Evolutionary ecology of the interactions between aphids and their parasitoids

Evolutionary consequences of loss of sexual reproduction on male-related traits in parthenogenetic lineages of the pea aphid

Transition from sexual reproduction to parthenogenesis constitutes a major life-history change with deep evolutionary consequences for sex-related traits, which are expected to decay. The pea aphid Acyrthosiphon pisum shows intraspecific reproductive polymorphism, with cold-resistant cyclically parthenogenetic (CP) lineages that alternate sexual and asexual generations and cold-sensitive obligately parthenogenetic (OP) lineages that produce only asexual females but still males. Here, the genotyping of 219 pea aphid lineages collected in cold-winter and mild-winter regions revealed contrasting population structures. Samples from cold-winter regions consisted mostly of distinct multilocus genotypes (MLGs) usually represented by a single sample (101 different MLGs for 111 samples) and were all phenotyped as CP. In contrast, fewer MLGs were found in mild-winter regions (28 MLGs for 108 samples), all but one being OP. Since the males produced by OP lineages are unlikely to pass on their genes (sexual females being rare in mild-winter regions), we tested the hypothesis that their traits could degenerate due to lack of selection by comparing male production and male reproductive success between OP and CP lineages. Male production was indeed reduced in OP lineages, but a less clear pattern was observed for male reproductive success: females mated with OP males laid fewer eggs (fertilized or not) but OP and CP males fertilized the same proportion of eggs. These differences may stem from the type of selective forces: male production may be counter-selected whereas male performances may evolve under the slower process of relaxed selection. The overall effective reproductive capacity of OP males could result from recent sex loss in OP lineages or underestimated reproductive opportunities.

Cereal Aphid Parasitoids in Europe (Hymenoptera: Braconidae: Aphidiinae): Taxonomy, Biodiversity, and Ecology

Cereals are very common and widespread crops in Europe. Aphids are a diverse group of herbivorous pests on cereals and one of the most important limiting factors of cereal production. Here, we present an overview of knowledge about the taxonomy, biodiversity, and ecology of cereal aphid parasitoids in Europe, an important group of natural enemies contributing to cereal aphid control. We review the knowledge obtained from the integrative taxonomy of 26 cereal aphid primary parasitoid species, including two allochthonous species (Lysiphlebus testaceipes and Trioxys sunnysidensis) and two recently described species (Lipolexis labialis and Paralipsis brachycaudi). We further review 28 hyperparasitoid species belonging to three hymenopteran superfamilies and four families (Ceraphronoidea: Megaspillidae; Chalcidoidea: Pteromalidae, Encyrtidae; Cynipoidea: Figitidae). We also compile knowledge on the presence of secondary endosymbionts in cereal aphids, as these are expected to influence the community composition and biocontrol efficiency of cereal aphid parasitoids. To study aphid-parasitoid-hyperparasitoid food webs more effectively, we present two kinds of DNA-based approach: (i) diagnostic PCR (mainly multiplex PCR), and (ii) DNA sequence-based methods. Finally, we also review the effects of landscape complexity on the different trophic levels in the food webs of cereal aphids and their associated parasitoids, as well as the impacts of agricultural practices and environmental variation.

Suitability and Profitability of a Cereal Aphid for the Parasitoid Aphidius platensis in the Context of Conservation Biological Control of Myzus persicae in Orchards

The use of cover crops can promote the abundance and early arrival of populations of natural enemies. Cereal cover crops between orchards rows could encourage the early arrival of the parasitoid Aphidius platensis, as they offer alternative winter hosts (e.g., Rhopalosiphum padi), enhancing the control of Myzus persicae in spring. However, the preference for and suitability of the alternative host must be addressed beforehand. To evaluate the potential of this strategy, we assessed host preference using behavioural choice tests, as well as no-choice tests measuring fitness traits, when developing on both host species. One source field for each aphid population from the above hosts was chosen. There was a clear choice for R. padi compared to M persicae, independently of the source, probably due to more defensive behaviours of M. persicae (i.e., kicks and escapes). Nevertheless, both aphid species were suitable for parasitoids’ development. The female progeny developed on R. padi were larger in size, irrespective of their origin. According to our results, in peach orchards with cereals sown between peach trees during the autumn, where we expect when R. padi populations will no longer be available during spring, A. platensis should be able to switch to M. persicae.

"Generalist" Aphid Parasitoids Behave as Specialists at the Agroecosystem Scale

The degree of trophic specialization of interacting organisms impacts on the structure of ecological networks and has consequences for the regulation of crop pests. However, it remains difficult to assess in the case of parasitoids. Host ranges are often established by listing host records from various years and geographic areas in the literature. Here, we compared the actual hosts exploited at a local farm-scale by aphid parasitoids (Hymenoptera: Aphidiinae), to the available species listed as hosts for each parasitoid species. We sampled aphids and their parasitoids in cultivated and uncultivated areas in an experimental farm from April to November 2014 and thereafter used DNA-based data to determine whether a differentiation in sequences existed. Twenty-nine parasitoid species were found on 47 potential aphid hosts. Our results showed that the great majority of the parasitoid tested used fewer host species than expected according to data published in the literature and parasitized a limited number of hosts even when other potential hosts were available in the environment. Moreover, individuals of the most generalist species differed in their DNA sequences, according to the aphid species and/or the host plant species. At a local scale, only obligate or facultative specialist aphid parasitoids were detected. Local specialization has to be considered when implementing the use of such parasitoids in pest regulation within agroecosystems.

Identification and Expression Profiling of Peripheral Olfactory Genes in the Parasitoid Wasp Aphidius ervi (Hymenoptera: Braconidae) Reared on Different Aphid Hosts

Generalist parasitoids of aphids, such as the wasp Aphidius ervi, display significant differences in terms of host preference and host acceptance, depending on the host on which they developed (natal host), which is preferred over a non-natal host, a trait known as host fidelity. This trait allows females to quickly find hosts in heterogeneous environments, a process mediated by chemosensory/olfactory mechanisms, as parasitoids rely on olfaction and chemical cues during host selection. Thus, it is expected that proteins participating in chemosensory recognition, such as odorant-binding proteins (OBPs) and odorant receptors (ORs) would play a key role in host preference. In this study, we addressed the effect of parasitoid reciprocal host switching between two aphid hosts (Sitobion avenae and Acyrthosiphon pisum) on the expression patterns of chemosensory genes in the wasp A. ervi. First, by using a transcriptomic approach based on RNAseq of A. ervi females reared on S. avenae and A. pisum, we were able to annotate a total of 91 transcripts related to chemoperception. We also performed an in-silico expression analysis and found three OBPs and five ORs displaying different expression levels. Then, by using qRT-PCR amplification, we found significant differences in the expression levels of these eight genes when the parasitoids were reciprocally transplanted from S. avenae onto A. pisum and vice versa. This suggests that the expression levels of genes coding for odorant receptors and odorant-binding proteins would be regulated by the specific plant–aphid host complex where the parasitoids develop (maternal previous experience) and that chemosensory genes coding for olfactory mechanisms would play a crucial role on host preference and host acceptance, ultimately leading to the establishment of host fidelity in A. ervi parasitoids.

The preference-performance relationship as a means of classifying parasitoids according to their specialization degree

Host range in parasitoids could be described by the preference-performance hypothesis (PPH) where preference is defined as host acceptance and performance is defined as the sum of all species on which parasitoid offspring can complete their life cycle. The PPH predicts that highly suitable hosts will be preferred by ovipositing females. However, generalist parasitoids may not conform to this hypothesis if they attack a large range of hosts of varying suitability. Under laboratory conditions, we tested the PPH relationship of three aphid parasitoids currently considered as generalist species (Aphelinus abdominalis, Aphidius ervi, Diaeretiella rapae). As expected, the three parasitoids species showed low selectivity, i.e., females stung all aphid species encountered (at least in some extent). However, depending on the parasitoid species, only 42%-58% of aphid species enabled producing parasitoid offspring. We did not find a correlation between the extent of preference and the performance of three generalist aphid parasitoids. For A. ervi, host phylogeny is also important as females showed higher attack and developmental rates on hosts closely related to the most suitable one. In addition, traits such as (a) the presence of protective secondary endosymbionts, for example, Hamiltonella defensa detected in Aphis fabae and Metopolophium dirhodum and (b) the sequestration of plant toxins as defense mechanism against parasitism, for example, in Aphis nerii and Brevicoryne brassicae, were likely at play to some extent in narrowing parasitoid host range. The lack of PPH relationship involved a low selectivity leading to a high adaptability, as well as selection pressure; the combination of which enabled the production of offspring in a new host species or a new environment. Testing for PPH relationships in parasitoids may provide useful cues to classify parasitoids in terms of specialization degree.

Consequence of emergence pattern on inbreeding risk in the aphid parasitoid Aphidius matricariae (Hymenoptera: Braconidae)

In insect parasitoids, mating strategy depends on mate availability and is influenced by the spatial and temporal emergence patterns of adults. For quasi-gregarious species, simultaneous emergence favors local mating and reduces search costs for partners while increasing the risk of inbreeding, particularly when only one female parasitizes the initial host patch. Consequently, in inbreeding sensitive species, mating on the place of adult emergence (patch mating) between siblings should be counter selected. In practice, the timing of male and female emergence and of their dispersal influences mate availability and can limit on patch mating. To test the role of these two factors, we analyzed the daily distribution of emergence and patch residence time of a cohort in the aphid parasitoid Aphidius matricariae (Hymenoptera: Braconidae). We observed that adult emergence is concentrated on the morning with males emerging on average before females with some overlaps. A more precise evaluation of emergence pattern within a brood suggests that brothers and sisters rarely emerge at the same time and rapid dispersal of males and females favors off-patch mating. The relationships between timing of emergence including differences between sex and consequences on inbreeding probability in these species are thus discussed.

Natural Occurrence of Secondary Bacterial Symbionts in Aphids from Tunisia, with a Focus on Genus Hyalopterus

Aphids (Hemiptera: Aphididae) can harbor two types of bacterial symbionts. In addition to the obligate endosymbiont Buchnera aphidicola Munson, Baumann and Kinsey 1991 (Enterobacteriales: Enterobacteriaceae), several facultative symbiotic bacteria, called secondary (S) symbionts, have been identified among many important pest aphid species. To determine interpopulational diversity of S-symbionts, we carried out a survey in a total of 18 populations of six aphid species collected from six localities in Tunisia, by performing a diagnostic polymerase chain reaction analysis of partial 16S-23S rRNA operon sequences. While 61.7% of individuals contained only Buchnera, three S-symbionts were found at different frequencies. Arsenophonus sp. Gherna et al. 1991 (Enterobacteriales: Enterobacteriaceae) was found in all species under study except for Acyrtosiphon pisum (Harris 1776) (Aphidinae: Macrosiphini); Serratia symbiotica Moran et al. 2005 (Enterobacteriales: Enterobacteriaceae) was present in all analyzed individuals of A. pisum but only sporadically in Aphis spiraecola (Patch 1914) (Aphidinae: Aphidini) and Hyalopterus amygdali (Blanchard 1840) (Aphidinae: Aphidini), while Hamiltonella defensa Moran et al. 2005 (Enterobacteriales: Enterobacteriaceae) was found in all analyzed individuals of one population of Aphis gossypii (Glover 1877) (Aphidinae: Aphidini) and sporadically in two populations of Hyalopterus. The lysogenic bacteriophage APSE-1 (A. pisum secondary endosymbiont, type 1) was detected in the three populations infected with H. defensa. This bacteriophage has been associated with moderate protection against braconid parasitoids in pea aphids. The high prevalence of Arsenophonus sp. in our samples is in accordance with previous studies indicating that, among gammaproteobacteria, this genus is one of the most widespread insect facultative symbionts.

Ecological specialization in Diaeretiella rapae (Hymenoptera: Braconidae: Aphidiinae) on aphid species from wild and cultivated plants

Diaeretiella rapae is an aphid parasitoid with potential for use in biological control strategies. However, several recent genetic studies have challenged the long held view that it is a generalist parasitoid. We investigated its ecological specialization and ability to use resources in cultivated and uncultivated areas. Ecological specialization would reduce its ability to exploit the diversity of aphid species, particularly in uncultivated areas, and to control pest aphids. Four D. rapae strains were studied, three reared on pest aphids on Brassicaceae and one strain on a non-pest aphid on Chenopodiaceae. For each strain, we performed host-switching experiments, with a total of six aphid species, five of which D. rapae parasitizes in France. We tested cross-breeding ability between strains to detect potential reproductive isolation linked to aphid host species in D. rapae. The strain reared on non-pest aphids was able to develop on aphid species from both cultivated and uncultivated plants. The strains reared on pest aphids, however, exclusively parasitized aphid species on cultivated Brassicaceae. In addition, reproductive isolation was detected between strains from uncultivated and cultivated plants. Thus, the D. rapae populations examined here appear to be showing ecological specialization or they may even be composed of a complex of cryptic species related to the aphid hosts. The role of Chenopodium album as a reservoir for D. rapae, by providing a habitat for non-pest aphids on which it can feed, appears to be severely limited, and thus its efficiency to maintain local populations of D. rapae in the vicinity of crops is questionable.

Host species exploitation and discrimination by animal parasites

Parasite species often show differential fitness on different host species. We developed an equation-based model to explore conditions favouring host species exploitation and discrimination. In our model, diploid infective stages randomly encountered hosts of two species; the parasite's relative fitness in exploiting each host species, and its ability to discriminate between them, was determined by the parasite's genotype at two independent diallelic loci. Relative host species frequency determined allele frequencies at the exploitation locus, whereas differential fitness and combined host density determined frequency of discrimination alleles. The model predicts instances where populations contain mixes of discriminatory and non-discriminatory infective stages. Also, non-discriminatory parasites should evolve when differential fitness is low to moderate and when combined host densities are low, but not so low as to cause parasite extinction. A corollary is that parasite discrimination (and host-specificity) increases with higher combined host densities. Instances in nature where parasites fail to discriminate when differential fitness is extreme could be explained by one host species evolving resistance, following from earlier selection for parasite non-discrimination. Similar results overall were obtained for haploid extensions of the model. Our model emulates multi-host associations and has implications for understanding broadening of host species ranges by parasites.This article is part of the themed issue 'Opening the black box: re-examining the ecology and evolution of parasite transmission'.

Evaluation of Aphis glycines as an Alternative Host for Supporting Aphelinus albipodus Against Myzus persicae on Capsicum annuum cv. Ox Horn and Hejiao 13

Bank plant systems provide effective biological control for pests infesting commercially important crops. Aphids cause physical damage to crops by feeding on the leaves, as well as transmitting damaging viral diseases. To develop a bank plant system to control aphids that damage vegetable crops, we initially reared the parasitoid Aphelinus albipodus (Hayat and Fatima) on the soybean aphid, Aphis glycines (Matsumura) reared on the soybean plant, Glycine max (L.) that was elected as the alternate host. Parasitoid adults that emerged from A. glycines were allowed to parasitize second instar nymphs of the aphid Myzus persicae (Sulzer) which were reared on sweet pepper and chili pepper leaves. The results showed that A. albipodus females feeding and parasitizing M. persicae nymphs reared on sweet pepper lived for 18.9 days, with an average fecundity of 337.3 progenies/female, while females feeding and parasitizing on M. persicae nymphs reared on chili pepper lived for 18.8 days, with an average fecundity of 356.2 progenies/female. There were no significant difference in the development time and reproduction of A. albipodus individuals parasitizing M. persicae nymphs reared on sweet pepper and chili pepper plants. The intrinsic rate of increase (r), net reproductive rate (R ₀), net aphid killing rate (Z ₀), and finite aphid killing rate (θ) of A. albipodus parasitizing sweet pepper and chili pepper M. persicae was 0.2258 days^-1, 171.7 progeny adults, 222.6 aphids, and 0.4048 and 0.2295 days^-1, 191.8 progeny adults, 243.3 aphids, and 0.4021, respectively. Our results suggested that A. glycines could serve as an effective alternative host for supporting A. albipodus against M. persicae infesting sweet pepper and chili pepper.

Brief Heat Stress Negatively Affects the Population Fitness and Host Feeding of Aphelinus asychis (Hymenoptera: Aphelinidae) Parasitizing Myzus persicae (Hemiptera: Aphididae)

Aphelinus asychis Walker (Hymenoptera: Aphelinidae), a polyphagous parasitoid, has been widely used as a biological control agent against Myzus persicae (Sulzer) (Hemiptera: Aphididae) and other aphid species. In order to ensure the successful biological control of M. persicae, we evaluated the influence of brief heat stresses (32.5, 35.0, 37.5, 40.0, and 41.5°C for 1 h), which occur frequently during the summer or under greenhouse conditions, on survival, longevity, host feeding, and parasitism of A. asychis that were reared on chili pepper at 25°C in the laboratory. Our results showed several major consequences on the parasitoid adults after brief heat stress. First, the survival of briefly heat stress-treated A. asychis female and male adults decreased significantly at ≥37.5°C and their longevity decreased as well. Second, the number of M. persicae nymphs infesting chili pepper killed through host feeding by the treated A. asychis females decreased significantly. Third, the cumulative number of mummified aphids and female progeny produced by the treated A. asychis also decreased significantly at ≥37.5°C. Our results indicated that a brief heat stress on A. asychis adults caused severe effects on major life history traits and total numbers of hosts killed by host feeding and parasitism, suggesting that high temperatures should be avoided to ensure the success of biological control of M. persicae and other aphid species using this parasitoid species.

Influence of temperature on patch residence time in parasitoids: physiological and behavioural mechanisms

Patch time allocation has received much attention in the context of optimal foraging theory, including the effect of environmental variables. We investigated the direct role of temperature on patch time allocation by parasitoids through physiological and behavioural mechanisms and its indirect role via changes in sex allocation and behavioural defences of the hosts. We compared the influence of foraging temperature on patch residence time between an egg parasitoid, Trichogramma euproctidis, and an aphid parasitoid, Aphidius ervi. The latter attacks hosts that are able to actively defend themselves, and may thus indirectly influence patch time allocation of the parasitoid. Patch residence time decreased with an increase in temperature in both species. The increased activity levels with warming, as evidenced by the increase in walking speed, partially explained these variations, but other mechanisms were involved. In T. euproctidis, the ability to externally discriminate parasitised hosts decreased at low temperature, resulting in a longer patch residence time. Changes in sex allocation with temperature did not explain changes in patch time allocation in this species. For A. ervi, we observed that aphids frequently escaped at intermediate temperature and defended themselves aggressively at high temperature, but displayed few defence mechanisms at low temperature. These defensive behaviours resulted in a decreased patch residence time for the parasitoid and partly explained the fact that A. ervi remained for a shorter time at the intermediate and high temperatures than at the lowest temperature. Our results suggest that global warming may affect host-parasitoid interactions through complex mechanisms including both direct and indirect effects on parasitoid patch time allocation.

Longitudinal clines in the frequency distribution of 'super-clones' in an aphid crop pest

Parthenogenesis is the main mode of reproduction of aphids. Their populations are therefore composed of clones whose frequency distribution varies in space and time. Previous population genetic studies on aphids have highlighted the existence of highly abundant clones ('super-clones'), distributed over large geographic areas and persisting over time. Whether the abundance of 'super-clones' results from their ecological success or from stochastic forces, such as drift and migration, is an open question. Here, we looked for the existence of clines in clonal frequency along a climatic gradient in the cereal aphid Rhopalosiphum padi (Linnaeus, 1758) and examined the possible influence of geographical distance and environmental variables in the buildup and maintenance of such clonal clines. We investigated the spatial distribution of the commonest genotypes of R. padi by sampling populations along an east-west transect in maize fields in the northern half of France in both spring and late summer. Individual aphids were genotyped at several polymorphic loci, allowing the assessment of frequency distributions of multilocus genotypes (MLGs) across the cropping season. We found several MLGs showing longitudinal clines in their frequency distribution in both spring and summer. In particular, two dominant asexual genotypes of R. padi showed inverted geographical clines, which could suggest divergent adaptations to environmental conditions. We concluded that while the distribution of some 'super-clones' of R. padi seems most likely driven by the action of migration and genetic drift, selection could be also involved in the establishment of longitudinal clines of others.

Ecological Interactions Affecting the Efficacy of Aphidius colemani in Greenhouse Crops

Aphidius colemani Viereck (Hymenoptera: Braconidae) is a solitary endoparasitoid used for biological control of many economically important pest aphids. Given its widespread use, a vast array of literature on this natural enemy exists. Though often highly effective for aphid suppression, the literature reveals that A. colemani efficacy within greenhouse production systems can be reduced by many stressors, both biotic (plants, aphid hosts, other natural enemies) and abiotic (climate and lighting). For example, effects from 3rd and 4th trophic levels (fungal-based control products, hyperparasitoids) can suddenly decimate A. colemani populations. But, the most chronic negative effects (reduced parasitoid foraging efficiency, fitness) seem to be from stressors at the first trophic level. Negative effects from the 1st trophic level are difficult to mediate since growers are usually constrained to particular plant varieties due to market demands. Major research gaps identified by our review include determining how plants, aphid hosts, and A. colemani interact to affect the net aphid population, and how production conditions such as temperature, humidity and lighting affect both the population growth rate of A. colemani and its target pest. Decades of research have made A. colemani an essential part of biological control programs in greenhouse crops. Future gains in A. colemani efficacy and aphid biological control will require an interdisciplinary, systems approach that considers plant production and climate effects at all trophic levels.

Could behaviour and not physiological thermal tolerance determine winter survival of aphids in cereal fields?

Traits of physiological thermotolerance are commonly measured in the laboratory as predictors of the field success of ectotherms at unfavourable temperatures (e.g. during harsh winters, heatwaves, or under conditions of predicted global warming). Due to being more complicated to measure, behavioural thermoregulation is less commonly studied, although both physiology and behaviour interact to explain the survival of ectotherms. The aphids Metopolophium dirhodum, Rhopalosiphum padi and Sitobion avenae are commercially important pests of temperate cereal crops. Although coexisting, these species markedly differ in winter success, with R. padi being the most abundant species during cold winters, followed by S. avenae and lastly M. dirhodum. To better understand the thermal physiology and behavioural factors contributing to differential winter success, the lethal temperature (physiological thermotolerance) and the behaviour of aphids in a declining temperature regime (behavioural thermotolerance) of these three species were investigated. Physiological thermotolerance significantly differed between the three species, with R. padi consistently the least cold tolerant and S. avenae the most cold tolerant. However, although the least cold tolerant of the study species, significantly more R. padi remained attached to the host plant at extreme sub-zero temperatures than S. avenae and M. dirhodum. Given the success of anholocyclic R. padi in harsh winters compared to its anholocyclic counterparts, this study illustrates that behavioural differences could be more important than physiological thermotolerance in explaining resistance to extreme temperatures. Furthermore it highlights that there is a danger to studying physiological thermotolerance in isolation when ascertaining risks of ectotherm invasions, the establishment potential of exotic species in glasshouses, or predicting species impacts under climate change scenarios.

Aphid-encoded variability in susceptibility to a parasitoid

BACKGROUND

Many animals exhibit variation in resistance to specific natural enemies. Such variation may be encoded in their genomes or derived from infection with protective symbionts. The pea aphid, Acyrthosiphon pisum, for example, exhibits tremendous variation in susceptibility to a common natural enemy, the parasitic wasp Aphidius ervi. Pea aphids are often infected with the heritable bacterial symbiont, Hamiltonella defensa, which confers partial to complete resistance against this parasitoid depending on bacterial strain and associated bacteriophages. That previous studies found that pea aphids without H. defensa (or other symbionts) were generally susceptible to parasitism, together with observations of a limited encapsulation response, suggested that pea aphids largely rely on infection with H. defensa for protection against parasitoids. However, the limited number of uninfected clones previously examined, and our recent report of two symbiont-free resistant clones, led us to explicitly examine aphid-encoded variability in resistance to parasitoids.

RESULTS

After rigorous screening for known and unknown symbionts, and microsatellite genotyping to confirm clonal identity, we conducted parasitism assays using fifteen clonal pea aphid lines. We recovered significant variability in aphid-encoded resistance, with variation levels comparable to that contributed by H. defensa. Because resistance can be costly, we also measured aphid longevity and cumulative fecundity of the most and least resistant aphid lines under permissive conditions, but found no trade-offs between higher resistance and these fitness parameters.

CONCLUSIONS

These results indicate that pea aphid resistance to A. ervi is more complex than previously appreciated, and that aphids employ multiple tactics to aid in their defense. While we did not detect a tradeoff, these may become apparent under stressful conditions or when resistant and susceptible aphids are in direct competition. Understanding sources and amounts of variation in resistance to natural enemies is necessary to understand the ecological and evolutionary dynamics of antagonistic interactions, such as the potential for coevolution, but also for the successful management of pest populations through biological control.

The evolutionary ecology of symbiont-conferred resistance to parasitoids in aphids

Aphids may harbor a wide variety of facultative bacterial endosymbionts. These symbionts are transmitted maternally with high fidelity and they show horizontal transmission as well, albeit at rates too low to enable infectious spread. Such symbionts need to provide a net fitness benefit to their hosts to persist and spread. Several symbionts have achieved this by evolving the ability to protect their hosts against parasitoids. Reviewing empirical work and some models, I explore the evolutionary ecology of symbiont-conferred resistance to parasitoids in order to understand how defensive symbiont frequencies are maintained at the intermediate levels observed in aphid populations. I further show that defensive symbionts alter the reciprocal selection between aphids and parasitoids by augmenting the heritable variation for resistance, by increasing the genetic specificity of the host-parasitoid interaction, and by inducing environment-dependent trade-offs. These effects are conducive to very dynamic, symbiont-mediated coevolution that is driven by frequency-dependent selection. Finally I argue that defensive symbionts represent a problem for biological control of pest aphids, and I propose to mitigate this problem by exploiting the parasitoids' demonstrated ability to rapidly evolve counteradaptations to symbiont-conferred resistance.

Identification of the main venom protein components of Aphidius ervi, a parasitoid wasp of the aphid model Acyrthosiphon pisum

Background

Endoparasitoid wasps are important natural enemies of the widely distributed aphid pests and are mainly used as biological control agents. However, despite the increased interest on aphid interaction networks, only sparse information is available on the factors used by parasitoids to modulate the aphid physiology. Our aim was here to identify the major protein components of the venom injected at oviposition by Aphidius ervi to ensure successful development in its aphid host, Acyrthosiphon pisum.

Results

A combined large-scale transcriptomic and proteomic approach allowed us to identify 16 putative venom proteins among which three γ-glutamyl transpeptidases (γ-GTs) were by far the most abundant. Two of the γ-GTs most likely correspond to alleles of the same gene, with one of these alleles previously described as involved in host castration. The third γ-GT was only distantly related to the others and may not be functional owing to the presence of mutations in the active site. Among the other abundant proteins in the venom, several were unique to A. ervi such as the molecular chaperone endoplasmin possibly involved in protecting proteins during their secretion and transport in the host. Abundant transcripts encoding three secreted cystein-rich toxin-like peptides whose function remains to be explored were also identified.

Conclusions

Our data further support the role of γ-GTs as key players in A. ervi success on aphid hosts. However, they also evidence that this wasp venom is a complex fluid that contains diverse, more or less specific, protein components. Their characterization will undoubtedly help deciphering parasitoid-aphid and parasitoid-aphid-symbiont interactions. Finally, this study also shed light on the quick evolution of venom components through processes such as duplication and convergent recruitment of virulence factors between unrelated organisms.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-342) contains supplementary material, which is available to authorized users.

A non-destructive method for testing two components of the behaviour of soil-applied agricultural chemicals over a long period

BACKGROUND

Owing to the complexity of soil composition, accurate predictions of both apoplastic systemicity of lipophilic xenobiotics and their leaching from the soil are made difficult. Therefore, a non-destructive method to assess directly these two components of the spatial behaviour of soil-applied phytochemicals is needed.

RESULTS

The plant selected was a dwarf tomato, which can exude an abundant apoplastic fluid through large stomata for several months. The feasibility and reliability of the method were assayed using three triazoles exhibiting different log D values. HPLC-MS analyses indicated that triadimenol (log D = 2.97) was clearly the most mobile compound within the apoplast, especially its diastereoisomer A. Propiconazole (log D = 3.65) and penconazole (log D = 4.64) exhibited a similar low systemicity. The data remained the same when the three fungicides were applied together on the soil. Long time-course studies (1.5 months) of penconazole behaviour indicated that, in contrast to leaching, which decrease sharply, root-to-shoot translocation remained almost unchanged during the whole experiment, in spite of the high lipophilicity of this fungicide.

CONCLUSION

This method must contribute to a better knowledge of the behaviour of commercial soil-applied phytochemicals. It can also be used to screen new xenobiotics within strategies to satisfy environmental requirements.

A universal method for the detection and identification of Aphidiinae parasitoids within their aphid hosts

Molecular methods are increasingly used to detect and identify parasites in their hosts. However, existing methods are generally not appropriate for studying complex host-parasite interactions because they require prior knowledge of species composition. DNA barcoding is a molecular method that allows identifying species using DNA sequences as an identification key. We used DNA amplification with primers common to aphid parasitoids and sequencing of the amplified fragment to detect and identify parasitoids in their hosts, without prior knowledge on the species potentially present. To implement this approach, we developed a method based on 16S rRNA mitochondrial gene and LWRh nuclear gene. First, we designed two primer pairs specific to Aphidiinae (Hymenoptera), the main group of aphid parasitoids. Second, we tested whether the amplified regions could correctly identify Aphidiinae species and found that 61 species were accurately identified of 75 tested. We then determined the ability of each primer pair to detect immature parasitoids inside their aphid host. Detection was earlier for 16S than for LWRh, with parasitoids detected, respectively, 24 and 48 h after egg injection. Finally, we applied this method to assess parasitism rate in field populations of several aphid species. The interest of this tool for analysing aphid-parasitoid food webs is discussed.

Aphid alarm pheromone: an overview of current knowledge on biosynthesis and functions

Aphids are important agricultural and forest pests that exhibit complex behaviors elicited by pheromonal signals. The aphid alarm pheromone--of which (E)-β-farnesene is the key (or only) component in most species--plays important roles in mediating interactions among individuals as well as multitrophic interactions among plants, aphids, and aphid natural enemies. Though many important questions remain to be answered, a large body of research has addressed various aspects of the biology, physiology, and ecology of aphid alarm pheromones. Here we review recent advances in our understanding of (a) the identity and composition of aphid alarm signals; (b) their biosynthesis and production; (c) their effects on conspecifics; (d) their role as cues for other insect species; and (e) their potential application for the management of pest organisms.

Effect of the Drosophila endosymbiont Spiroplasma on parasitoid wasp development and on the reproductive fitness of wasp-attacked fly survivors

In a previous study, we showed that Spiroplasma, a maternally transmitted endosymbiotic bacterium of Drosophila hydei, enhances larval to adult survival of its host when exposed to oviposition attack by the parasitoid wasp Leptopilina heterotoma. The mechanism by which Spiroplasma enhances host survival has not been elucidated. To better understand this mechanism, we compared the growth of wasp larvae in Spiroplasma-infected and uninfected hosts. Our results indicate that wasp embryos in Spiroplasma-infected hosts hatch and grow normally for ~2 days, after which their growth is severely impaired, compared to wasps developing in uninfected hosts. Thus, despite their reduced ability to complete development in Spiroplasma-infected hosts, developing wasps may exert fitness costs on their hosts that are manifested after host emergence. The severity of these costs will influence the degree to which this protective mechanism contributes to the long-term persistence of Spiroplasma in D. hydei. We therefore examined survival to 10-day-old adult stage and fecundity of Spiroplasma-infected flies surviving a wasp treatment. Our results suggest detrimental effects of wasp attack on longevity of Spiroplasma-infected adult flies. However, compared to Spiroplasma-free flies exposed to wasps, Spiroplasma-infected flies exposed to wasps have ~5 times greater survival from larva to 10 day-adult. The relative fecundity of wasp-attacked Spiroplasma-infected females was ~71% that of un-attacked Spiroplasma-free females. Our combined survival and female fecundity results suggest that under high wasp parasitism, the reproductive fitness of Spiroplasma-infected flies may be ~3.5 times greater than that of uninfected females, so it is potentially relevant to the persistence of Spiroplasma in natural populations of D. hydei. Interestingly, Spiroplasma-infected males surviving a wasp attack were effectively sterile during the 3-day period examined. This observation is consistent with the expectation that, as a maternally transmitted symbiont, there is little selective pressure on Spiroplasma to enhance the reproductive fitness of its male hosts.

The conflicting relationships between aphids and men: a review of aphid damage and control strategies

In this review, after giving some figures on the economic impact of aphids on agricultural production, we describe the different mechanisms leading to yield losses (direct damage due to sieve drain and plant reaction, indirect damage, often the most important, due to virus transmission). Then, after a history of chemical control and of its limits, the main control strategies (chemical control with decision rules, plant resistance, biological control, farming practices) are reviewed in the light of an integrated pest management approach. Several topics tackled in this article are exemplified for cereal aphids, which are among the most important in Europe as direct feeders and virus vectors.