Host‐Plant Specialization in Western Palearctic Anthidine Bees (Hymenoptera: Apoidea: Megachilidae)

To determine the extent of host—plant specialization, the pollen sources of the 72 anthidiine species (family Megachilidae, subfamily Megachilinae, tribe Anthidiini) of Europe, North Africa, and Asia Minor were investigated by microscopic analysis of ≈ 1800 pollen loads of females. By this measure, 31 species (43%) were oligolectic (relatively specialized to pollen source) at the level of plant tribe, plant subfamily, or plant family. Exclusive pollen sources of these bees throughout their geographic ranges are flowers of the Cardueae (Compositae), the Asteroideae (Compositae), the Papilionoideae (Leguminosae), the Lamioideae (Labiatae), the Nepetoideae (Labiatae), the Dipsacaceae, or the Campanulaceae. Thirteen species (18%) were found to exhibit a strong, but not exclusive, preference for the Papilionoideae (Leguminosae), the Labiatae, the Cardueae (Compositae), and Zygophyllum (Zygophyllaceae), respectively, while 25 species (35%) proved to be more markedly polylectic, visiting the flowers of up to 17 different plant families for pollen. The plants exploited by three species (4%) are insufficiently known. By far the most important pollen sources of the anthidiine bees as a whole are the Compositae (41.7%) followed by the Leguminosae (23.1%) and the Labiatae (13.0%). The phylogenetic relationships of the anthidiine bees were estimated by a cladistic analysis based on 115 characters to trace possible evolutionary patterns of diet composition. Based on the estimated phylogeny, at least eight shifts of oligoleges between different plant taxa and six transitions between oligolecty and polylecty appear to have occurred. Four transitions were from oligolecty to polylecty whereas two transitions are of unknown direction, both directions being equally parsimonious. Assuming that the ancestral state in the anthidiine bees was oligolectic, the present distribution of oligolectic and polylectic species can be explained solely by switches from the oligolectic to the polylectic habit and by shifts of oligoleges between different plant taxa. Three of four transitions from oligolecty to polylecty are accompanied by a reduction in bee body size. The significance of this size reduction with respect to the polylectic habit is discussed. The oligolectic anthidiine species visit significantly fewer flower species for pollen during a single foraging bout than the polylectic species. On average, 1.4 plant species were recorded in the loads of specialists compared to 2.2 for generalists. Two monophyletic groups of bees belonging to the genus Anthidium are equipped with a pollen—collecting apparatus consisting of specialized hairs localized either on the face or on the underside of the thorax. It is used to remove pollen from the raised anthers of flowers of the Labiatae and the Scrophulariaceae and to brush pollen from the flat inflorescences of some Compositae, respectively. The observation of flower—visiting females of several anthidiine species revealed that pollen uptake is far from an accidental process. Basic pollen—harvesting techniques have reached a high degree of efficiency: females of a given bee species worked flowers of a certain plant species in a fixed manner, the structures used for pollen uptake from the same flower type were found to be largely the same among anthidiine bees of different taxonomic groups, and no distinct differences with respect to the basic pattern of pollen removal from flowers of the same architecture were obvious when comparing oligolectic and polylectic species. On the other hand, polyleges showed a high intraspecific flexibility regarding the organs used for pollen uptake from flowers of a different architecture.