Cleptoparasitic: Hosts; Unknown in Britain. Specimens from north-west England and south Wales belong to a large race (females 12-14 mm in length, males 11-12 mm), whereas the Kent population consists of smaller individuals (females 9-11 mm inlength, males 9-10 mm). It is possible that the latter race is cleptoparasitic on Megachile dorsalis, a view also expressed by F. Smith (1876) (he considered Coelioxys mandibularis to be no more than a form of C. elongata and cited the host as Megachile argentata, a misidentification of M. dorsalis). Both sexes of the larger race were noted flying around the nesting burrows of Megachile maritima on the Wallasey sandhills, Merseyside, by Arnold (1906). Hallett (1928) considered that this species seems to take the place in south Wales which is occupied by Coelioxys conoidea in the south of England, as a cleptoparasite of M. maritima (C. conoidea, however, has since been found in both Dyfed and Mid Glamorgan). D.B. Baker’s West Sussex specimen was flying with M. dorsalis. M. Edwards (pers. comm.) has observed C. mandibularis flying about the burrows of M. dorsalis at Whitford Burrows, Glamorgan.
Banaszak & Romasenko (1998), copied by Stanisavljević (2000), list the following possible or probable hosts of this species: Megachile circumcincta, M. centuncularis, M. brevicornis, M. argentata, M. versicolor, M. pyrenaea and Osmia (=Anthocopa) papaveris
A female C. mandibularis was observed running up and down a wooden post at Wallasey (Gardner, 1901b), and this behaviour may indicate that it was searching for a nest of a host species which normally nests in wood, rather than in soil.
The following Megachilinae have been recorded as hosts or possible hosts elsewhere in Europe: Megachile brevicornis (Banaszak & Romasenko, 1998); Megachile circumcincta (Jørgensen, 1921, van der Zanden, 1982; Banaszak & Romasenko, 1998); M. centuncularis (Bischoff, 1927; Banaszak & Romasenko, 1998); M. versicolor (Janzon & Svensson,1984; Banaszak & Romasenko, 1998; Peeters, Raemakers & Smit, 1999); M. dorsalis (Jørgersen 1921, van der Zanden, 1982; [as M. argentata] Banaszak & Romasenko, 1998; Peeters, Raemakers & Smit, 1999); M. pyrenaea (Banaszak & Romasenko, 1998) and Hoplitis leucomelana (van der Zanden, 1982; Peeters, Raemakers & Smit, 1999). Hoplitis papaveris [as Anthocopa papaveris] (Banaszak & Romasenko, 1998); Hoplitis villosa [as Osmia villosa](Peeters, Raemakers & Smit, 1999).