Pollen and leaf plant-hosts of Megachile (Austromegachile) susurrans (Megachilidae) in a temperate city: a leafcutter bee linked to papilionoid legumes?

The Megachilidae are among the bees that most depend on plant resources, since they need them both for food provisioning and for nest building. However, pollen and leaf specializations in the large genus Megachile have been largely unexplored, both in natural and urban habitats. Here, their botanical origin was studied in more than 20 cells from eight nests of Megachile susurrans in a temperate city of Argentina during four consecutive summers when adults were active, identifying a strong link for pollen (Styphnolobium japonicum) and leaf (Robinia pseudoacacia) hosts, caused by local preference. They were identified at light and stereoscopic microscopes comparing to reference collections from pollen and leaves surrounding the nesting area. Pollen resources were almost exclusively from S. japonicum, and three nests also contained important quantities of Syagrus romanzoffiana and Punica granatum, the remaining five only S. japonicum. The nest architecture was the typical for most Megachile, with cylindrical brood cells made of leaf pieces of two shapes disposed in linear series. Each nest had two to four cells composed only of leaf pieces of the exotic R. pseudoacacia. The abundant pollen of flower types with diverse configurations indicates versatile foraging behavior: keel flowers (Styphnolobium, Fabaceae Papilionoideae), multiple stamen brush flowers (Punica, Punicaceae) and open small radially symmetric flowers (Syagrus, Arecaceae). Although only three families were abundant in the diet, its composition with exotic species “unknown to bees” and flower type versatility suggest polylecty. Megachile susurrans was also generalist in the choice of nesting substrates, and generalist with occasional (local) preference in leaf and pollen hosts. The link to any papilionoid legume might also extend to its whole geographical distribution.