Levels of clonal mixing in the black bean aphid Aphis fabae, a facultative ant mutualist

Aphids are a worldwide pest and an important model in ecology and evolution. Little is known, however, about the genetic structure of their colonies at a microgeographic level. For example, it remains largely unknown whether most species form monoclonal or polyclonal colonies. Here, we present the first detailed study on levels of clonal mixing in a nonsocial facultative ant mutualist, the black bean aphid Aphis fabae. In contrast to the earlier suggestion that colonies of this species are generally monoclonal, we found that across two subspecies of the black bean aphid, A. fabae cirsiiacanthoidis and A. fabae fabae, 32% and 67% of the aphid colonies were in fact polyclonal, consisting of a mix of up to four different clones, which resulted in an overall average relatedness within colonies of 0.90 and 0.79 in the two subspecies. Data further show that the average relatedness in A. f. cirsiiacanthoidis remained relatively constant throughout the season, which means that clonal erosion due to clonal selection more or less balanced with the influx of new clones from elsewhere. Nevertheless, relatedness tended to decrease over the lifetime of a given colony, implying that clonal mixing primarily resulted from the joining of pre‐existing colonies as opposed to via simultaneous host colonisation by several foundresses. Widespread clonal mixing is argued to affect the ecology and evolution of the aphids in various important ways, for example with respect to the costs and benefits of group living, the evolution of dispersal and the interaction with predators as well as with the ant mutualists.     

How symbiotic bacteria influence plant utilisation by the polyphagous aphid, Aphis fabae

To explore the effect of rearing-plant species on the contribution of the symbiotic bacterium, Buchnera, to aphid performance, larvae of Aphis fabae that contained the bacteria (symbiotic aphids) and larvae experimentally deprived of the bacteria (aposymbiotic aphids) were reared on 16 plant species. Mortality of aphids was low on most plant species. The relative growth rate (RGR) of the larvae varied with plant species, and was generally depressed by elimination of the bacteria; the mean values of RGR varied between 0 and 0.29 μg μg⁻¹ day⁻¹ for symbiotic aphids and 0 and 0.17 μg μg⁻¹ day⁻¹ for aposymbiotic aphids. The extent to which RGR was depressed by aposymbiosis varied significantly between plant species, suggesting that aphid host plant may influence the contribution of the bacteria to plant utilisation. It is proposed that the bacteria may be particularly important on plants with phloem sap of high amino acid content of low quality, i.e. low concentrations of essential amino acids. Received: 18 August 1996 / Accepted: 13 January 1997     

Phloem amino acids and the host plant range of the polyphagous aphid, Aphis fabae

This study investigated the relationship between the essential amino acid requirement of the aphid Aphis fabae Scop. and the phloem sap amino acid composition of its host plants. The dietary amino acid requirement of A. fabae varied between clones. One or more of the eight clones of A. fabae tested displayed depressed larval survival, larval growth rate, or r_m on diets lacking histidine, methionine, threonine, and valine, but none of the other five essential amino acids. The required amino acids corresponded closely to the essential amino acids that varied in relative concentrations among 16 plant species tested: histidine, threonine, tryptophan, and valine. It is suggested that the interclonal variation in the dietary requirements of an aphid species may contribute to the intraspecific variation in plant utilisation patterns. The phloem sap amino acid composition and sucrose : amino acid ratio did not differ consistently between host plant species of A. fabae and non‐host species, indicating that phloem amino acid composition is not an important factor in determining the host plant range of this aphid species.     

The photoperiodic control of wing development in the black bean aphid,Aphis fabae

Newly born presumptive gynoparae of Aphis fabae were transferred from their prenatal short-day rearing conditions (light-dark 12:12, 15°C) to a variety of postnatal photoperiodic regimes. Long days prevented wing formation and the majority of aphids developed into apterous or alate-apterous intermediate adults. Continued short days resulted, almost exclusively, in winged adults. The photoperiodic-response curve (T = 24) revealed a critical photoperiod of light-dark 13.5:10.5 and further investigations showed that morphogenesis depended more upon night- than day-length. Maximal apterization occurred with 8 or 9 h dark regardless of the length of the photophase but at photophases of 12 h or less the critical night length was reduced to 8.5 h. Night-interruption experiments revealed two peaks of photosensitivity when the scotophase was 12–14 h long but only a single peak was seen with longer dark periods. A series of experiments using early night interruptions followed by extended dark revealed apparent critical night lengths which decreased as the interruption was placed later in the scotophase. Resonance experiments involving 12 or 16 h photophases and extended scotophases at 15°C revealed long-day effects with scotophases shorter than critical and short-day effects with longer dark periods. However, similar regimes at 20°C produced three peaks of apterization 24 and 20 h apart indicating the possibility of a circadian element involved in the photoperiodic response. The results are compared with the photoperiodic responses of other insects.     

The influence of facultative endosymbionts on honeydew carbohydrate and amino acid composition of the black bean aphid Aphis fabae

The facultative endosymbionts Hamiltonella defensa and Regiella insecticola are commonly found in aphids. They are linked with various ecological benefits but generally occur at low prevalence, which indicates a possible harbouring cost. Little is known about how the presence of facultative endosymbionts is reflected in honeydew composition. Honeydew is the key mediator of the mutualism between aphids and their tending ants. The present study examines whether endosymbionts have an influence on aphid honeydew quality by comparing the amino acid and carbohydrate concentrations between infected and uninfected aphids. To this end, two genetic lines of the aphid Aphis fabae Scopoli are experimentally infected with different strains of Hamiltonella and Regiella. Infected aphids are shown to have reduced concentrations of amino acids in the honeydew compared with uninfected aphids. However, the presence of endosymbionts has no effect on the absolute amount of carbohydrates produced. Nevertheless, interclonal variation in honeydew composition between aphid genotypes is observed for both carbohydrate and amino acid production. These results imply that the nutritional value of honeydew depends on aphid genotype, as well as on the presence of secondary bacterial endosymbionts, which suggests that there is a physiological cost of harbouring endosymbionts and which could also impact aphid attractiveness to tending ants.     

Electrophysiological characterisation of olfactory sensilla in the black bean aphid, Aphis fabae

Electrophysiological responses of three different olfactory sensilla (proximal primary rhinaria (PPR), distal primary rhinaria (DPR) and secondary rhinaria (SR)) to the sex pheromone components, nepetalactol, nepetalactone, and a plant volatile, (E)-2-hexenal, were investigated in four different morphs of the black bean aphid, Aphis fabae Scopoli. The DC responses recorded directly from the antennal sensilla and termed electrosensillograms (ESGs) were generally much larger (up to 14 mV) than electroantennogram (EAG) responses (up to 2.5 mV). Characteristic morph-specific response profiles to these compounds were observed in each type of rhinarium and response waveforms were different between (E)-2-hexenal and the sex pheromone components. (E)-2-Hexenal elicited the largest responses at PPR, while nepetalactol and nepetalactone elicited the largest responses at SR in gynoparae and males. Nepetalactol and nepetalactone also showed significant activities on DPR and PPR in all morphs. In contrast, (E)-2-hexenal had almost no activity on SR. However, almost all of the SR investigated in males and gynoparae were sensitive to both nepetalactol and nepetalactone. A small sub-set of male SR responded mainly to nepetalactol. SR of winged virginoparae did not respond to the sex pheromone compounds. Paraffin oil (the solvent control) also elicited significant responses at PPR in virginoparae but not in other morphs. In a further experiment, SR of winged virginoparae showed no response to 30 other plant volatile compounds or the alarm pheromone component, (E)-beta-farnesene. Nepetalactol and nepetalactone had similar dose-response profiles in the SR of gynoparae. The results indicate that SR in males and gynoparae are highly specialised to detect sex pheromone compounds, while the DPR and PPR are relatively broadly tuned to both plant volatiles and sex pheromone components. The presence of SR in winged virginoparae that are not responsive to sex pheromone components, alarm pheromone, or any of the plant volatile compounds tested may indicate a possible role of these sensilla to detect, as yet, unknown compound(s) with a high specificity. The present study also suggests that PPR may play a role in detecting paraffin-related compounds such as cuticular hydrocarbons of plants.     

Juvenile Hormone and host-plant colonization by the black bean aphid, Aphis fabae

Abstract Host-plant colonization by winged, summer forms of Aphis fabae Scop, involves the resumption of embryogenesis and larviposition, leading eventually to flight muscle degeneration. Topical application of Juvenile Hormone I to young adults which (a) had access to the host plant, (b) were starved, (c) were starved and treated with precocene III, or (d) were decapitated shortly after the final ecdysis suggests that embryogenesis and flight muscle histolysis may be stimulated by an increase in Juvenile Hormone titres after settling. The duration of the pre-reproductive period and the initial reproductive rate were not significantly affected, possibly because other neuroendocrine factors are involved in parturition.     

Photoperiodic induction of winged females in the black bean aphid. Aphis fabae

Abstract. The Photoperiodic of winged females (alatae) in the black bean aphid, Aphis fabae Scop. (Homopetera: Aphididae), is investigated in detail with emphasis on the interaction of the maternal and embryonic/young larval photoperiodic clocks. Previous work had shown that in uncrowded conditions the induction of gynoparae (winged females that produce sexual females) requires both prenatal and postnatal exposure to long-night (12 h) Photoperidic cycles: present results show that sole postantal exposure to long nights of any lenght does not induce wing formation in early-born aphids.
When aphids were exposed to experimental light-dark cycles postanatally only, their daughters developed as alate in long nights and as apterae in short nights: the critical night lenght (CNL) was 11:1 h. Additional prenatal exposure to experimental regimes resulted in a significantly shorter CNL (10.6 h). This difference could be accounted for by the fact that more experimental light-dark cycles were experienced in the latter case.
Apterous aphids transferred from LD 16:8 h to LD 12:12 h as either third-or fourth-stadium larvae, or young adults, switched for aptera-production to alata-production. The transition form aptera- to alata-production was rather abrupt in third-stadium transfers but more gradual when transfers occurred as fourth-stadium larvae and adults. Moreover, s the number of days required for 50% of the aphids to become alata-producers increased from 7–8 in third-stadium transfers, to 9–10 and 11–12 in the later transfers.     

The effect of juvenile hormone on host-plant preference in the black bean aphid, Aphis fabae

ABSTRACT. When offered the choice between a germinating tick bean (summer host) and a detached spindle leaf (winter host) adult gynoparae of Aphis fabae Scop. showed a distinct preference for spindle, as judged by site of larviposition. However, when offered the same alternatives, all four larval instars showed a settling preference for bean. A switch in host preference therefore occurs at or shortly after final (fourth) moult. Fifth instar, supernumerary larvae induced by juvenile hormone (JH I) treatment also preferred bean, indicating that the host preference switch at the fourth moult had been inhibited. Larval/adult intermediate forms produced by JH I treatment, but which were capable of parturition, showed a significant increase in the acceptability of bean as a larviposition site. Acceptability was proportional to the degree of juvenilization. Thus, although larviposition is an adult prerogative, JH I can influence the preferred site. This may, however, be an indirect effect which reflects a juvenilized settling preference.     

The cost of ant attendance and melezitose secretion in the black bean aphid Aphis fabae

1. The aphid Aphis fabae (Scopoli) is facultatively tended by Lasius niger (Linnaeus) ants. Previously, we found that A. fabae colonies can be made up of several clones, and that clones display significant differences in the composition of their honeydew sugars, especially in the amount of the ant attractant sugar melezitose that they produce. 2. These clonal differences could greatly impact the strength of the mutualistic interaction with ants as well as the aphids' fitness. 3. Hence, the aim of this study was to compare the fitness of different A. fabae clones that differed in their melezitose secretion, and whether or not they were tended by ants. 4. Individual fitness indices, colony growth, and alate production of single‐clone aphid colonies were analysed. 5. The results demonstrate that the fitness consequences of ant attendance critically depend on an interaction between levels of melezitose production. In particular, we show that high‐melezitose secreting clones produce fewer alates and hence might have a lower dispersal ability in the presence of ants. 6. Furthermore, these data confirm previous evidence that ant attendance is costly and results in the production of fewer apterae.     

Maternal reproductive decisions are independent of feeding in the black bean aphid,Aphis fabae

Aphids are phloem feeders and an important assumption has been that reproduction is initiated only after phloem ingestion. Here we investigate the plant tissue location of parturition cues in winged and wingless, summer virginoparae and autumn migrants (gynoparae) of the black bean aphid, Aphis fabae. These seasonal forms have different host preferences. Using electrical penetration graph (EPG: to observe activity of the mouthparts) and video-monitoring procedures we demonstrate that the time to first parturition after host-plant contact is significantly shorter than the time to first registered phloem contact in the summer winged form. In gynoparae, the time to first parturition does not significantly differ from time to first phloem contact but is shorter than time to first phloem ingestion. Times to first parturition, first registered phloem contact and first phloem ingestion do not differ significantly in the summer wingless form. Simultaneous EPG and video recording procedures show that a high proportion of individuals of all morphs (45-70%) initiate reproduction before sustained phloem activities (salivation/ingestion). The only behaviours that all individuals demonstrate before first parturition are ‘non-penetration’ (aphid on plant surface with mouthparts outside plant) and stylet ‘pathway activity’, including secretion of gelling saliva and penetration of the non-vascular (epidermis and mesophyll) cells. A short period of penetration of the peripheral plant tissues (five cell punctures per individual) by aphids tethered and monitored by EPG decreases the time to first parturition of the winged summer form when subsequently placed on a Parafilm sachet containing 15% sucrose solution. This treatment also significantly increases the incidence of reproduction and individual reproductive output of gynoparae over a 24-h period. No detectable effects of tissue penetration on subsequent reproductive output are observed in the wingless summer form. Additionally, EPGs reveal that a number of aphids of all morphs display xylem ingestion, which occurs predominantly before initiation of phloem feeding but is not necessary to initiate parturition. It is concluded that aphids are likely to detect parturition cues during stylet punctures of cells within peripheral tissue layers, before reaching the phloem vessels.     

Lipid composition in long-day and short-day forms of the black bean aphid,Aphis fabae

Long-day reared winged (alate) virginoparae from laboratory stock cultures which had been reared throughout larval development on bean stipules were significantly smaller (0.46+/-0.02 mg; mean+/-SEM) than short-day-reared gynoparae (0.69+/-0.04 mg; the winged autumn migrant) which completed development on intact beans. When winged virginoparae were raised from the third stadium on bean seedlings they grew larger (0.86+/-0.02 mg) but the gynoparae contained proportionally more total lipid (12.1+/-0.4%, gynoparae; 7.4+/-0.6%, stipule-reared virginoparae; 9.2+/-0.8%, seedling-reared virginoparae). Wingless aphids (apterae) were heavier, whether reared in short (0.99+/-0.03 mg) or long days (0.95+/-0.04 mg) but the lipid content was low (4.5+/-0.7% and 4.9+/-0.6%, respectively). The triacyl-, diacylglycerol and the phospholipid contents followed this trend but analysis of the fatty acid moieties of the triacylglycerides showed phenotypic differences. The ratios of myristic acid:palmitic acid were significantly higher in the winged forms than the wingless forms and were much higher in the gynoparae than the winged virginoparae. Short-day-reared wingless females also had a higher myristic acid:palmitic acid ratio than long-day-reared apterae, possibly reflecting the embryonic gynoparae maturing in their ovaries.     

Migratory and targeted flight in seasonal forms of the black bean aphid, Aphis fabae

ABSTRACT. Adult winged forms of Aphis fabae Scop., the spring migrants (fundatrigeniae; alate virginoparae), the summer migrants (alate virginoparae) and the autumn migrants (gynoparae), were flown in an automated vertical wind tunnel, with a small green target presented every 60 s. On average, targeted flight developed in the spring and summer migrants after 19 and 15 min, respectively. However, in autumn migrants targeted flight occurred after 184 min of flight. No evidence of target approach was found up to 30 min prior to this time, although some movement towards the target was observed in the 30 min before the first response of autumn migrants, suggesting that a gradual change from migratory to targeted flight occurred. No correlation was found between aphid weight and the time to first response for spring or summer migrants, but a positive correlation was found for autumn migrants. Flight patterns of autumn migrants, in the absence of a target, are described and compared with flight patterns previously reported for summer migrants.     

The effect of certain antibiotics on reproduction of the black bean aphid, Aphis fabae Scop.

The antibiotic terramycin at 0.2% concentration was consistently effective in greatly reducing the populations of Aphis fabae on Vicia faba when sprayed on larvae on and off the plant. The insects were not killed, but their fertility was decreased by over 97% in the first generation, leading to total sterility in the second generation. At 0.01% concentration, terramycin was not effective and only moderately so at 0.1% Larval development was invariably delayed by 1 day and the weight and size of the resulting adults were significantly reduced. After the most effective treatments, the adults were very inactive in their feeding and excretion, and sometimes became completely sterile. Even when the aphids reproduced, they ceased to do so after 6–10 days in different terramycin treatments and frequently the larvae were born dead. The affected insects could not, apparently, regain their reproductive capacity even after their transfer to untreated plants. When the insects were sprayed on the plants, terramycin in water was as effective as with surfactants: but when the insects were sprayed off the plants an oxyethyl phenol derivative (NP 10) greatly increased the efficiency of terramycin: surfactants alone had no effect on the insects. The other four antibiotics, namely tetracycline, aureomycin, chloramphenicol and reverin were not effective at the 0.2% concentration when the plants alone were sprayed before introducing the apids. The addition of glycerin to the antibiotics and keeping plants in a high humidity for 24 hr. after the treatment also had no effect.     

Large interclone differences in melezitose secretion in the facultatively ant-tended black bean aphid Aphis fabae

Many aphids are known to engage in a trophic mutualism with ants, whereby the aphids secrete sugary-rich honeydew which is collected by the ants for food, and the ants, in exchange, protect the aphids against natural enemies. Previous results, however, suggest that the production of some of the honeydew sugars, such as the ant-attractant trisaccharide melezitose, may induce an indirect cost to the aphids. This led us to believe that large differences in the nature of the secreted honeydew might exist, due to some clones capitalizing more or less on their mutualistic interaction with ants, or due to some “cheater” clones foregoing the production of particular sugars, instead taking advantage of the ant-attracting effect of other non sugar-deficient clones, co-occurring on the same plant. Here we present data on clonal variation in the composition of honeydew of the black bean aphid Aphis fabae which confirm this prediction. In particular, our results show that there was large interclone variation in the amount of glucose, melezitose and total sugar produced. The variation in the production of melezitose, however, showed particularly large differences, with 54% (7 out of 13) of the clones screened being virtually deficient for the production of this sugar, irrespective of whether the aphid colonies were ant-tended or not. The consequences of this finding in the context of the evolution and maintenance of the ant–aphid mutualism, as well as the adaptive benefits of oligosaccharide synthesis in aphids and other insects are discussed.     

A potent, morph-specific parturition stimulant in the overwintering host plant of the black bean aphid, Aphis fabae

Abstract. The black bean aphid, Aphis fabae Scopoli, has a host‐alternating life cycle. Winged female autumn migrants (gynoparae) develop on numerous summer host plants but as adults will only colonize the winter host (spindle, Euonymus europaeus L.). When stylet activities of gynoparae were electrically recorded during access to a spindle leaf, the insects spent the majority (75%) of the 6‐h experimental period penetrating the plant surface and a large proportion of it (44%) ingesting from either phloem or xylem vessels. Most (95%) gynoparae initiated reproduction on spindle, producing 4.15 ± 0.59 offspring per adult by the end of the experiment (mean ± SEM). By contrast, gynoparae placed on a seedling of their natal, summer host (broad bean, Vicia faba L.) penetrated the plant for only 39% of the available time, rarely ingested plant sap and never reproduced. The number of nymphs deposited on spindle leaves was not correlated with the occurrence or duration of ingestion from vascular tissues, suggesting that parturition stimulants are detected before feeding, probably during penetration of nonvascular cells. Presentation of an aqueous spindle extract to the aphids in artificial feeding chambers showed that water‐soluble spindle factors evoke stimulation of parturition by gynoparae in 72‐h bioassays. The stimulant was extremely potent, remaining active until the total extracted material was diluted to less than 10 p.p.m. Stylet activities and reproductive responses were also evaluated for summer winged females (alate virginoparae), which have a broad host range, and will colonize both bean and spindle under laboratory conditions. On both of these plant species, virginoparae often ingested plant sap and deposited nymphs during the 6‐h electrical recording experiment (producing 4.60 ± 0.48 offspring on bean; 2.70 ± 0.35 on spindle: mean ± SEM), but no significant correlations were found between reproduction and the occurrence or duration of particular stylet activities. Aqueous host‐plant extracts had no effect on the numbers of offspring deposited by virginoparae in artificial feeding chambers, showing that this form of the aphid is not responsive to the spindle‐derived parturition stimulant. The results highlight the need for more information on the factors determining host acceptance and parturition by polyphagous aphid phenotypes.     

Phenology of the black bean aphid,Aphis fabae,on organic crops and effect of parasitoid communities

1. Parasitoids represent the third trophic level in plant-insect food webs. They develop in or on herbivorous insects at the second trophic level, which in turn feed on plants that constitute the first trophic level. This food web system might be affected by agricultural landscape and practices.
2. We studied the phenology and population density of Aphis fabae (Hemiptera: Aphididae) and determined the impact of parasitoids in organic fields of beans and beet plants.
3. Twenty leaves per field were collected weekly. For each leaf, the total number of aphids, apterous and alate morphs, the number of mummies, and mortality of aphids were recorded.
4. Our results showed that the phenology of A. fabae varied throughout the season with maximal numbers observed at the beginning of summer. In addition, the production of alate morphs and the proportion of parasitism were positively correlated to an increase in aphid densities.
5. Six parasitoid species were recorded attacking A. fabae in the field, with Lysiphlebus fabarum and Binodoxys angelicae (Hymenoptera: Aphidiinae) being the most abundant parasitoids.
6. Field characteristics and availability of host plants in the surroundings affected A. fabae density. Lysiphlebus fabarum and B. angelicae appeared to be good biocontrol candidates against A. fabae.

     

Field evaluation of resistance to black bean aphid, Aphis fabae, in close relatives of the Faba bean, Vicia faba

A field assessment of 26 accessions of Vicia narbonensis and three of V. johannis confirmed previous laboratory studies demonstrating higher levels of resistance to Aphis fabae in these two wild species compared to the closely related crop, Vicia faba. Accessions of V. johannis were significantly more resistant than most accessions of V. narbonensis for all resistance indices measured except survival of aphid nymphs. Plant growth stage significantly affected levels of resistance in both Vicia species, resistance being moderate at pre-bud stage, decreasing on flowering and rising again at pod fill and onset of leaf senescence. Significant intraspecific variability in aphid resistance was found only within the 26 accessions of V. narbonensis, var. serratifolia being more resistant than var. narbonensis. Possible resistance factors and the agronomic potential of these two wild relatives of Faba bean are considered.