Introduction biological control againt Sonchus oleraceus (Asteraceae) : An integrative approach to address the invasion process and contribute to biocontrol agent selection through ecological network analysis.

Les invasions biologiques sont un des facteurs majeurs de perte de la biodiversité. Face à cette menace, plusieurs approches sont utilisées pour contrôler l’expansion des populations d’espèces invasives. La lutte biologique par introduction s’appuie sur la sélection et l’introduction des ennemis naturels spécialistes de l’espèce invasive, appelés agents de lutte biologique, depuis l’aire d’origine vers l’aire d’invasion pour limiter les populations invasives sous un seuil de nuisibilité écologiquement et économiquement acceptable. Ces travaux de thèse se place dans un contexte de lutte biologique par introduction contre le laiteron maraîcher, Sonchus oleraceus (Asteraceae) en Australie. Dans une visée appliquée de gestion de S. oleraceus, nous avons alors adopté une démarche multidisciplinaire pour adresser plusieurs étapes critiques inhérentes au programme de lutte biologique, concernant 1) l’identification de la cible, 2) la compréhension du processus d’invasion et 3) l’anticipation des risques associés aux lâchers des agents via l’étude des réseaux écologiques. L’identification correcte de la cible étant un prérequis pour la recherche des agents de lutte, nous avons défini un caractère diagnostic fiable et pratique à utiliser lors des prospections, à savoir l’ornementation des akènes. Ensuite, nous avons évalué si l’évolution rapide des traits chez S. oleraceus était un mécanisme qui pouvait avoir contribué au succès d’invasion. Nous avons montré que les populations invasives étaient plus performantes que les natives, et que le compromis qui existe entre l’allocation des ressources à la croissance et la reproduction avait évolué, conduisant les populations invasives à investir davantage dans les fonctions de croissance pour un investissement équivalent aux populations natives dans les fonctions de reproduction. Ces résultats ouvrent la voie à d’autres études utiles pour déterminer les conséquences de ces changements pour les futures populations d’agents de lutte. De plus, nous avons abordé de façon préliminaire le rôle du relâchement de la pression d’herbivorie dans le succès d’invasion. Par des inventaires réalisés en Europe et en Australie, nous avons mis en avant un déficit d’ennemis naturels spécialistes dans l’aire d’introduction. Les introductions délibérées d’agents de lutte ne sont cependant pas anodines et un des challenges en lutte biologique réside dans la capacité à anticiper les risques pour la communauté indigène de l’aire d’introduction. Dans le dernier axe de cette thèse, nous avons exploré l’intérêt de l’analyse des réseaux d’interactions écologiques pour aider à la sélection d’agents de lutte. Pour cela une approche solide et transposable combinant technologies moléculaires et données d’observations a été développée pour reconstruire avec fiabilité et une résolution élevée les interactions trophiques entre plantes, herbivores et ennemis naturels. L’analyse de réseau, nous a permis d’identifier 37 espèces utilisant S. oleraceus en aire native et d’apporter des précisions sur la gamme d’hôte écologique de ces herbivores conduisant à 1) cibler ou exclure certains agents de lutte candidats et 2) questionner l’existence de complexes d’espèces lié à la plante hôte, nécessitant confirmation. Par ailleurs, nous avons révélé l’utilisation de ces herbivores par une large gamme d’ennemis naturels, ce qui présente des implications pour l’évaluation des risques indirects. En somme, cette thèse a participé à renforcer les connaissances quant aux processus d’invasion impliqués dans la colonisation du laiteron maraîcher en Australie, justifiant en partie la stratégie de lutte adoptée, et a montré le potentiel de l’analyse des réseaux écologiques pour complémenter les démarches classiques de sélection d’agents présentant un minium de risqueBiological invasions are one of the main driver of biodiversity loss. Several strategies are employed to tackle the expansion of invasive species populations. Among them, introduction biological control is based on selection and release of specialist natural enemies, called biological control agents, from native range to introduced range to decrease the invasive population densities below an ecological and economical acceptable threshold. This PhD thesis is part of a biological control program targeting the common sow thistle, Sonchus oleraceus (Asteracae), an invasive species in Australia. We employed a multidisciplinary approach to address several critical steps of the program, directed towards applied management perspectives, regarding 1) the identification of the target plant, 2) the understanding of the invasion process, and 3) the prediction of risks associated to biocontrol agents release through ecological networks analysis. The accurate identification of the target plant is a prerequisite for the survey of biocontrol agent candidates. As a first step, we thus defined a reliable and convenient diagnostic character to be used in the field, which is the achene ornamentation. Then, we assessed whether rapid evolution could be one of the process that could have contributed to S. oleraceus invasion success. We showed that introduced plants outperformed native ones, and detected a significant shift in the relationship (trade-off) between growth and reproduction, introduced populations tended to invest more in growth that native ones for an equivalent investment into reproductive functions. More investigations would be necessary to determine whether those changes would affect future biocontrol agents. Post-introduction evolution seemed however moderated, and preliminary results obtained through filed surveys realised in Europe and Australia tended to indicate that enemy release had also probably benefited S. oleraceus in Australia. However, the deliberated introduction of biocontrol agents poses risks to the recipient community, and their anticipation is an ongoing challenge in introduction biological control. In the last part of this thesis, we explore the interest of ecological network analysis in supporting biological control selection. Therefore, a powerful and transferable approach based on the combination of molecular tools and observational data have been developed to reconstruct with high levels of taxonomic resolution and interaction recovering, trophic links between plants, herbivores and natural enemies. Network analyses, allowed to determine 37 species using S. oleraceus as food plant in the native range. We were able to detail their field host range, 1) leading to the selection and exclusion of candidate biocontrol agents, and 2) raising the question of potential cryptic host races, that needs to be confirmed. We revealed a wide range of natural enemy species using S. oleraceus herbivores, which is of particular interest for indirect risk assessment. Ecological analysis proved to be complementary to classical specificity tests and has a great potential to support biocontrol agent selection. In conclusion, this PhD thesis strengthened our knowledge on invasion processes involved in the colonisation of the common sow thistle in Australia, partly justifying the control strategy used, and demonstrated the potential offered by ecological network analysis in supporting the selection of minimal-risk agents

Jouer avec les pollinisateurs

International audienceDans le cadre de rencontres avec le grand public ou avec des enfants, l'utilisation de jeux peut permettre une meilleure compréhension de processus écologiques ou de méthodes scientifiques complexes. Nous vous présentons 2 jeux de vulgarisation scientifique que nous avons récemment développés. Le premier traite de réseaux plantes-pollinisateurs et de métabarcoding, et permet d'aborder la notion d'ADN environnemental gr ce à un petit «Laboratoire d'extraction ADN ». Après une explication mimée de visites d'une fleur par différents pollinisateurs et de la technique d'extraction de l'ADN, les participants sont invités à trier des séquences d'ADN représentées par des colliers de perles et de réfléchir à la cause de leur présence sur la fleur de démonstration. Le second est une expérimentation des relations entre la structure des paysages (configuration et composition) et la diversité des pollinisateurs. A partir de dalles représentant différents milieux à deux saisons, de ressources de nidification et de ressources florales, les participants doivent expérimenter les déplacements de plusieurs espèces d'apoïdes différant par leur spécialisation alimentaire, leur type de nidification et leurs capacités de dispersion. Ils sont amenés à réfléchir à comment modifier les paysages pour améliorer leur capacité d'accueil pour les pollinisateurs

Characterising ecological interaction networks to support risk assessment in classical biological control of weeds

International audienceA key element in weed biological control is the selection of a biological control agent that minimizes the risks of non-target attack and indirect effects on the recipient community. Network ecology is a promising approach that could help decipher tritrophic interactions in both the native and the invaded ranges, to complement quarantine-based host-specificity tests and gain insights on potential interactions of biological control agents. This review highlights practical questions addressed by networks, including 1) biological control agent selection, based on specialization indices, 2) risk assessment of biological control agent release into a novel environment, via particular patterns of association such as apparent competition between agent(s) and native herbivore(s), 3) network comparisons through structural metrics, 4) potential of network modelling and 5) limits of network construction method

Détection des abeilles sauvages et domestiques par ADN environnemental

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Characterising the herbivore community and its impact on Sonchus oleraceus (Asterales: Asteraceae) in its invaded range in Australia

International audienceSonchus oleraceus is an annual species native to Eurasia and Northern Africa that has become a major weed of cultivated fields and ruderal areas in Australia. Populations are difficult to manage in cropping systems because of the development of resistance to common herbicides. Biological control is being investigated as an additional tactic for managing the weed. A literature review was conducted to identify the phytophagous arthropod species already associated with the species in Australia to guide such a biological control programme. To identify opportunities for biological control agents to aid in management, we undertook field surveys across Queensland and South Australia in different environments. We also investigated factors that may influence their performance in Australia. Both the literature review and field surveys identified 21 arthropod species associated with S. oleraceus in Australia, most of them being generalist species and pests of exotic origin. Capitula were the most damaged plant part while stems were relatively free from insects, except aphids. The field surveys recorded an unexpected new interaction between S. oleraceus and the gall midge, Contarinia jongi (Diptera: Cecidomyiidae). This association was surprising as the midge, only known to develop on Alstroemeria (Liliales: Alstroemeriaceae), a very distant relative to S. oleraceus (Asterales: Asteraceae), was reported in Australia only a few years ago under greenhouse conditions. The midge and the moth Eublemma cochylioides (Lepidoptera: Erebidae) were the two species that occurred most frequently in developing flower head samples. We considered their infestation rate as a proxy of herbivory and tested whether the environment surrounding the plant may influence herbivory. Both E. cochylioides and C. jongi showed the highest infestations in ruderal sites compared with the sites located in conventional farming areas. We discuss the implications of our results in the context of selecting and releasing candidates for the biological control of the weed, especially in agricultural landscapes

An innovative approach combining metabarcoding and ecological interaction networks for selecting candidate biological control agents

International audienceClassical biological control (CBC) can be used to decrease the density of invasive species to below an acceptable ecological and economic threshold. Natural enemies specific to the invasive species are selected from its native range and released into the invaded range. This approach has drawbacks, despite the performance of specificity tests to ensure its safety, because the fundamental host range defined under controlled conditions does not represent the actual host range in natura, and these tests omit indirect interactions within community. We focus on Sonchus oleraceus (Asteraceae), a weed species originating from Western Palearctic that is invasive in many countries and notably in Australia. We explore how analyses of interaction network within its native range can be used to (a) inventory herbivores associated to the target plant, (b) characterize their ecological host ranges and (c) guide the selection of candidate biocontrol agents considering interactions with species from higher trophic levels. Arthropods were collected from plant community sympatric to S. oleraceus, in three bioclimatic regions, and interactions were inferred by a combination of molecular and morphological approaches. The networks reconstructed were structured in several trophic levels from basal species (plant community), to intermediate and top species (herbivorous arthropods and their natural enemies). The sub-network centred on S. oleraceus-related interactions contained 116 taxa and 213 interactions. We identified 47 herbivores feeding on S. oleraceus, 15 of which were specific to the target species. Some discrepancies with respect to published findings or conventional specificity tests suggested possible insufficient sampling effort for the recording of interactions or the existence of cryptic species. Among potential candidate agents, six exhibited interactions with natural enemies. Synthesis and applications. Adopting a network approach as prerequisite step of the classical biological control programme can provide a rapid screening of potential agents to be tested in priority. Once ecological host range defined, we suggest that priority should be given to agent used by a minimum species, and, when they exist, to agents that possess enemies from the most distant taxonomical group from those occurring in the range of introduction

Feasibility of classical biological control of Sonchus oleraceus in Australia

International audienceSonchus oleraceus L. (Asteraceae), an annual species native to Eurasia and northern Africa, is among the most widely distributed plant species on Earth. In Australia, S. oleraceus, is a common weed in disturbed areas such as crop fields, pastures, gardens and roadsides. In agricultural settings, it can dominate fallows and cultivated fields where it uses stored soil moisture and reduces crop yield. This weed has also developed herbicide resistance, predicating the need for alternative management solutions. In this context, we undertook field surveys and preliminary host range studies in the native range of S. oleraceus to determine the feasibility of developing classical biological control solutions for Australia. Fifty-nine phytophagous arthropod species were recorded and nine pathogenic fungi were recovered from disease symptoms. Four arthropod species were selected for initial host-specificity testing based on information available in the literature. Preliminary host-specificity tests were also performed with representative isolate(s) of six of the pathogenic fungi. All these candidate agents were shown in the tests to affect key native species in Australia in the same subtribe as S. oleraceus (i.e. Sonchus hydrophilus and Actites megalocarpus). The results of our investigations suggest that classical biological control may not be a feasible option for the management of S. oleraceus in Australia, and that alternative integrated weed management tactics may need to be pursued to mitigate the impacts of this weed