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Studies on pathogenic agents of cultivated plants are generally organised on a binomial basis with examination of a host plant and specific parasites or pests. As cassava in Africa has few important pests and they display a limited range of biological features and relations with the host, it was feasible for ORSTOM (L'Institutfrançaise de recherche scientifique pour le developpement en coopératon) researchers to study each of them over a period of 25 years in Côte d'lvoire, Congo and Togo and such work is currently in progress in Benin and France. The diseases and pests concerned are African cassava mosaic disease and its whitefly vector (Bemisia tahaci), cassava bacterial blight caused by Xanthomonas campestris pv manihotis, cassava mealybug (Phenacoccus manihoti), and cassava green mite (Mononychelus progresivus). Research initially concerned only specific cassava-pathogen or cassava-pest combinations, without attention being paid to the system as a whole, despite obvious epidemic convergences resulting from a common environment, analogies between effects on the host plant and probable interactions between the various pests of the same crop. A biocenotic approach was developed to integrate the various research activities. It is aimed at analysis of the functioning of plant-pathogen and plant-pest systems and enables the design and evaluation of options for integrated management. 'Transversal' comparisons of cassava parasite models cover pathogenic agents (pests and diseases) and their variability, their biological cycles and the climatic factors affecting them, relations with the host (genetic variability, physiology and crop management), the effect of entomophagous species and entomopathogens on phytophagous pests and the integration potential of modelling. The ORSTOM studies show the complexity of the ecology of pathogenic agents and pests in the cassava biocenosis. The extent of the cassava cropping area and the variety of production situations, the variability of pests (phytophagous and entomophagous organisms) and diseases and the many interactions between the factors regulating epidemic mechanisms leads to a complex mosaic whose structure is described. The various sequences of analysis of a system of parasite constraints are shown in matrix form (variability of pathogenic agents, vectors and phytophagous organisms, host-parasite relation modes, epidemiology and population dynamics, modelling) for the various organisms investigated to date. The analysis reveals the fundamental achievements and gaps in knowledge and also the most appropriate areas for combining approaches. The body of knowledge assembled at different sites, during different periods and concerning a variety of organisms and the analysis of this knowledge show that it is not necessary at this stage to collect new information but to make a synthesis (modelling) that will open up original lines of research with combinations of approaches and solutions.Key Words: Cassava, pests and diseases, epidemiology and population dynamics, life cycles, host-parasite relations, natural enemies, vectors, modelling, integrated pest management, Africa Les travaux sur les agents pathogènes des plantes cultivées s'organisent le plus souvent selon une approche binomiale qui prend en compte une plante et un parasite ou une plante et un ravageur. Dans le cas du manioc en Afrique, pour des ennemis peu nombreux et qui offrent une palette très diverse de traits biologiques et de relations avec la plante, les recherches ont été conduites depuis près d'un quart de siècle par les chercheurs de l'ORSTOM (L'Institut française de recherche scientitique pour le developpement en coopératon) en Côte d'lvoire, au Congo et au Togo et sont actuellement poursuivies au Bénin et en France. Il s'agit de la mosaïque africaine du manioc et de son insecte vecteur Bemisia tabaci, de la bactériose vasculaire dont l'agent est Xanthomonas campestris pv manihotis, de la cochenille du manioc Phenacoccus manihoti, et de l'acarien vert, Mononychellus progresivus. Les travaux se sont d'abord attachés aux seuls couples manioc-maladie ou manioc-ravageur, sans considérer le système parasitaire dans son ensemble en dépit de convergences évidentes dues à un environnement common, à des analogies d 'effet sur la plante hôte et à des interactions probables entre les différents ennemis de la même culture. Nous avons ici élaboré une approche biocénotique qui permet d'intégrer différentes actions de recherche, orientée vers l'analyse du fonctionnement des systèmes plante-parasites et plante-ravageurs et qui permet de formuler et d'évaluer les options de gestion intégrée de cette biocénose parasitaire. La comparaison "transversale" des modèles parasitaires du manioc aborde successivement les agents pathogènes (maladies et ravageurs) et leurs variabilités, leurs cycles biologiques et les facteurs environnementaux qui les conditionnent, les relations avec la plante (variabilité génétique, physiologie et conduite de la culture), l'incidence des entomophages et entomopathogènes sur les phytophages et les perspectives d 'intégration offertes par la modélisation. Les études menées par l'ORSTOM mettent en évidence la complexité de l'écologie des ravageurs et des agents pathogènes de la biocénose manioc. L'étendue de l'aire de culture du manioc, la diversité des situations de production, la variabilité des parasites (maladies, phytophages etentomophages), la multiplicité des modes de relation entre la plante et ses satellites (parasites, phytophages, entomophages et vecteurs ), le foisonnement des interactions entre les facteurs qui règlent les mécanismes épidémiques, font que la connaissance se présente sous l'aspect d'une mosaïque dont nous faisons apparaître la structure. Nous avons représenté de façon matricielle les différentes séquences d'une analyse d'un système de contraintes parasitaires (variabilité des agents pathogènes, des vecteurs et des phytophages; modalités des relations hôte-parasite; épidémiologie et dynamique des populations; modélisation) en regard des différents organismes jusqu'ici etudiés. L'analyse révèle à la fois les acquis fondamentaux et les lacunes de la connaissance, mais aussi les domaines privilégiés d 'une intégration des démarches. La masse des connaissances, obtenues dans des sites différents, à des périodes différentes et sur des organismes variés, ainsi que l'analyse qui en est faite, montre qu'il n'est pas nécessaire à ce stade de recueillir de nouvelles informations, mais de réaliser une synthèse (modéllsation) qui ouvrira des voies de recherche originales dans le sens de l'intégration des approches et des solutions à apporter.Mots Clés: Manioc, parasites et ravageurs, épidémiologie et dynamique des populations, cycles biologiques, relations hôte-parasite, ennemis naturels, vecteurs, modélisation, gestion intégrée, Afrique

International audience; BACKGROUND: While resistance against insecticides is widely known in pest arthropods, it remains poorly known in non-target arthropods of the same agrosystems. This may be of crucial importance in the context of organic pest management or integrated pest management. First, stopping of pesticide pressure during farm conversion may lead to important rearrangements of non-target communities due to fitness cost of resistance in populations of some species. Second, resistant biological agents may be useful to farms with low synthetic pesticide use. Communities of mesostigmatid mites, encompassing numerous predatory species, are supposed to be involved in important ecological processes in both crop soils and animal litter/manure.RESULTS: Here we provide a tarsal contact method for assessing resistance in different populations from various species of mesostigmatid mites. Analyses of data from repeated tests on three populations from different mesostigmatid families proved the method to be robust and able to generate consistent and reliable mortality percentages according to insecticideconcentration.CONCLUSION: Our bioassay system allows for both one-shot estimate of pyrethroid sensitivity in mite populations and estimation of how it changes over time, making possible survival analyses and assessment of recovery from knockdown. The rating system retained makes it possible to score response to insecticides in a consistent and standard way in species from different mesostigmatid families

Environmental problems, at local scale as well as global scale, are now considered as key issues and scientists are encouraged to be part of the process to address these issues. For the last decades, scholars have been focusing on the study of interactions between social dynamics and ecological processes and produced a set of concepts and scientific discourses aiming at framing the analysis of socio-ecological dynamics and eventually at orienting interventions. Scientific discourses are produced by scholars who belong to different groups (resilience, vulnerability, political ecology, commons, robustness ¿) which identity go beyond disciplines, methods, frameworks and concepts and include a collective history on the evolution of ideas and research organization. Different research units based in Montpellier (France) have been conducting research on socio-ecological systems (SES) to understand many types of relationships, including those between agriculture and biodiversity, policies and landscapes dynamics, watershed management, ecosystem management and health risk. A project named SETER (Socio-Ecological Theories and Empirical Research) was elaborated aiming at assessing the relevance and the complementarities of theoretical frameworks by applying and testing them on several empirical research case studies developed by the participating research units based in Montpellier. We have confronted different scholars, holding the flags of different schools of thought, to the same concrete issues. The purpose of this assessment was to clarify the respective potential of the different theoretical frameworks and to provide the basis for new conceptualizations of socio-ecological systems dynamics and management. Although some authors call for an integrated framework and some advocate for the diversity of explanations, both agree on the fact that there is a need for clarification for a better scientific debate and better interactions with the managers, stakeholders and interested people. The scientific debate remains obscure when it is based on abstract developments. This report presents the material of this experiment, the interaction process and the lessons on the schools of thought and methodologies. Firstly, we introduce briefly the different schools based on the literature and on the presentations given by the invited scholars. Then we introduce the four case studies which were analyzed by the invited scholars and report on the interactions and their results. Then we present a discussion proposing a classification of the different perception of change which crosses the different schools.

Human-environment interactions are studied by several groups of scholars who have elaborated different approaches to describe, analyze, and explain these interactions, and eventually propose paths for management. The SETER project (Socio-Ecological Theories and Empirical Research) analyzed and compared how "flag-holders" of distinct school of thought in human-environment scholarship approached a number of empirical problems of environmental management. This paper presents the findings from this experiment by concentrating on how representatives of four schools of thought approached one of these case studies: the plant health crisis in greenhouse tomato production in south of France. Our analysis suggests that these approaches share a common conceptual vocabulary composed of four explanatory elements of change (Power, Incentives, System and Adaptation-PISA). We argue that what distinguishes these schools from one another is the syntax the "rules" by which researchers in each of the sub-disciplines tend to organize the components of this shared conceptual vocabulary. In other words, the schools under scrutiny are differentiated not so much by what they speak of, but rather in what order, or hierarchy, do they tend to rank the importance and/or the sequence of each of these concepts in human-environment explanations. The results of our experiment support the view that communication and cooperation across the diverse human-environment traditions is possible and productive. At the same time, however, we argue that it is the distinctiveness of the claims yielded by these different schools of thought that augment our collective understanding of complex socio-ecological problems. Attempts to integrate these perspectives in one unitary approach would undermine the intellectual wealth necessary to meet the challenges of the Anthropocene.

Cannibalism and the effects of host plant, sex, time and food resources on its expression were studied for the zoophagous mirid Macrolophus pygmaeus Wagner (Hemiptera: Miridae). Cannibalistic behaviour was studied by offering 5 conspecific larvae (first instar) to newly emerging adults. Four treatments were studied: without water, with water only, with a host plant (tobacco) and with both a host plant and prey (eggs of Ephestia kuehniella). Cannibalism was observed in all treatments. In the "host plant + eggs of E. kuehniella" treatment, very few individuals displayed cannibalistic behaviour. The proportion of cannibalism was only reduced when eggs of E. kuehniella were offered. Water (free or via a host plant) was very important for both survival and feeding. The cannibalistic behaviour of M. pygmaeus should be taken into account when planning a release strategy in the context of biological control.

International audience; The consumption of plants by the zoophytophagous bug Macrolophus pygmaeus Rambur (Heteroptera : Miridae) in relation to the presence or absence of prey is monitored for 7 days. Tobacco plants enriched with N-15, a stable, nonradioactive isotope of nitrogen, are used for characterization. In the absence of prey, the relationship between time and N-15 content (which equates to plant feeding) is linear, with a constant daily accumulation of N-15; however, if prey are available, the mathematical relationship becomes curvilinear. From day 1 to day 5, feeding on plants is independent of the consumption of prey. The rate of N-15 accumulation decreases in the presence of prey after day 5, whereas the number of prey eaten remains unchanged.

L_objectif de ce travail est d_étudier l_infl uence des températures hivernales sur la survie des B. tabaci adultes. Les individus sont soumis à un cycle de températures qui correspond aux températures observées pendant les 24 heures d_une des journées les plus froides de l_hiver du sud de la France (Perpignan). L_évolution de ces températures est reproduite en conditions contrôlées, et les individus sont exposés à un, trois ou quatre de ces cycles. Après un cycle de 24 heures, le taux de survie moyen des individus est de 73,5% alors que celui du lot témoin est de 85%. Après une exposition à 3 cycles, le taux moyen de survie reste encore relativement élevé (44,4%). Les chances de survivre à 4 cycles de 24 heures est de 7,4%. Le Nord méditerranéen semble être une zone tampon pour le maintien en extérieur des populations de B. tabaci car ni les températures minimales régulièrement enregistrées, ni leur durée, ne sont clairement limitantes. Survival of Bemisia tabaci (Homoptera: Aleyrodidae) on the winter conditions of the north zone Mediterranean Sea. The aim of this job was to study the influence of the winter temperatures on the survival of B. tabaci adults. The individuals are subjected to a cycle of temperatures which corresponds to temperatures noticed during 24 hours by one of the most cold days of the winter of the south of France (Perpignan). The evolution of these temperatures is reproduced in controlled conditions, and the individuals were subjected to one, three or four of these cycles. After a cycle of 24 hours, the rate of survival of the individuals is 73,5 % while that of the control is 85%. After an exposure of 3 cycles, the rate of survival remained high (44,4 %). The probability to survive to 4 cycles of 24 hours is 7,4 %. The Mediterranean North seems to be a buffer zone for the establishment of the populations of B. tabaci because neither the regularly recorded minimal temperatures nor their length seem to be limiting factors.

This presentation will present the SETER project. For the last twenty years, different research units participating in this project have been developing research activities on socio-ecological systems aiming at understanding relationships between agriculture and biodiversity, policies and landscapes dynamics, watershed management, ecosystem management and health risk, etc. At the same time, diverse schools of thought have developed theories and frameworks to analyse the dynamics and management of socio-ecological systems (Resilience,Vulnerability, Political ecology). The objective of this project is to assess the relevance and the complementarities of these theoretical frameworks by applying and testing them in the case of four empirical research case studies (Wetlands of the Rhone river delta, Locust control in Sahelian region, Multiple uses of Sahelian ecosystems, Management of Bemisia-phytovirus risk.) The lessons from this project will also provide the basis for new conceptualisations of socio-ecological systems dynamics and management. During summer 2009 senior researchers (Lance Gunderson, Allyson Quinlan, Colin Polsky, Marco Janssen, Paul Robbins, Nancy Peluso, Tom Bassett, Tor Benjaminssen) were hosted in Montpellier (France) for a common stay during which they interacted among them and with the participating units in Montpellier. Each case study was analysed from a different angle leading to progress in the understanding of the dynamics and management of the socio-ecological system under study. The different view points were discussed. This paper will present the preliminary results of this scientific dialog. (Texte intégral)

This paper present an international collaborative effort, the SETER project "Socio-Ecological Theories and Empirical research", hosted by the Center for International Co-operation in Agronomic Research for Development (CIRAD) and funded by the Agropolis Foundation. This project brought together researchers in the fields of common pool resource research, hazards and vulnerability, political ecology, and resilience to address multiple ongoing empirical research problems through a comparative lens. The objective of this project is to assess the relevance and the complementarities of these theoretical frameworks by applying and testing them in the case of four empirical research case studies (Wetlands of the Rhone river delta, Locust control in Sahelian region, Multiple uses of Sahelian ecosystems, Management of Bemisia-phytovirus risk) The lessons from this project will also provide the basis for new conceptualisations of socio-ecological systems dynamics and management.

Whiteflies and whitefly-transmitted viruses are some of the major constraints on European tomato production. The main objectives of this study were to: identify where and why whiteflies are a major limitation on tomato crops; collect information about whiteflies and associated viruses; determine the available management tools; and identify key knowledge gaps and research priorities. This study was conducted within the framework of ENDURE (European Network for Durable Exploitation of Crop Protection Strategies). Two whitefly species are the main pests of tomato in Europe: Bemisia tabaci and Trialeurodes vaporariorum. Trialeurodes vaporariorum is widespread to all areas where greenhouse industry is present, and B. tabaci has invaded, since the early 1990’s, all the subtropical and tropical areas. Biotypes B and Q of B. tabaci are widespread and especially problematic. Other key tomato pests are Aculops lycopersici, Helicoverpa armigera, Frankliniella occidentalis, and leaf miners. Tomato crops are particularly susceptible to viruses causingTomato yellow leaf curl disease (TYLCD). High incidences of this disease are associated to high pressure of its vector, B. tabaci. The ranked importance of B. tabaci established in this study correlates with the levels of insecticide use, showing B. tabaci as one of the principal drivers behind chemical control. Confirmed cases of resistance to almost all insecticides have been reported. Integrated Pest Management based on biological control (IPM-BC) is applied in all the surveyed regions and identified as the strategy using fewer insecticides. Other IPM components include greenhouse netting and TYLCD-tolerant tomato cultivars. Sampling techniques differ between regions, where decisions are generally based upon whitefly densities and do not relate to control strategies or growing cycles. For population monitoring and control, whitefly species are always identified. In Europe IPM-BC is the recommended strategy for a sustainable tomato production. The IPM-BC approach is mainly based on inoculative releases of the parasitoids Eretmocerus mundus and Encarsia formosa and/or the polyphagous predators Macrolophus caliginosus and Nesidiocoris tenuis. However, some limitations for a wider implementation have been identified: lack of biological solutions for some pests, costs of beneficials, low farmer confidence, costs of technical advice, and low pest injury thresholds. Research priorities to promote and improve IPM-BC are proposed on the following domains: (i) emergence and invasion of new whitefly-transmitted viruses; (ii) relevance of B. tabaci biotypes regarding insecticide resistance; (iii) biochemistry and genetics of plant resistance; (iv) economic thresholds and sampling techniques of whiteflies for decision making; and (v) conservation and management of native whitefly natural enemies and improvement of biological control of other tomato pests.

International audience

Dans le cadre d'une approche systémique des crises phytosanitaires (cas emblématique du risque TYLCV-Bemisia), une plateforme prototype de formalisation et de simulation a été dédiée à l'intégration des composantes épidémiologiques, biotechniques, socio-économiques, réglementaires et organisationnelles. La plateforme est en cours de réalisation suivant une méthodologie qui permet de la concevoir en partenariat avec des chercheurs de champs disciplinaires divers et différents acteurs afin qu'elle puisse répondre à la fois aux questions relatives à l'émergence du risque et à sa gestion en termes de gouvernance territoriale.

Whiteflies and whitefly-transmitted viruses present some of the most intractable constraints to European tomato production. The main objectives of the Tomato Case Study (TCS) were to: identify where and why whiteflies were a major limitation, collect information related to whiteflies and associated viruses; establish which management tools are available; identify key knowledge gaps and research priorities. Two whitefly species are pests of the tomato in Europe. Bemisia tabaci is widely distributed, Trialeurodes vaporariorum is ubiquitous. Biotypes B and Q of B. tabaci are widespread and problematic. Tomato crops are particularly susceptible to Tomato yellow leaf curl disease (TYLCD) and high incidences were associated to its vector, B. tabaci. Unlike other tomato pest species, the ranked importance of B. tabaci correlated with levels of insecticide use, showing B. tabaci to be one of the principal drivers behind chemical control. Confirmed cases of resistance have been reported to almost all insecticides. PM based on biological control (IPM-BC) is applied in all the surveyed regions and was identified as the strategy consuming fewer insecticides. Other IPM components include greenhouse netting and TYLCD-tolerant tomato cultivars. Sampling techniques differ between regions, decisions are generally based upon whitefly densities and do not relate to control strategies or growing cycles. IPM-BC is the recommended strategy for a sustainable agriculture. However, some limitations for a wider implementation such as lack of biological solutions for some pests, costs of benefits, low farmer confidence, costs of technical advice and low pest injury thresholds were identified. Research priorities to promote IPM-BC are proposed.

Calacarus heveae Feres, 1992 (Eriophyidae) is a mite described from specimens collected on rubber tree (Hevea brasiliensis, Euphorbiaceae) in the northwest region of the state of São Paulo. This mite prefers the adaxial face of the folioles which it can turn dry, yellowish and brownish; it can also cause leaf fall. The aim of this work was to analyze the distribution of C. heveae on rubber trees, to select the most representative sampling unit and to develop a sampling plan to determine the populational fluctuation. This study was conducted with clones PB 260 and IAN 873, in Itiquira and Pontes e Lacerda, respectively, both in the state of Mato Grosso. In Itiquira, significant differences were observed in four occasions in relation to the average number of mites per leaf in the different plant strata. In the samplings carried out in Pontes e Lacerda, no significant differences were observed between strata in relation to that parameter. Only in Itiquira, in one occasion, a significant difference between strata was verified in relation to the proportion of infested leaves. No significant differences were verified in relation to the average number of mites per leaf and proportion of leaves infested by C. heveae at different depths in the canopy. Calacarus heveae exhibits aggregated distribution in the field. To estimate the density of C. heveae, numeric and sampling plans were developed.

National audience; Bemisia tabaci, une espèce cosmopolite et complexe. Les aleurodes sont vecteurs de plus de 114 virus phytopathogènes, qui présentent une grande variabilité (risque constant de nouvelles viroses) et dont une centaine (111) est inféodée à Bemisia tabaci. Les premiers indices qui ont permis de suggérer l’existence de différents biotypes chez B. tabaci (espèce décrite en 1889) proviennent de la mise en évidence de gamme d’hôtes (espèces et variétés végétales) spécifiques chez des populations d’origines africaines ou américaines. La caractérisation de ces biotypes, au sein de populations identiques sur le plan morphologique, s’est avérée indispensable pour comprendre (i) l’invasion de plusieurs pays par des populations de B. tabaci qui se caractérisaient par un fort potentiel biotique par rapport à d’éventuelles populations indigènes, (ii) des phénomènes de phytotoxicité sur cucurbitacées, et (iii) des manifestations de résistance aux insecticides. Des tests basés sur des analyses iso-enzymatiques puis sur l’analyse du génome (séquences répétées de l’ADN appelées « microsatellites » et gène mitochondrial) ont permis de distinguer à travers le monde une vingtaine de biotypes qualifiés par une lettre de A à N. Les risques sont décuplés du fait de la conjonction entre la variabilité génétique et des potentialités adaptatives du vecteur B. tabaci d’une part, et de la grande variabilité des virus incriminés d’autre part. Les auteurs considèrent comme point de départ de cette dramatique escalade l’explosion en Californie en 1991 du biotype B de B. tabaci (perte de 500 millions de dollars sur les seules cultures hivernales dans cet état pendant l’hiver 1991). Le biotype Q achève sa colonisation de l’arc nord méditerranéen avec son introduction en France et l’apparition du TYLCV. Parallèlement, il vient d’apparaître sur le continent américain où il a été décelé durant l’hiver 2004-2005 sur des plants de Poinsettia commercialisés à Tucson (Arizona) puis détecté en 2005 dans au moins 17 états des USA et dans un site à Mexico ainsi qu’au Guatemala

Correspondance: bonato@mpl.ird.fr; National audience; Le mode de reproduction de Bemisia tabaci (Gennadius) par parthénogenèse a déjà été signalé par quelques auteurs sans qu’aucune précision du biotype ne soit indiquée. Or Bemisia tabaci, actuellement présente sur tous les continents, englobe un complexe de biotypes morphologiquement identiques uniquement distinguables par leurs caractéristiques biologiques et génétiques (comportement et gamme alimentaire, potentialité de vection, taux de croissance, interfécondité, etc.). Les populations appartenant au biotype Q n’ont pas un potentiel biotique très élevé comparativement aux autres biotypes (notamment le B) en revanche elles ont la particularité de présenter un haut de degré de résistance aux insecticides (les néonicotinoïdes et le pyroproxyfen). La première caractérisation du biotype Q a été réalisée sur des échantillons provenant de sud de l’Espagne et du Portugal (Guirao et al. 1997). Des prospections successives ont montré que ce biotype était également présent en Tunisie (Chermitti et al. 1997), au Maroc (Monci et al. 2000), en Egypte (De Barro et al. 2000), en Israël (Horowitz et al. 2003) et dans le sud de l’Italie (Demichelis et al. 2000; Simon et al. 2003). A cause de sa très grande polyphagie et de sa facilité à transmettre une large gamme de phytovirus à un nombre élevé de plantes-hôtes, le biotype Q est considéré actuellement comme particulièrement dangereux (Muñiz 2000; Navas-Castillo et al. 2000). Malgré l’étendue et la gravité du biotype Q dans le bassin méditerranéen, il n’existe actuellement aucune étude publiée sur la mise en évidence expérimentale de la reproduction asexuée de ce ravageur. Le but de ce travail est donc de montrer expérimentalement l’existence d’une telle reproduction dont les implications sont particulièrement importantes sur la biologie des populations.

National audience

Un comptage des oeufs et des formes mobiles (immatures et adultes) du phytoséiide Amblyseius manihoti présents sur la plante entière a été réalisé pendant dix mois (soit 90 plantes analysées) dans un champ de manioc de la région de Bahia (Brésil). Les proportions d'oeufs, d'immatures et d'adultes observées au champ sont comparées aux distributions d'âge stable théoriques (oeufs: 58 %, immatures: 24 %, adultes: 18 %). La comparaison des distributions théoriques aux observées permet de spéculer sur l'influence de facteurs tels que la fécondité, la mortalité et la migration sur la structure d'âge des populations.