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

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

Distribution in time and space of resistance to the pathogenic fungus Erynia neoaphidis in the pea aphid Acyrthosiphon pisum

Two biotypes of pea aphid, Acyrthosiphon pisum, have been recognized in Australia, by their susceptibility (S) or resistance (R) to certain isolates of the fungal pathogen Erynia neoaphidis. The responses of these two biotypes to 40 isolates of the fungus have shown that the R form is largely confined to two States, New South Wales and Victoria, but appears to have recently spread into Queensland and Tasmania. There is no evidence to suggest it occurs outside Australia. Sequential sampling of two field populations of pea aphids during 1981 and 1982 showed that the proportion of R form remained stable at 10.7±3.0 and 14.6±2.6% (mean±standard error) for the two populations. Glasshouse competition experiments run at the comparatively high temperature of 25°C resulted in the R form becoming dominant even when the initial ratio was 4:1 in favour of the S form. The ecological and genetical implications are discussed.

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Résumé Deux types biologiques d'Acyrthosiphon pisum ont été définis, en Australie, suivant leur sensibilité (S) ou leur résistance (R) à certains isolats du champignon pathogène Erynia neoaphidis. Les réponses des deux biotypes à 40 isolats du champignon ont montré que la forme R est essentiellement cantonnée à deux états: Nouvelle Galle due Sud et Victoria, mais a récemment gagné le Queensland et la Tasmanie. Aucun élément ne fait dire qu'elle existe hors d'Australie. Des échantillonnages séquentiels des deux populations de pucerons dans la nature en 1981 et 1982 ont montré que la proportion de la forme R est restée stable à 10,7 et 14,62 dans les deux populations. Des expériences de compétition en serre à la température relativement haute de 25°C ont rendu la forme R dominante, même quand le rapport initial était de 4/1 en faveur de la forme S. La discussion porte sur les conséquences écologiques et génétiques.

     

Population level effects of cadmium and the insecticide imidacloprid to the parasitoid, Aphidius ervi after exposure through its host, the pea aphid, Acyrthosiphon pisum (Harris)

The objective of this study was to assess the influence of toxic substances with different modes of action on a two-species system: an aphid-specific parasitoid, Aphidius ervi Haliday, feeding on the pea aphid, Acyrthosiphon pisum (Harris). The instantaneous rate of population increase (r_i) was used as a measure of population level toxic effect in this study. The toxicants evaluated were imidacloprid, a nonpersistent neurotoxic insecticide, and cadmium, a chronic pollutant with a tendency to accumulate. We evaluated the effects of cadmium and imidacloprid on A. pisum and A. ervi because both toxicants can occur together in polluted areas where crops are grown. Cadmium (200 or 400 mg kg⁻¹ dry weight soil) and imidacloprid (4 or 40 g a.i. ha⁻¹) were applied to soil contained in plastic pots in which broad bean plants, Vicia faba L., were grown. Results of this study indicated that cadmium at the concentrations tested, reduced population growth rate of the pea aphid. Imidacloprid also reduced aphid growth rate, but only at the highest concentration tested (40 g a.i. ha⁻¹). Combinations of cadmium and imidacloprid had the greatest impact on aphid growth rate. Imidacloprid alone had no effect on population growth rate of the parasitoid. However, cadmium alone or in combination with imidacloprid had a negative impact on A. ervi by reducing population growth rate 77%. These results indicate that negative impacts on parasitoids may occur in areas where cadmium contamination is present and imidacloprid is used to control aphids.     

Extraordinary proliferation of microorganisms in aposymbiotic pea aphids,Acyrthosiphon pisum

Aposymbiotic pea aphids, which were deprived of their intracellular symbiotic bacterium, Buchnera, exhibit growth retardation and no fecundity. High performance liquid chromatographic (HPLC) analysis revealed that these aposymbiotic aphids, when reared on broad bean plants, accumulated a large amount of histamine. To assess the possibility of extraordinary proliferation of microorganisms other than Buchnera, we enumerated eubacteria and fungi in aphids using the real-time quantitative PCR method that targets genes encoding small-subunit rRNAs. The result showed that these microorganisms were extremely abundant in the aposymbiotic aphids reared on plants. Microbial communities in aposymbiotic aphids were further profiled by phylogenetic analysis of small-subunit rDNAs. Of 172 nonchimeric sequences of fungal 18S rDNAs, 138 (80.2%) belonged to the phylum Ascomycota. Among them, 21 clustered within a monophyletic group consisting of insect-pathogenic fungi and yeast-like symbionts of homopteran insects. Thirty-one (18.0%), two (1.2%), and one (0.6%) clones were clustered within the Basidiomycota, Zygomycota, and Oomycota, respectively. Of 167 nonchimeric sequences of eubacterial 16S rDNAs, 84 (50.3%) belonged to the gamma-subdivision of Proteobacteria to which most primary endosymbionts of insects and prolific histamine producers belong. Forty (24.0%), 25 (15.0%), 10 (6.0%), and five (3.0%) clones were clustered within alpha-Proteobacteria, Cytophaga-Flavobacterium-Bacteroides (CFB) group, Actinobacteria, and beta-Proteobacteria, respectively. Three had no phylogenetic association with known taxonomic divisions. None of the sequences studied in this study coincided exactly with those deposited in GenBank.     

Juvenile hormone effects on polymorphism in the pea aphid, Acyrthosiphon pisum

When reared in short days (LD 12:12) at 15°C, apterous Acyrthosiphon pisum gave birth to sexual females (oviparae) exclusively for the first eight days of larviposition. After this time they switched to the production of parthenogenetic females (viviparae). Topical application of juvenile hormones I, II and III to fourth instar or adult ovipara-producers induced the precocious appearance of parthenogenetic females in the progeny sequence. Various forms intermediate between oviparae and viviparae were also produced and repetitive JH-I treatments resulted in a few alatiform progeny. However, many of the JH induced apterous, parthenogenetic females appeared to be normal viviparae and were capble of reproduction. Thus, prenatal treatment of oviparous embryos with JH diverts development towards the viviparous form. JH-I treatment of long-day reared A. pisum had no effect on the type of progeny produced.

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Effets de l'hormone juvénile sur le polymorphisme d'Acythosiphon pisum

Résumé Quand il est élevé sous jours courts (LN 12/12) à 15°C, le type anglais vert d'Acyrthosiphon pisum ne donne naissance qu'à des femelles sexuées (ovipares) pendant la première partie de sa période de reproduction. Ensuite quelques types intermédiaires ovipares/vivipares peuvent apparaitre avant que les pucerons ne bifurquent spontanément vers la production de femelles parthénogénétiques (vivipares). L'application cutanée d'hormones juvéniles (JH I, II, et III) aux larves de quatrième stade ou à des adultes producteurs d'ovipares provoque l'apparition prématurée d'intermédiaires et de vivipares dans la descendance. Les différentes formes intermédiaires produites par des applications répétées de J.H. comprenaient des types ailés ou partiellement ailés. Cependant, les vivipares aptères induits par J.H. étaient morphologiquement normaux et beaucoup étaient capables de se reproduire. Des traitements semblables aux J.H. de vivipares élevées en jours longs (LN 16/8) n'ont pas eu d'effets sur le type de la descendance.On ne sait pas si l'action de JH exogène sur l'induction des vivipares est direct ou indirect. La reprogrammation des embryons, autrement destinés à se développer comme ovipares, est examinée en relation avec notre connaissance du contrôle endocrine du polymorphisme des pucerons.

     

Performance of pea aphid (Acyrthosiphon pisum) clones on host plants and synthetic diets mimicking the same plants phloem amino acid composition

The performances of three clones of pea aphids, with different host affiliations, were evaluated on four host plants species and on four artificial diets. The amino acid compositions of the diets mimicked those of the phloem sap of the respective host plants. The total concentration of amino acids was the same in all the diets. The pea aphid clones performance were significantly affected by amino acid composition of the diets in different ways, implying physiological and/or behavioural differences among coexisting pea aphid clones in response to amino acids in artificial diets. The observed differences in performance on diets between clones were not related to host plant affiliations. Thus, even if the variation in amino acid composition in phloem sap among the host plants affects the pea aphid clones when tested on artificial diets, this variation has no observable effect on pea aphid performance on natural host plants.     

Comparative Prey Suitability ofOstrinia nubilalisEggs andAcyrthosiphon pisumforColeomegilla maculata

The effects ofOstrinia nubilalis(Hübner) (Lepidoptera: Pyralidae) eggs andAcyrthosiphon pisum(Harris) (Homoptera: Aphididae), when provided as single prey species and in combination, on life history characteristics ofColeomegilla maculataDeGeer (Coleoptera: Coccinellidae) larvae and adults were quantified. Preimaginal development was not influenced by the larval prey regime; development at 26 ± 1°C was completed in approximately 13.5 days onO. nubilaliseggs,A. pisum,orA. pisumalternated daily withO. nubilaliseggs. The resulting adults weighed 13.0, 10.7, and 12.5 mg when reared onO. nubilaliseggs,A. pisum,andA. pisumalternated daily withO. nubilaliseggs, respectively. Eighteen percent of the individuals died when reared onA. pisum,28% died when reared onO. nubilaliseggs, and 22% died when fedA. pisumalternated daily withO. nubilaliseggs. Seven adult diet combinations, based on diet regimes of larvae and adults, did not cause significant differences in preoviposition period, interoviposition period, and the number of days on which eggs were laid. Total fecundity was influenced both by larval and adult diet. The diet that resulted in highly fecund females wasA. pisumalternated daily withO. nubilaliseggs for larvae andO. nubilaliseggs for adults. FemaleC. maculatafedO. nubilaliseggs had the highest intrinsic rate of increase and net reproductive rate.     

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

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

Instar-specific defense of the pea aphid,Acyrthosiphon pisum: Influence on oviposition success of the parasiteAphelinus asychis (Hymenoptera: Aphelmidae)

The parasite Aphelinus asychisWalker (Hymenoptera: Aphelinidae) oviposits in all four instars of the pea aphid, Acyrthosiphon pisum(Harris) (Homoptera: Aphididae). Searching females display a highly stereotyped sequence of behaviors when encountering a host. Once recognized, an aphid is examined and probed by the wasp with the everted ovipositor prior to oviposition. Oviposition success is influenced by aphid behavior that is related to aphid size and expressed through instarspecific escape and defense reactions. Being smaller and less able to defend themselves, first and early-second instars of pea aphid are more susceptible to successful parasitism than third and fourth instars, in that order. Observed patterns of preference by Aphelinus females for particular aphid species and instars reflect the outcome of behavioral interactions between the hosts and the parasites, rather than preference in the strict sense.     

Protein toxicity to aphids: Anin vitro test onAcyrthosiphon pisum

Recent progress in plant transformation for insect resistance has increased the interest in the potential toxicity of proteins towards insect pests. While studies have been targeted to a large array of insect species, phloem-feeding Homoptera have not been investigated yet. The paper describes a routine test for screening toxicity and growth inhibition of purified proteins in artificial diets onAcyrthosiphon pisum (Harris). Twenty-five commercially available proteins of different classes were tested and compared to some non-protein chemicals (an insecticide, an antibiotic …).A. pisum proved to be very sensitive to all proteases tested and to some venoms with general cytolytic properties. A plant lectin, concanavalin A, displayed significant toxicity and growth inhibition, while various proteins such as a soybean proteinase inhibitor, a chitinase, and bovine serum albumin showed measurable impairments of growth only at higher dose (≥250 μg.ml⁻¹). Some proteins were without short-term effect onA. pisum physiology. The influence of these results on aphid-plant interactions are discussed.

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Résumé L'effet de protéines alimentaires sur les insectes phloémophages, dont les pucerons, n'a jamais été étudié. Nous proposons ici un test biologique standardisé sur milieu artificiel permettant d'analyser les effets de différentes classes de protéines sur la physiologie d'A. pisum. La validité de ce test est éprouvée (protocole, reproductibilité) et les différentes données récoltées (mortalité et inhibition de croissance) permettent de définir des paramètres toxicologiques tels que concentration létale 50 ou concentration inhibitrice 50. Cette caractérisation toxicologique a été réalisée sur 25 protéines appartenant à des classes différentes, ainsi que plusieurs substances non protéiques utilisées comme témoin de toxicité (insecticide, antibiotique, inhibiteur de synthèse protéique et glucoside phénolique). Les regroupements de protéines par proximités de profils toxicologiques ont été corrélés aux activités biochimiques des différentes protéines. Les implications de ces résultats sur les interactions plante-puceron sont discutées, ainsi que le potentiel d'une stratégie de création de variétés transgéniques résistantes aux pucerons.

     

Insecticidal properties of Sclerotinia sclerotiorum agglutinin and its interaction with insect tissues and cells

This project studied in detail the insecticidal activity of a fungal lectin from the sclerotes of Sclerotinia sclerotiorum, referred to as S. sclerotiorum agglutinin or SSA. Feeding assays with the pea aphid (Acyrthosiphon pisum) on an artificial diet containing different concentrations of SSA demonstrated a high mortality caused by this fungal lectin with a median insect toxicity value (LC₅₀) of 66 (49–88) μg/ml. In an attempt to unravel the mode of action of SSA the binding and interaction of the lectin with insect tissues and cells were investigated. Histofluorescence studies on sections from aphids fed on an artificial liquid diet containing FITC-labeled SSA, indicated the insect midgut with its brush border zone as the primary target for SSA. In addition, exposure of insect midgut CF-203 cells to 25 μg/ml SSA resulted in a total loss of cell viability, the median cell toxicity value (EC₅₀) being 4.0 (2.4–6.7) μg/ml. Interestingly, cell death was accompanied with DNA fragmentation, but the effect was caspase-3 independent. Analyses using fluorescence confocal microscopy demonstrated that FITC-labeled SSA was not internalized in the insect midgut cells, but bound to the cell surface. Prior incubation of the cells with saponin to achieve a higher cell membrane permeation resulted in an increased internalization of SSA in the insect midgut cells, but no increase in cell toxicity. Furthermore, since the toxicity of SSA for CF-203 cells was significantly reduced when SSA was incubated with GalNAc and asialomucin prior to treatment of the cells, the data of this project provide strong evidence that SSA binds with specific carbohydrate moieties on the cell membrane proteins to start a signaling transduction cascade leading to death of the midgut epithelial cells, which in turn results in insect mortality. The potential use of SSA in insect control is discussed.     

Acyrthosiphon pisum performance in response to the sugar and amino acid composition of artificial diets, and its relation to lucerne varietal resistance

In order to understand the mechanisms of resistance of two lucerne cultivars (susceptible Resistador and resistant Lahontan clones) to a French biotype of Acyrthosiphon pisum Harris, several biological parameters of this aphid were investigated on sap-copied holidic diets differing either by their amino acid or sucrose concentrations or by the relative proportions of some amino acids.We recorded the quantitative influence of nutrient levels and amino acid profiles on adult survival and reproduction, as well as on larval growth and development. The differences in A. pisum performances on artificial diets may partly by explained by variable ingestion rates, but also by differences in metabolic efficiency of the ingested nutrients, which greatly depends on amino acid composition.Although holidic diet experiments partially confirm the biological assays of resistance on plants, they also point out a lack in our understanding of the factors present in the sap which are involved in phagostimulation in vivo, thus preventing a complete interpretation of observed resistance.

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Résumé Sur des milieux holidiques se différenciant par leur concentration en acides aminés ou en saccharose ou par leur spectre en acides aminés et copiant la sève de deux variétés de luzerne (sensible et résistante) nous avons étudié différents paramètres biologiques d'un biotype d'Acyrthosiphon pisum Harris afin de mieux comprendre les mécanismes de résistance à ce puceron.Que ce soit pour le reproduction et la survie des adultes ou pour la croissance et le développment larvaire, nous montrons l'influence spécifique de la concentration en saccharose et en acides aminés ainsi que celle du spectre en acides aminés des régimes artificiels. Ces différences de performances de A. pisum peuvent en partie être expliquées par les différences d'ingestion. Néanmoins pour des rapport saccharose/acides aminés identiques et pour des niveaux d'ingestion égaux, il existe selon les milieux de grandes différences de taux de reproduction et donc d'efficacités métaboliques.Les milieux copiant la sève de variétés sensible et résistante permettent jusqu'à présent d'expliquer une partie des observations faites sur végétal, mais démontrent que l'ignorance de certains facteurs phagostimulants présents dans la plante empêche une interprétation complète des effets observés.

     

Juvenile hormone titres and winged offspring production do not correlate in the pea aphid, Acyrthosiphon pisum

Pea aphids, Acyrthosiphon pisum, reproduce parthenogenetically and are wing-dimorphic such that offspring can develop into winged (alate) or unwinged (apterous) adults. Alate induction is maternal and offspring phenotype is entirely determined by changes in the physiology and environment of the mother. Juvenile hormones (JHs) have been implicated in playing a role in wing differentiation in aphids, however until recently, methods were not available to accurately quantify these insect hormones in small insects such as aphids. Using a novel LC-MS approach we were able to quantify JH III in pea aphids that were either producing a high proportion of winged morphs among their offspring or mainly unwinged offspring. We measured JH III titres by pooling the hemolymph of 12 or fewer individuals (1 μL hemolymph) treated identically. Levels of JH ranged from 30 to 163 pg/μL. While aphids in the two treatments strongly differed in the proportion of winged morphs among their offspring, their JH III titres did not differ significantly. There was also no correlation between JH III titre and the proportion of winged offspring in induced aphids. This supports earlier findings that wing dimorphism in aphids may be regulated by other physiological mechanisms.     

Effect of Adult Experience on in-Flight Orientation to Plant and Plant–Host Complex Volatiles inAphidius erviHaliday (Hymenoptera, Braconidae)

The effect of adult experience on in-flight orientation to plant–host complex volatiles byAphidius erviHaliday was studied in a wind tunnel bioassay, usingAcyrthosiphon pisum(Harris), maintained on broad bean plants (Vicia faba) as host. A short oviposition experience (15 s) on the plant–host complex (PHC) was sufficient to induce a drastic decrease of flight propensity and stimulated a foraging behavior characterized by intense walking activity. However, flight activity resumed to normal levels 1 h after the oviposition experience on the PHC occurred. For parasitoids conditioned on the PHC for at least 1 min the recorded proportion making oriented flights to the PHC was significantly higher than that for naive females. In contrast, oviposition experience in the absence of plant material did not influence theA. erviflight response. Oviposition attempts on aphid dummies without egg release did not reduce flight activity. WhenA. ervifemales were exposed to glass beads coated withAc. pisumcornicle secretion, a priming effect was observed, resulting, compared with naive females, in a significantly higher rate of oriented flights to the PHC. In contrast, oviposition attempts visually induced by colored aphid dummies did not influence flight behavior. A strong reaction to volatile cues from uninfested plants was induced by oviposition experience on newly infested broad bean plants. This appears to be a case of associative learning. In fact, uninfested broad bean plants are basically unattractive to naiveA. ervifemales. The results demonstrate that adult experience has a considerable influence onA. ervibehavior and may have important implications for biological control of natural pest aphid populations.     

Learning by the parasitoid wasp, Aphidius ervi (Hymenoptera: Braconidae), alters individual fixed preferences for pea aphid color morphs

Learning, defined as changes in behavior that occur due to past experience, has been well documented for nearly 20 species of hymenopterous parasitoids. Few studies, however, have explored the influence of learning on population-level patterns of host use by parasitoids in field populations. Our study explores learning in the parasitoid Aphidius ervi Haliday that attacks pea aphids, Acyrthosiphon pisum (Harris). We used a sequence of laboratory experiments to investigate whether there is a learned component in the selection of red or green aphid color morphs. We then used the results of these experiments to parameterize a model that examines whether learned behaviors can explain the changes in the rates of parasitism observed in field populations in South-central Wisconsin, USA. In the first of two experiments, we measured the sequence of host choice by A. ervi on pea aphid color morphs and analyzed this sequence for patterns in biased host selection. Parasitoids exhibited an inherent preference for green aphid morphs, but this preference was malleable; initial encounters with red aphids led to a greater chance of subsequent orientation towards red aphids than predicted by chance. In a second experiment, we found no evidence that parasitoids specialize on red or green morphs; for the same parasitoids tested in trials separated by 2 h, color preference in the first trial did not predict color preference in the second, as would be expected if they differed in fixed preferences or exhibited long-term (> 2 h) learning. Using data from the two experiments, we parameterized a population dynamics model and found that learning of the magnitude observed in our experiments leads to biased parasitism towards the most common color morph. This bias is sufficient to explain changes in the ratio of aphid color morphs observed in field sites over multiple years. Our study suggests that for even relatively simple organisms, learned behaviors may be important for explaining the population dynamics of their hosts.