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.     

Fitness effects of two facultative endosymbiotic bacteria on the pea aphid,Acyrthosiphon pisum,and the blue alfalfa aphid,A. kondoi

The effects of two bacterial endosymbionts, designated PASS and PAR, were evaluated on the pea aphid, Acyrthosiphon pisum (Harris) (Hemiptera:Aphididae), in which they occur facultatively, and on the blue alfalfa aphid, A. kondoi Shinji, in which these bacteria have not been found in natural populations. Subclones of pea aphids and blue alfalfa aphids, derived from parent aphid clones that did not contain PASS or PAR, were infected with one or both bacteria, generating PASS- and/or PAR-positive subclones with minimal genetic differences from the parent clones. Under laboratory conditions at 20 °C, PAR consistently reduced the fecundity (by between 19 and 60%) of subclones derived from three different parent pea aphid clones on bur clover, Medicago hispida Gaertn. PAR had intermediate effects on pea aphids reared on sweet pea, Lathyrus odoratus L., and had no significant effect on pea aphids on alfalfa, Medicago sativa L. The effect of PASS was either neutral or negative, depending on parent clone as well as host plant. Also at 20 °C, PASS reduced fecundity (70–77%) and longevity (40–48%), and increased the age of first reproduction (by up to 1.5 days) of blue alfalfa aphid reared on alfalfa and clover. PAR had a less dramatic effect (e.g., 30–39% reduction in fecundity) on these traits of blue alfalfa aphid. In contrast, PAR and PASS increased the fitness of pea aphid subclones of one parent clone reared for three generations at 25 °C on each of the three test plants. Without facultative bacteria, fecundity of the parent clone was reduced to a mean total of < 6 offspring per adult at this elevated temperature, but with PASS or PAR, mean total fecundity of its subclones was > 35. However, this ameliorative effect of facultative bacteria at 25 °C was not found for two other sets of parent clones and their derived subclones. Alate production in pea aphids was significantly increased in large populations of two PASS- and PAR-positive subclones relative to their parent clones. Attempts to transmit PASS or PAR horizontally, i.e., from aphid to aphid via feeding on host plants (bur clover), were unsuccessful.     

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.     

Aphid clonal resistance to a parasitoid fails under heat stress

Parasitoid virulence and host resistance are complex interactions depending on metabolic rate and cellular activity, which in aphids additionally implicate heritable secondary symbionts among the Enterobacteriaceae. As performance of the parasitoid, the aphid host and its symbionts may differentially respond to temperature, the success or failure of aphid parasitism is difficult to predict when temperature varies. We tested the hypothesis that resistance of the pea aphid Acyrthosiphon pisum to the parasitoid Aphidius ervi, which is linked to aphid secondary symbionts, may depend on temperature in several resistant and non-resistant aphid clonal lineages of different geographic origin and of known bacterial symbiosis, using experiments in controlled environments. Complete immunity to A. ervi at 20 degrees C in three different aphid clones whose symbiosis is characterized by the possession of Hamiltonella defensa reversed to high susceptibility at 25 degrees C and especially 30 degrees C, suggesting that the aphid's immune responses to the establishment and early development of the parasitoid is strongly reduced at moderately high temperatures. There was no evidence that a pea aphid control genotype that was susceptible to A. ervi at 20 degrees C could become more resistant as temperature increases, as has been suggested for insect fungal pathogens. By contrast, our results suggest that aphid clonal resistance to A. ervi and related parasitoids is characteristic of cool temperature conditions, similar to various other fitness attributes of aphids. Based on evidence that H. defensa symbionts characterized all three A. ervi resistant pea aphid clones studied, but was absent in control aphids that remained susceptible at all temperatures, we suggest that secondary symbiosis plays a key role in the heat sensitivity of aphid clonal resistance. Our study may also indicate that aphid natural control of variably susceptible host populations by aphid parasitoids is more likely at moderate to high temperatures.     

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     

Ecological specialization correlates with genotypic differentiation in sympatric host-populations of the pea aphid

The pea aphid, Acyrthosiphon pisum, encompasses distinct host races specialized on various Fabaceae species, but the extent of genetic divergence associated with ecological specialization varies greatly depending on plant and geographic origins of aphid populations. Here, we studied the genetic structure of French sympatric pea aphid populations collected on perennial (pea and faba bean) and annual (alfalfa and red clover) hosts using 14 microsatellite loci. Classical and Bayesian population genetics analyses consistently identified genetic clusters mostly related to plant origin: the pea/faba bean cluster was highly divergent from the red clover and the alfalfa ones, indicating they represent different stages along the continuum of genetic differentiation. Some genotypes were assigned to a cluster differing from the one expected from their plant origin while others exhibited intermediate genetic characteristics. These results suggest incomplete barriers to gene flow. However, this limited gene flow seems insufficient to prevent ecological specialization and genetic differentiation in sympatry.     

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.

     

Impact of environmental stress on aphid clonal resistance to parasitoids: Role of Hamiltonella defensa bacterial symbiosis in association with a new facultative symbiont of the pea aphid

Resistance to endoparasitoids in aphids involves complex interactions between insect and microbial players. It is now generally accepted that the facultative bacterial symbiont Hamiltonella defensa of the pea aphid Acyrthosiphon pisum is implicated in its resistance to the parasitoid Aphidius ervi. It has also been shown that heat negatively affects pea aphid resistance, suggesting the thermosensitivity of its defensive symbiosis. Here we examined the effects of heat and UV-B on the resistance of A. pisum to A. ervi and we relate its stability under heat stress to different facultative bacterial symbionts hosted by the aphid. For six A. pisum clones harboring four different facultative symbiont associations, the impact of heat and UV-B was measured on their ability to resist A. ervi parasitism under controlled conditions. The results revealed that temperature strongly affected resistance, while UV-B did not. As previously shown, highly resistant A. pisum clones singly infected with H. defensa became more susceptible to parasitism after exposure to heat. Interestingly, clones that were superinfected with H. defensa in association with a newly discovered facultative symbiont, referred to as PAXS (pea aphid X-type symbiont), not only remained highly resistant under heat stress, but also expressed previously unknown, very precocious resistance to A. ervi compared to clones with H. defensa alone. The prevalence of dual symbiosis involving PAXS and H. defensa in local aphid populations suggests its importance in protecting aphid immunity to parasitoids under abiotic stress.     

Inhibited Long-Distance Movement of Potato Leafroll Virus to Tubers in Potato Genotypes Expressing Combined Resistance to Infection, Virus Multiplication and Accumulation

Plants of two potato clones which, in preliminary greenhouse assessments, showed resistance to multiplication and accumulation of potato leafroll virus (PLRV) were graft or aphid inoculated with the virus and grown in the greenhouse; plants of a moderately susceptible cultivar were used for comparison in all experiments. A high concentration of aphid‐borne inoculum was used to ensure strong infection pressure. Clone M62759 appeared to be highly resistant to PLRV infection, whereas clone PS1706 was more susceptible. Both clones expressed a high level of resistance to virus multiplication, when primary or secondary infection was assayed by enzyme‐linked immunosorbent assay. Moreover, PLRV was detected in only few or none of the progeny plants of clone M62759, which thus strongly inhibited virus transport to tubers. The study on PLRV translocation from aphid‐inoculated shoots to uninoculated shoots sprouted from the same tubers showed that no specific mechanisms are likely to impair PLRV movement through the tubers of the resistant genotypes. These results indicate that three valuable components of the resistance to PLRV are probably closely linked in the genotype, a combination that seems to occur rather rarely in potato clones. Nevertheless, selecting potato genotypes for the complex resistance to PLRV may prove to be a worthwhile part of breeding programmes, provided that the genetic mechanisms governing particular types of resistance are better recognized.     

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.     

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.     

Role of phloem sap quality and exudation characteristics on performance of pea aphid grown on lucerne genotypes

The phloem sap of two lucerne (Medicago sativa L.) genotypes inducing differences in pea aphid,Acyrthosiphon pisum Harris (Homoptera: Aphididae) performances (weight, fecundity, survival), was collected by stylectomy (radio frequency microcautery). Sugars and amino acids were assayed. No significant difference in their concentrations could be established between genotypes. It is thus unlikely that a substantial part of the varietal resistance of these lucerne genotypes to pea aphid could be related to the content in sugars or amino acids. The same conclusion was drawn for the sugar/amino acid ratio. The amino acid profiles were globally similar. They nevertheless allowed a discrimination between the two genotypes on a few amino acids (alanine, leucine, isoleucine, arginine, ornithine) only. Moreover, the degree of resistance can be related neither to GABA concentration, nor to that of sulfur amino acids, because these constituants were either absent, or in trace amounts, whatever the lucerne genotype taken into account. Stylectomy parameters revealed some interesting differences in the sap exudation of the two genotypes. Average volume of exudation by stylet and average duration of exudation were respective 2.8 and 2.3 times higher in the susceptible genotype. These results suggest a reduced flow of phloem sap, after puncturing of the sieve-tubes by the aphid stylets on the resistant genotype; this could result in a greater difficulty in phloem sap ingestion by aphids settled on the resistant genotype and thus in suboptimal performances.     

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-based divergence in populations of the pea aphid: insights from nuclear markers and the prevalence of facultative symbionts

In North America, the pea aphid Acyrthosiphon pisum encompasses ecologically and genetically distinct host races that offer an ideal biological system for studies on sympatric speciation. In addition to its obligate symbiont Buchnera, pea aphids harbour several facultative and phylogenetically distant symbionts. We explored the relationships between host races of A. pisum and their symbiotic microbiota to gain insights into the historical process of ecological specialization and symbiotic acquisition in this aphid. We used allozyme and microsatellite markers to analyse the extent of genetic differentiation between populations of A. pisum on pea, alfalfa and clover in France. In parallel, we examined: (i) the distribution of four facultative symbionts; and (ii) the genetic variation in the Buchnera genome across host-associated populations of A. pisum. Our study clearly demonstrates that populations of A. pisum on pea, clover and alfalfa in France are genetically divergent, which indicates that they constitute distinct host races. We also found a very strong association between host races of A. pisum and their symbiotic microbiota. We stress the need for phylogeographic studies to shed light on the process of host-race formation and acquisition of facultative symbionts in A. pisum. We also question the effects of these symbionts on aphid host fitness, including their role in adaptation to a host plant.     

Influence of temperature on pea aphid Acyrthosiphon pisum (Hemiptera: Aphididae) resistance to natural enemy attack

The ability to resist or avoid natural enemy attack is a critically important insect life history trait, yet little is understood of how these traits may be affected by temperature. This study investigated how different genotypes of the pea aphid Acyrthosiphon pisum Harris, a pest of leguminous crops, varied in resistance to three different natural enemies (a fungal pathogen, two species of parasitoid wasp and a coccinellid beetle), and whether expression of resistance was influenced by temperature. Substantial clonal variation in resistance to the three natural enemies was found. Temperature influenced the number of aphids succumbing to the fungal pathogen Erynia neoaphidis Remaudière & Hennebert, with resistance increasing at higher temperatures (18 vs. 28 degrees C). A temperature difference of 5 degrees C (18 vs. 23 degrees C) did not affect the ability of A. pisum to resist attack by the parasitoids Aphidius ervi Haliday and A. eadyi Stary, González & Hall. Escape behaviour from foraging coccinellid beetles (Hippodamia convergens Guerin-Meneville) was not directly influenced by aphid clone or temperature (16 vs. 21 degrees C). However, there were significant interactions between clone and temperature (while most clones did not respond to temperature, one was less likely to escape at 16 degrees C), and between aphid clone and ladybird presence (some clones showed greater changes in escape behaviour in response to the presence of foraging coccinellids than others). Therefore, while larger temperature differences may alter interactions between Acyrthosiphon pisum and an entomopathogen, there is little evidence to suggest that smaller changes in temperature will alter pea aphid-natural enemy interactions.     

The oviposition behavior ofAphidius ervi andMonoctonus paulensis (Hymenoptera: Aphididae) encountering different host species (Homoptera: Aphididae) in sequential patches

Two-day-old mated females ofAphidius ervi Haliday andMonoctonus paulensis (Ashmead) were each provided with two sequential host patches. Patches were comprised of plastic petri dishes containing either 15 pea aphids,Acyrthosiphum pisum (Harris), or 15 alfalfa aphids,Macrosiphum creelii Davis. Both wasp species parasitized more hosts in patches containing pea aphids than in those containing alfalfa aphids, regardless of sequence. Females ofA. ervi also laid more eggs per aphid in patches containing pea aphids than in patches containing alfalfa aphids. When both patches contained alfalfa aphids,M. paulensis females parsitized more aphids in the second patch than in the first. Fewer alfalfa aphids were parasitized in the second patch when the first patch contained pea aphids, and fewer eggs were laid per alfalfa aphid. Parasitoid females of both species exhibited consistently higher rates of oviposition into their preferred host species and adjusted their reproductive allocation to hosts and host patches as a function of their experience in previous patches.     

Response of the melon aphid, Aphis gossypii, to host-plant resistance: evidence for high adaptive potential despite low genetic variability

In agrosystems, pests are submitted to strong human-imposed selective pressures to which they sometimes adapt rapidly, either through selection of genotypes resulting from mutation and/or recombination events, or through phenotypic plasticity. Understanding how insects respond to such selective pressures is of great importance for sustainable pest management strategies, such as the use of resistant plants. In this study, we investigated the genetic and phenotypic variability of anholocyclic Aphis gossypii Glover (Hemiptera: Aphididae) strains, in response to the resistance gene Vat that is present in melon crops. Forty-nine aphid colonies were sampled on several melon crops in southern France, genotyped using 15 microsatellite loci, and tested in phenotypic experiments using Vat or non- Vat melons. The level of genetic polymorphism between these colonies was low, as only seven multilocus genotypes were detected. In contrast, the phenotypic variability for life-history and behavioral traits between colonies, including those sharing the same genotype, was unexpectedly high, with a continuum of response to the Vat gene from complete susceptibility to strong virulence. The low genetic polymorphism associated with a strong phenotypic variability highlights the high adaptive potential of A. gossypii and the major role of environmental cues in shaping phenotypic responses of this aphid to pest management strategies.     

Genotypic variation among different phenotypes within aphid clones.

Most aphid species Hemiptera: Aphididae are parthenogenetic between periods of sexual reproduction. They are also highly polyphenic, with different adult morphs occurring in the life cycle, piz. winged, wingless, asexual and sexual. It is assumed that aphids born in a parthenogenetic clonal lineage are genetically identical regardless of the final adult form with the exception of sexual forms). Using the randomly amplified polymorphic DNA-polymerase chain reaction (RAPD-PCR) we have found that different asexual adult phenotypes winged and wingless of some clones of two cereal aphid species (the grain aphid, Sitobion avenae (F.) and the bird-cherry aphid. Rhopalosiphum padi (L.) may be distinguished by the presence or absence of one or more RAPD-PCR bands. In three of nine clones examined, such differences were found, and Southern blotting and hybridization of the discriminating bands confirmed these to be of aphid origin, rather than due to endosymbiotic bacteria or contaminating fungi. The main 248 and 296 bp bands, in the two species respectively, were sequenced and found to be A/T rich. The smaller band showed 57% homology with white striated muscle over a stretch of 90 bp. Genomic DNA treated with dimethyl sulphoxide to remove secondary structures still showed differences in RAPD-PCR profiles between winged and wingless morphs within the unusual clones. This discovery may be widespread and therefore it is important to understand the phenomenon in relation to clonal organisms.