DARNIS Margaux

Subject thesis: Identify likely pathways for the spread of hemipteran-vectored plant pathogens for better prophylaxis

Abstract: The aim of this thesis is to identify the likely trajectories of dissemination of insect vectors of phytopathogens with high economic impact, which could inform early monitoring systems. Statistical analysis will first be used to explore the links between vector dynamics (total and specific abundances) and the abiotic environment of the plot (local and distant climatic variables), in order to formulate hypotheses as to the relative importance of short/long-distance dispersal. On the basis of these hypotheses, we will then use modeling to design plausible scenarios concerning vector arrival patterns (timing, near or distant origin). These scenarios will then be validated using various complementary approaches (population genetics and dynamics, molecular epidemiology). Three pathosystems will be studied in parallel: Prunus-phytoplasma-psyllid, melon-virus CABYV-aphids and melon-virus WYMV-aphids. These complementary biological models represent the diversity of epidemic situations in the fields(perennial/annual crops) and constitute a continuum of pathogen/vector interactions, starting from a pathogen with a long to very long retention time in the insect (a few days to a few months) or, on the contrary, a very short retention time (a few hours), multiplying or not in its vector. In this way, it will be possible to test the importance of taking these factors into account in the development of risk indicators. Given the general nature of the dissemination trajectory design stage, the questions may be extended to other biological models studied as part of the BEYOND project (the Citrus-liberibacter-psylla model, Huanglongbing-HLB disease, for example), depending on the results and the student's interest. Epidemic scenarios have been proposed for the Prunus-phytoplasm (persistent multiplying) model. The aim here is to refine these scenarios by investigating the origin of the infective psyllids arriving in orchards in the Rhône Valley in early spring. For the melon-CABYV (persistent-non-multiplying virus) model, the main vector in France is the aphid Aphis gossypii, and its abundance at the start of the crop partially explains viral dynamics. However, the question remains as to where the life cycle of aphids arriving in a melon plot takes place. Nearby? In which case, dispersal would be short-distance, and prophylaxis could be considered on a local scale. On one or more distant sites? In this case, dispersal would be over a longer distance, and prophylaxis would have to be considered on a large-scale territory, and certainly anticipated at a very early stage. The approach here will be to search for probable flight trajectories, taking into account air mass movements and historical data on A. gossypii abundance in Avignon, and then to validate hypotheses by targeted sampling of potential source points. The same approach could be used for the melon-WMV model (non-persistent-non-multiplying virus vectored by more than 35 aphid species), refocusing the analysis of probable dispersion scenarios on short times compatible with the retention time of this virus (less than 30 hours).