Lipid composition of the pea aphid,Acyrthosiphon pisum (Harris) (homoptera: Aphididae), reared on host plant and on artificial media

Polar and neutral lipids and their constitutive fatty acids were quantified in the pea aphid, Acyrthosiphon pisum (Harris), grown on host plant or on a lipid free artificial diet. The results were compared to determine if lipids were involved in the suitability of the diet for continuous rearing of this A. pisum biotype. For apterous adults grown on plants, the lipids were characterized by a low amount of neutral lipids (2.5% weight/fresh weight) almost entirely (96.4%) composed of hexanoyl and sorboyl dimyristin. These storage lipids were higher in the alatae (3.8%), probably correlated with potential flight activity. The phospholipid amounts were identical in these two morphs (1.3–1.4% weight/fresh weight), comprised mainly of phosphatidylethanolamines (52%) and phosphatidylcholines (40.6%). These phospholipids contained a still unidentified fatty acid, with a retention time close to that of linolenic acid and synthesized by the aphid or its bacterial symbionts (not found in plants). The apterous adult aphids reared on an artificial diet showed an accumulation of neutral lipids (8.9% for the first generation); this increase was shown to be slightly greater for the hexanoyl and sorboyl triglycerides. In contrast, the phospholipids decreased in aphids reared on an artificial diet (1.1% and 0.9%, respectively, for first and second generation), correlated with a phospholipid fatty acid profile significantly deficient in C18:3 and in the unidentified peculiar fatty acid. These phospholipids are essential components of biological membranes and a diet-driven deficient synthesis in some of their components may result in the observed symptoms. © 1992 Wiley-Liss, Inc.     

Effect of entomopathogenic Aspergillus strains against the pea aphid,Acyrthosiphon pisum (Hemiptera: Aphididae)

Aphids (Homoptera: Aphididae) are sap-sucking insect pests that feed on several plants of agronomical importance. Entomopathogenic fungi are valuable tools for potential aphid control. As part of a selection process, laboratory bioassays were carried with five different concentrations of Aspergillus clavatus (Desmazières), Aspergillus flavus (Link) and Metarhizium anisopliae ((Metschnikoff) Sorokin) spores against the pea aphid, Acyrthosiphon pisum (Harris). Aspergillus isolates induced higher mortalities than M. anisopliae, which is a well-known entomopathogen in the literature. Lethal concentrations (LC₅₀ and LC₉₀) were 1.23 × 10³ and 1.34 × 10⁷ spores/ml for A. flavus, 4.95 × 10² and 5.65 × 10⁷ spores/ml for A. clavatus, and 3.67 × 10³ and 9.71 × 10⁷ spores/ml for M. anisopliae 5 days after treatment. Mycelia development and sporulation on adult cadavers were observed 48 h after incubation. The intrinsic growth rate of A. pisum decreased with increased spore concentration for all fungal strains, suggesting an increase in pathogen fitness related to a consumption of host resources. In conclusion, Aspergillus species could be useful in aphid control as pest control agents despite their saprophytic lifestyle. This is also to our knowledge the first report of A. clavatus and A. flavus strains pathogenic to aphids.     

Temporal changes in host adaptation in the pea aphid, Acyrthosiphon pisum

Abstract.

• 1 Temporal changes in host adaptation were followed in a local population of the pea aphid, Acyrthosiphon pisum. Aphid clones were collected in one alfalfa and one clover field at three different times. In the spring, first-generation females were collected. Later, in the autumn, females belonging to the last parthenogenetic generation were collected. Lastly, sexual females were collected after mating in autumn and allowed to produce eggs which were hatched. The performance was evaluated on alfalfa and clover. The spring-collected individuals were also assessed on peas.
• 2 On the overwintering hosts clover and alfalfa, the clones performed best on the plant of origin, i.e. negative correlations in performance. Correlations between performance on the temporary summer host, pea, and that on clover/alfalfa were weak or nonsignificant.
• 3 Significant variation in host performance was found within both host fields at spring, which is a prerequisite for changes in clone composition due to selection/migration.
• 4 The clones from alfalfa showed an increase in mean performance on alfalfa between spring and autumn, whereas no changes among the clones from the clover field were observed. This difference in seasonal response between the two fields could have been the result of larger variation in performance among the alfalfa clones and/or a differential tendency to migrate among clones in both fields.
• 5 After sexual recombination in the autumn, mean performance in the alfalfa field returned to the spring level, probably as a result of emergence of new genetic combinations. In the clover field, mean performance did not change significantly over time.

     

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

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

Effect of temperature and cultivar on pea aphid, Acyrthosiphon pisum (Hemiptera: Aphididae) life history

Life history parameters of the pea aphid, Acyrthosiphon pisum were studied at five constant temperatures on two cultivars of peas, Scout and Sancho. The development and mortality of juveniles and the life-span, age-specific fecundity and survivorship of adult aphids were recorded and used to construct life tables. The juvenile development period (from birth to adulthood) was longest at 11.9 degrees C (16.8 days on cv. Scout and 16.2 days on cv. Sancho) and shortest at 26.7 degrees C (8.5 days on cv. Scout and 8.8 days on cv. Sancho). At all temperatures, except 26.7 degrees C, juveniles developed faster on cv. Sancho than on cv. Scout. On both pea varieties juvenile mortality was highest at temperatures above 19.6 degrees C and lowest at 19.6 degrees C. Highest cumulative juvenile mortality was recorded on cv. Scout at 26.7 degrees C when only 9% of aphids survived from birth to reproductively mature adults. Fecundity rates were unaffected by temperature in the range tested on cv. Sancho but increased with increasing temperatures between 11.9 and 19.6 degrees C on cv. Scout. These differences in life history parameters were reflected in the population growth (rm) of aphids on both pea cultivars which increased with increasing temperatures between 11.9 and 23.1 degrees C on cv. Sancho and 11.9 and 19.6 degrees C on cv. Scout, declining thereafter. Population growth was consistently greater at all temperatures for aphids reared on cv. Sancho than those reared on cv. Scout.     

Bacteria from the alimentary canal of the pea aphid,Acyrthosiphon pisum (Harr.) (Homoptera,Aphididae)

Bacteria isolated from the alimentary canal of the pea aphid, Acyrthosiphon pisum, were identified mainly as species of the genera Sarcina, Micrococcus, Achromobacter, and Flavobacterium. Starch was rapidly hydrolysed by all species of Sarcina and Flavobacterium and by most species of Achromobacter and slowly by some species of Micrococcus. All of the types of bacteria found in the alimentary tract of the pea aphid were represented in the bacterial flora on the leaves of the pea plants on which the aphids were reared. It is suggested that these organisms become established in the gut after being picked up from the leaf surface while the aphid is probing for the phloem bundles.     

Intracellular symbiotes of the pea aphid,Acyrthosiphon pisum

Histological and ultrastructural studies on the mycetome of the pea aphid, Acyrthosiphon pisum, disclosed two types of symbiotes. The more common primary symbiotes were oval in shape and were found in large mycetocytes making up the bulk of the mycetome. The secondary symbiotes were smaller, rod-shaped, and were restricted to an apparently syncytial sheath partially enclosing the primary mycetocytes. Extensive rough endoplasmic reticulum and Golgi occurred in the sheath but not in the primary mycetocytes. Lysosomal breakdown occurred in both primary and secondary symbiotes but the two processes differed markedly. In the primary mycetocytes, a small number of symbiotes were broken down individually to form small, compact residual bodies. In the sheath, breakdown of secondary symbiotes was more extensive: large numbers were broken down within cytolysomes.     

Rapid response to intraclonal selection in the pea aphid (Acyrthosiphon pisum)

Summary Pea aphids show intraclonal variability in antipredator behaviour. Among the offspring of a single parthenogenetically reproducing female, some individuals drop from the plant in response to alarm pheromone while others remain on the plant. We demonstrate that this intraclonal behavioural variability can be altered by selection. The proportion of aphids dropping in response to alarm pheromone was significantly greater in lines in which this behaviour was selected than in clonally identical lines in which the opposite phenotype was favoured. This change occurred within one generation and could not be attributed to grand-maternal effects, nor to environmental effects. These results demonstrate the ability of clonal aphids to adapt to changes in the environment within a single generation.     

The secondary endosymbiotic bacterium of the pea aphid Acyrthosiphon pisum (Insecta: homoptera)

The secondary intracellular symbiotic bacterium (S-symbiont) of the pea aphid Acyrthosiphon pisum was investigated to determine its prevalence among strains, its phylogenetic position, its localization in the host insect, its ultrastructure, and the cytology of the endosymbiotic system. A total of 14 aphid strains were examined, and the S-symbiont was detected in 4 Japanese strains by diagnostic PCR. Two types of eubacterial 16S ribosomal DNA sequences were identified in disymbiotic strains; one of these types was obtained from the primary symbiont Buchnera sp., and the other was obtained from the S-symbiont. In situ hybridization and electron microscopy revealed that the S-symbiont was localized not only in the sheath cells but also in a novel type of cells, the secondary mycetocytes (S-mycetocytes), which have not been found previously in A. pisum. The size and shape of the S-symbiont cells were different when we compared the symbionts in the sheath cells and the symbionts in the S-mycetocytes, indicating that the S-symbiont is pleomorphic under different endosymbiotic conditions. Light microscopy, electron microscopy, and diagnostic PCR revealed unequivocally that the hemocoel is also a normal location for the S-symbiont. Occasional disordered localization of S-symbionts was also observed in adult aphids, suggesting that there has been imperfect host-symbiont coadaptation over the short history of coevolution of these organisms.     

Oral activity of FMRFamide-related peptides on the pea aphid Acyrthosiphon pisum (Hemiptera: Aphididae) and degradation by enzymes from the aphid gut

Insect myosuppressins and myosuppressin analogues were tested for oral toxicity against the pea aphid Acyrthosiphon pisum (Harris) by incorporation into an artificial diet. Acyrthosiphon pisum myosuppressin (Acypi-MS) and leucomyosuppressin (LMS) had significant dose-dependent effects (0.1-0.5μg peptide/μl diet) on feeding suppression, mortality, reduced growth and fecundity compared with control insects, but Acypi-MS was more potent than LMS. One hundred percent of aphids had died after 10days of feeding on 0.5μg Acypi-MS/μl diet whereas 40% of aphids feeding on 0.5μg LMS/μl diet were still alive after 13days. Myosuppressins were degraded by aphid gut enzymes; degradation was most likely due to a carboxypeptidase-like protease, an aminopeptidase and a cathepsin L cysteine protease. The estimated half-life of Acypi-MS in a gut extract was 30min, whereas LMS was degraded more slowly (t?=54min). No toxicity was observed when the analogues δR(9) LMS and citrolline(9) Acypi-MS or FMRFamide were fed to the pea aphid. These findings not only help to better understand the biological effects of myosuppressins in aphids but also demonstrate the potential use of myosuppressins in a strategy to control aphid pests.     

Spiroplasma symbiont of the pea aphid, Acyrthosiphon pisum (Insecta: Homoptera)

From a laboratory strain of the pea aphid, Acyrthosiphon pisum, we discovered a previously unknown facultative endosymbiotic bacterium. Molecular phylogenetic analysis based on 16S ribosomal DNA revealed that the bacterium is a member of the genus Spiroplasma. The Spiroplasma organism showed stable vertical transmission through successive generations of the host. Injection of hemolymph from infected insects into uninfected insects established a stable infection in the recipients. The Spiroplasma symbiont exhibited negative effects on growth, reproduction, and longevity of the host, particularly in older adults. Of 58 clonal strains of A. pisum established from natural populations in central Japan, 4 strains possessed the Spiroplasma organism.     

The economics of escape behaviour in the pea aphid,Acyrthosiphon pisum

Summary Pea aphids have several alternative responses to the detection of alarm pheromone produced by conspecifics. One of these, dropping from the feeding site to the ground, is potentially costly owing to the risk of desiccation-induced mortality on the ground before another host plant can be reached. Both dropping and walking from the feeding site incur a cost due to lost feeding opportunity. The aphids' decision as to which anti-predator tactic to use should be sensitive to the costs of their behaviour. Consequently, aphids should be less likely to drop when the risk of desiccation is higher, and less likely to drop or walk when the lost opportunity cost is higher. We tested these predictions by manipulating climatic severity (temperature and humidity) and host quality, respectively. As predicted, aphids are less likely to drop or walk in response to pheromone when feeding on high quality than on low quality hosts, and less likely to drop when the environment is hot and dry than when it is more benign. The latter is true whether the aphids are feeding on real or simulated leaves. Since all aphids were of the same clone, these results show that individual aphid genotypes possess the ability to adaptively modify their escape behaviour with changes in prevailing conditions. A number of other behavioural observations in the aphid literature may be interpreted in an economic or cost-benefit framework. The approach holds considerable promise for understanding many aspects of the anti-predator behaviour of aphids and other animals.     

Evaluation of four co-occurring ladybirds for use as biocontrol agents of the pea aphid,Acyrthosiphon pisum (Homoptera: Aphididae)

The potential of four co-occurring ladybirds, Coccinella septempunctata (L.), Coccinella transversalis (F.), Cheilomenes sexmaculata (F.), and Propylea dissecta (Mulsant), as predators of the pea aphid, Acyrthosiphon pisum (Harris) was investigated. Larger ladybirds were more voracious while smaller ones had higher consumption indices and were more efficient converters of food.     

Behavioural correlates of genetic divergence due to host specialization in the pea aphid, Acyrthosiphon pisum

Host specialization plays a central role in the diversification of herbivorous insects and yet we know very little about the evolution of this trait. Populations of the pea aphid, Acyrthosiphon pisum (Harris), are specialized and locally adapted to either alfalfa or clover. Preference for either plant produces assortative mating, and gene flow between alfalfa and clover populations is consistently restricted (Via, 1991a,b, 1994). Behavioural studies of freely-moving aphids on the plant surface for 30 min suggest that host preference is chemically mediated, involves chemoreception, and is most likely due to secondary compounds located in the epidermal or mesophyll cells. Pea aphids do not distinguish between hosts and non-hosts at a distance but determine whether the plant is suitable or not after only a short probe. Thirty-minute recordings of the activities of aphid stylets using an electrical monitoring system (EPG), where aphids are attached to a gold wire with silver paint, provide a different picture, suggesting that EPG experiments do not accurately reflect natural behaviour during the first 30 min of the aphid-plant interaction.     

Characterisation of immune responses in the pea aphid, Acyrthosiphon pisum

The innate immune system of insects provides effective defence against a range of parasites and pathogens. The pea aphid, Acyrthosiphon pisum, is a novel study system for investigating host-parasite interactions due to its complex associations with both well-characterised bacterial symbionts and a diversity of pathogens and parasites, including several important biological control agents. However, little is known about the cellular and humoral immune responses of aphids. Here we identify three morphologically distinct types of haemocytes in circulation that we name prohemocytes, granulocytes and oenocytoids. Granulocytes avidly phagocytose Gram negative Escherechia coli and Gram positive Micrococcus luteus while oenocytoids exhibit melanotic activity. Prohaemocytes increase in abundance immediately following an immune challenge, irrespective of the source of stimulus. Pea aphids form melanotic capsules around Sephadex beads but do not form cellular capsules. We also did not detect any antimicrobial peptide activity in the haemolymph using zone of inhibition assays. We discuss these results in relation to recent findings from the pea aphid genome annotation project that suggest that aphids have a reduced immune gene repertoire compared to other insects.