Renter: Existing cavities. The females construct the nests in existing cavities in stones. The cells are pot-like, with all the walls made of petal fragments (often Linum) . The nests are usually located in broad cavities, the cells forming a lump. There are between 8-15 cells in a nest. Banaszak & Romasenko (1989)
Praz (2017) reports that Megachile giraudi appears to nest specifically in holes in rocks and to use petals for cell construction (Westrich 1989, and references therein)