation in host performance was found within both host fields at spring, which is a prerequisite for changes in clone composition due to selection/migration.
• 4 The clones from alfalfa showed an increase in mean performance on alfalfa between spring and autumn, whereas no changes among the clones from the clover field were observed. This difference in seasonal response between the two fields could have been the result of larger variation in performance among the alfalfa clones and/or a differential tendency to migrate among clones in both fields.
• 5 After sexual recombination in the autumn, mean performance in the alfalfa field returned to the spring level, probably as a result of emergence of new genetic combinations. In the clover field, mean performance did not change significantly over time.

     

Genetic variation in the effect of a facultative symbiont on host-plant use by pea aphids

Ecological specialisation on different host plants occurs frequently among phytophagous insects and is normally assumed to have a genetic basis. However, insects often carry microbial symbionts, which may play a role in the evolution of specialisation. The bacterium Regiella insecticola is a facultative symbiont of pea aphids (Acyrthosiphon pisum) where it is found most frequently in aphid clones feeding on Trifolium giving rise to the hypothesis that it may improve aphid performance on this plant. A study in which R. insecticola was eliminated from a single naturally infected aphid clone supported the hypothesis, but a second involving two aphid clones did not find the same effect. We created a series of new pea aphid–R. insecticola associations by injecting different strains of bacteria into five aphid clones uninfected by symbionts. For all aphid clones, the bacteria decreased the rate at which aphids accepted Vicia faba as a food plant and reduced performance on this plant. Their effect on aphids given Trifolium pratense was more complex: R. insecticola negatively affected acceptance by all aphid clones, had no effect on the performance of four aphid clones, but increased performance of a fifth, thus demonstrating genetic variation in the effect of R. insecticola on pea aphid host use. We discuss how these results may explain the distribution and frequency of this symbiont across different aphid populations. Julia Ferrari and Claire L. Scarborough contributed equally to the work.     

Diversity of bacteria associated with natural aphid populations

The bacterial communities of aphids were investigated by terminal restriction fragment length polymorphism and denaturing gradient gel electrophoresis analysis of 16S rRNA gene fragments generated by PCR with general eubacterial primers. By both methods, the gamma-proteobacterium Buchnera was detected in laboratory cultures of six parthenogenetic lines of the pea aphid Acyrthosiphon pisum and one line of the black bean aphid Aphis fabae, and one or more of four previously described bacterial taxa were also detected in all aphid lines except one of A. pisum. These latter bacteria, collectively known as secondary symbionts or accessory bacteria, comprised three taxa of gamma-proteobacteria (R-type [PASS], T-type [PABS], and U-type [PAUS]) and a rickettsia (S-type [PAR]). Complementary analysis of aphids from natural populations of four aphid species (A. pisum [n = 74], Amphorophora rubi [n = 109], Aphis sarothamni [n = 42], and Microlophium carnosum [n = 101]) from a single geographical location revealed Buchnera and up to three taxa of accessory bacteria, but no other bacterial taxa, in each aphid. The prevalence of accessory bacterial taxa varied significantly among aphid species but not with the sampling month (between June and August 2000). These results indicate that the accessory bacterial taxa are distributed across multiple aphid species, although with variable prevalence, and that laboratory culture does not generally result in a shift in the bacterial community in aphids. Both the transmission patterns of the accessory bacteria between individual aphids and their impact on aphid fitness are suggested to influence the prevalence of accessory bacterial taxa in natural aphid populations.     

The use of visual cues in host evaluation by aphidiid wasps I. Comparison between threeAphidius parasitoids of the pea aphid

Host evaluation behaviour was examined in three species of aphid parasitoids,Aphidius ervi haliday,A. pisivorus Smith, andA. smithi Sharma & Subba Rao (Hymenoptera: Aphidiidae). Parasitoids were provided under laboratory conditions with three kinds of hosts representing two aphid species: (green) pea aphid,Acyrthosiphon pisum (Harris), and green and pink colour morphs of the alfalfa aphid,Macrosiphum creelii Davis. Females of all threeAphidius species distinguished between aphids on the basis of colour, movement, and host species. Patterns of host acceptance by parasitoids were species-specific. InA. ervi, host preference was the same in light and dark conditions: pea aphid>green alfalfa aphid≫pink alfalfa aphid. In contrast,A. pisivorus attacked and accepted pea aphid and green alfalfa aphid equally in the light and preferred both of these over pink alfalfa aphid; however, it made no distinction between pea aphid and pink alfalfa aphid in the dark. Females ofA. smithi attacked all three kinds of hosts (pea aphid>green alfalfa aphid≫pink alfalfa aphid) but apparently laid eggs only in pea aphid. The frequencies of attack and oviposition by all wasps were higher on ‘normal’ pea aphids than on those anaesthetized with CO₂. Host recognition is confirmed by chemical cues in the aphid cuticle that are detected during antennation, and host acceptance is dependent on an assessment of host quality during ovipositor probing.     

Host selection and performance of the giant willow aphid, Tuberolachnus salignus Gmelin – implications for pest management

1 The recent increase in planting of selected willow clones as energy crops for biomass production has resulted in a need to understand the relationship between commonly grown, clonally propagated genotypes and their pests. 2 For the first time, we present a study of the interactions of six willow clones and a previously unconsidered pest, the giant willow aphid Tuberolachnus salignus. 3 Tuberolachnus salignus alatae displayed no preference between the clones, but there was genetic variation in resistance between the clones; Q83 was the most resistant and led to the lowest reproductive performance in the aphid 4 Maternal effects buffered changes in aphid performance. On four tested willow clones fecundity of first generation aphids on the new host clone was intermediate to that of the second generation and that of the clone used to maintain the aphids in culture. 5 In the field, patterns of aphid infestation were highly variable between years, with the duration of attack being up to four times longer in 1999. In both years there was a significant effect of willow clone on the intensity of infestation. However, whereas Orm had the lowest intensity of infestation in the first year, Dasyclados supported a lower population level than other monitored clones in the second year.     

Behavioural correlates of genetic divergence due to host specialization in the pea aphid, Acyrthosiphon pisum

Host specialization plays a central role in the diversification of herbivorous insects and yet we know very little about the evolution of this trait. Populations of the pea aphid, Acyrthosiphon pisum (Harris), are specialized and locally adapted to either alfalfa or clover. Preference for either plant produces assortative mating, and gene flow between alfalfa and clover populations is consistently restricted (Via, 1991a,b, 1994). Behavioural studies of freely-moving aphids on the plant surface for 30 min suggest that host preference is chemically mediated, involves chemoreception, and is most likely due to secondary compounds located in the epidermal or mesophyll cells. Pea aphids do not distinguish between hosts and non-hosts at a distance but determine whether the plant is suitable or not after only a short probe. Thirty-minute recordings of the activities of aphid stylets using an electrical monitoring system (EPG), where aphids are attached to a gold wire with silver paint, provide a different picture, suggesting that EPG experiments do not accurately reflect natural behaviour during the first 30 min of the aphid-plant interaction.     

Does variation in host plant association and symbiont infection of pea aphid populations induce genetic and behaviour differentiation of its main parasitoid, Aphidius ervi?

The host-associated differentiation (HAD) hypothesis states that higher trophic levels in parasitic associations should exhibit similar divergence in case of host sympatric speciation. We tested HAD on populations of Aphidius ervi the main parasitoid of the pea aphid Acyrthosiphon pisum, emerging from host populations specialized on either alfalfa or red clover. Host and parasitoid populations were assessed for genetic variation and structure, while considering geography, host plant and host aphid protective symbionts Regiella insecticola and Hamiltonella defensa as potential covariables. Cluster and hierarchical analyses were used to assess the contribution of these variables to population structure, based on genotyping pea aphids and associated A. ervi with microsatellites, and host aphid facultative symbionts with 16S rDNA markers. Pea aphid genotypes were clearly distributed in two groups closely corresponding with their plant origins, confirming strong plant associated differentiation of this aphid in North America. Overall parasitism by A. ervi averaged 21.5 % across samples, and many parasitized aphids producing a wasp hosted defensive bacteria, indicating partial or ineffective protective efficacy of these symbionts in the field. The A. ervi population genetic data failed to support differentiation according to the host plant association of their pea aphid host. Potential for parasitoid specialization was also explored in experiments where wasps from alfalfa and clover aphids were reciprocally transplanted on alternate hosts, the hypothesis being that wasp behaviour and parasitic stages should be most adapted to their host of origin. Results revealed higher probability of oviposition on the alfalfa aphids, but higher adult emergence success on red clover aphids, with no interaction as expected under HAD. We conclude that our study provides no support for the HAD in this system. We discuss factors that might impair A. ervi specialization on its divergent aphid hosts on alfalfa and clover.     

Seasonal variation in the photoperiodic responses of a pea aphid population: evidence for long-distance movements between populations

Summary The purpose of the study was to quantify long distance movements in populations of pea aphid, Acyrthosiphon pisum (Harris), by estimating origins and distances travelled by immigrants into a southern Manitoba population. A strong relationship was demonstrated between latitude of origin and photoperiods at which pea aphid populations are stimulated to produce the diapause stage (Smith 1987). Therefore, the approach was to use photoperiodic response as a physiological marker to identify the source of immigrant aphids. The responses of 89 clones from Glenlea, Manitoba (49°38N), sampled 5 times over 2 seasons, were measured. One sample of clones collected the first season had photoperiodic responses similar to those of a population about 300 km to the south, and significantly different from clones collected in spring of the same year at the same site. Weather analysis corroberates that the migrants were probably carried into Manitoba on a southerly flow of air during the previous 24 to 36 h.     

Impact of a parasitoid on the bacterial symbiosis of its aphid host

Embryo production in aphids is absolutely dependent on the function of symbiotic bacteria, mainly Buchnera, and the growth and development of koinobiont parasitoids in aphids requires the diversion of nutrients from aphid embryo production to the parasitoid. The implication that the bacterial symbiosis may be promoted in parasitized aphids to support the growing parasitoid was explored by analysis of the number and biomass of mycetocytes, and the aphid cells bearing Buchnera, in the pea aphid Acyrthosiphon pisum Harris (Hemiptera: Aphididae) parasitized by the wasp Aphidius ervi Haliday (Hymenoptera: Braconidae). Aphids hosting a young larval parasitoid bore more mycetocytes of greater total biomass, and embryos of lower biomass than unparasitized aphids. Furthermore, one of the three aphid clones tested, which limited teratocyte growth (giant cells of parasitoid origin having a trophic role), bore smaller mycetocytes and larger embryos, than one or both of the two aphid clones with greater susceptibility to the parasitoid. These data suggest that susceptibility of the aphid‐Buchnera symbiosis to parasitoid‐mediated manipulation may, directly or indirectly, contribute to aphid susceptibility to parasitoid exploitation.     

Acceptance of low-saponin lines of alfalfa with varied phenolic concentrations by pea aphid (Homoptera: Aphididae)

This research aims to examine the effect of phenolics on pea aphid (Acyrthosiphon pisum) (Homoptera: Aphididae) development and feeding behaviour, on leaves of selected low-saponin lines of Radius alfalfa (Medicago sativa). There was a slight, negative correlation (Spearman rank correlation r _s = −0.80) between concentrations of saponins and phenols. Lines with higher concentrations of saponins had less phenolics. Levels of phenolics in low-saponin lines of alfalfa cv. Radius were related to their acceptance by the pea aphid. Our data revealed an inverse relationship between level of phenolics and the aphid abundance and its biology on studied alfalfa lines. Larval development of the pea aphid was longer, reproduction period was shorter, and the fecundity was lower on low-saponin lines with higher level of phenolics. There were observed some tendencies in the pea aphid feeding behaviour on these lines: prolonging the probing of the peripheral tissues (epidermis and mesophyll) and shortening the period of phloem sap ingestion. The better hosts for the pea aphid were low-saponin lines with low levels of phenolic compounds.     

Interspecific and intraspecific variation in the performance of three pest aphid species on five grain legume hosts

The suitability of five grain legume species (narrow-leafed lupin, chickpea, faba bean, field pea, lentil) as hosts for three aphid species (green peach aphid, cowpea aphid, bluegreen aphid) was evaluated by measuring the mean relative growth rate (MRGR) and survivorship of nymphs over a 5 day period. For each aphid species, intraspecific (interclonal) variation was also determined by independently measuring the performance of 30 clones collected from a variety of hosts and from different parts of the Western Australia (WA) wheatbelt. The suitability of the grain legumes varied among aphid species. Chickpea was not a suitable host for any of the aphids tested. Averaged over all clones, lentil and faba bean were the most suitable hosts for cowpea aphid, and narrow-leafed lupin was the most suitable host for green peach aphid. Field pea was a suitable host for all three species, but only at a suboptimal level. Cowpea aphid showed the greatest amount of intraspecific variation, with significant variation in MRGR among clones on all hosts except chickpea and significant variation in survivorship on chickpea and lupin. For green peach aphid, there was significant variation in MRGR among clones on field pea and lupin, but in survivorship on lupin only. Bluegreen aphid clones showed significant variation only for MRGR on faba bean and lupin. There were positive correlations in performance of green peach aphid clones on faba bean and lentil, and of cowpea aphid clones on faba bean and lentil. Bluegreen aphid clones showed a negative correlation in performance on field pea and faba bean. These results show the importance of screening cultivars against a wide variety of aphid clones when assessing aphid susceptibility in breeding programmes. The implications of these results on the adaptability of parthenogenetic aphids are discussed.