Wire effects on aphids during electrical recording of stylet penetration

Aphids are attached to a thin flexible goldwire (20–25 m) with conductive silver paint when stylet penetration is recorded electrically. The paint solvents may be toxic and the electrical current passing through the aphid when it is connected to the recording circuit may cause electrical effects. The attachment itself may also disturb normal behaviour; the tether effect. Absence of direct toxic effects is inferred from equal longevity and fecundity in silver paint treated aphids without wires as in untreated insects. Wiring caused a marked reduction of fecundity and longevity whereas also the differences in aphid reactions to host and nonhost plants were significantly reduced when they had been attached to the wires. However, qualitative differences in penetration behaviour, such as the occurrence of sieve element penetration, remained distinct. Since electrical effects are presumably negligible, the effects are supposed to be mainly due to tethering. A special experimental procedure was developed to allow comparison of penetration patterns in free and wired aphids. This type of control experiment should be executed when interpreting results from electrical recording, especially in studies on acceptability of crop plant varieties.

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Résumé Lorsque la pénétration du stylet était enregistrée électriquement les pucerons étaient fixés à un fil d'or fin (20 à 25 m) par une peinture conductrice d,argent. Les solvants de la peinture peuvent être toxiques et le courant électrique passant dans le puceron peut provoquer des effets électriques quand il est relié au circuit d'enregistrement. La fixation elle-même peut perturber le comportement normal. Comme les longévités des pucerons libres et fixéx étaient voisines, un effet toxique est invraisemblable mais la fécondité des pucerons fixés était nettement diminuée. La vixation a réduit significativement les différences entre les réactions avec plantes hôtes ou non. Puisque les effectsélectriques sont présumés négligeables (Tjallingii, 1985a), ces conséquences sont supposées dues principalement à la fixation. Un dispositif expérimental particulier a été mis en place pour permettre une comparaison de la pénétration des stylets chez les pucerons libres et fixes. Ce type de contrôle expérimental doit avoir lieu pour pouvoir interpréter les résultats d'enregistrement électriques, particulièrement lors d'études sur la sélections de variétés de plantes cultivées.

     

Role of micromorphological leaf traits and molecular data in taxonomy of three sympatric white oak species and their hybrids (Quercus L.)

Hybridisation and introgression occur with high frequency in the genus Quercus and interspecific hybrid individuals show patterns of morphological traits that might be influenced in different ways. Micromorphological leaf traits appear to be positive and stable in Quercus species, and by combining genetic and micromorphological analyses, it is possible to compare the patterns of variation in micromorphological leaf traits of pure and hybrid individuals. Trichomes and stomatal traits were examined using scanning electron microscopy at 150–2000 × magnification in sympatric oak species collected in a natural deciduous wood. Q. frainetto, Q. petraea and Q. pubescens appear to have a relatively predictable complement of trichome types. Both the pattern and quantitative values of each micromorphological trait examined (stomata and trichomes) have an important role in identifying hybrids and pure species; putative hybrids show a pattern of trichomes that is a combination of the parental types. These results, combined with the fact that micromorphological traits generally exhibit higher consistency, indicate that this source of information can be an excellent clue to hybridisation and introgression and useful in taxonomical, systematic and evolutionary studies on the European white oaks.     

寄主植物-蚜虫-天敌三重营养关系的化学生态学研究进展

综述了寄主植物－蚜虫－天敌三重营养关系的化学生态学研究,重点阐述了3个研究热点：①植物挥发性物质在蚜虫及其天敌选择寄主行为过程中的作用;②蚜虫信息素和蜜露对蚜虫天敌寄主选择行为的影响;③植物挥发性物质对蚜虫信息系作用的影响。对寄主植物－蚜虫－天敌三重营养关系的全面了解,将为蚜虫的综合治疗提供新思维。     

香港地区蚜虫物种多样性

蚜虫是半翅目中一类体型较小的昆虫,广布于世界各地,其中绝大部分是农林害虫。目前,国内许多地区已开展蚜虫物种多样性和区系研究,但香港地区因蚜虫研究基础薄弱,至今尚未开展过系统研究。作者研究和整理了英国自然历史博物馆和中国科学院动物研究所动物标本馆已有的香港地区蚜虫标本,并参考大量相关文献,从分类阶元、特有物种、动物地理区划和寄主植物四个方面分析了香港地区蚜虫物种多样性。研究发现香港地区已知蚜虫7科44属70种,分别占中国已知科、属、种的53.85%、16.60%和6.83%,多样性较为丰富,其中有香港地区特有种6种。香港地区在动物地理区划中位于东洋界华南区闽广沿海亚区,蚜虫物种以分布于东洋界、古北界 东洋界、东洋界 澳洲界成分为主。现记载香港地区共有蚜虫的寄主植物30科58属,其中蚜科的寄主植物最为丰富,其次为扁蚜科和斑蚜科;寄主植物中寄生蚜虫类群最多的3个科分别为:禾本科、桑科和樟科。虽然香港地区面积仅占中国总面积的万分之一,但该地区复杂的地形地貌、适宜的气候环境以及丰富的植被共同构成了有利于蚜虫生存的小生境,因此蚜虫物种多样性较高,这也是我国南方地区蚜虫物种丰富的鲜明体现。     

Molecular data support a rapid radiation of aphids in the Cretaceous and multiple origins of host alternation

Many aphids display a remarkably complex life cycle of host alternation, in which cyclical parthenogenesis is combined with the obligate use of two unrelated host plants. We used mitochondrial ribosomal DNA (partial 12S and 16S) sequences to reconstruct the phylogeny of aphids, to determine how many origins of host alternation and correlated major host-plant shifts have occurred. Our results agreed with previous morphological studies in that species clustered with good support at the level of tribes. There was little well-supported phylogenetic structure at levels deeper than tribes, however, except for the monophyly of two subfamilies, Aphidinae and Lachninae. We argue that aphids experienced a rapid radiation at the tribal level, after host shifting from gymnosperms to angiosperms. A rapid radiation is consistent with aphid fossils, which record the presence of few subfamilies in the late Cretaceous, but most extant tribes by the early Tertiary. Plant fossils also record host plants of aphid tribes diversifying during this time. A hypothesized mechanism by which host alternation has evolved (fundatrix specialization), coupled with the rapid radiation, implies that this life cycle may have originated as often as in the ancestor of each tribe that displays it. We also consider, however, an alternative hypothesis of fewer origins. The basal radiation of Aphididae was dated from molecular sequences to have occurred at approximately 80–150 Mya.     

Rodent acorn selection in a Mediterranean oak landscape

Quercus suber, Quercus ilex and Quercus coccifera (Cork, Holm and Kermes oaks, respectively) are common evergreen oak species that coexist in the landscapes of the western part of the Mediterranean basin. Rodents are the main acorn predators and thus one of the main factors for understanding recruitment patterns in oaks. In this paper we analyse to what extent mice prefer acorns from one oak species over another in three oak species studied using acorn removal experiments and video tape recordings. Twenty labelled acorns from each of the three Quercus species (60 acorns) were placed in 40 cm×40 cm quadrats on each plot. Because selection might vary as a result of the vegetation context, we performed the trials in the five main vegetation types within the study area (four replicates in each vegetation type) in order to control for habitat influences on rodent acorn preferences (a total of 20 plots). The removal of 1,200 acorns occurred within 68 days. Mice removed 98.7% of the acorns. Q. ilex acorns were preferred over Q. suber and Q. coccifera in all vegetation types except in pine forest, where no acorn preferences were detected. Acorn removal rates differed with vegetation type, correlating positively with shrub cover. The distance at which acorns were displaced by rodents (mean =4.6 m±5.1 SD) did not differ between acorn species, but varied among vegetation types. Bigger acorns of Q. coccifera were selected only after Q. ilex and Q. suber acorns were depleted, while no size selection was detected for the latter two species. Thus, we conclude that rodents show preference for some oak acorns and that landscape context contributes significantly to rodent activities and decisions.     

Mapping resistance to cereal aphids in barley

 A set of 150 doubled-haploid (DH) barley (Hordeum vulgare L.) lines derived from the cross of Harrington/TR306 was used to determine the number and chromosomal location of quantitative trait loci (QTLs) controlling resistance to cereal aphids. The experiments were conducted under natural infestation in the field during two growing seasons: 1994 and 1995. Aphid resistance was measured by counting the number of aphids per plot. Counts were made on a weekly basis. Each year at the time of maximum aphid density there was an obvious difference in reaction between the parental genotypes. The DH lines showed continuous variation for aphid density. Simple interval mapping and simplified composite interval mapping revealed that the principal QTL determining cereal aphid resistance is on the distal region of the short arm of chromosome 1. This aphid-resistance QTL is linked with a heading-date QTL. At the time of highest aphid infestation, this QTL accounted for 31% and 22% of the total variance of aphid density in 1994 and 1995, respectively. A QTL on chromosome 5 was also detected but only by simplified composite interval mapping. However, the largest consistent effect was due to the QTL on the short arm of chromosome 1. This QTL may be a useful target for marker-assisted selection for adult plant cereal aphid resistance in barley. Received: 10 September 1996/Accepted: 11 October 1996     

The integrative effects of population density, photoperiod, temperature, and host plant on the induction of alate aphids in Schizaphis graminum

The wheat aphid Schizaphis graminum (Rondani) displays wing dimorphism with both winged and wingless adult morphs. The winged morph is an adaptive microevolutionary response to undesirable environmental conditions, including undesirable population density, photoperiod, temperature, and host plant. Here we studied the integrative effects of population density, photoperiod, temperature, and host plant on the induction of alate aphids in S. graminum. The present results show that these four factors all play roles in inducing alate aphids in S. graminum but population density is the most important under almost all circumstances. In importance, population density is followed by photoperiod, host plant, and temperature, in that order. These results indicate that ambient environmental factors are highly important to stimulation of alate aphids in S. graminum, especially when population density reaches 64 individuals per leaf.     

Honeydew of aphids as a source of sugar for Phlebotomus ariasi

Wild-caught Phlebotomus ariasi Tonnoir, starved until their sugar meals had been digested, were caged for 24 h with plants or plants infested with aphids and then tested for fructose, a constituent of plant sap. No evidence was found that the flies took sap directly from nine types of plants present in their habitat. About two-thirds of flies caged with oak (Quercus ilex L.) infested with an aphid [Lachnus roboris (L.)] and about a fifth kept with leaves of the french bean (Phaseolus vulgaris L.) infested with an aphid took honeydew. Experiments with three other aphid species gave negative results. Of sandflies caught in an oak tree infested with aphids, half of the females and three-quarters of the males contained fructose. Tests on females caught in a house and grouped according to state of engorgement showed that, having taken blood, the females stop taking sugar until the bloodmeal is completely digested. It is predicted that honeydews are important in the development of Leishmania infantum Nicolle in the gut of P. ariasi.     

Contrasting population structure and demographic history of cereal aphids in different environmental and agricultural landscapes

Genetic diversity of populations has important ecological and evolutionary consequences, whose understanding is fundamental to improve the sustainability of agricultural production. Studies of how differences in agricultural management and environment influence the population structure of insect pests are central to predict outbreaks and optimize control programs. Here, we have studied the population genetic diversity and evolution of Sitobion avenae and Sitobion miscanthi, which are among the most relevant aphid pests of cereals across Europe and Asia, respectively. We have used genotyping by sequencing (GBS) to identify genome‐wide single nucleotide polymorphisms (SNPs) to infer the geographic structure and migration patterns. In the present study, we show that the population structure in present‐day populations is different from that described in previous studies, which suggest that they have evolved recently possibly as a response to human‐induced changes in agriculture. This study shows that S. avenae in England is predominantly parthenogenetic and there has been a demographic and spatial expansion of a single genetic cluster, which could correspond with the insecticide resistance superclone identified in previous studies. Conversely, in China, S. miscanthi populations are mostly cyclical parthenogenetic, with one sexual stage in autumn to produce overwintering eggs, and there are six genetically differentiated subpopulations and high genetic differentiation between geographic locations, which suggests that further taxonomical research is needed. Unlike S. avenae in England, there is no evidence for insecticide resistance and there is no predominance of a single lineage in S. miscanthi in China.     

Suitability of native and ornamental oak species in California for Agrilus auroguttatus

Goldspotted oak borer, Agrilus auroguttatus Schaeffer (Coleoptera: Buprestidae), is a new invasive species in southern California, USA. The extent of the host range of this insect is not known, but this knowledge will have a major impact on assessment of the risks that this pest poses to oaks [Quercus spp. (Fagaceae)]. We conducted laboratory tests to determine the potential suitability of native and ornamental oak species for larvae and adults of A. auroguttatus. We infested 179 cut logs (from 163 different trees) with eggs or larvae, measured neonate survival and, after 5 months, counted feeding galleries, and noted the proportion of galleries with late instars. Initial larval survival was generally high when larvae penetrated the phloem (range: 62–98% among oak species), and low by the time larvae began to feed at the phloem/xylem interface (range: 0–25% among oak species). The majority of larvae that established a visible feeding gallery survived to the fourth instar (total of 73% for all oak species). Larval galleries were established with greater frequency in red oaks (Section Lobatae) compared with other oaks (19 vs. 7 or 4%). All red oaks tested (Q. agrifolia Née, Q. kelloggii Newberry, and Q. wislizeni A. DC.) were likely suitable hosts for larvae. Larvae were apparently able to feed on some of the other oaks (Q. chrysolepis Leibmann, Q. suber L., Q. lobata Née, and Q. douglasii Hook & Arn), although it remains unclear whether these species would be preferred hosts under natural conditions. Adult longevity and fecundity varied little by species of oak foliage fed to adults. The host range of A. auroguttatus is likely limited by suitability of oak species for the larval rather than the adult life stage. Results support published field observations that red oaks are more suitable hosts than white oaks.     

Kin structure provides no explanation for intruders in social aphids

Nontraditional social organisms have received increasing attention in recent years, because they present opportunities to study the convergent properties of social evolution. Some aphid species are social, occurring in dense clones with specialized morphs that attack predators and parasites. Little is known about how social aphid colonies resolve conflicts of interest when clonal barriers break down. Pemphigus obesinymphae is a North American gall-forming social aphid that produces both nymphal defenders that protect natal clones, and specialized intruders that invade other nearby clones on their host plants. We tested the hypothesis that clones are arranged on their host plants in spatial clusters of related family groups, such that intruders would be biased towards movement within kin groups. Movement within and not between kin groups would then provide insight into the nature of conflict in this social aphid. We sampled eight sites in the eastern United States and in Arizona, and used eight microsatellite markers to estimate pairwise relatedness between spatial groups. We found little evidence of deviation from random distributions of genotypes on their host plants. Evidently, Pem. obesinymphae intruders typically exploit unrelated clones, and spatial orientation provides no solution to the problem of 'polyclonality' in this species. We discuss implications of this result for our understanding of cooperation and conflict in social aphids.     

Behavioural response of aphids to the alarm pheromone component (E )-β-farnesene in the field

The alarm response of aphids to (E)‐β‐farnesene was examined for 59 species in the subfamilies Aphididae, Callaphididae, Chaitophoridae and Lachnidae, in the field. Forty‐one aphid species responded to (E)‐β‐farnesene at a dosage of 1 ng and/or 10 ng; however, 18 species did not. Myrmecophilous aphids did not react as extensively or strongly as non‐myrmecophilous aphids. The response of aphids to (E)‐β‐farnesene was related to host‐plant species.     

Terpenes in Pistacia plants: A possible defence role for monoterpenes against gall-forming aphids

A re-investigation of Pistacia vera from southern Italy afforded, in addition to known compounds, two new monoterpenes (+)-9,10-cyclopropylterpine-     

Use it or lose it: reproductive implications of ecological specialization in a haematophagous ectoparasite

Using experimentally induced disruptive selection, we tested two hypotheses regarding the evolution of specialization in parasites. The 'trade-off' hypothesis suggests that adaptation to a specific host may come at the expense of a reduced performance when exploiting another host. The alternative 'relaxed selection' hypothesis suggests that the ability to exploit a given host would deteriorate when becoming obsolete. Three replicate populations of a flea Xenopsylla ramesis were maintained on each of two rodent hosts, Meriones crassus and Dipodillus dasyurus, for nine generations. Fleas maintained on a specific host species for a few generations substantially decreased their reproductive performance when transferred to an alternative host species, whereas they generally did not increase their performance on their maintenance host. The results support the 'relaxed selection' hypothesis of the evolution of ecological specialization in haematophagous ectoparasites, while suggesting that trade-offs are unlikely drivers of specialization. Further work is needed to study the extent by which the observed specializations are based on epigenetic or genetic modifications.     

Genomewide signatures of selection in Epichloë reveal candidate genes for host specialization

Host specialization is a key process in ecological divergence and speciation of plant‐associated fungi. The underlying determinants of host specialization are generally poorly understood, especially in endophytes, which constitute one of the most abundant components of the plant microbiome. We addressed the genetic basis of host specialization in two sympatric subspecies of grass‐endophytic fungi from the Epichloë typhina complex: subsp. typhina and clarkii. The life cycle of these fungi entails unrestricted dispersal of gametes and sexual reproduction before infection of a new host, implying that the host imposes a selective barrier on viability of the progeny. We aimed to detect genes under divergent selection between subspecies, experiencing restricted gene flow due to adaptation to different hosts. Using pooled whole‐genome sequencing data, we combined F_ST and D_XY population statistics in genome scans and detected 57 outlier genes showing strong differentiation between the two subspecies. Genomewide analyses of nucleotide diversity (π), Tajima's D and dN/dS ratios indicated that these genes have evolved under positive selection. Genes encoding secreted proteins were enriched among the genes showing evidence of positive selection, suggesting that molecular plant–fungus interactions are strong drivers of endophyte divergence. We focused on five genes encoding secreted proteins, which were further sequenced in 28 additional isolates collected across Europe to assess genetic variation in a larger sample size. Signature of positive selection in these isolates and putative identification of pathogenic function supports our findings that these genes represent strong candidates for host specialization determinants in Epichloë endophytes. Our results highlight the role of secreted proteins as key determinants of host specialization.     

Earlywood vessel size of oak as a potential proxy for spring precipitation in mesic sites

Aim In this study, we evaluate the importance of the mean earlywood vessel size of oaks as a potential proxy for climate in mesic areas. Location The study was conducted in Switzerland at three forest sites dominated by oak (Quercus petraea and Q. pubescens). The three sites were in different climatic zones, varying mainly in terms of precipitation regime. Methods Three 50‐year‐long site chronologies of mean earlywood vessel size and tree‐ring widths were obtained at each site and related to monthly meteorological records in order to identify the main variables controlling growth. The responses of mean vessel size to climate were compared with those of the width variables to evaluate the potential climatic information recorded by the earlywood vessels. Results The results show that the mean vessel size has a different and stronger response to climate than ring‐width variables, although its common signal and year‐to‐year variability are lower. This response is better in particular at mesic sites, where it is linked to precipitation during spring, i.e. at the time of vessel formation, and is probably related to the occurrence of only a few processes controlling vessel growth, whereas radial increment is controlled by multiple and varying factors. Main conclusions The mean earlywood vessel size of oak appears to be a promising proxy for future climate reconstructions of mesic sites, where radial growth is not controlled by a single limiting factor.     

The efficiency of a new lightweight suction sampler for sampling aphids and their predators in arable land

A petrol driven machine, normally used to collect leaf litter, was modified to be used as a suction sampler for polyphagous predators in cereals and grassy habitats and aphids in cereals. Recovery efficiency of Araneae, Carabidae and Staphylinidae did not differ significantly between winter sampling from Agrostis stolonifera, Dactylis glomerata, Festuca rubra, Holcus lanatus or Lolium perenne although the structure of these grasses was very different. Summer sampling of aphids and their predators in winter wheat showed that the new sampler captured significantly more target organisms per unit area than did a traditional suction sampler. The machine was also lighter, cheaper and much easier to use than a traditional machine.