Solitary Hymenoptera aculeata inhabiting trap-nests in Lithuania: nesting cavity choice and niche overlap

The study has been carried out during the years 1989-2002 in five localities of Lithuania, using bundles of 25-35 internode fragments of Phragmites australis stems as trap-nests. We obtained 3237 nests of 33 identified solitary wasp and bee species. The dominant species were as follows: Symmorphus allobrogus (2030 nests), Ancistrocerus antilope (260), S. murarius (222), Discoelius zonalis (139), S. bifasciatus (115), TRypoxylon figulus (99), S. debilitatus (52), A. trifasciatus (40), T. clavicerum (39 nests), T. minus (36), S. crassicornis (36), and S. gracilios (33). The other 21 wasp and bee species built less than 30 nests each. We appraised the differences on the species preference for the inner diameter of the trap-nest, height above the ground, orientation of the wall, on which the trap-nest was situated, and the nest entrance direction. The studied species formed groups with median nesting cavity diameter of 2.5-4.0 mm (Chelostoms maxillosum, Hylaeus communis, Passaloecus corniger, S. bifasciatus, S. debilitatus, T. clavicerum, T. minus), 4.6-5.2 mm (A. trifasciatus, D. dufourii, S. allobrogus, S. gracilis, T. figulus), and 5.5-6.5 mm (A. antilope, A. nigricornis, D. zonalis, S. crassicornis, S. murarius, Osmia rufa), possibly having synusial dependence on exit holes of particular xylophagous beetles. Most wasp species had wide range of the used nesting cavity diameter and considerable niche overlap. However, some congeneric species seem to avoid competition by different nesting cavity diameter preferences, as D. dufourii and D. zonalis, or S. allobrogus and S. bifasciatus. On the other hand, some other congeneric species that use the same nesting cavity diameter (S. allobrogus and S. gracilis, S. bifasciatus and S. debilitatus, S. crassicornis and S. murarius) coexist in the same habitat due to other niche differences, as different prey.