Résumé
In recent decades, the number of introduced species has increased due to the development of human activities such as trades and transports. Insect species developing within fruits or seeds are particularly difficult to detect and may have a high invasive potential. The insect genus Megastigmus is a group of micro-hymenoptera that mostly develop in seeds. Several species of Megastigmus have been accidentally introduced in different parts of the world through the exchange of seeds carrying hidden Megastigmus larvae. Among these species, two were introduced in France a century apart: the Douglas fir seed Chalcid, Megastigmus spermotrophus, was introduced from North America to Europe at the end of XIXth century, and the cedar seed chalcid, Megastigmus schimitscheki, was observed for the first time in southeastern France in the early 1990’s. To disentangle the evolutionary history of these introductions, we used two complementary approaches: (i) reviewing available historical data on the traceability of imported seed lots, and (ii) studying the patterns of genetic diversity of the invasive wasps in their native range compared to the invasive range. We combined mitochondrial sequences and nuclear microsatellites to characterize several populations from both the areas, and our results suggest that we can trust the historical data for the Douglas-fir seed chalcid (i.e. sourced from the North American coast) even if some genetic data indicate that the history in the introduced range is likely complex. On the other hand, genetic results demonstrate an unambiguously Cypriot origin for the cedar seed chalcid, although the available historical data strongly suggested that Turkey could be the most plausible source area. We conclude that historical data may provide incomplete, sparse or misleading information on invasion history, and genetic analyses are essential to reconstruct the whole history.