The biodiversity of Hymenoptera at Watchung Reservation, a heterogeneous, 800-ha urban green space in densely populated Union County, New Jersey, USA, has been studied since 2000. In 2003-2005, yellow pan traps, supplemented by pitfalls, baits and hand collecting, were used to sample at three conifer sites that vary in site history, proximity to roadways, and pest activity. Over 6000 specimens representing at least 159 species in 24 families were collected. Parasitic wasps (especially Diapriidae and Ichneumonidae) and ants (Formicidae) and were the most abundant taxa, with 1000-2500 specimens collected in each case. For ants, species composition but not richness varied among sites as a function of distance. Rarefaction curve analysis indicated that the epigaeic component of the ant fauna was well sampled. In contrast, diapriid species composition and weekly trap catch varied between adjacent sites. These results suggest that, for some small invertebrates, microhabitat differences can strongly affect community composition, and even small urban green spaces can harbor diverse assemblages. In 2006, hornets and yellowjackets (Vespidae) were sampled with Malaise and baited dome traps to examine the effect of social insect activity on parasitic wasp diversity. Preliminary results suggest that vespid abundance depresses the diversity of some parasitic taxa. This may be particularly the case in small urban preserves, where competition for insect herbivore hosts and prey is likely to be intense.