Herein we describe the nests (including construction, closure, orientation, and depth of cells) of the bee Osmia (Ozbekosmia) avosetta Warncke found nesting near Antalya, Turkey, and Sepidan, Iran. Cells are unusual in that they are lined by two layers of colorful flower petals that sandwich a thin middle layer of mud. Analyses of pollen taken from scopal hairs of specimens from the Turkish site were identified as solely from Onobrychis viciifolia Scop. (Fabaceae) whereas those from the Iranian site were from a related plant, Hedysarum elymaiticum Boiss. and Hausskn. These facts coupled with analyses of scopal pollen from 11 other sites in Turkey, Jordan, and Syria strongly suggest that this bee is oligolectic with respect to the plant tribe Hedysareae.
The egg and last larval instar of Osmia avosetta are described. The presence of an egg taken from a cell and provisionally identified as belonging to Sapyga pulcherrima Morawitz suggests that this cleptoparasite may have this bee as one of its hosts.
In addition, we report new information on and review published accounts concerning the use of whole petals or large petal pieces in the construction of cell walls of osmiine bees. Only Osmia (Ozbekosmia) avosetta and species of Osmia (Tergosmia) have three-layered cell walls with the middle layer made of mud. Recorded also are the similarities and differences exhibited in pollen and petal preferences and nest characteristics of species in these two related subgenera.