It is a cleptoparasite of Megachile willughbiella (Kirby), M. circumcincta (Kirby) and Anthophora furcata (Panzer). It may also parasitise M. centuncularis (Linnaeus) and A. bimaculata (Panzer). The females of all but one British species of Coelioxys have a pointed sixth tergite and fifth sternite which are apparently used to cut open the cell wall or cap of the host species. An egg is laid in this slit with either at least one third protruding through into the cell or laid directly onto the host egg. There is currently no work to suggest which method C. rufescens employs. In general, Coelioxys larvae kill, and in some species eat, the host egg immediately after hatching. Pupation occurs within a cocoon spun within the host cell where the larva overwinters as a pre-pupa, prior to final pupation, presumably in spring of the following year.