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.     

The reactions of two cereal aphid parasitoids, Aphidius uzbekistanicus and A. ervi to host aphids and their food-plants

ABSTRACT. A Y-tube olfactometer was used to test the reactions of the hymenopteran cereal aphid parasitoids Aphidius uzbekistanicus Luzhetski and A. ervi Haliday to odours from aphids and their host plants. Only females responded to aphids but both sexes responded to plant odours. A. uzbekistanicus responded to the cereal aphids Sitobion avenae (F.) and Metopolophium dirhodum (Walker) whilst A. ervi , which has a broad host range, responded to M. dirhodum and the pea aphid, Acyrthosiphon pisum. Female A. uzbekistanicus responded to wheat leaves only but males responded to a range of plant material. Both male and female A. ervi responded to wheat and bean leaves. The failure of A. ervi to respond to either nettle aphids, Microlophium carnosum (Bukt.), or nettle leaves, despite its frequent parasitization of this aphid in the field, suggests the existence of more than one race of the parasitoid and casts doubts on the usefulness of alternative hosts as reservoirs for A. ervi in integrated control programmes. Males of both species responded to their respective females suggesting the presence of a sex specific attractant.     

Evidence for the existence of a species closely related to the cereal aphid parasitoid Aphidius rhopalosiphi De Stefani-Perez based on host ranges, morphological characters, isoelectric focusing banding patterns, cross-breeding experiments and sex pheromone specificities (Hymenoptera, Braconidae, Aphidiinae)

Abstract. Host transfer trials with six strains of the cereal aphid parasitoid Aphidius rhopalosiphi De Stefani-Perez revealed the presence of two differing host ranges: three strains parasitized Sitobion uvenae (F.) and S.fragariae (Wlk.) exclusively, whereas the others successfully developed in both Sitobion species and also in Metopolophium dirhodum (Wlk.), M.festucue cerealiurn Stroyan, Rhopalosiphum padi (L.) and Diuraphis muehlei Börner. The two host range groups were poorly distinguishable by means of morphological characters. In con trast, isoelectric focusing proved to be a much better tool for discrimi nation. Reproductive isolation between the two host range groups was clear but not complete, as evidenced by crossing experiments with laboratory-bred and field-collected material. Under field conditions, males showed a specific response to virgin females of the two groups. Hence, the sex pheromones of the females were obviously different. Considering these results, a species ( Aphidius sp. A) closely related to A.rhopalosiphi seems to occur. As a detailed study of morphological characters is still lacking, Aphidius sp. A is not yet formally established here. Due to its dependence upon Sitobion spp., Aphidius sp. A may be an economically important parasitoid of the pest aphid S.avenae .     

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.     

农业景观格局与麦蚜密度对其初寄生蜂与重寄生蜂种群及寄生率的影响

农业景观格局与过程能够强烈影响寄生蜂对寄主的寻找及寄生作用,寄主密度亦是影响寄生蜂分布的重要因素,然而农业景观的格局和寄主密度对寄生蜂寄生率的相互影响是一项值得研究的工作.在简单与复杂2种麦田农业景观结构下,调查了麦蚜的分布格局与2种寄主密度下麦蚜的初寄生率与重寄生率,分析了景观结构对麦蚜密度的影响、景观格局与麦蚜密度对寄生蜂寄生率与重寄生率的影响及交互作用.结果表明:景观结构的复杂性对麦蚜分布和寄生蜂初寄生率与重寄生率的影响均不明显,但寄主密度与景观结构的复杂性对寄生蜂的影响存在着明显的交互作用,寄主密度与寄生率呈正相关,寄主密度较低时烟蚜茧蜂为优势种,寄主密度较高时燕麦蚜茧蜂为优势种.麦蚜初寄生蜂与重寄生蜂对寄主密度的反应与其形态学、体型大小以及生活史特征相关,初寄生蜂与重寄生蜂的群落组成显著影响其对麦蚜的寄生率,而与景观结构的复杂性关系不大.     

Patterns of host selection by four species of aphidiid (Hymenoptera) parasitoids: influence of host switching

Abstract.

• 1 We tested switching behaviour in four species of aphidiid parasitoids, using a two-aphid experimental system consisting of second-instar nymphs of pea aphid (Acyrthosiphon pisum (Harris)) and alfalfa aphid (Macrosiphum creelii Davis) feeding on broad beans in the laboratory.
• 2 Aphidius ervi Haliday, A.pisivorus Smith, A.smithi Sharma & Subba Rao, and Pram pequodorum Viereck showed an innate preference for pea aphid when both host species were provided in equal numbers.
• 3 Wasps encountered both aphid species equally but differed in their acceptance of alfalfa aphid. Females of A.pisivorus and P.pequodorum accepted alfalfa aphids when few pea aphids were available, but A. smithi always concentrated attacks on pea aphid. Aphidius ervi super-parasitized an increasing proportion of pea aphids as their availability declined.
• 4 Switching to the alfalfa aphid occurred in A.ervi and P.pequodorum (but not in A.pisivorus and A.smithi) under the condition of a 1:3 ratio of pea aphids:alfalfa aphids. Wasps did not switch when more pea aphids than alfalfa aphids were provided (3:1 ratio).
• 5 Alfalfa aphids were more likely than pea aphids to escape from parasitoid attack.
• 6 Switching to the most abundant host may not be adaptive in these four species of aphid parasitoids. A foraging wasp incurs a potentially higher cost in lost opportunity time when attacking (and failing to oviposit in) alfalfa aphids. In addition, alfalfa aphids may have lower host quality than pea aphids, a difference that could influence offspring fitness.

     

The influence of temperature on size as an indicator of host quality for the development of a solitary koinobiont parasitoid

The influence of aphid size on the host quality assessment and progeny performance of aphidiine parasitoids was examined using the mealy plum aphid parasitoid, Aphidius transcaspicus Telenga (Hymenoptera: Braconidae) and the black bean aphid, Aphis fabae Scopoli (Homoptera: Aphididae), as a readily acceptable alternate host. Aphid size in relation to stage of development was manipulated by rearing synchronous aphid cohorts at either 15 or 30 °C. At 15 °C, 2nd instar aphids were approximately the same size as 4th instar aphids reared at 30 °C. Cohorts of 30 aphids from each instar, reared at each temperature, were exposed to parasitism by a single parasitoid female for a period of 5 h. Overall susceptibility to parasitism did not vary between aphid cohorts, but the parasitoid response to aphid size differed significantly between rearing temperatures for both progeny sex ratio (parent female assessment of host quality) and larval growth and development (host suitability for parasitoid development). For aphids reared at 15 °C, the proportion of female progeny and emerging adult size for the parasitoid increased linearly with aphid size at the time of attack, while development time remained constant. In contrast, for aphids reared at 30 °C, the proportion of female progeny, emerging adult size, and the development time of the parasitoid all declined with aphid size at the time of attack. The contrasting responses of the parasitoid to host size for aphids reared at the two temperatures suggest that host quality is only indirectly related to aphid size among aphidiine parasitoids. The possible effects of higher temperatures on nutritional stress, obligate endosymbionts, and future growth potential of the aphids are discussed as explanations for the variation in host quality for parasitoid development.     

Aspects of host—parasitoid interactions of two aphid parasitoids, Aphidius urticae (Haliday) and Aphidius uzbeckistanicus (Luzhetski) (Hymenoptera, Aphidiidae)

Abstract. 1. Various aspects were studied of two host-parasitoid interactions: Aphidius urticae parasitizing the cocksfoot aphid Hyalopteroides humilis and Aphidius uzbeckistanicus parasitizing the cereal aphid Metopolophmm dirhodum. The work formed part of a study on the population dynamics of grassland and cereal aphids in Berkshire, England.
2. A.uzbeckistanicus had a higher instantaneous attack rate but a slightly longer handling time than A.urticae. In both cases the functional response was curvilinear.
3. A.urticae was specific to H.humilis and failed to develop in M.dirhodum. A.uzbeckistanicus could develop successfully in H.humilis but the functional response was sigmoid and the progeny were less fecund than those reared on the natural host. This species showed a clear preference for M.dirhodum when supplied with mixtures of M.dirhodum and H.humilis.
4. A.uzbeckistanicus had a faster rate of development and a lower developmental threshold than A.urticae , a situation paralleled by their natural hosts.
5. The results are discussed in relation to field observations. A.uzbeckistanicus may have considerable potential for reducing cereal aphid numbers. However, its preference for M.dirhodum mitigates against a reservoir being maintained in the economically unimportant grassland aphid, H.humilis.     

Flying with a 'death sentence' on board: electrophoretic detection of braconid parasitoid larvae in migrating winged grain aphids, Sitobion avenae (F.)

Polyacrylamide gel electrophoresis of enzymes (carboxylesterases) was used for the first time to monitor rates of parasitism in airborne alate (winged) grain aphid, Sitobion avenae (F.) population samples collected by suction trapping in Hertfordshire, UK. Using previously described electrophoretic 'keys', the species of hymenopterous parasitoids present in individual aphids were identified and found to be Aphidius ervi (Haliday) and/or Aphidius rhopalosiphi (De Stephani Perez) (Braconidae). Entomophthoralean fungal infection was also detected using this approach. Aphidiid wasp parasitism was detected from early June to mid-August and fungal infection from late June to late July. The results are discussed in relation to parasitoid population structure and dynamics, especially (i) the fact that winged aphids passively transport the early stages of their braconid parasitoids and fungal pathogens, potentially to newly-founded colonies, which may directly impact on the dual aphid-parasitoid populations genetics; and (ii) the approach used to collect and assay parasitised and fungal infected aphids involving both suction trapping and electrophoretic testing may have potential in assessing the level and efficacy of these biological control agents in integrated pest management (IPM) schemes to combat cereal aphid outbreaks.     

Non‐consumptive effects of a natural enemy on a non‐prey herbivore population

1. Predator–prey interactions have traditionally focused on the consumptive effects that predators have on prey. However, predators can also reduce the abundance of prey through behaviourally‐mediated non‐consumptive effects. For example, pea aphids (Acyrthosiphon pisum Harris) drop from their host plants in response to the risk of attack, reducing population sizes as a consequence of lost feeding opportunities. 2. The objective of the present study was to determine whether the non‐consumptive effects of predators could extend to non‐prey herbivore populations as a result of non‐lethal incidental interactions between herbivores and foraging natural enemies. 3. Polyculture habitats consisting of green peach aphids (Myzus persicae Sulzer) feeding on collards and pea aphids feeding on fava beans were established in greenhouse cages. Aphidius colemani Viereck, a generalist parasitoid that attacks green peach aphids but not pea aphids, was released into half of the cages and the abundance of the non‐host pea aphid was assessed. 4. Parasitoids reduced the population growth of the non‐host pea aphid by increasing the frequency of defensive drops; but this effect was dependent on the presence of green peach aphids. 5. Parasitoids probably elicited the pea aphid dropping behaviour through physical contact with pea aphids while foraging for green peach aphids. It is unlikely that pea aphids were responding to volatile alarm chemicals emitted by green peach aphids in the presence of the parasitoid. 6. In conclusion, the escape response of the pea aphid provided the opportunity for a parasitoid to have non‐target effects on an herbivore with which it did not engage in a trophic interaction. The implication is that natural enemies with narrow diet breadths have the potential to influence the abundance of a broad range of prey and non‐prey species via non‐consumptive effects.     

The effects of a plant defence priming compound, β-aminobutyric acid,on multitrophic interactions with an insect herbivore and a hymenopterous parasitoid

Biocontrol of aphids by natural enemies is utilized in many organic and integrated pest management schemes. β-aminobutyric acid (BABA), a non-protein amino acid, is a plant defence primer that suppresses growth of some insect herbivores when applied as a root drench. This investigation examined how applying BABA to host plants via the roots may impact on a parasitoid wasp of aphids. Female Aphidius ervi (Haliday) did not discriminate against pea aphids (Acyrthosiphon pisum (Harris)) reared on BABA-treated beans (Vicia faba L.) or show any modified responses to volatiles released from BABA-treated plants. BABA reduced the size of emerging wasps, primarily by inhibiting the growth of the host aphid. Metabolomic analysis revealed BABA in both aphids and emergent wasps indicating some potential for direct physiological inhibition to have occurred. Survival of the parasitoids was only reduced at doses of BABA likely to produce phytotoxic effects in many plant species, thus there may be potential to incorporate plant defence primers like BABA into integrated pest management practices. However, the precise mechanisms of BABA-inhibition of insects still require elucidation.     

Presence of an unsuitable host diminishes the competitive superiority of an insect parasitoid: a distraction effect

Competitive interactions between parasitoid species are traditionally evaluated when they compete for a single host species. Yet, the presence of additional host species can alter competitive interactions, even if the host is unsuitable for parasitoid development. In alfalfa of the mid-western USA, a native parasitoid species, Praon pequodorum, was once a dominant natural enemy, but it has become rare since the introduction of another parasitoid, Aphidius ervi. Despite A. ervi’s competitive superiority for their most common host, the pea aphid Acyrthosiphum pisum, P. pequodorum still persists at low densities. We performed a suite of laboratory and field studies to determine if the presence of an alternative host, the spotted alfalfa aphid Therioaphis maculata, may mitigate A. ervi’s competitive superiority and facilitate P. pequodorum’s persistence. We show that spotted alfalfa aphids reduce the foraging efficiency of both parasitoid species for pea aphids, despite spotted alfalfa aphids being an unsuitable host. This decrease in efficiency, however, was not symmetrical; the presence of spotted alfalfa aphids had a greater detrimental effect on A. ervi foraging for pea aphids. This might facilitate the persistence of the competitively inferior P. pequodorum. Our study suggests that indirect effects generated by the presence of alternative hosts are important for understanding parasitoid–host dynamics and overall insect community structure.     

Is the response of aphids to alarm pheromone stable?

Aphid taxa are characterized by a number of biological features, such as their feeding behaviour and host selection, which it is generally accepted are affected by keeping them for several generations under standard conditions in a laboratory. Analyses of three strains of the green pea aphid, Acyrthosiphon pisum (Harris, 1776), reared in culture for long periods, indicate that other characters are also affected. For example, the response of these aphids to alarm pheromone is dramatically reduced. This raises an interesting question regarding the mechanism by which it occurs and has consequences when aphids from laboratory cultures are used for studies in ecology and applied biology and especially the long‐term effectiveness of crop plants genetically engineered to produce EBF as a means of controlling aphids.     

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 specialization in habitat specialists and generalists

Generalists and specialists use different cues to find their habitat and essential resources. While generalists have the advantage of exploiting a wider range of resources, they are predicted to be less efficient in using one particular resource compared to specialists. The level of specialization of parasitoids can be either at the habitat or at the host level; strategies used by either type are expected to differ. We examined interactions between three aphid parasitoid species that are a habitat specialist Aphidius rhopalosiphi, a habitat generalist Aphidius ervi, and a host generalist Praon volucre on three cereal aphids, Sitobion avenae, Metopolophium dirhodum and Rhopalosiphum padi. We compared total parasitism rate across behavioral and physiological variation in a non-choice test. Next, we addressed total parasitism in two phases to examine: (1) the response of parasitoids to different hosts through the behavioral sequence from antennation through oviposition, and (2) the physiological suitability of different hosts for oviposition and larval development. Parasitization typically involved the following behavioral steps: (1) antennal contact, (2) abdominal bending, and (3) ovipositor insertion (acceptance). A. rhopalosiphi had the same number of antennal contacts with the three aphids but showed fewer instances of abdominal bending towards R. padi. Pre-contact host preference was found for A. ervi but it did not correspond to the level of acceptance. The number of antennal contacts by P. volucre corresponded to the parasitization level of the aphid species but more mummies were produced on M. dirhodum than on R. padi. These results suggest that parasitoid species that are habitat specialists react similarly to the different host species present in the same habitat, whereas generalist species exhibit clear preferences during host selection. Preferences were, however, not always related to host suitability.     

Variability in characters commonly used to distinguish Aphidius species (Hymenoptera: Aphidiidae)

Abstract Characters traditionally used to separate some Aphidius species commonly parasitizing pea and cereal aphids are shown to be more variable than previously supposed. Aphidius ervi, picipes and rhopalosiphi (Hymenoptera: Aphidiidae) were cultured and the variability of the numbers of antennal segments, sculpture on the anterolateral area of the petiole and the tentorial index were recorded. The number of antennal segments in each of the three species varied by four in males and by three in females. The numbers of ridges on the anterolateral area of the petiole of A.rhopalosiphi varied widely from six to eighteen. The tentorial index range from 0.35–0.62, 0.34–0.54 and 0.35–0.60 for A.ervi, picipes and rhopalosiphi respectively, compared with previously published ranges of 0.30–0.40, 0.28–0.37 and 0.30–0.35. A.rhopalosiphi and uzbekistanicus cannot be distinguished reliably by the wing-vein length ratios previously used.     

The influence of light environment on host colour preference in a parasitoid wasp

1. Visual chromatic cues and contrast effects are widely used by insects in behaviours involving host/prey/mate‐finding and recognition. However, naturally changing light conditions may challenge the visual perception of cues for these organisms. 2. We used the host/parasitoid system Acyrthosiphon pisum/Aphidius ervi to determine if apparent visual preference of the wasp for green over pink host aphids was a visually based choice or a post‐attack mechanism based on host susceptibility depending on anti‐parasitoid symbiotic bacteria. 3. The study tested the ability of the wasp to recognise and attack pea aphid clones expressing variation based on colour and/or symbionts under a broad range of LED‐controlled light environments mimicking natural variations. 4. Results showed that the amount of reflected light of pink morphs was about half that of the green morphs in the cyan‐green components. Both host colours were recognised and attacked under all tested light conditions, even red light (660 nm). The previously reported preference of A. ervi for green pea aphids, was clear only for naive females given a choice between two aphid colours under all light environments, but quickly disappeared. 5. Wasps showed no tendency of avoiding oviposition in clones with defensive symbionts. 6. These findings suggest that variable rates of pea aphid parasitism by A. ervi in fields do not depend on host colour discrimination, but rather on susceptibility variation among aphid clones in allowing larval development after egg‐laying. Further studies should consider deeper investigation of the impact of red lights used in modified light environments in greenhouses and the proportion of host colour morph available.     

Behavioural differences between Aphidius ervi populations from two tritrophic systems are due to phenotypic plasticity

The Palaeoarctic parasitoid Aphidius ervi Haliday (Hymenoptera, Aphidiidae) parasitises legume aphids in its region of origin. In Chile, it parasitises both legume and cereal aphids. This special situation was studied at two levels: (i) the host searching behaviour of A. ervi from two different tritrophic systems (Acyrthosiphon pisum on alfalfa and Sitobion avenae on wheat) was investigated in dual choice tests in a wind tunnel between odours from both A. pisum-alfalfa host plant complex (HPC) and S. avenae-wheat HPC, and (ii) the genetic structure of A. ervi populations from both sources using molecular markers. Responses of A. ervi females to volatile olfactory cues emanating from A. pisum-alfalfa HPC and S. avenae-wheat HPC were significantly higher towards the HPC on which they were reared during the last generation before experimentation, regardless of the origin of the parasitoid. As previously described for this parasitoid species, oviposition experience was also of major relevance in the preferences of female parasitoids. On the other hand, variation in mitochondrial DNA segments and RAPD-PCR polymorphism using total DNA showed the absence of host-based population structure and a high genetic homogeneity between these A. ervi populations. These results reject the possible existence of different host-strains of this parasitoid in Chile.     

Complex interactions between a plant pathogen and insect parasitoid via the shared vector-host: consequences for host plant infection

Plant viruses modify the development of their aphid vectors by inducing physiological changes in the shared host plant. The performance of hymenopterous parasitoids exploiting these aphids can also be modified by the presence of the plant pathogen. We used laboratory and glasshouse microcosms containing beans (Vicia faba) as the host plant to examine the interactions between a plant virus (pea enation mosaic virus; PEMV) and a hymenopterous parasitoid (Aphidius ervi) that share the aphid vector/host Acyrthosiphon pisum. Neither PEMV-infection of V. faba, nor the carriage of PEMV virions by A. pisum, affected the growth or morphology of the aphid, or the oviposition behaviour and development of A. ervi. The presence of developing Aphidius ervi larvae within Acyrthosiphon pisum did not affect the ability of the aphids to transmit PEMV. However, by reducing their longevity, parasitism ultimately decreased the time viruliferous aphids were able to inoculate plants. In terms of virus dispersal, parasitized aphids exhibited more movement around experimental arenas than unparasitized controls, causing a slight increase in the proportion of beans infected with PEMV. Exposure to adult Aphidius ervi caused Acyrthosiphon pisum to rapidly drop off bean plants and disperse to new hosts, resulting in considerably higher plant infection rates (70%) than that seen in control arenas (25%). The results of this investigation demonstrate that when parasitoids are added to a plant-pathogen-vector system, benefits to the host plant due to reduced herbivore infestation must be balanced against the consequences of parasitoid-induced aphid dispersal and a subsequent increase in the level of plant infection.     

The maintenance of intraspecific biodiversity: the interplay of selection on resource use and on natural enemy resistance in the pea aphid

Biodiversity has both intraspecific and interspecific components, and speciation is the process through which the former is converted to the latter. Ecological factors can cause population divergence and differentiation. In this paper, we investigate the interplay between top-down effects from natural enemies and bottom-up effects from host plants in an aphid model system. Pea aphids, Acyrthosiphon pisum, are known to form host-adapted races on Trifolium and Medicago. Here, replicate clones of pea aphids collected from a broader set of five host plant genera are screened for their performance on the same set of host plants and also for their resistance to four natural enemies: the parasitoids Aphidius eadyi and Aphidius ervi, and the entomopathogenic fungi Pandora (=Erynia) neoaphidis and Zoophthora phalloides. The populations showed clear adaptation to their host plant from which they were collected, but they also performed well on Vicia. Performance on the other three plant species was poor. The aphid population collected from Lotus was significantly better at defending itself against the parasitoid A. eadyi, and there was a tendency for the clones from Trifolium to be resistant to the pathogen P. neoaphidis. These patterns highlight the importance of understanding the ecological processes influencing speciation in the context of the web of ecological adaptations within which a species is embedded.