Search

Models of coexistence of sexual and asexual lineages in aphids assume that obligate parthenogenetic lineages predominate in areas with mild winter climate because of their high reproductive output, while sexual lineages predominate in areas with severe winter because they produce eggs resistant to frost. To validate this hypothesis in natural conditions, the reproductive mode of populations of the aphid Sitobion avenae was assessed in two very contrasting climatic situations, Romania (severe winter) and Western France (mild winter). To achieve this, reproductive modes were inferred from both (1) the population composition in sexual and asexual forms in autumn, and (2) the genetic structure of Romanian and French populations of S. avenae using microsatellite markers. Romanian populations encompassed a high proportion of sexual forms and were characterised by a very high genotypic diversity and low linkage disequilibrium. In constrast, the French population showed frequent linkage disequilibria, low genetic diversity, and high level of clonal amplification with two asexual genotypes representing over 60% of the sample. In agreement with the model's predictions, these results clearly indicate that sexual reproduction in S. avenae is predominant under the continental climate of Romania, while asexual lineages prevail under the oceanic climate of Western France.

Parthenogenesis is the main mode of reproduction of aphids. Their populations are therefore composed of clones whose frequency distribution varies in space and time. Previous population genetic studies on aphids have highlighted the existence of highly abundant clones (‘super-clones’), distributed over large geographic areas and persisting over time. Whether the abundance of ‘super-clones’ results from their ecological success or from stochastic forces, such as drift and migration, is an open question. Here, we looked for the existence of clines in clonal frequency along a climatic gradient in the cereal aphidRhopalosiphum padi(Linnaeus, 1758) and examined the possible influence of geographical distance and environmental variables in the buildup and maintenance of such clonal clines. We investigated the spatial distribution of the commonest genotypes ofR. padiby sampling populations along an east–west transect in maize fields in the northern half of France in both spring and late summer. Individual aphids were genotyped at several polymorphic loci, allowing the assessment of frequency distributions of multilocus genotypes (MLGs) across the cropping season. We found several MLGs showing longitudinal clines in their frequency distribution in both spring and summer. In particular, two dominant asexual genotypes ofR. padishowed inverted geographical clines, which could suggest divergent adaptations to environmental conditions. We concluded that while the distribution of some ‘super-clones’ ofR. padiseems most likely driven by the action of migration and genetic drift, selection could be also involved in the establishment of longitudinal clines of others.

Aset of 39 F1 Sitobion avenae clones was obtained by selfing a poorly efficient BYDV-PAV vector clone. These clones were genetically typed by 11 microsatellite loci, and tested for BYDV-PAV4 transmission to barley. The 39 clones displayed a continuum in transmission percentages, from 0% to 88% with a significant clone effect. From this set, two highly efficient (HEV) and two poorly efficient (PEV) vectoring clones were more precisely characterized for transmission of two other PAV isolates. The molecular bases of the lower transmissibility of BYDV-PAV4 by PEV clones and of the aphid vectoring properties were investigated respectively by comparing the sequences corresponding to structural proteins (CP and RTD) of BYDV, and by using proteomic analysis of aphids in two dimensional electrophoresis (2-DE) with immobilized pH gradients (IPG) after an improved protein extraction. Four residues specific to BYDV-PAV4 located in the CP sequence (A(24) and L(130)) or in the RTD region (M(334) and S(456)) could be responsible for the lower transmissibility of this isolate by PEV clones. Among a total of 2150 well-resoluted spots scored on S. avenae proteinic pattern, only twelve proteins were qualitatively or quantitatively different between clones. Four out of them discriminated HEV and PEV groups.

As French populations of the aphid Sitobion avenae exhibit a range of reproductive modes, this species provides a good opportunity for studying the evolution of breeding system variation. The present analysis combined ecological and genetic investigations into the spatial distribution of variation in reproductive mode. Reproductive mode was characterized in 277 lineages of S. avenae from France, and these aphids were scored for five microsatellite loci. The analyses revealed strong geographical partitioning of breeding systems, with obligate asexuals mostly restricted to the south of France, while lineages producing sexual forms were more common in the north. Contrary to what might be anticipated for organisms with frequent parthenogenesis, there was substantial genic and genotypic diversity, even in the obligately asexual lineages. More than 120 different genotypes were detected among the 277 aphid lineages, with an average of 5.9 alleles per locus (range four to 16) and heterozygosity of 56.7%. As with previous studies of allozyme variation in aphids, most loci showed heterozygote deficits, and disequilibrium was common among allelic variants at different loci, even after removal of replicate copies of genotypes that might have been derived through clonal reproduction. Our results suggest that selection is important in structuring reproductive systems and genetic variation in French S. avenae. Canonical correspondence analysis was employed to examine the associations between genotypic and phenotypic variables, enabling the identification of alleles correlated with life-history traits.

E. SADEGHI, C. A. DEDRYVER*and J. P. GAUTHIERINRA, Laboratoire de Zoologie, domaine de la Motte, 35653 Le Rheu cedex, FranceVector efficiency of 20Rhopalosiphum padi clones, originating from Europe, North America and NorthAfrica and exhibiting different types of life cycle, was evaluated by transmitting a French BYDV-PAVisolate to barley plants under five different acquisition/inoculation sequences (AAP/IAP). Differencesbetween clones in transmission efficiency were found only when a short AAP was followed by a long IAP(6h/120h) and, to some extent, when a long AAP (48h) was followed by a short IAP (6h), but no differenceswere found when the conditions for virus transmission were optimal, i.e. long AAP followed by long IAP(48h/120h). There were no differences in transmission rates by clones of different geographical origins andwith different life cycles. As a consequence, clonal variation is probably of little importance in the vectoraspects of the epidemiology of PAV serotypes transmitted by R. padi, but the availability of a range of clonesexhibiting transmission differences under limiting AAP or IAP conditions could be of interest for studies ofvirus–vector relationships.INTRODUCTIONPopulations of a given aphid species consist ofnumerous clonal lines which may reproduce parth-enogenetically all the year round (obligate parth-enogenetic clones) or for at least several monthsduring the year (cyclical parthenogenetic clones).Most of such clones were shown to differ in mol-ecular characters (Carvalho et al., 1991; Black etal., 1992; Simon et al., 1996) and/or biologicaltraits, such as ability to produce sexual morphs(Blackman, 1971; Simon et al., 1991), host-plantpreference (Bournoville, 1971; Via, 1991; Caillaudet al., 1995) and virus transmission.Assessing intraspecific variations for virus trans-mission may be of importance in studies of bothepidemiology and virus transmission mechanisms.Seasonal prevalence of efficient or inefficient virustransmitting clones in a given area could greatlyaffect the course of an epidemic, and a comparisonof a range of genetically distinct clones for virustransmission could be very useful in evaluating themolecular sites of virus specificity in aphids.Intraspecific variation has been investigated fornon-persistently (stylet-borne) transmitted viruses(Sohi & Swenson, 1964; Upreti & Nagaich, 1971b;Thottappilly et al., 1972, 1977; Jurik et al., 1980;Singh et al., 1983; Lupoli et al., 1992) and persis-tently (circulative) transmitted viruses (Bjo¨rling O Saksena et al., 1964; Rochow& Eastop, 1966; Robert & Maury, 1970; Upreti N Lupoli et al.,1992) a continuum in virus transmission percent-ages was found, suggesting a polygenic determin-ism of this character.BYD luteoviruses are transmitted persistently byvarious aphid species with a more or less strongvector specificity (Rochow, 1969, 1970a, b). Moststudies of aphid–BYDV relationships have beenfocused on variation in species-specific transmis-sion among aphid species but clonal variations forserotype transmission were demonstrated even inthe earlier studies. In the USA, Rochow (1960)reported strains of the greenbug Schizaphis grami-num differing in their ability to transmit a BYDV-SGV isolate and four biotypes of the corn leaf aphidRhopalosiphum maidis were proved to differ intheir ability to transmit the AG-1 strain of BYDV(Saksena et al., 1964). Rochow & Eastop (1966)

Holocyclic individuals of the aphid Rhopalosiphum padi reproduce by cyclical parthenogenesis, while anholocyclic individuals are obligate parthenogens. Although admixed on the secondary hosts in summer, holocyclic and anholocyclic populations occur separately on the primary and secondary hosts during winter and spring. In this study, we compared the genetic diversity and population structure of holocyclic and anholocyclic populations collected in spring in the northern half of France. We also analysed the genetic composition of summer populations of R. padi on its secondary hosts. In spring, holocyclic populations were in Hardy-Weinberg equilibrium at individual loci and had a relatively high genotypic diversity. Conversely, anholocyclic populations deviated from Hardy-Weinberg equilibrium and often consisted of a single clone. Moreover, these populations showed very low mean heterozygosities compared with holocyclic populations. Analysis of summer populations suggested that in regions with cold winters, summer populations were largely recruited from holocyclic clones, whereas in areas with mild winters, summer populations were mainly derived from anholocyclic clones. These results permit an assessment of the geographical distribution of the two modes of reproduction on a large scale. The reasons for the diminished heterozygosity of anholocyclic populations are also discussed in relation to the mechanisms which may induce transitions to asexuality in aphids.

Summary During the years 1989–1992 cereal aphids were caught alive in a low level (1.5 m high) suction trap operated in Le Rheu (Brittany, France) and tested for BYDV transmission. In most cases comparisons with data collected simultaneously by a 12.2 m suction trap operating in the same site resulted in good relationships between weekly catches at both heights. Results from transmission tests showed that: (i) the two main BYDV vectors were Rhopalosiphum padi and Metopolophium dirhodum during the years of experiment; (ii) PAV and MAV were the commonest viruses and RPV was relatively scarce; (iii) during spring M. dirhodum appeared to be the most important MAV vector and nearly as good a PAV vector as R. padi; (iv) during autumn R. padi was the only vector of the three viruses with mixed transmission allowing it to transmit also MAV probably by heteroencapsidation. To give an indication of the risk of infection, infectivity indices were calculated by multiplying the numbers of aphids caught by the 12.2 m suction trap by the proportion that were infective. These infectivity indices agreed with field records of primary infections.

Summary The effect of wheat resistance in lines of Triticum monococcum L., on the reproductive performances of the cereal aphid (Sitobion avenae F.) was investigated. Aphids were reared from birth to adult moult either on resistant or susceptible wheat lines, and transferred as apterae to both host genotypes. The influence of these transfers on the subsequent adult weight, gonad status and reproductive performances was evaluated. Aphids transferred from resistant to susceptible plants proved able to compensate for their poor nymphal growth, mainly through additional embryo growth and an increase in the number of matured embryos within the first 10 days of their adult life. Most aphids transferred from susceptible to resistant plants died within the first week following the transfer. Their most advanced embryos matured and were born, but subsequent embryo growth was quickly reduced. The reproductive strategies adopted by S. avenae when facing plant resistance, and the hypothesis of a resistant mechanism based on a poor nutritional state of the resistant plants are discussed.

Multivariate analysis methods were used to distinguish between the three female alate morphs of Rhopalosiphum padi (L.) (emigrants, alate exules and gynoparae). Fourteen morphometric descriptors and seven anatomical parameters indicative of their potential fecundity were considered. Three populations or clones from distinct areas were also taken into account. The three alate morphs were separated with between 93% and 100% success. Some simplified criteria allowing distinction are described, and as an example one of them was applied to R. padi alatae caught in three suction traps. The reliability of the samples analysed is discussed. Practical use of identification techniques described may have implications for forecasting (e.g. Barley Yellow Dwarf Virus Disease), and for improving knowledge of aphid population biology. Resume Recherche de criteres morphometriques et anatomiques de differenciation des trois formes femelles ailees (emigrants, exules ailes et gynopares) du puceron des cereales Rhopalosiphum padi Des techniques d'analyse multivariee ont ete appliquees sur des donnees morphometriques et anatomiques obtenues pour les trois formes femelles ailees (emigrants, exules ailes et gynopares) de Rhopalosiphum padi (L.) dans le but de les differencier. Pour cela des parametres de la fecondite potentielle (nombre d'ovarioles, nombre d'embryons) et des variables morphologiques (mensurations, nombre de rhinaries) ont ete evalues pour chacun des morphes sur des individus de trois populations ou clones d'origines geographiques distinctes (Angleterre, Normandie, Bretagne). Les analyses factorielles discriminantes realisees sur ces donnees nous ont permis de separer avec un taux de succes compris entre 93% et 100% les individus selon leur morphe. A partir de celles-ci nous avons pu proposer des criteres susceptibles de differencier ces trois formes ailees. Le rapport ARS/TARS2 (longueur de l'article apical du rostre/longueur du deuxieme segment du tarse posterieur) semble tres bien discriminer les emigrants des exules ailes et a ete applique sur des ailes de R. padi captures par deux pieges a succion francais et un suisse. La separation des gynopares des exules ailes est par contre moins simple. La representativite de l'echantillon analyse a ete discutee. Les possibilites d'application des methodes d'identification des morphes de R. padi sur les ailes captures aux pieges a succion ont ete envisagees dans le cadre de la prevision des risques d'infection des cereales par le virus de la Jaunisse Nanisante de l'Orge (BYDV) et de l'amelioration des connaissances en Biologie des Populations aphidiennes.

This study aimed at evaluating the influence of intrinsic factors (clone, morph) on the parameters of the intrinsic rate of increase of Sitobion avenae (F.) and Rhopalosiphum padi (L.). For each species, apterous and alate exules of three clones originating from French oceanic regions were compared at 20 °C. The clonal factor had a significant effect on both time from birth until onset of reproduction (TBR) and intrinsic rate of increase (r). This effect was lower than that of the morph. Moreover, in the case of R. padi, a strong clone-morph interaction was observed. Both apterous and alate morphs were shown to have a higher rate of increase in R. padi than in S. avenae. Various models were fitted to aphid daily rate of reproduction. Both types of model and quality of fit depended mainly on the morph. Resume Les effets du clone et du morphe sur les parametres du taux d'accroissement de deux especes de pucerons des cereales, Sitobion avenae et Rhopalosiphum padi Cette etude a cu pour but d'evaluer l'importance de facteurs intrinseques (clone et morphe) sur les parametres du taux d'accroissement de Sitobion avenae (F.) et Rhopalosiphum padi (L.). Pour chaque espece, trois clones originaires de regions oceaniques francaises ont ete compares a 20 °C pour les morphes exules ailes et exules apteres. L'effet du clone est significatif sur l'âge reproducteur (duree comprise entre la naissance du puceron et le debut de sa phase de reproduction) et sur le taux intrinseque d'accroissement naturel. Il est plus faible que celui du morphe avec lequel il interagit dans le cas de R. padi. Les differences interspecifiques relevees mettent en evidence les capacites d'accroissement superieures de R. padi par rapport a celles de S. avenae, dans le cas des ailes comme dans celui des apteres. Enfin, differents modeles ont ete appliques aux donnees de reproduction journaliere des pucerons. Le type et la qualite des ajustements varient essentiellement en fonction du morphe.

As pasture grasses are preponderant in the agriculture of western France, their role as reservoirs of barley yellow dwarf virus (BYDV) and its aphid vectors has been studied from 1984 to 1986. Aphids were observed on most crops of five pasture grass species (brome grass, cocksfoot, tall fescue, Italian and perennial ryegrass) but in very low numbers. Rhopalosiphum padi was the most numerous species. The incidence of BYDV was high, with levels of infection ranging from 6 to 80% depending on the species of pasture grasses. Fescue pastures were highly infected with BYDV, whereas the virus was not detected in cocksfoot pastures. PAV-, RPV-, MAV-like isolates of BYDV were detected in pasture grasses, but RPV- and MAV-like isolates were only detected from ryegrass and brome grass, respectively. The role of these grasses as source of viruliferous aphids infecting cereals is discussed.

Population dynamics of Padi avenae (PAV), Macrosiphum avenae (MAV), and Rhopalosiphum padi (RPV) virus serotypes of Barley yellow dwarf virus (BYDV) and of their main aphid vectors were studied in winter barley (Hordeum vulgare) fields for three successive years in western France. An epidemiological model of the spread of viruses in the field was developed based on vector populations as forcing variables and the population dynamics of each virus serotype. This model accurately simulated the kinetics of the epidemic for PAV serotypes, which are the most common ones. For RPV and to some extent for MAV, the results were less satisfactory. The occurrence and spread of PAV and MAV serotypes in the field was clearly and easily related to that of their main vector species. Conversely, the spread of RPV serotypes showed no consistent relationships with the dynamics of their vectors. Incidence of PAV in 1989 to 1990 and 1990 to 1991 showed a bimodal distribution, with maximums in fall (December) and spring (May) that were linked to fall infestations by R. padi and spring infestations by three (R. padi, Sitobion avenae, and Metopolophium dirhodum) or two (S. avenae and M. dirhodum) aphid species. In 1991 to 1992, the PAV infection curve was monomodal and mainly due to a primary spread of the virus by very large populations of alate R. padi. MAV incidence was low in fall and winter and reached a maximum in spring 1990 and 1991 related to the occurrence of S. avenae and M. dirhodum. RPV incidence was low every year, despite the abundance of its vector, R. padi. Mixed infections were more frequent than expected by chance and were assumed to be partly related to heterologous encapsidation. The occurrence of each serotype is discussed in relation to the time of crop infection and possible damage.

SUMMARY In the Rennes basin, Rhopalosiphum padi is anholocyclic and represents more than 90% of suction trap catches of potential vectors of barley yellow dwarf virus (BYDV) during autumn. From 1983 to 1987 the possibility of predicting the risk of BYDV infection of batches of barley test seedlings (sampling units) exposed each week from September to December near a 12.2 m high suction trap was investigated. Three kinds of variables were checked as possible predictors: weekly mean or maximum temperatures; weekly catches of R. padi (including or excluding males); and percentage of sampling units infested by aphids. Three contrasting examples were observed: during the first three years (1983–1985), infection was high and its change with time followed temperature, aphid catches and plant infestation changes; in 1986, high numbers of aphids caught and a high proportion of plants infested resulted in only low infection and in 1987, both infestation and infection were very low. Simple linear regression analysis showed that the more reliable predictors of infection were the proportion of infested plants and to a lesser extent the numbers of trapped aphids. Multiple linear regressions including either of the three groups of ‘predicting’ variables did not result in any improvement in the prediction. At a practical level, the use of counts of aphid catches would seem a better compromise between accuracy and consistency of prediction and ease of gathering data than that of plant infestation but any significant improvement of the prediction should be sought in an early estimate of the amount of virus available to aphids before they colonise the plants.

International audience; In Chile, the aphid Sitobion avenae is of recent introduction, lives on cultivated and wild Poaceae, and is thought to reproduce by permanent parthenogenesis. In order to study the genetic variability and population structure of this species, five microsatellite loci were typed from individual aphids collected from different cultivated and wild host plants, from different geographical zones, and years. Chilean populations showed a high degree of heterozygosity and a low genetic variability across regions and years, with four predominant genotypes representing nearly 90% of the sample. This pattern of low clonal diversity and high heterozygosity was interpreted as the result of recent founder events from a few asexually reproducing genotypes. Most geographical and temporal variation observed in the genetic composition resulted from fluctuations of a few predominant clones. In addition, comparisons of the genotypes found in Chile with those described in earlier surveys of S. a! venae populations in Western Europe led us to identify 'superclones' with large geographical distribution and high ecological success, and to make a preliminary exploration of the putative origin(s) of S. avenae individuals introduced to Chile.

Aphids are particularly interesting models in the study of genetic and demographic components of plant adaptation because of their breeding system which combines parthenogenesis and sexual reproduction (i.e. cyclical parthenogenesis), and the frequent emergence of host-adapted races reported in this group. In this paper, patterns of host adaptation were assessed on local populations of the aphid Sitobion avenae by following their demographic and genetic structure in a maize field for two consecutive years. The existence of putative generalist (polyphagous) or specialized (host-adapted) genotypes was also investigated by comparing the genotypic distribution of this aphid on maize and other cultivated host plants, using five microsatellite loci. Although population dynamics revealed strong variation in aphid abundance during the colonization period on maize, two genotypes identified at seven additional microsatellite loci were predominant and exhibited stable frequencies over cropping season and between years. Based on present and earlier studies, these two prevalent genotypes were shown to survive on different host plants other than maize, to colonize large geographical zones and to persist parthenogenetically for several years. All these data strongly suggest that these two genotypes are asexual generalist clones that could have been favoured by agricultural practices encountered in western Europe. Besides these two clones, a continual replacement of rare genotypes was observed on maize in both years. Hypotheses involving selection via aphid-plant interactions and natural enemies were proposed for explaining the disappearance of these genotypes on maize.

As French populations of the aphid Sitobion avenae exhibit a range of reproductive modes, this species provides a good opportunity for studying the evolution of breeding system variation. The present analysis combined ecological and genetic investigations into the spatial distribution of variation in reproductive mode. Reproductive mode was characterized in 277 lineages of S. avenae from France, and these aphids were scored for five microsatellite loci. The analyses revealed strong geographical partitioning of breeding systems, with obligate asexuals mostly restricted to the south of France, while lineages producing sexual forms were more common in the north. Contrary to what might be anticipated for organisms with frequent parthenogenesis, there was substantial genic and genotypic diversity, even in the obligately asexual lineages. More than 120 different genotypes were detected among the 277 aphid lineages, with an average of 5.9 alleles per locus (range four to 16) and heterozygosity of 56.7%. As with previous studies of allozyme variation in aphids, most loci showed heterozygote deficits, and disequilibrium was common among allelic variants at different loci, even after removal of replicate copies of genotypes that might have been derived through clonal reproduction. Our results suggest that selection is important in structuring reproductive systems and genetic variation in French S. avenae. Canonical correspondence analysis was employed to examine the associations between genotypic and phenotypic variables, enabling the identification of alleles correlated with life-history traits.

A descriptive epizootiological model is proposed and parameterized to predict the dynamics of the host-pathogen system “Cereal aphid — entomophthoralean fungusErynia neoaphidis”. The model partitions the host population into distinct classes corresponding to fungal development stages. The dynamics of each class density are described by a set of non-linear ordinary differential equations with the reciprocal of estimated parameter giving average transit or developmental times. The model is compared to the biological data. The experiment was performed under controlled conditions of 20°C and RH >95% using the cereal aphidSitobion avenae as the host. The stability of the model was examined and indicated periodic oscillations, i.e. epizootic intervals separated by longer enzootic periods. In addition, Lotka-Volterra mass action assumption for transmission rate was tested using the data.

International audience

The development and reproduction of 60 clones of Sitobion avenae (Fabricius), collected in the Rennes Basin, were compared on a resistant Triticum monococcum (Linnaeus) line (Tm44) and a susceptible Triticum aestivum (Linnaeus) cultivar (Arminda). All clones had lower larval survival and mean fecundity when reared on Tm44 in comparison with Arminda. They all performed equally well on Arminda whereas there was a marked and continuously distributed variation in performance parameters among clones maintained on Tm44. The plant species, from which clones originated, significantly affected aphid performances on resistant Tm44. A more detailed experiment was carried out with four clones, shown to differ in their level of fitness on Tm44. Their performances were compared on resistant (Tm44 and Tm46) and susceptible (Tm47 and Arminda) wheat genotypes. On the basis of larval development time and intrinsic rate of natural increase (rm), two types of response were distinguished among the four clones: clones Sa1 and Sa39 appeared to be less affected by Tm44 and Tm46 resistance than Sar2 and Sa48. Moreover, Tm46 was much less resistant to clone Sal than was Tm44. These results are discussed in relation to the origins of the variation in performance of S. avenae on resistant wheat, and their implications in plant breeding for resistance to S. avenae.

Summary The incidence and distribution of the three principal isolates of barley yellow dwarf virus (PAV, RPV and MAV) are described in winter cereal crops, cereal (stubble) regrowth and grasses from 11 sites in western France and south-west England during 1987 and 1988. Isolates were identified by indirect ‘sandwich’ ELISA using the monoclonal antibodies MAC91, MAC92 and MAFF2. More virus infection occurred in all localities and in most of the plant communities sampled, with the exception of perennial grass leys, in 1987 than in 1988. All three isolates were widespread. MAV was associated more with sites further north and PAV more with those further south. The geographical distribution of RPV was less variable. Underlying these trends, the relative abundance of isolates differed considerably between habitats. RPV always predominated in perennial grass leys and MAV in most cereal crops, although in the latter MAV was less prevalent in 1987 than in 1988. The greatest regional difference was found in stubble regrowth where PAV predominated in France but MAV predominated in England. Grasses from field margins (only sampled in England) were mainly infected by MAV and RPV. The implications of these findings for the epidemiology of BYDV are discussed, especially the roles of different host plant communities or habitats in the annual infection cycle of small-grain cereals.

Deux experimentations ont ete faites en serre, dans le but de declencher des epizooties aEntomophthora surAphis fabae Scop. par dissemination de pucerons vivants, infectes parE. fresenii Nowal. dans des colonies saines et par brumisation. Un relevement moyen de l'humidite relative ambiante, obtenu par brumisation, meme assez faible (10%), est suffisant pour avancer la saturation nocturne et retarder la baisse matinale de l'humidite relative. Ceci, ainsi que le maintien d'eau liquide sur les plantes pendant une grande partie de la journee, a permis le declenchement des processus epizootiques. Durant la 1re experimentation, la croissance des colonies est arretee 13 j apres la distribution de l'inoculum et les populations sont detruites a 80% 27 j apres cette distribution. Durant la 2e experimentation la croissance des colonies est arretee 6 j apres la distribution de l'inoculum et les populations sont detruites a 90% 24 j apres cette distribution. Les meilleurs resultats (2e experimentation) ont ete obtenus avec une serre completement fermee et un inoculum distribue tres peu de temps apres l'installation des pucerons. Les roles respectifs de l'eau libre et de l'humidite relative ambiante sur l'epizootiologie du champignon sont discutes, ainsi que les problemes poses par l'influence de la brumisation sur les champignons phytopathogenes, par l'action des fongicides sur lesEntomophthora, et par la compatibilite de telles methodes de lutte avec les traitements insecticides.

Plusieurs especes d'Entomophthora participent a la regulation des populations d'Aphis fabae Scop. sur feverole dans l'Ouest de la France. Les roles respectifs de chaque espece ont ete evalues en 1971 et de 1973 a 1975. Entomophthora fresenii Nowak. est l'espece qui se dissemine le plus rapidement et le plus completement dans les parcelles, et qui est la plus agressive au niveau des colonies. Ces deux caracteres en font l'agent pathogene dominant d'A. fabae dans l'Ouest, 3 annees d'observations sur 4.Entomophthora aphidis Hoffman, bien que present tous les ans n'est le pathogene le plus efficace qu'en 1973.Entomophthora thaxteriana Petch etEntomophthora planchoniana Cornu n'apparaissent pas chaque annee et, lorsqu'ils sont presents, n'interviennent que pour une faible part dans le developpement des mycoses. Des facteurs inherents a l'agent pathogene (specificite, caracteres du cycle) ainsi que des facteurs ecologiques (climatiques) sont evoques pour expliquer la predominance d'E. fresenii.

En Bretagne, les mycoses aEntomophthora ont une incidence sur les populations d'Aphis fabaeScop., non seulement sur Feverole (Vicia faba L. var.minor), mais egalement sur Fusain (Evonymus europaeus L.) et Boule-de-neige (Viburnum opulus L. var.sterilis). Alors que sur hote secondaire elles donnent lieu a des epizooties, elles ne detruisent a la meme date qu'une faible partie des aphides sur les hotes primaires. L'objet de la presente note est de comparer le deroulement de la maladie sur ces deux types de plantes et de discuter de son incidence sur la structure des populations.

Aphid counts were made on a maize field over three years (1985–1987) in western France. Metopolophium dirhodum Wlk. and Sitobion avenae F. were present from early June. Numbers were maximal in late July and the species disappeared in August. Rhopalosiphum padi L. numbers fluctuated differently each year. In 1985 and 1986 they were very low in the first part of July, then increased appreciably to the end of August, reaching very high levels in September 1985. In 1987, R. padi colonised the field from July, it reached a maximum in August, but was no longer abundant in September–October. For all three aphid species there was a good correlation between weekly counts on maize and corresponding suction trapping of alates. Percentage of maize infected by barley yellow dwarf virus (BYDV) was also recorded over three years. Levels varied between 20–30 % in 1985 and were 0 % and 1 % in 1986 and 1987 respectively. BYDV infection pathways for maize, together with the role of this crop in the annual cycle of the aphid vector and the virus are discussed. Zusammenfassung Zur Beziehung zwischen der Fluktuation von Getreideblattlausen in Maisfeldern Westfrankreichs und der Epidemiologie des BYD-Virus In den Jahren 1985 bis 1987 wurde die Populationsdichte der Blattlause in westfranzosischen Maisfeldern ermittelt. Wahrend dieser drei Jahre fand man Metopolophium dirhodum Wlk. und Sitobion avenae F. von Anfang Juni bis Ende August, mit einem Maximum im Juli. 1985 und 1986 war der Befall durch Rhopalosiphum padi L. wahrend der ersten Sommerhalfte gering, erhohte sich aber betrachtlich ab Ende August und erreichte seinen Hohepunkt im September. 1987 fand man R. padi jedoch bereits ab Juli und der Befallshohepunkt war im August. Im September/Oktober waren dagegen nur noch wenige dieser Blattlause zu finden. Fur alle drei Arten bestand eine enge Korrelation zwischen der Anzahl der in den Maisfeldern gefundenen Blattlause und der in den “Saugfallen” gefangenen Tiere. Zusatzlich wurde der Prozentsatz der durch BYDV befallenen Maispflanzen in den drei Jahren ermittelt. Er betrug 1985 20–30%, 1986 0% und 1987 1%. Die Bedeutung der Maispflanzen fur den Zyklus sowohl der Blattlause als auch der Viren wird diskutiert.

L'etude comparee de l'action limitative de 3 especes d'Entomophthoraceae sur les pucerons des cereales a ete effectuee dans l'Ouest de la France (Bassin de Rennes essentiellement) de 1975 a 1978. La quasi-totalite des cas de mycose sont dus aErynia neoaphidisRemaud. & Henn.,Conidiobolus obscurus (Hall & Dunn) Remaud. & Keller etEntomophthora planchonianaCornu.

On a compare les modalites de la production de formes sexuees par deux ou trois clones de Sitobion avenae F. originaires de regions oceaniques de la France. Deux experimentations ont ete faites, l'une en conditions climatiques constantes et l'autre en conditions simulant un climat d'automne. Les trois clones sont genetiquement holocycliques et deux d'entre eux, etudies plus en detail, different par certaines modalites de leur production de sexues, l'un formant en particulier des femelles ovipares a 10 °C/12 h de photophase et l'autre a 15 °C/12 h de photophase. Au contraire, ces clones conservent des caracteres generaux identiques, ils forment des andropares apteres et des gynopares ailes. Ils paraissent ainsi moins degrades par rapport au type primitif heteroe-cique qu'un autre clone etudie en Grande-Bretagne. Les limits d'extrapolation de ces resultats a ce qui se passe dans la nature sont discutees.