Generalist Aphid Research Articles

Interactions of elevated carbon dioxide and temperature with aphid feeding on transgenic oilseed rape: Are Bacillus thuringiensis (Bt) plants more susceptible to nontarget herbivores in future climate?

AbstractClimate change factors such as elevated carbon dioxide (CO2) and temperature typically affect carbon (C) and nitrogen (N) dynamics of crop plants and the performance of insect herbivores. Insect‐resistant transgenic plants invest some nutrients to the production of specific toxic proteins [i.e. endotoxins from Bacillus thuringiensis (Bt)], which could alter the C–N balance of these plants, especially under changed abiotic conditions. Aphids are nonsusceptible to Lepidoptera‐targeted Bt Cry1Ac toxin and they typically show response to abiotic conditions, and here we sought to discover whether they might perform differently on compositionally changed Bt oilseed rape. Bt oilseed rape had increased N content in the leaves coupled with reduced total C compared with its nontransgenic counterpart, but in general the C : N responses of both plant types to elevated CO2 and temperature were similar. Elevated CO2 decreased N content and increased C : N ratio of both plant types. Elevated temperature increased C and N contents, total chlorophyll and carotenoid concentrations under ambient CO2, but decreased these under elevated CO2. In addition, soluble sugars were increased and starch decreased by elevated temperature under ambient but not under elevated CO2, whereas photosynthesis was decreased in plants grown under elevated temperature in both CO2 levels. Myzus persicae, a generalist aphid species, responded directly to elevated temperature with reduced developmental time and decreased adult and progeny weights, whereas the development of the Brassica specialist Brevicoryne brassicae was less affected. Feeding by M. persicae resulted in an increase in the N content of oilseed rape leaves under ambient CO2, indicating the potential of herbivore feeding itself to cause allocation changes. The aphids performed equally well on both plant types despite the differences between C–N ratios of Bt and non‐Bt oilseed rape, revealing the absence of plant composition‐related effects on these pests under elevated CO2, elevated temperature or combined elevated CO2 and temperature conditions.

Ecological and life-history notes on syrphid predators of woolly apple aphid in Virginia, with emphasis on Heringia calcarata

Three species of hover fly commonly prey on woolly apple aphid, Eriosoma lanigerum, in Virginia, USA apple orchards. Larvae of Heringia calcarata are specialized predators of this pest, while Eupeodes americanus and Syrphus rectus are generalist aphid predators. The developmental duration of the immature stages of H. calcarata was determined under laboratory conditions, revealing a generation time of 19–20 d at 25°C. Descriptions of the larval, pupal and adult stages of H. calcarata are reported. Potted apple trees infested with arboreal colonies of woolly apple aphid and deployed in an orchard in Virginia were used as sentinels to measure seasonal changes in the relative abundance of the three syrphid species, based on the number of unhatched eggs deposited during weekly, 48-h exposure intervals from April to October, 2003–2005. Similar trends in the relative abundance of each species were recorded across all years. Eupeodes americanus was recorded first, showing a pronounced peak between mid-April and mid-May, followed by a prolonged period during which it was absent or present in very low numbers and then a much smaller peak in September and October. First records of H. calcarata occurred slightly later than for E. americanus. Early peaks of H. calcarata abundance typically occurred in May and June and tended to be smaller than those of E. americanus. Heringia calcarata eggs were recovered throughout most of each season. The potential role of predation by aphidophagous hover flies on the suppression of woolly apple aphid outbreaks in eastern apple orchards is discussed.

Contrasting effects of natural habitat loss on generalist and specialist aphid natural enemies

The greater susceptibility of higher trophic levels to habitat loss has been demonstrated to disrupt important trophic interactions such as consumer control of prey populations. This pattern is predicted to break down for generalist species that can use matrix habitats, yet empirical studies comparing generalist and specialist enemy pressure in response to natural habitat loss are lacking. Here we examined the effects of landscape simplification resulting from habitat conversion to agriculture on nettles, Urtica dioica, their specialized aphid herbivore, Microlophium carnosum, and associated natural enemies that varied broadly in their degree of specialization. Both nettles and their specialized aphid herbivore were significantly more abundant in complex than simple landscapes. Different enemy groups showed contrasting responses. Aphid specialists (parasitic wasps and cecidomyiid midges) reached higher densities in complex than simple landscapes, and this effect was primarily related to shifts in local resource abundance (i.e. nettle aphid densities). In contrast, densities of generalists (coccinellid beetles and spiders) were significantly higher in simple landscapes, presumably due to spillover of generalists from surrounding cropland habitats. Natural enemy-prey ratios did not differ significantly across landscape types for specialist groups but were significantly higher in simple than complex landscapes for generalist groups, suggesting that enemy pressure on nettle aphids likely increases with landscape simplification. This was supported by our finding that aphid population growth rates were lower in simple than complex landscapes, and declined significantly with increasing coccinellid densities. Thus, in marked contrast to previous work, our results suggest that natural habitat loss may augment rather than disrupt consumer–prey interactions, and this will depend greatly on the degree of specialization of functionally dominant natural enemies.

Contrasting effects of natural habitat loss on generalist and specialist aphid natural enemies

The greater susceptibility of higher trophic levels to habitat loss has been demonstrated to disrupt important trophic interactions such as consumer control of prey populations. This pattern is predicted to break down for generalist species that can use matrix habitats, yet empirical studies comparing generalist and specialist enemy pressure in response to natural habitat loss are lacking. Here we examined the effects of landscape simplification resulting from habitat conversion to agriculture on nettles, Urtica dioica, their specialized aphid herbivore, Microlophium carnosum, and associated natural enemies that varied broadly in their degree of specialization. Both nettles and their specialized aphid herbivore were significantly more abundant in complex than simple landscapes. Different enemy groups showed contrasting responses. Aphid specialists (parasitic wasps and cecidomyiid midges) reached higher densities in complex than simple landscapes, and this effect was primarily related to shifts in local resource abundance (i.e. nettle aphid densities). In contrast, densities of generalists (coccinellid beetles and spiders) were significantly higher in simple landscapes, presumably due to spillover of generalists from surrounding cropland habitats. Natural enemy‐prey ratios did not differ significantly across landscape types for specialist groups but were significantly higher in simple than complex landscapes for generalist groups, suggesting that enemy pressure on nettle aphids likely increases with landscape simplification. This was supported by our finding that aphid population growth rates were lower in simple than complex landscapes, and declined significantly with increasing coccinellid densities. Thus, in marked contrast to previous work, our results suggest that natural habitat loss may augment rather than disrupt consumer–prey interactions, and this will depend greatly on the degree of specialization of functionally dominant natural enemies.

Transcriptional responses of Arabidopsis thaliana ecotypes with different glucosinolate profiles after attack by polyphagous Myzus persicae and oligophagous Brevicoryne brassicae

Plants are equipped with a range of defence mechanisms against herbivorous insects. In cruciferous species, jasmonic acid, salicylic acid, and ethylene along with glucosinolates and their hydrolysis products play important roles in plant protection and plant-insect communication. In turn, a number of herbivores have adapted to plants that contain glucosinolates. As a result of adaptation to their host plants, specialized insects may elicit different plant-inducible responses than generalists. Oligonucleotide microarrays and qRT-PCR analysis were used to characterize transcriptional profiles of Arabidopsis thaliana plants in response to infestation with a generalist aphid, Myzus persicae, or a cruciferous plant specialist, Brevicoryne brassicae. To find possible differences and similarities in molecular responses between plants differing in predominant glucosinolate hydrolysis products, three ecotypes of A. thaliana were chosen: Wassilewskija (Ws), Cape Verde Islands (Cvi), and Landsberg erecta (Ler), which, respectively, produce mainly isothiocyanates, epithionitriles, and nitriles. In all three ecotypes, general stress-responsive genes, genes belonging to octadecanoid and indole glucosinolate synthesis pathways were induced upon both generalist and specialist attack. By contrast, transcription of myrosinases, enzymes hydrolysing glucosinolates, was suppressed. The induction of the jasmonic acid synthesis pathway was strongest in Cvi, while the up-regulation of the indole glucosinolate synthesis pathway was highest in Ler, suggesting a slightly different defence strategy in these two ecotypes. Specialist and generalist infestations caused statistically significant differential regulation of 60 genes in Ws and 21 in Cvi. Among these were jasmonic acid and tryptophan synthesis pathway enzymes, and pathogenesis related protein (PR1). Insect no-choice experiments revealed lowered fitness of B. brassicae on Ler and Cvi in comparison to Ws, but no ecotype-dependent change in fecundity of M. persicae. Targeted studies employing constructs of GUS reporter gene under the control of promoters from CYP79B2 and CYP79B3 genes showed insect-specific induction of the indole glucosinolates synthesis pathway.

Life table parameters affecting the population development of the woolly beech aphid, Phyllaphis fagi

AbstractA detailed study of the basic biological parameters affecting the population development of the woolly beech aphid, Phyllaphis fagi L. (Homoptera: Drepanosiphidae), was performed under field and laboratory conditions in order to obtain information about factors responsible for outbreak situations in forest nurseries. In the nurseries, the aphid eggs have found to hatch before budburst. The newly hatched nymphs were highly active in searching for feeding places, which resulted in higher nymph mortality in the first generation than in the following generations. Ten aphid generations were recorded in the nursery during one growing season. In the laboratory, development time, nymph mortality, reproduction, and adult lifetimes were assessed for P. fagi at four constant temperatures, i.e., 12, 15, 20, and 25 °C through four successive generations. Both temperature and aphid generation were found to affect life table parameters. The first generation, however, was less susceptible to changing temperatures than the following generations. Phyllaphis fagi reproductive effort was more dependent on temperature than on aphid generation. No significant difference was found in reproductive effort between apterous and alate females. The potential for outbreak situations therefore exists throughout the summer. The formation of winged morphs was not restricted to a few generations but rather continued for several months, with activity peaking in all years in mid‐June. This had implications for migration to new seedlings in the nurseries. The highest rm value was obtained at 20 °C. Nymphs born at higher temperatures were sometimes born deformed and unable to survive.

Contrasting performances of generalist and specialistMyzus persicae(Hemiptera: Aphididae) reveal differential prevalence of maternal effects after host transfer

Transgenerational maternal effects on performance (rm) after host transfer were evaluated in the generalist aphid Myzus persicae s.s., and in its subspecies specialized on tobacco, M. persicae nicotianae Blackman. We tested whether the performance of these taxa, when reared separately on optimal and suboptimal hosts (as sources of different maternal background) and then transferred to optimal hosts, experienced variations along four successive generations. Additionally, to compare the tolerance of both taxa to stress following host transfers, developmental instability (fluctuating asymmetry and body abnormalities) along the four generations was assessed. Taxon, rearing host, and generation affected the performance after host transfer. In the generalist, there was a significant improvement of rm along generations when transferred from suboptimal to optimal host and a significant decrease when transferred from optimal to optimal host; in the specialist, no increase or decrease occurred in any host transfer treatment. Transfer from suboptimal to optimal hosts caused higher losses of remaining replicates along generations than transfers from optimal to optimal hosts, and the specialist showed higher losses than the generalist. The only significant effect detected in comparisons involving fluctuating asymmetry values was that of taxon on length of siphunculi. Frequency of body abnormalities was not affected by treatments. Collectively, these results show a transgenerational weakening of maternal effects in the generalist but not in the specialist aphid, and suggest that rearing the latter in a suboptimal host causes not easily reversible changes that further give rise to constraints in performance.

Involvement of the xyloglucan endotransglycosylase/hydrolases encoded by celery XTH1 and Arabidopsis XTH33 in the phloem response to aphids

During infestation, phloem-feeding insects induce transcriptional reprogramming in plants that may lead to protection. Transcripts of the celery XTH1 gene, encoding a xyloglucan endotransglycosylase/hydrolase (XTH), were previously found to accumulate systemically in celery (Apium graveolens) phloem, following infestation with the generalist aphid Myzus persicae. XTH1 induction was specific to the phloem but was not correlated with an increase in xyloglucan endotransglycosylase (XET) activity in the phloem. XTH1 is homologous to the Arabidopsis thaliana XTH33 gene. XTH33 expression was investigated following M. persicae infestation. The pattern of XTH33 expression is tightly controlled during development and indicates a possible role in cell expansion. An xth33 mutant was assayed for preference assay with M. persicae. Aphids settled preferentially on the mutant rather than on the wild type. This suggests that XTH33 is involved in protecting plants against aphids; therefore, that cell wall modification can alter the preference of aphids for a particular plant. Nevertheless, the ectopic expression of XTH33 in phloem tissue was not sufficient to confer protection, demonstrating that modifying the expression of this single gene does not readily alter plant-aphid interactions.

Parallels and contrasts between symbiotic bacteria and bacterial-derived organelles: evidence from Buchnera, the bacterial symbiont of aphids

Bacteria of the genus Buchnera (γ-Proteobacteria, allied with Escherichia coli) occur exclusively in symbiosis with aphids. Buchnera is advantageous to aphids because it over-produces essential amino acids, which are made available to the insect tissues. The congruence of the phylogenies of Buchnera and aphids, together with dating from the aphid fossil record, suggests that Buchnera has been transmitted vertically down the aphid generations for 160–280 million years. Despite this long history in symbiosis, the Buchnera genome does not exhibit the reduction and simplification characteristic of bacteria-derived organelles. The principal genetic reorganisation, relative to E. coli, relates to amino acid over-production. Buchnera in advanced aphid families has genes for enzymes in the synthesis of two essential amino acids, tryptophan and leucine, amplified on plasmids. The Buchnera genome is also characterized by high AT content and high rate of nonsynonymous substitutions in protein-coding genes. The latter may include deleterious mutations that accumulate because (as a consequence of vertical transmission) Buchnera lacks the opportunity for recombination.

Fungal endosymbionts affect aphid population size by reduction of adult life span and fecundity

The symbiosis between grasses and endophytic fungi is a common phenomenon and can affect herbivore performance through acquired, chemical plant defence by fungal alkaloids. In laboratory experiments, two species of common grass aphids, Rhopalosiphum padi and Metopolophium dirhodum were tested, in a population experiment (on four plant cultivars) and individually (on one plant cultivar) for the effects of the endophyte, Neotyphodium lolii, that forms symbiotic associations with perennial ryegrass Lolium perenne. In the population experiment that lasted for four aphid generations both aphid species showed decreased population sizes when feeding on each of the four endophyte-infected cultivars. Individuals of R. padi tested individually showed reduced adult life span and fecundity when feeding on infected plants. Individuals of M. dirhodum showed no response in any of the traits measured. This suggests that R. padi individuals are more sensitive to endophyte infection than M. dirhodum individuals. However, all infected grass cultivars reduced population sizes of both aphid species over four generations. Therefore, fungal endophytes can reduce populations of aphid herbivores independent of plant cultivars. Die Symbiose zwischen Gräsern und endophytischen Pilzen ist ein häufig auftretendes Phänomen, wobei die Wirtspflanze durch die vom Endophyten produzierten Alkaloide einen Schutz gegen Herbivorenfrass erhält. In Laborexperimenten wurden die Auswirkungen des endophytischen Pilzes Neotyphodium lolii, welcher mit dem Gras Lolium perenne assoziiert ist, auf zwei häufige Grasblattlausarten untersucht. Im ersten Experiment wurden die Blattlauspopulationen auf vier unterschiedlichen Grassorten jeweils mit und ohne Endophytenbefall getestet. Im zweiten Experiment wurden Entwicklungszeit, Lebensspanne, Fekundität und Körpergrösse der Blattläuse, Rhopalosiphum padi und Metopolophium dirhodum, auf infizierten und Pilz-freien Gräsern einer Sorte untersucht. Das Populationsexperiment dauerte vier Blattlausgenerationen und zeigte für beide Blattlausarten reduzierte Populationsgrössen auf infizierten Grassorten. Individuell getestet hatte nur R. padi eine reduzierte Lebensspanne und Fekundität auf infizierten Pflanzen, wohingegen M. dirhodum keine signifikante Reaktion auf die Infektion der Futterpflanze zeigte. Rhopalosiphum padi-Individuen scheinen daher sensitiver auf die Endophyteninfektion zu reagieren als M. dirhodum. Auf allen Grassorten beeinflusste indessen Pilzinfektion die Populationsgrössen von Aphiden negativ. Endophyten können also Aphidenpopulationen unabhängig von der Grassorte reduzieren.

Transfer of endophyte‐origin defensive alkaloids from a grass to a hemiparasitic plant

AbstractPlants growing in natural environments experience myriad interactions with a diverse assemblage of pathogens, parasites and mutualists. Many of these interactions involve symbiotic bacteria and fungi, but they also include macroparasitic plants. In this study, we investigated the interactions among a host grass (Lolium pratense, ex., Festuca pratensis), its symbiotic endophytic fungus (Neotyphodium uncinatum), a root hemiparasitic plant (Rhinanthus serotinus) of the host grass and a generalist herbivore (aphid Aulacorthum solani) of the hemiparasite. We demonstrate that the hemiparasitic plant acquires defending mycotoxins produced by the endophytic fungus living within their shared host grass. The uptake of defensive mycotoxins from the endophyte‐infected host grass enhances the resistance of the hemiparasitic plant to the generalist aphid herbivore. Endophyte infection increases the performance of the hemiparasitic plant, but reduces the growth of the host grass. In other words, the mutualistic endophytic fungus becomes parasitic in the presence of the hemiparasitic plant. Our results suggest that the outcomes of grass–endophyte interactions are conditional on the complexity of community‐level interactions; thus, the outcome of multispecies interactions may not be predictable from pair‐wise combinations of species.

Effect of Nitrogen Fertilizer on the Intrinsic Rate of Increase ofHysteroneura setariae(Thomas) (Homoptera: Aphididae) on Rice (Oryza sativaL.)

We studied the intergenerational effects of nitrogen (N) fertilizer applications to rice plants on the size, fecundity, and intrinsic rate of increase (rm) of Hysteroneura setariae (Thomas). Treatments consisted of rearing aphids on rice plants receiving 0, 50, 100, and 150% of the standard N fertilizer rates. Adult females produced on the 0 and 150% treatments were transferred to new plants at 0 or 150% N levels to create a matrix of four treatments: 0–0, 0–150, 150–0, and 150–150%. Second-generation aphids produced on 150% N plants had significantly higher survivorship, fecundity, and rm regardless of the N treatment of plants that parents fed on. N levels had no observed effect on head width. Aphids reared on 0% N plants whose parents were also reared on 0% N plants had significantly shorter body lengths than aphids on all other treatments, including the 150–0% treatment, indicating that the effect of a high N diet was expressed in a second generation of aphids. Results are discussed in the context of aphid population outbreaks in rice fields and in greenhouses.

Host selection by the generalist aphid Myzus persicae (Hemiptera: Aphididae) and its subspecies specialized on tobacco, after being reared on the same host

Decision-making during host selection by phytophagous insects has proved to be related to host range, with specialists taking faster decisions than generalists; however, this pattern fails to materialize in some host selection studies performed with aphids. Differences found in testing designs point to rearing effects on aphid host selection. To test whether specialization patterns derive from the nature of the aphid or as a consequence of rearing environment, host selection behaviours were compared between the generalist Myzus persicae (Sulzer) s.s. and its subspecies specialized on tobacco when reared on a common host and offered the choice of an alternative host and a non-host plant. Pre-alighting (host finding and attraction towards host volatiles) and post-alighting (leaf surface exploration and probing) behaviours did not differ between the generalist and the tobacco-specialist, except in the allocation of time to probing behaviour; furthermore, all specialists chose the host on which they performed best. Thus, although the specialist was not faster than the generalist, it showed a higher level of commitment to its preferred host plant.

Foraging behaviour at the fourth trophic level: a comparative study of host location in aphid hyperparasitoids

AbstractIn studies of foraging behaviour in a multitrophic context, the fourth trophic level has generally been ignored. We used four aphid hyperparasitoid species: Dendrocerus carpenteri (Curtis) (Hymenoptera: Megaspilidae), Asaphes suspensus Walker (Hymenoptera: Pteromalidae), Alloxysta victrix (Westwood) (Hymenoptera: Alloxystidae) and Syrphophagus aphidivorus (Mayr) (Hymenoptera: Encyrtidae), to correlate their response to different cues with their ecological attributes such as host range and host stage. In addition, we compared our results with studies of primary parasitoids on the same plant–herbivore system. First, the olfactory response of females was tested in a Y‐tube olfactometer (single choice: plant, aphid, honeydew, parasitised aphid, aphid mummy, or virgin female parasitoid; dual choice: clean plant, plant with aphids, or plant–host complex). Second, their foraging behaviour was described on plants with different stimuli (honeydew, aphids, parasitised aphids, and aphid mummies). The results indicated that olfactory cues are probably not essential cues for hyperparasitoid females. In foraging behaviour on the plant, all species prolonged their total visit time and search time as compared to the control treatment (clean plant). Only A. victrix did not react to the honeydew. Oviposition in mummies prolonged the total visit time because of the long handling time, but the effect of this behaviour on search time could not be determined. No clear correlation between foraging behaviour and host stage or host range was found. In contrast to specialised primary aphid parasitoids that have strong fixed responses to specific kairomones and herbivore‐induced synomones, more generalist aphid hyperparasitoids seem to depend less on volatile olfactory stimuli, but show similarities with primary parasitoids in their use of contact cues while searching on a plant.

Selection of Nothofagus Host Trees by the Aphids Neuquenaphis staryi and Neuquenaphis edwardsi

Leaf volatiles were collected from three Nothofagus species growing in close proximity in Los Ruiles National Reserve, Chile. The volatile preparation from leaves of No. alessandrii were attractive to the specialist aphid, Neuquenaphis staryi, but not to the generalist aphid, Ne. edwardsi, while the volatile preparations of No. dombeyi and No. glauca were attractive to Ne. edwardsi, but not to Ne. staryi. This reflects the pattern of aphid/host-plant associations. Alpha-Agarofuran was found to occur in all leaf volatile preparations and was shown by electroantennography and olfactometry to be attractive for both Neuquenaphis spp., suggesting it may be the Nothofagus host-recognition factor for Neuquenaphis. The factor(s) mediating Ne. stayi's specialization on No. alessandrii remain to be identified.

Reproductive response of generalist and specialist aphid morphs with the same genotype to plant secondary compounds and amino acids

Many studies have paid particular attention to the role of either secondary plant compounds or amino acids as determinants of host-plant range in phytophagous insects. Here we examine the relative importance of both of these classes of compound in host acceptance by generalist and specialist morphs of the black bean aphid, Aphis fabae, that are morphologically similar and genetically identical. Eleven secondary plant compounds and six amino acids with known biological activity in aphids are presented to insects within an artificial membrane system as single compounds, mixtures of same-class compounds and combined mixtures of the two classes of compound. It is found that 1) when specific single secondary compounds and amino acids are presented to generalist and specialist morphs of A. fabae, differential responses are exclusively consistent with plant-use strategy for amino acids but not for secondary compounds, 2) neither secondary compound nor amino acid mixtures give reproductive responses entirely consistent with plant range, but the response to secondary compounds is broadly consistent with plant range whereas the response to amino acids is not, 3) when secondary compounds and amino acid mixtures are combined, the response to secondary compounds generally dominates that to amino acids. Some scenarios of plant-range determination by secondary plant compounds and amino acids, suggested by results, are discussed.

Elucidation of the Transmission Patterns of an Insect-Borne Bacterium

Quantitative data on modes of transmission are a crucial element in understanding the ecology of microorganisms associated with animals. We investigated the transmission patterns of a gamma-proteobacterium informally known as pea aphid Bemisia-like symbiont (PABS), also known as T-type, which is widely but not universally distributed in natural populations of the pea aphid, Acyrthosiphon pisum. The vertical transmission of PABS to asexual and sexual morphs and sexually produced eggs was demonstrated by a diagnostic PCR-based assay, and the maximum estimated failure rate was 2%. Aphids naturally lacking PABS acquired PABS bacteria administered via the diet, and the infection persisted by vertical transmission for at least three aphid generations. PABS was also detected in two of five aphid honeydew samples tested and in all five siphuncular fluid samples tested but in none of 15 samples of salivary secretions from PABS-positive aphids. However, PABS-negative aphids did not acquire PABS when they were cocultured with PABS-positive aphids; the maximal estimated level of horizontal transmission was 18%. A deterministic model indicated that the force of infection by a horizontal transmission rate of 3% is sufficient to maintain a previously described estimate of the prevalence of PABS-positive aphids (37%), if the vertical transmission rate is 98%. We concluded that PABS infections in A. pisum can be maintained by high vertical transmission rates and occasional horizontal transmission, possibly via the oral route, in the absence of selection either for or against aphids bearing this bacterium.

Decision making by generalist and specialist aphids with the same genotype

One idea to explain the high incidence of specialisation in phytophagous insects is that their nervous systems are simple and unable to efficiently recognise multiple host plants. Here this ‘neural limitations’ hypothesis is tested using a generalist (the summer winged virginopara) and specialist (the autumn gynopara) morph of a single clone of the black bean aphid, Aphis fabae, that are almost identical morphologically and have the same genotype. Electrical monitoring of stylet behaviour and non-invasive behavioural observations are used to compare initial plant recognition, first reproduction, ‘first registered phloem contact’ and ‘phloem acceptance’ of the generalist and specialist on their specific host plants (generalist—herbaceous hosts, Beta vulgaris, Papaver dubium, Rumex obtusifolius, Vicia faba; specialist—woody host, Euonymus europaeus) and on the same host ( E. europaeus). Additional analyses of behaviour over the first few minutes of plant contact are carried out for three mutual non-host plants: Euonymus alatus, Prunus padus and Brassica pekinensis. Few examples are found where the host-selection behaviour of the specialist on its host plant is more efficient than the generalist across its hosts and there is little difference in the behaviour of the different forms on the same host. Indeed there are several instances where the generalist is behaviourally more efficient than the specialist. These include absolute time to first reproduction by the generalist on various herbaceous hosts and the same host, E. europaeus, absolute time to first registered phloem contact by the generalist on P. dubium, and incidence of phloem acceptance by the generalist on several of its herbaceous hosts. Rejection of non-hosts is carried out more efficiently by the specialist gynopara on P. padus and B. pekinensis but not on E. alatus. For generalist and specialist morphs of the same A. fabae genotype, therefore, broad host range does not appear to be associated with a reduced ability to make plant-use decisions.

Influence of prey host plant on a generalist aphidophagous predator: Episyrphus balteatus (Diptera: Syrphidae)

Secondary plant metabolites (allelochemicals) play a major role in plant-insect interactions. Glucosinolates (GLS) and their degradation products from Brassica species are attractants and feeding stimulants for Brassicaceae specialist insects but are generally repellent and toxic for generalist herbivores. The impact of these compounds on crucifer specialist insects are well known but their effect on generalist predators is still not well documented. The influence of the prey's host plant on both development and reproduction of an aphidophagous beneficial, the hoverfly Episyrphus balteatus, was determined using the cabbage aphid, Brevicoryne brassicae (a specialist) and the peach aphid Myzus persicae (a generalist) reared on two crucifer plants, Brassica napus and Sinapis alba containing low and high GLS levels respectively.The prey and its host plant differently influenced life history parameters of E. balteatus. The predator's rates of development and survival did not vary when it fed on the generalist aphid reared on different host plants. These rates decreased, however, when the predator fed on the specialist aphid reared on the host plant with high GLS content plant versus the host plant with lower GLS content. This aphid host plant combination also negatively affected hoverfly reproduction; lower fecundity was observed. As a result, the fitness of the hoverfly was strongly affected. This study illustrates the importance of tritrophic relations in pest management involving predators. The host plant of the prey can have a major influence on the potential of a biological agent to control herbivore species such as aphids.

Lack of detectable genetic recombination on the X chromosome during the parthenogenetic production of female and male aphids.

We used polymorphic microsatellite markers to look for recombination during parthenogenetic oogenesis between the X chromosomes of aphids of the tribe Macrosiphini. We examined the X chromosome because it comprises approximately 25 % of the genome and previous cytological observations of chromosome pairing and nucleolar organizer (NOR) heteromorphism suggest recombination, although the same is not true for autosomes. A total of 564 parthenogenetic females of Myzus clones with three distinct reproductive modes (cyclical parthenogenesis, obligate parthenogenesis and obligate parthenogenesis with male production) were genotyped at three informative X-linked loci. Also, parthenogenetically produced males from clones encompassing the full range of male-producing reproductive strategies were genotyped. These included 391 Myzus persicae males that were genotyped at three X-linked loci and 538 males from Sitobion clones that were genotyped at five informative X-linked loci. Our results show no departure from clonality in parthenogenetic generations of aphids of the tribe Macrosiphini: no recombinant genotypes were observed in parthenogenetically produced males or females.