Renter: Existing cavities. Utilises the old gall-chambers of the fly Lipara lucens on Common Reed, (Phragmites australis) .
Throughout its European range this species is mainly associated with the vacated, spindle-shaped galls of the chloropid fly Lipara lucens which it uses as a nest site. These conspicuous galls are formed on the apices of the flower stems of the Phragmites, their development inhibiting flowering. When freshly formed and occupied by Lipara lucens larvae (one per gall) the ensheathing leaves are green, but by the following summer these are dead and have assumed a brown colour. It is only the vacated galls that are utilised as a nest site, these usually being recognised by their frayed tips.
Details of the life cycle and nesting habits of H. pectoralis (including illustrations of the nest and its contents) have been described in detail by Else (1995). Photographs of opened nests, portraying both cells and nest plug, appear in Imms (1971), Else, Felton & Stubbs (1978), and Müller, Krebs & Amiet (1997).
A specimen has also been reared by J.P. Field (pers. comm.) from a bundle of cut, dead Phragmites stems suspended as trap-nests from a pole within a reed bed. R.C.L. Perkins (1900) describes the species as burrowing into dry reed stems, and Janvier (1972a) also mentions broken Phragmites stems as a nest site.
In Germany, it has been shown that Hylaeus pectoralis nests in Lipara lucens galls can tolerate several days immersion in water (P. Westrich, pers. comm.).