Renter: Existing cavities. The females construct the nests in existing cavities in a variety of substrata and will readily take to trap nests. The nest is linear, and all the walls are made of leaf fragments. There is an observation (Gurtbir, 1916) which suggests that occasionally, the cells may have no true walls, but only partitions (Banaszak & Romasenko, 1989; Stanisavljević, 2000).
Praz (2017) reports that Megachile rotundata nests primarily in existing cavities such as stems or beetle burrows and uses leaf discs to build its brood cells, and occasionally petal fragments (Westrich 1989, and references therein)