Summer Host Plants Research Articles

Ecological divergence despite common mating sites: Genotypes and symbiotypes shed light on cryptic diversity in the black bean aphid species complex.

Different host plants represent ecologically dissimilar environments for phytophagous insects. The resulting divergent selection can promote the evolution of specialized host races, provided that gene flow is reduced between populations feeding on different plants. In black bean aphids belonging to the Aphis fabae complex, several morphologically cryptic taxa have been described based on their distinct host plant preferences. However, host choice and mate choice are largely decoupled in these insects: they are host-alternating and migrate between specific summer host plants and shared winter hosts, with mating occurring on the shared hosts. This provides a yearly opportunity for gene flow among aphids using different summer hosts, and raises the question if and to what extent the ecologically defined taxa are reproductively isolated. Here, we analyzed a geographically and temporally structured dataset of microsatellite genotypes from A. fabae that were mostly collected from their main winter host Euonymus europaeus, and additionally from another winter host and fourteen summer hosts. The data reveals multiple, strongly differentiated genetic clusters, which differ in their association with different summer and winter hosts. The clusters also differ in the frequency of infection with two heritable, facultative endosymbionts, separately hinting at reproductive isolation and divergent ecological selection. Furthermore, we found evidence for occasional hybridization among genetic clusters, with putative hybrids collected more frequently in spring than in autumn. This suggests that similar to host races in other phytophagous insects, both prezygotic and postzygotic barriers including selection against hybrids maintain genetic differentiation among A. fabae taxa, despite a common mating habitat.

Cattle Graze Central US Milkweeds at Least as Much as Grasses, Even Under Patch-Burn-Grazing Management

Most researchers assume cattle avoid milkweeds due to toxic compounds. However, observations suggest cattle graze common milkweed (Asclepias syriaca L.), an important summer host plant for monarch butterflies (Danaus plexippus L.) east of the Rocky Mountains, as well as showy milkweed (Asclepias speciosa Torr.). This directly relates to efforts to add over 1 billion stems of milkweed for monarch butterfly conservation. We sampled whether cattle regularly graze common/showy milkweed in a central Nebraska restored prairie patch-burn-grazing system with patches burned in the year of the main study (2018) or previous years (2017–2016). We also sampled burned control grasslands without livestock. This sampling therefore compared three different types of burn-graze management. Cow-calf pairs were put onto the grasslands in April, and we sampled every 2 wk through September. We found that wild grazers (e.g., deer) were not grazing common/showy milkweed stems in control grasslands, whereas by August > 73% of stems had been grazed in the patch-burn-grazed grasslands—with no noticeable negative effects on cattle. Also, cattle grazing of common/showy milkweed did not differ significantly in different patches, even though big bluestem grass (Andropogon gerardii Vitman) was grazed significantly more in patches burned in 2018. Finally, milkweed abundance in control grasslands ranged from 34- to 185-fold higher than the average milkweed abundance in patch-burn-grazed grasslands. These results suggest that cattle regularly graze common/showy milkweed and that patch-burn-grazing does not increase milkweed abundance when cattle begin grazing in April. The good news of these results is that cattle safely grazed low amounts of common/showy milkweed, suggesting control efforts for these milkweeds are often unnecessary. Future work should examine how to increase milkweed abundance without affecting cattle weight gain, possibly by deferring grazing in some areas during critical milkweed growth periods.

The Effect of Wind Speed on Male Potato Aphid, Macrosiphum euphorbiae, Responses to Primary Host Plant Volatiles and Female Sex Pheromone.

Simple SummaryWe demonstrate that male potato aphids respond more to a combination of volatiles from the host plant on which the species overwinters and the female sex pheromone than to the host plant alone. In both cases, the level of response declines as wind speed increases, but the higher attraction to the combined odour sources is maintained. These findings are discussed within the context of mate location by male aphids, which are insects that have little control over flight direction.In fall, alate males of the potato aphid, Macrosiphum euphorbiae (Thomas), migrate from their summer (secondary) host plants, such as potatoes, to primary host plants, such as roses, where they mate with wingless oviparae who produce the overwintering egg stage. Males are weak fliers and generally walk towards a pheromone source under windy conditions, so we tested the hypothesis that upwind walking behaviour in response to wind velocity would be affected by the volatile cues present. We compared male responses to the odour of a rugosa rose cutting alone and to the combination of host plant volatiles and the female sex pheromone under a range of wind speeds in a laboratory walking bioassay. The proportion of males responding decreased as the wind speed increased, but at all wind velocities, the responses to the combined odours were higher than to the host plant alone. However, at any given wind velocity, the speed at which responding aphids moved was not influenced by the odour source. These findings support the idea that host plant volatiles serve as long-distance cues for males and that the female sex pheromone is used once on the host plant.

Microbiome diversity of cotton aphids (Aphis gossypii) is associated with host alternation

Aphids are infected by a series of bacteria that can help them survive on specific host plants. However, the associations between aphids and these bacteria are not clear, and the bacterial communities in many aphid species are poorly characterized. Here, we investigated the bacterial communities of cotton aphids (Aphis gossypii) on 2 representative winter host plants and transferred to 3 summer host plants by 16S rDNA sequencing using the Illumina MiSeq platform. Our results revealed that the bacterial communities varied among cotton aphids on hibiscus, cotton aphids on pomegranate, cotton aphids on cotton transferred from hibiscus, cotton aphids on muskmelon transferred from hibiscus, cotton aphids on cucumber transferred from hibiscus,. The diversity and richness of the bacterial communities were significantly higher in aphids on muskmelon and aphids on cucumber than in the other treatments. There were two main factors influencing the distribution of internal bacterial OTUs revealed by principal component analysis, including the differences among Punicaceae, Malvaceae and Cucurbitaceae. There were 28 bacterial communities with significant differences between two arbitrary treatments, which could be grouped into 6 main clusters depending on relative abundance. Moreover, our results indicated that in addition to the obligate endosymbiont Buchnera, with a dominant position (> 52%), A. gossypii also harbored 3 facultative endosymbiotic bacteria (Serratia, Arsenophonus, and Wolbachia) and 3 possibly symbiotic bacteria (Acinetobacter, Pantoea, and Flavobacterium). There were several correspondences between the symbiotic bacteria in cotton aphids and the specific host plants of the aphids. This study provides a better understanding of the interactions among symbiotic bacteria, aphids and host plants, suggesting that the selection pressure on aphid bacterial communities is likely to be exerted by the species of host plants.

Evaluation of Host Stinging Behavior of the Larval Ectoparasitoid Diglyphus isaea on Some Selected Host Plants

Background: In parasitoid insects, beside parasitism, there are two other behaviors that may increase leaf miner mortality: Feeding on the host (host feeding) and, host penalization without oviposition or feeding (host stinging).
 Objectives: The present study aimed to evaluate (host stinging) behavior of the larval ectoparasitoid D. isaea in field on some selected host plants.
 Methods: The present study was carried out in Alejelat region. Four selected host plants were studied, chick pea (Cicer arietinum) and broad bean (Vecia faba) as a Winter host plants and, Cow pea (Vigna unguiculate) and kidney bean (Phaseolus vulgaris) were selected as summer host plants during the growing seasons 2018 and 2019. Samples were kept in plastic bags and transferred to be examined in the laboratory. Number of living L. trifolii larvae, and number of killed larvae according to feeding (no oviposition) by D. isaea were counted and recorded.
 Results: On cow pea host stung larvae recorded (156 individuals/ 100 infested leaflets) occurred in 10th of August, with (r = +0.69**) between host stung larvae and the population of L. trifolii recorded). On kidney bean host stung larvae recorded (101 individuals/ 100 infested leaflets) occurred in 24th of August, with (r = + 0.85**) value between host stung larvae and the population of L. trifolii recorded. On the other hand, the percentage of host stung larvae ranged between (5.67% and 28.22%) and (15.88% and 42.30%) for cow pea and kidney bean respectively. On broad bean host stung larvae recorded (159 individuals/ 100 infested leaflets), with (r = +0.59*) between host stung larvae and the population of L. trifolii .On chick pea host stung larvae recorded (82 individuals/ 100 infested leaflets) with (r = +0.20ns) between host stung larvae and the population of L. trifolii. On the other hand, percentages of host stung larvae ranged between (8.45% and 36.60%) and (21.80% and 87.65%) for broad bean and chick pea respectively.
 Conclusion: The highest percentage of host stung larvae recorded on chick pea 44.65% followed by kidney bean 24.21%, broad bean 18.45% and cow pea 12.97% respectively. On the other hand, the correlation coefficient value (r) between numbers of L. trifolii larvae and, host stung larvae recorded the highest on kidney bean +0.85** followed by cow pea +0.69**, Broad bean +0.59* and Chick pea +0.20ns respectively.

The chemoreceptors and odorant binding proteins of the soybean and pea aphids

We examined the genome of the soybean aphid, Aphis glycines, and an updated genome assembly of the pea aphid, Acyrthosiphon pisum, for members of the three major families of chemoreceptors, the Odorant Receptors (ORs), Gustatory Receptors (GRs) and Ionotropic Receptors (IRs), as well as the Odorant Binding Proteins (OBPs). The soybean aphid has 47 ORs, 61 GRs, 19 IRs, and 10 OBPs, compared with 87 ORs, 78 Grs, 19 IRs, and 18 OBPs in the pea aphid, with variable numbers of pseudogenes in the OR and GR families. Phylogenetic analysis reveals that while all of the IRs are simple orthologs between these two species, the OR, GR, and OBP families in the pea aphid have experienced major expansions of particular gene lineages and fewer losses of gene lineages. This imbalance in birth-and-death of chemosensory genes has led to the larger pea aphid gene repertoire, which might be related to the broader host range of pea aphids versus the specialization of soybean aphids on a single summer host plant. Examination of the expression levels of these chemosensory genes in parthenogenetic and sexual females and males of pea aphids revealed multiple genes that are differentially expressed in sexual females or males and might be involved in reproductive biology. Examination of the soybean aphid genes in parthenogenetic females under multiple stressors revealed multiple genes whose expression levels changed with heat or starvation stress, the latter potentially important in finding new food sources.

Cover Caption

Insect ScienceVolume 24, Issue 3 CONTENTSFree Access Cover Caption First published: 25 May 2017 https://doi.org/10.1111/1744-7917.12388AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract Host alternation has significant consequences for the diversification and success of the superfamily of aphids. We explored the underlying molecular mechanism through a large-scale gene expression analysis on the mealy aphid Hyalopterus persikonus, a worldwide pest to stone fruits, collected from its winter and summer host plants. More than four times as many unigenes of the mealy aphid were significantly upregulated on summer host. Putative salivary gland expressed genes and genes encoding secretory proteins were given special attentions (see pages 431–442). Photo provided by Le Kang. Volume24, Issue3June 2017 RelatedInformation

Large-scale gene expression reveals different adaptations of Hyalopterus persikonus to winter and summer host plants.

Host alternation, an obligatory seasonal shifting between host plants of distant genetic relationship, has had significant consequences for the diversification and success of the superfamily of aphids. However, the underlying molecular mechanism remains unclear. In this study, the molecular mechanism of host alternation was explored through a large-scale gene expression analysis of the mealy aphid Hyalopterus persikonus on winter and summer host plants. More than four times as many unigenes of the mealy aphid were significantly upregulated on summer host Phragmites australis than on winter host Rosaceae plants. In order to identify gene candidates related to host alternation, the differentially expressed unigenes of H. persikonus were compared to salivary gland expressed genes and secretome of Acyrthosiphon pisum. Genes involved in ribosome and oxidative phosphorylation and with molecular functions of heme-copper terminal oxidase activity, hydrolase activity and ribosome binding were potentially upregulated in salivary glands of H. persikonus on the summer host. Putative secretory proteins, such as detoxification enzymes (carboxylesterases and cytochrome P450s), antioxidant enzymes (peroxidase and superoxide dismutase), glutathione peroxidase, glucose dehydrogenase, angiotensin-converting enzyme, cadherin, and calreticulin, were highly expressed in H. persikonus on the summer host, while a SCP GAPR-1-like family protein and a salivary sheath protein were highly expressed in the aphids on winter hosts. These results shed light on phenotypic plasticity in host utilization and seasonal adaptation of aphids.

Disentangling the visual cues used by a jumping spider to locate its microhabitat.

Many arthropod species have evolved to thrive only on the leaves of a particular species of plant, which they must be capable of finding in order to survive accidental displacement, developmental transitions or the changing of the seasons. A number of studies have tested whether such species select leaves to land or oviposit on based on their color, shape or size. Unfortunately, many studies did not control for correlates of these characters, such as the brightness of different colors, the areas of different shapes, and the level of ambient illumination in the vicinity of different sizes of leaves. In the present study, we tested for leaf color, shape and size preferences in a leaf-dwelling jumping spider (Lyssomanes viridis) with known summer and winter host plants, while controlling for these correlates. First, color preferences were tested outdoors under the natural illumination of their forest habitat. Lyssomanes viridis did not prefer to perch on a green substrate compared with various shades of gray, but did prefer the second darkest shade of gray we presented them with. Of the green and gray substrates, this shade of gray's integrated photon flux (350-700 nm), viewed from below, i.e. the spider's perspective in the arena, was the most similar to that of real leaves. This relationship also held when we weighted the transmitted photon flux by the jumping spiders' green photopigment spectral sensitivity. Spiders did not prefer the star-like leaf shape of their summer host plant, Liquidambar styraciflua, to a green circle of the same area. When given a choice between a L. styraciflua leaf-shaped stimulus that was half the area of an otherwise identical alternative, spiders preferred the larger stimulus. However, placing a neutral density filter over the side of the experimental arena with the smaller stimulus abolished this preference, with spiders then being more likely to choose the side of the arena with the smaller stimulus. In conclusion, L. viridis appears to use ambient illumination and possibly perceived leaf brightness but not leaf shape or color to locate its microhabitat. This calls for a careful re-examination of which visual cues a variety of arthropods are actually attending to when they search for their preferred host species or microhabitat.

Host Plant Volatiles and the Sexual Reproduction of the Potato Aphid, Macrosiphum euphorbiae.

In late summer, heteroecious aphids, such as the potato aphid, Macrosiphum euphorbiae, move from their secondary summer host plants to primary host plants, where the sexual oviparae mate and lay diapausing eggs. We tested the hypothesis that volatiles of the primary host, Rosa rugosa, would attract the gynoparae, the parthenogenetic alate morph that produce oviparae, as well as the alate males foraging for suitable mates. In wind tunnel assays, both gynoparae and males oriented towards and reached rose cuttings significantly more often than other odour sources, including potato, a major secondary host. The response of males was as high to rose cuttings alone as to potato with a calling virgin oviparous female. These findings are discussed within the seasonal ecology of host alternating aphids.

Mitochondrial COI and morphological specificity of the mealy aphids (Hyalopterus ssp.) collected from different hosts in Europe (Hemiptera, Aphididae)

Forty three European population samples of mealy aphids from various winter and summer host plants were attributed to respective species of Hyalopterus by means of their partial sequences of mitochondrial COI gene. Used Hyalopterus samples emerged as monophyletic relative to outgroup and formed three major clades representing three host specific mealy aphid species in the Neighbor joining, Maximum parsimony, Maximum likelihood and Bayesian inference trees. Hyalopterus pruni and Hyalopterus persikonus emerged as a sister species, whilst Hyalopterus amygdali was located basally. Samples representing different clades in the molecular trees were used for canonical discrimination analysis based on twenty two morphological characters. Length of the median dorsal head hair enabled a 97.3 % separation of Hyalopterus amygdali from the remaining two species. No single character enabled satisfactory discrimination between apterous viviparous females of Hyalopterus pruni and Hyalopterus persikonus. A modified key for the morphological identification of Hyalopterus species is suggested and their taxonomic status discussed.

Host acceptance by aphids: Probing and larviposition behaviour of the bird cherry-oat aphid, Rhopalosiphum padi on host and non-host plants

The probing and larviposition behaviour of the bird cherry-oat aphid, Rhopalosiphum padi on summer and winter host plants were investigated using electrical penetration graph (EPG) coupled with simultaneous video recording. In this way the precise probing history prior to parturition can be monitored and the location of possible reproductive stimulants identified. On the host plant, all gynoparae (autumn winged females that give birth to sexual females on bird cherry, Prunus padus, the primary host) and 55% of winged virginoparae (summer females which produce further virginoparae on barley, Hordeum vulgare, a secondary host) initiated larviposition before phloem contact. However, 90% of wingless virginoparae (on barley) contacted the phloem before first larviposition whilst 10% did not. Thus, phloem contact does not appear to be a pre-requisite for these aphid forms to initiate reproduction.

Biology of Trioza apicalis - a review

The morphology and biology of immature stages and the distinguishing characters of generations in summer are briefly reviewed. The time of migration of the overwintered generation and the use of the median flight time are given. The longevity of overwintered adults, and the start and course of oviposition by overwintered females were observed mainly during June and July. The interval between adult emergence and flights from carrots appears to be very short, 2–3 days. Departure from developmental summer host plants culminates at the beginning of September. The sex ratio of the overwintered generation is not persistent, since it is usually female-biased. In emerged adults in summer the ratio is 50:50, with weak protandry. The systemic action of sucking for the curling of leaves was found by Láska already in 1964. Inliterature records, both subspecies of Daucus carota dominate as developmental hosts, more recently Coriandrum sativum was shown to be more suitable than Petroselinum hortense. Norway spruce, and to a lesser extent other conifers, are predominantly mentioned as overwintering shelter plants. It is not clear how overwintering occurs in regions without coniferous forests, and only undefined shrubs or trees are given in the literature. The first damage was reported inDenmark about in 1896, spreading from about 1918 over just a few years to the other Fennoscandian countries, and later in some other European countries. The spring migration is discussed, which occurs in central and northern Europe on the same date, perhaps explicable by the orientation of this pest or its particular populations to long days, despite the climate and phenology of each region. It is hypothesised that a particularly aggressive race evolved inSjaelland (Denmark) at the turn of the nineteenth and twentieth century, which later spread to other regions of northern and centralEurope. 

Occurrence of cotton aphids (Aphis gossypii) and lady beetles (Hamonia axyridis) on Hibiscus syriacus Linne: Are the aphids a pest of cucurbits?

AbstractPopulations of cotton aphid on Hibiscus syriacus increased rapidly from 17 to 24 May 2007, and then decreased as its predator, the lady beetle Hamonia axyridis, increased in number. There was a 10 day time lag between peak populations of aphids and lady beetles. The infestation of aphids on H. syriacus produced some damage, but H. syriacus recovered soon after the lady beetles arrived. Cotton aphid clones from H. syriacus were transferred to other summer host plants: to five different vegetables on two dates, and to cucumber on three dates. Apart from one case where reproduction occurred on eggplant, most H. syriacus aphid clones did not survive on the vegetables. The cotton aphid on H. syriancus was prey and a food source for H. axyridis and acted to conserve natural enemies.

Honeydew production and honeydew sugar composition of polyphagous black bean aphid, Aphis fabae (Hemiptera: Aphididae) on various host plants and implications for ant-attendance

The black bean aphid, Aphis fabae, is polyphagous and its life cycle involves seasonal migration between summer and winter host plants. The aphids are regularly tended by honeydew-collecting ants. This study investigates whether differences exist in honeydew production and honeydew sugar composition for A. fabae subspecies feeding on various host plants and whether such differences reflect differences in the intensity of ant-atttendance (Lasius niger).A. f. fabae feeding on the perennial summer host, Tanacetum vulgare, produced twice the amount of honeydew (ca. 110 µg per aphid . h-1) than when feeding on the annual host plants Vicia faba or Chenopodium album. Honeydew production of A. f. cirsiiacanthoides feeding on the creeping thistle Cirsium arvense was the highest measured in this study (ca. 150 µg per aphid . h-1). Total sugar concentration in the honeydew of A. f. fabae did not differ when feeding on various summer hosts, whereas the honeydew of A. f. cirsiiacanthoides on C. arvense contained a significantly higher amount of total sugars. The trisaccharide melezitose was the dominant sugar in all the honeydew samples, except for the honeydew of A. f. fabae and A. f. evonymi feeding on the woody winter host plant Evonymus europaeus. The highest proportion of melezitose (80% of total sugars) was found in the honeydew of A. f. cirsiiacanthoides feeding on C. arvense. In this subspecies, the intensity of ant-attendance was also highest. The results confirm our hypothesis, that the sugar richness of the honeydew (rate of honeydew secretion × total sugar concentration) along with the presence of the attractant sugar melezitose are the critical factors in determining the extent of ant-attendance.In A. f. fabae feeding on the spindle tree E. europaeus, the total sugar concentration of the honeydew as well as the sugar composition differed significantly between generations.

Life history pattern, host plants, and habitat as determinants of population survival of Pieris napi oleracea interacting with an introduced braconid parasitoid

Abstract Pieris napi oleracea Harris is a native pierid butterfly whose range in New England has decreased since the 1869 invasion of an exotic congener, Pieris rapae (L.). Populations have disappeared from all but a few locations in western Massachusetts, but persist more widely in northern Vermont. Parasitism by the introduced braconid wasp Cotesia glomerata (L.) has been postulated as responsible for this decline. Field exposures of cohorts of P. napi larvae in 2002 showed that, on the summer host plant Sisymbrium officinale (L.) Scoop., parasitism rates of second generation butterfly larvae were very high (100% in Massachsusetts and 66% in Vermont). Direct observations of host plant and habitat use patterns by P. napi females in northern Vermont in 2000 showed that this species changes habitats between the first and second generation, with over 95% of eggs of the first generation of P. napi being laid in wooded habitats on the crucifer Cardamine (formerly Dentaria) diphylla (Michx.) Wood. Second generation butterflies fly in meadows and oviposit on S. officinale and other crucifers in full-sun habitats. The second flight of P. napi is, however, only a partial generation, with many first generation pupae entering diapause. The proportion of butterflies entering diapause increases if eggs are laid on C. diphylla plants at or past the flower bud stage. Of first instar P. napi reared outdoors on pre-flower-bud C. diphylla, 63% (n=64) entered diapause in Massachusetts, but 86% (n=119) did so in northern Vermont. Higher rates of diapause in early larval groups in northern Vermont, plus lower rates of parasitism of second generation larvae in meadows, as compared to Massachusetts, indicate that the impact of C. glomerata on total population survival is lower in Vermont and is likely the reason P. napi has persisted in northern Vermont, but not in western Massachusetts. This case illustrates how non-target effects of parasitoids may be mediated by habitat and life history features of populations of affected insects, which may differ geographically.

Seasonal Migration of Apolygus spinolae (Hemiptera: Miridae) between Grapevines and Herbaceous Plants

This study was conducted to verify the seasonal migration of Apolygus spinolae (Meyer-Dür) between grapevines and herbaceous plants. Overwintering eggs were hidden in the hair layer under grapevine bud scales. A. spinolae adults were captured on sticky traps in the grapevine yard from late spring to early summer, dwindled through the summer, and captured again in late fall. However, adults were observed from early summer in herbaceous plant fields. A. spinolae adults were abundant during the summer of July and August in the herbaceous field, and thereafter its density decreased through fall. A few or no A. spinolae was found on mesh-netted grapevines after the installation year of the mesh-net, which indicated that A. spinolae adults migrating to lay overwintering eggs during the autumn could not land at the grapevines because of the mesh-net. Damaged shoots by A. spinolae were concentrated near the edge of grapevine yards bordering the herbaceous plants. This distribution pattern of shoot damage was believed to be related to an oviposition behavior of A spinolae, reflecting that adults migrating from herbaceous plants lay eggs more frequently in grapevines adjacent to the summer host plants. Seasonal occurrence of A. spinolae in grapevine yards was suggested as follows: A. spinolae overwinter as eggs in dormant buds on grapevines and hatch in the spring. Nymphs feed on grapevines then develop to adults (spring population), and migrate to herbaceous plants. A. spimolae spends the summer on the herbaceous hosts (summer population). Then, adults migrate back to grapevines in late autumn and lay overwintering eggs.

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.

The chemical ecology of aphid host alternation: How do return migrants find the primary host plant?

The life cycle of some aphid species involves seasonal switches between unrelated summer (secondary) and winter (primary) host plants. Many of these “host-alternating” species, belonging to the sub-family Aphidinae, produce two return migrant forms on secondary host plants in autumn. Winged females (gynoparae) are produced first; these locate the primary host and deposit their sexual female offspring (oviparae). Later, males are produced on the secondary host and these locate the primary host independently before mating with the oviparae. The mechanisms of primary-host location by gynoparae and males are reviewed in this paper. Studies with several aphid species indicate that both forms are able to respond to volatile cues released by their specific primary host plant. Plant odours may also enhance or modify the responses of return migrants to the sex pheromone released by mature oviparae. Aphids are also able to sample non-volatile plant chemicals after landing, but there have been very few detailed investigations of the behaviour of return migrants at the primary-host-plant surface. Recent experiments with gynoparae of the black bean aphid, Aphis fabae Scopoli, show that these insects detect primary-host-specific cues during stylet penetration of peripheral plant tissues, and these stimuli promote settling and reproduction. Similar behavioural studies with males are required to shed light on the processes of speciation and reproductive isolation in host-alternating aphids.

Nutritional quality of phloem sap in relation to host plant-alternation in the bird cherry-oat aphid

Host alternation in aphids has been attributed to complementary growth of host plants, or more specifically to seasonal changes in the nitrogen quality of the phloem sap. In this report, seasonal fluctuation of free amino acids in phloem of the winter and summer host plants (Prunus padus, bird cherry and Hordeum vulgare, barley) of Rhopalosiphum padi (the bird cherry-oat aphid) were investigated in the context of aphid growth and behaviour. Phloem was collected from the cut stylets of aphids taken from plants that were grown outdoors. The total concentration of amino acids in P. padus phloem increased between bud break and late flush (spring), decreased in mature leaves (summer), and increased again in early senescent leaves (autumn). In H. vulgare, however, amino acid concentration fluctuated less from seedlings to flowering. Spring aphids from P. padus grew rapidly on this host from bud break to late flush, but died on mature and early senescent leaves. Summer aphids from H. vulgare grew as fast on this host as spring aphids did on flush leaves of P. padus. Sexual females grew more slowly than other generations and nearly as well on mature as on early senescent P. padus leaves. As judged by aphid growth and phloem nitrogen quality, P. padus during spring equals the summer host H. vulgare. However, the lower growth rates of R. padi on mature and senescent leaves of P. padus appear only loosely correlated with phloem amino acid concentrations. Therefore, factors influencing aphid nutrition, or ecology, other than seasonal changes in phloem sap amino acid concentration may explain the existence of host alternation in R. padi.