[Ragweed (Ambrosia artemisiifolia L.): expansion history of a "biological pollutant" in France].

In France, common ragweed (Ambrosia artemisiifolia L.) is an invasive species, which most probably originates from North America. This plant is responsible for human health problems as the pollen causes allergic rhinitis and seasonal asthma; in addition, it engenders agronomical problems as the efficient herbicide treatments are few. Consequently, various departments of the Rhône-Alpes region set up eradication programs for common ragweed. The species is distributed over a large range of ecological environments (road margins, embankments, river beds) and does not seem to be dependent on soil properties. Its ability to occupy different environments varies with the geographic location. Common ragweed was cultivated in botanical gardens during the XVIIIth century but seems to have arrived in France in seed lots during the XIXth century. It therefore began its "invasion" as a crop weed. Because of its late emergence date (late March), common ragweed is most frequently found in spring crops as well as during the inter-crop season. Its "natural" dispersal mechanisms are rudimentary; its seeds are probably dispersed mostly during the transport of material (soil, gravel, compost...), irrigation and especially via harvest combines. The development history of this species in France is scarcely known. The examination of herbarium collections helped to partially reconstruct the history of the species. According to the first results, the species arrived in several locations and at different dates in France. However, common ragweed spread most successfully in the Lyons region because of reasons still unknown.