Rhopalum nigrinum KIESENWETTER, 1849; Crabro kiesenwetteri MORAWITZ, 1866
A small, stem nesting solitary wasp. Identification keys are given in Lomholdt (1984) (as R. nigrinum), Richards (1980) and Yeo & Corbet (1995).
Infrequently recorded from a few wetland localities in East Anglia, with very few modern records. Overseas this species is found throughout Europe and eastwards to Japan.
Listed as Vulnerable (RDB 2) in Shirt (1987) and by Falk (1991). Possibly under-recorded due to its habitat preferences.
Associated with lush marshes and reed beds, both in fresh and (in Scandinavia) brackish water sites.
Univoltine; on the wing from June to August.
From studies of the species in Japan, R. gracile stocks its nest cells with a mixture of small flies (Chironomidae, Psychodidae, Dolichopodidae and Tephritidae) and Psocoptera (Lomholdt 1984).
No nest sites have been recorded from Britain. In Scandinavia, the species nests in the stems of reeds; in Japan nests have also been found in lyme-grass (Leymus arenarius) and goldenrod (Solidago occidentalis) (Lomholdt 1984).
Angelica species.
None recorded, though Lomholdt (1984) suggests the generalist chrysid wasp Trichrysis cyanea may be a parasite.