Abstract
Biological invasions are a concern because of their increase and their environmental, economic and human health consequences. To become invasive, a population must (i) be introduced, (ii) established itself and (iii) proliferates. Each of these three steps constitutes a challenge, and the processes involved are still poorly understood. This thesis describes a set of research actions which aims at understanding the worldwide successful invasion of the Asian ladybird Harmonia axyridis.We first investigated the introduction step by retracing invasion routes of H. axyridis using microsatellite markers and the ABC (Approximate Bayesian Computation) method. We have shown that the oldest invasive population in the eastern North America acted as a bridgehead of the worldwide invasion by becoming the source of the European, the South American and the African outbreaks. We also found evidence for a genetic admixture event in Europe with a biological control strain.Second, we explored the establishment step. We have shown that invasive populations of H. axyridis endured a genetic purge which significantly reduced adverse effects associated with inbreeding depression. In addition, the admixture event in Europe likely brought phenotypic benefits to this invasive population.We then studied several aspects of the proliferation step. We found that European populations had evolved towards higher dispersal abilities on the invasion front. Moreover, our results show that the use of a flightless biocontrol strain which is still sold in France may have a positive impact on the expansion of the invasive population through heterosis or increased of genetic variance.Finally, we discuss the importance of studying in detail a model species such as H. axyridis to improve our general understanding of the eco-evolutionary mechanisms involved in biological invasions.