1. Old-growth forests have features that endow them with an extraordinary ecological value. These forests are sources of habitat diversity and, consequently, biodiversity, which makes them a basic objective of conservation programs. Insects have been traditionally used as indicators of forest condition.
2. The aim of this study is to uncover patterns of Hymenoptera abundance and diversity, and their relationship with structural features in old-growth forests. We use pan traps to sample the community of flying Hymenoptera in two old-growth forest types (silver fir and mountain pine) with important structural differences.
3. Compared to other surveys of local Hymenoptera communities, our sampling yielded an extremely high number of species (630).
4. At the plot level, the two forest types showed important differences in family richness and diversity, but not in abundance or in species richness and diversity. However, variability in species richness was higher among pine than silver fir plots, leading to overall higher species richness in the former.
5. Species composition also differed between pine than silver fir forests, and these differences were related to important structural differences between the two forest types.
6. Canonical correspondence and multiple regression analysis yielded contrasting habitat requirements among Hymenoptera families and functional groups (bees, sawflies, parasitic wasps and predatory wasps).
7. We conclude that flying Hymenoptera communities can be used as good indicators of forest structure, habitat complexity and conservation status.