Cleptoparasitic: Hosts: In Guernsey, the species has been found with Megachile dorsalis (Luff, 1907), its presumed host on this island. The New Forest specimen is probably a vagrant or an introduction; M. leachella is almost entirely coastal in Britain with no New Forest records.
In Germany, Westrich (1989) cites both M. dorsalis [as leachella] and M. pilidens (a species not recorded from Britain) as hosts. Van der Zanden (1982) gives only M. leachella as the host in the Netherlands. Banaszak & Romasenko (1989)Stanisavljević (2000)list M. leachella [as M. argentata], M. pilidens and M. apicalis as hosts.