Host regulation by the aphid parasitoid Aphidius ervi: the role of teratocytes

The biochemical profile and metabolism of Acyrthosiphon pisum(Harris) (Homoptera, Aphididae) are markedly altered and redirected in response to parasitization by the endophagous braconid Aphidius erviHaliday (Hymenoptera, Braconidae). In the present study, the role played in the host regulation process by teratocytes, cells deriving from the dissociation of the embryonic membrane of the parasitoid, is taken into consideration. The protein synthesis activity of these cells of embryonic origin is analysed in vitroand an essential characterization of those proteins de novosynthesized and released in the incubation medium is provided. Teratocytes, obtained by dissecting parasitized host aphids, 3, 4 and 5 days after parasitoid oviposition, were incubated in vitroand, at the end of the incubation period, were separated from the medium for SDS-PAGE analysis of both cellular and secreted proteins. Various cellular proteins were more abundant as the time between parasitization and teratocyte collection increased. Furthermore, two proteins, showing an approximate molecular mass of 15 kD (p15) and 45 kD (p45) respectively, were abundantly secreted in the incubation medium by 5 day-old teratocytes. Incubations in presence of ³⁵S radiolabelled amino acids indicated that p15 and p45 are both synthesized by A. erviteratocytes. The amino acid composition of these two proteins was similar to that reported for other insect proteins with a demonstrated nutritional function. The p45 protein was found to be glycosylated. A tentative physiological model describing the host regulation role played by different parasitoid-derived factors is proposed.     

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.     

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.     

Altered dispersal behaviour in parasitised aphids: parasitoid-mediated or pathology?

1. Several hypotheses concerning modified dispersal behaviour in aphids parasitised by aphidiine wasps (Hymenoptera: Braconidae: Aphidiinae) were tested in the laboratory. Behavioural changes may be host-mediated, parasitoid-mediated, or a by-product of trauma and pathology. 2. Mummification site varied with parasitoid species. Pea aphids (Acyrthosiphon pisum) parasitised by Aphidius ervi, Aphidius pisivorus, Monoctonus paulensis, and Praon pequodorum mummified near the aphids’ preferred feeding sites on bean plants, but those parasitised by Ephedrus californicus often died and mummified outside the colony, away from the plants. 3. Parasitism by E. californicus had a progressive effect on the behaviour of pea aphids. Approaching death, aphids lost motor control and frequently dropped off the host plant when disturbed. Dropped aphids were unable to return to the feeding site and mummified elsewhere. The proportion of aphids mummifying outside the colony increased with mummy density. 4. Mummification site was not influenced by the presence within the same colony of aphids parasitised by different species of aphidiine wasps. 5. The evidence does not support the hypothesis that mummification site selection in E. californicus is determined by a host- or a parasitoid-mediated change in aphid dispersal behaviour. Association-specific differences in the dynamics of larval development and growth between aphidiine species provide an equally valid and possibly more general explanation of mummification behaviour.     

Possible bases of pseudoparasitism in Spodoptera littoralis larvae stung by Microplitis rufiventris

The effects of host age and parasitoid female age on the occurrence of 'Pseudoparasitism', using the Spodoptera littoralis-Microplitis rufiventris host-parasitoid system were investigated. The first four larval instars of the host are not equally suitable for parasitoid development. The proportion of pseudoparasitized hosts significantly increases when: (1) the age of the female parasitoid increases; (2) oviposition occurs mostly in fourth instar larvae; (3) a later age of the host instar is used; (4) the mandibles of the newly hatched parasitoid larvae mistakenly attack host interior organs (e.g. Malpighian tubules); and (5) an imperfect growth pattern of teratocytes occurs. The reluctance of female wasps to parasitize fourth instar host larvae is not due to the thickness of host cuticle but possibly due to the unfavourable physiological state of the host larvae. The age of host larvae at the time of parasitization may influence the adverse effects of parasitoid factors (e.g. polydnavirus, venom and teratocytes) on the growth of host larvae. It is suggested that females of M. rufiventris are able to determine the suitability of a potential host instar for the development of their offspring. The cell diameter of M. rufiventris teratocytes increases with increasing age of host larvae at the time of oviposition. The association within the host of living parasitoid larvae and functional teratocytes may be important for the survival of each other and consequently for successful parasitism.     

Parasitoids induce production of the dispersal morph of the pea aphid, Acyrthosiphon pisum

In animals, inducible morphological defences against natural enemies mostly involve structures that are protective or make the individual invulnerable to future attack. In the majority of such examples, predators are the selecting agent while examples involving parasites are much less common. Aphids produce a winged dispersal morph under adverse conditions, such as crowding or poor plant quality. It has recently been demonstrated that pea aphids, Acyrthosiphon pisum, also produce winged offspring when exposed to predatory ladybirds, the first example of an enemy‐induced morphological change facilitating dispersal. We examined the response of A. pisum to another important natural enemy, the parasitoid Aphidius ervi, in two sets of experiments. In the first set of experiments, two aphid clones both produced the highest proportion of winged offspring when exposed as colonies on plants to parasitoid females. In all cases, aphids exposed to male parasitoids produced a higher mean proportion of winged offspring than controls, but not significantly so. Aphid disturbance by parasitoids was greatest in female treatments, much less in male treatments and least in controls, tending to match the pattern of winged offspring production. In a second set of experiments, directly parasitised aphids produced no greater proportion of winged offspring than unparasitised controls, thus being parasitised itself is not used by aphids for induction of the winged morph. The induction of wing development by parasitoids shows that host defences against parasites may also include an increased rate of dispersal away from infected habitats. While previous work has shown that parasitism suppresses wing development in parasitised individuals, our experiments are the first to demonstrate a more indirect influence of parasites on insect polyphenism. Because predators and parasites differ fundamentally in a variety of attributes, our finding suggests that the wing production in response to natural enemies is of general occurrence in A. pisum and, perhaps, in other aphids.     

Effects of the parasitization of a braconid, Apanteles, on the blood of its host, Pieris

Parasitization of a braconid wasp, Apanteles glomeratus, of larvae of a common cabbage butterfly, Pieris rapae crucivora, caused changes in differential haemocyte count (DHC), total haemocyte count (THC), and encapsulative capacity against dead eggs of Apanteles in the fourth and fifth instar host larvae.However, no correlation could be found between the number of Apanteles eggs deposited and THC of the middle fourth instar host larvae or between the number of parasitoid larvae and specific gravity of the haemolymph from the late fifth instar host larvae.From the changes in DHC and in THC of both non-parasitized and parasitized Pieris larvae, an increase in the number of plasmatocytes of non-parasitized Pieris larvae in the early fourth instar period was supposed to be due to transformation of prohaemocytes into plasmatocytes, and a low population of plasmatocytes of parasitized larvae in the comparable period was assumed to be due to a suppression of transformation of prohaemocytes by some factor released from the parasitoid eggs.Failure of the parasitized fourth instar Pieris larvae to encapsulate injected dead eggs of Apanteles indicated that the parasitoid embryos were, in some way, actively inhibiting the encapsulation reactions of the host.The increase in THC of the parasitized fifth instar larvae could not be ascribed to a decrease in the volume of host haemolymph. Rather it could be interpreted by a suppression of adhesive capacity of haemocytes in the host haemocoel to tissue surfaces.Reduced encapsulative capacity of the parasitized fifth instar larvae might be attributed either to a depression of the adhesive activity of plasmatocytes resulting from a depletion of energy source for haemocytes in the host haemolymph by parasitization, or from an active suppression of adhesiveness of the plasmatocytes by secretions from ‘giant cells’ (teratocytes) originated from the parasitoid.     

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.     

Teratocytes of the solitary endoparasitoid Meteorus gyrator (Hymenoptera: Braconidae): morphology, numbers and possible functions

Abstract. Laboratory studies investigated the development of teratocytes derived from the eggs of the parasitoid Meteous gyrator (Thun.) in its host, the tomato moth Lacanobia oleracea (L.). At hatching, each parasitoid egg produced an average of approximately 1000 teratocytes, but this number declined to approximately 400 during the course of parasitism. The teratocytes increased in size markedly, such that 7 days after egg hatch their mean diameter was approximately four times that of the cells immediately after dissociation. The haemolymph of parasitized hosts had reduced phenoloxidase activity, and teratocytes inhibited phenoloxidase activity when coincubated with plasma from nonparasitized hosts. The injection of teratocytes into nonparasitized fifth‐instar L. oleracea larvae suppressed growth and induced a supernumerary moult in some larvae. A number of parasitism‐specific proteins were detected in the haemolymph of parasitized hosts, and incubation of teratocytes in culture media indicated that these cells were a source of at least two of these proteins.     

Covariance of adult size and development time in the parasitoid waspAphidius ervi in relation to the size of its host,Acyrthosiphon pisum

Summary Adult size (in terms of dry weight; DW) and development time (T _p ) of the solitary parasitoidAphidius ervi varied when reared in different nymphal instars of its host, apterous virginoparae of the pea aphid (Acyrthosiphon pisum). Parasitoid DW increased with an increase in the DW of the host at parasitization, from the first to the third aphid instar. Female wasps gained 1.1 times more in DW than their male counterparts in all four host classes, butT _p did not significantly differ between the sexes. Parasitoid DW was consistently more variable thanT _p . The two traits covaried positively with an increase in host size from the first to the third instar, but they varied independently in parasitoids from fourth-instar hosts. The host size (and stage) at the time of parasitization imposes constraints on the growth and development of immatureA. ervi that are reflected in the pattern of covariation between DW andT _p . When growing in aphids below a certain size threshold, parasitoids can maximize fitness by a trade-off between DW andT _p . Consequently, the assumption implicit in host-size models of parasitoid oviposition decisions — that females incur a relatively greater reduction in size (used as an index of fecundity) than males when developing in poor quality hosts — can be falsified.     

Development of the aphid parasitoid,Aphidius ervi, in asexual and sexual females of the pea aphid,Acyrthosiphon pisum, and the blackberry-cereal aphid,Sitobion fragariae

The aphid parasitoid,Aphidius ervi Haliday, overwinters in larval diapause. The possibility that the parasitoid might prefer sexual (oviparae) rather than asexual females (virginoparae) as overwintering hosts (oviparae predominate in autumn when host numbers are generally declining) was tested by comparing these aphid morphs as potential hosts. Two host species were also examined, the pea aphid,Acyrthosiphon pisum (Harris), and the blackberry aphid,Sitobion fragariae (Walker). The parasitoids took longer to develop inS. fragariae than inA. pisum but the development of non-diapausingA. ervi was similar in sexual and asexual females. This observation, together with the greater variation in the duration of the different parasitoid stadia inS. fragariae, indicated that the parasitoid is specialized on the pea aphid. In photophases of 12 h and longer, the proportion ofA. ervi entering diapause inA. pisum oviparae was higher than in virginoparae. The critical daylength (where 50% of parasitoids entered diapause) was therefore longer in oviparae (12.6 h) than in virginoparae (11.7 h) with the inference that parasitoids developing in the oviparae would enter diapause earlier in the field. InS. fragariae, critical day-lengths were similar in both aphid morphs. The duration of diapause was unaffected by host morph and emergence in short days (10:14 L:D) occurred over a long period (c. 60 days).     

Host Age Preference Behavior in <Emphasis Type="Italic">Aphidius ervi</Emphasis> Haliday (Hymenoptera: Aphidiidae)

Many parasitoid species have preference for certain stages of hosts to parasitize but the underlying behavioral mechanisms of such preference are still poorly understood, making it difficult to evaluate host-parasitoid interactions and their effects on the success of biological control programs. Here, we report our work on a parasitoid Aphidius ervi Haliday on the pea aphid Acyrthosiphon pisum (Harris). We show that with the increase of host age, female parasitoids are more likely to encounter and to attack their hosts but the hosts develop increasingly greater defensive capabilities. Encounter almost always triggers attack attempt; however, increasing attack attempts do not proportionally lead to ovipositor probings and increasing ovipositor probings do not proportionally translate into ovipositions. These asymmetric responses may be interpreted as that A. ervi females prefer to parasitize older aphids for higher fitness return but those aphids can better defend themselves, and as a consequence, A. ervi females may achieve the highest gain by attacking aphids of intermediate ages. We suggest that A. ervi females forage in a manner consistent with the optimal foraging theory, trading off host handling time with fitness returns.     

Larvae of an endoparasitoid,Cotesia kariyai (Hymenoptera: Braconidae), feed on the host fat body directly in the second stadium with the help of teratocytes

Larvae of a gregarious endoparasitoid, Cotesia kariyai (Watanabe), grew rapidly during the second stadium in the host. The fat body of a Pseudaletia host parasitized by C. kariyai was completely consumed by 10 d, just before larval emergence. It seemed hard to explain the growth of the second instar parasitoids and the rapid consumption of the fat body only by ingestion of hemolymph converted from the fat body or other organs of the host. Paraffin sections of the parasitized host revealed that many teratocytes were attached to the surface of the fat body in many sites and destroyed the fat body tissue locally. Zymography of proteins released from the teratocytes revealed that the teratocytes 4 to 9 days after parasitization showed collagenase activity (as a gelatinase). Further, 1st instar parasitoids which were transplanted together with teratocytes into unparasitized hosts preconditioned with C. kariyai polydnavirus (CkPDV) plus venom, grew normally to the 2nd stadium. Abnormal growth of parasitoid larvae was observed when parasitoid larvae were transplanted without teratocytes. These results suggest that the teratocytes attach to the outer sheath of the fat body, secrete an enzyme that makes a hole in the matrix of the fat body, thus allowing the second instar parasitoid to ingest the content of the fat body.     

Aphidius ervi teratocytes release an extracellular enolase

We report the cloning of a gene and the characterization of the encoded protein, which is released by the teratocytes of the parasitoid Aphidius ervi in the haemocoel of the host aphid Acyrthosiphon pisum. The studied protein was identified by LC-MS/MS, and the gathered information used for isolating the full length cDNA. The corresponding gene was made of 3 exons and 2 introns, and was highly expressed in the adult wasps and in parasitized hosts. The translation product, which was named Ae-ENO, showed a very high level of sequence identity with insect enolases. In vivo immunodetection experiments evidenced Ae-ENO localization in round spots, present in the teratocytes and released in the host haemocoel. Moreover, strong immunoreactivity was detected on the surface of A. ervi larvae and of host embryos. Ae-ENO expressed in insect cells was not secreted in the medium, indicating the occurrence in the teratocytes of an unknown pathway for Ae-ENO release. The recombinant protein produced in bacteria under native conditions was a dimer, with evident enolase activity (K_m = 0.086 ± 0.017 mM). Enolase is a well known enzyme in cell metabolism, which, however, is associated with a multifunctional role in disease, when present in the extracellular environment, on the surface of prokaryotic and eukaryotic cells. In these cases, the enolase mediates the activation of enzymes involved in the invasion of tissues by pathogens and tumour cells, and in the evasion of host immune response. The possible role played by Ae-ENO in the host regulation process is discussed in the light of this information.     

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.     

Oviposition behaviour of the aphid parasitoid,Aphidius ervi: Are wet aphids recognized as host?

Females of the central European population of the aphid parasitoid, Aphidius ervi, did not attack wet pea aphids (Acyrthosiphon pisum) that were washed previously with water. After 1 hour, this phenomenon disappeared and A. ervi attacked washed hosts to the same degree as dry ones. Similarly, A. ervi attacked dead aphids killed in liquid nitrogen readily if they were dry but not if they were wet. This effect was also reversible and disappeared after 1 h. When A. ervi females were foraging on broad beans (Vicia faba), they laid significantly more eggs into dry aphids than into wet aphids. Resource utilization of wet aphids, however, was significantly lower in this design than in Petri dishes, due to a changed drop-off behaviour of the aphid. We conclude that females did not use visual cues for host recognition but instead relied on chemical cues. These cues may be covered by a thin water layer directly after aphids became wet. Our results also demonstrate the importance of abiotic factors for the estimation of the reproductive success of parasitoids in the field.     

Host suitability and fitness-related parameters of Campoletis sonorensis (Hymenoptera: Ichneumonidae) as a parasitoid of the cabbage looper,Trichoplusia ni (Lepidoptera: Noctuidae)

The seven age-classes of Trichoplusia ni (Hübner) larvae evaluated in this study as hosts of Campoletis sonorensis indicates that early 2nd larval instar (3–5 day-old larvae) of T. ni represents the most suitable host stage for the development of the larval endoparasitoid C. sonorensis. The higher suitability of early 2nd larval instar of T. ni resulted in more parasitised larvae, a higher rate of successful parasitoid emergence, a higher rate of female progeny, and a lower rate of immature parasitoid mortality. The fitness gain of C. sonorensis on late 1st larval instar (2 day-old larvae) and late 2nd larval instar –early 3rd instars (6–8 day-old larvae) stages of T. ni is negatively affected by the trade-offs between the different physiological and behavioral characteristics influencing their suitability as hosts of C. sonorensis.     

Stage dependent influences of polydnaviruses and the parasitoid larva on host ecdysteroids

In the solitary egg-larval parasitoid Chelonus inanitus (Braconidae) both polydnavirus and the parasitoid larva manipulate host development. Parasitization leads to a premature drop in juvenile hormone titre and a precocious onset of metamorphosis in the 5th larval instar. The C. inanitus bracovirus (CiBV) alone causes a reduction in host ecdysteroid titres at the pupal cell formation stage and prevents pupation. Here we report three new findings. (1) We show that parasitization causes a reduction in haemolymph ecdysteroid titre immediately after the moult to the 5th instar; similarly low values were seen in nonparasitized larvae after the moult to the 6th instar. These data along with parasitoid removal experiments indicate that the low ecdysteroid titre after the moult is a very early sign of the upcoming metamorphosis. (2) In vitro experiments with prothoracic glands and brain extracts showed that CiBV affects both prothoracic glands and prothoracicotropic hormone after the stage of pupal cell formation. (3) In the haemolymph of parasitized larvae the ecdysteroid titre increased in the late cell formation stage, i.e. immediately before egression of the parasitoid. In vitro experiments showed that late 2nd instar parasitoids release ecdysteroids and are thus very likely responsible for the rise in host ecdysteroids.     

Metabolic and symbiotic interactions in amino acid pools of the pea aphid,Acyrthosiphon pisum,parasitized by the braconid Aphidius ervi

Aphidius ervi Haliday (Hymenoptera, Braconidae) is an endophagous parasitoid of the pea aphid, Acyrthosiphon pisum (Harris) (Homoptera, Aphididae). This parasitoid strongly redirects host reproduction and metabolism to favour nutrition and development of its juvenile stages. Parasite-regulated biosynthesis and mobilization of nitrogen metabolites determine a significant increase of host nutritional suitability. The aim of the present study was mainly to investigate the temporal changes of A. pisum amino acid pools, as affected by A. ervi parasitism, and to assess the role of the aphid bacterial endosymbiont Buchnera in determining the observed changes. In parasitized aphids, we observed a very significant increase in total free amino acids, compared with synchronous non-parasitized controls, starting from day 4 after parasitization (+51%). This trend culminated with more than doubling the control value (+152%) on day 6 after parasitization. However, a significant “parasitism” effect was observed only for 10 of the 28 amino acids detected. Tyrosine accumulation was the most prominent parasitoid-induced alteration, with a fourfold increase over control levels registered on day 6. In parasitized hosts, the amino acid biosynthetic capacity of Buchnera was unaltered, or even enhanced for the phenolic pool, and contributed greatly to the definition and maintainance of host free amino acid pools. The hypertyrosinemic syndrome was not dependent on food supply of the aromatic nucleus but was induced by parasitism, which likely enhanced the aromatic shuttle mediating phenylalanine transfer from bacteria to the host tissues, where tyrosine conversion occurs. This process is likely associated with a selective disruption of the host’s functions requiring tyrosine, leading to the remarkable accumulation of this amino acid. The possible mechanisms determining these parasitism-induced host alterations, and their nutritional significance for the developing parasitoid larva, are discussed.     

Does mother really know best? Oviposition preference reduces reproductive performance in the generalist parasitoid Aphidius ervi

The reproductive success of female parasitoids is dependent on their ability to accurately assess the suitability of a host for larval development. For generalist parasitoids, which utilize a broad range of species and instars as hosts, a set of assessment criteria determines whether a host is accepted or rejected. The suitability of a host, however, can only be imperfectly assessed by the female parasitoid, which can result in the selection of lesser quality hosts for oviposition. In this study we explored the disparity between host quality and host preference using the generalist koinobiotic parasitoid Aphidius ervi Haliday (Hymenoptera: Aphidiidae) and the host Aulacorthum solani (Harris) (Homoptera: Aphididae), the foxglove aphid. The second instar hosts produced the highest level of reproductive success, while third and fourth instars resulted in a substantially reduced reproductive performance. When given a choice of host instars, parasitoids preferred the older hosts for oviposition disregarding their reduced suitability for larval development. Results are discussed in context of mechanisms involved in A. ervi host selection and biases in the criteria used to assess hosts that may arise when parasitoids transfer host species between generations.