Abstract :
[en] The Mons and Gembloux laboratories study the wild bees of Belgium and France more than 30 years, beside special contributions for other countries. A first report, published as early as 1980, listed 13 threathened wild bees species in Belgium and N. published as early as 1980, listed 13 threathened wild bees species in Belgium and N. France. In 1991, this research team published a comprehensive report about the faunistic drift in Apoidea of Belgium by comparing the relative numbers of species before 1950 and since then. The change in the abundance was estimated by the Stroot & Depiereux statistical method. Of 360 species, 91 are decreasing (25.2%), 145 are stable (40.2%), 39 are expanding (10.8%), and 85 have an indeterminable status (rare species: 23.5%). The authors compared different hypotheses that could explain this global regression. As the regression mostly affected long-tongued species, it is likely due to the fall in availability of plants with long corollae (e.g. Lamiaceae, Fabaceae, Scrophulariaceae). The strong relative regression of cleptoparasites could be seen as the result of an absolute numerical decrease of all Apoidea. The relative regression of species nesting under ground could be explained by the lessened availability or suitability of open areas due to afforestation, urbanisation or agricultural intensification. The strong regression of long-tongued species seriously threatens the maintain of an appropriate pollination level of wild and cultivated plants. The present situation of the survey indicates that numerous species are strongly regressing, not only in the cultivated plains of Belgium and surrounding countries but also at the continental level (typical examples are Bombus cullumanus, Bombus confusus, Bombus sylvarum, Megachile parietina). Everywhere, the species associated with Leguminous plants seem to be at highest risk. On the other hand, some species are stable, despite of their extreme localisation (a good example is Xylocopa cantabrita). A few species are also known to be expanding (a typical case is Bombus semenoviellus).
