Early arriving saproxylic beetles (Coleoptera) and parasitoids (Hymenoptera) in low and high stumps of Norway spruce

Abstract

Stumps constitute a large wood resource in managed forests, but have generally been little studied in terms of insect biodiversity. Basic knowledge of the stump fauna is necessary to e.g. assess the value of other dead wood, such as high stumps created for conservation purposes. This study focuses on early arriving bark- and wood-boring beetles and associated insect enemies (predatory Coleoptera, Heteroptera, and parasitic Hymenoptera) in fresh stumps of Norway spruce Picea abies. Ordinary low stumps were compared with mechanically created 4 m high stumps. Wood samples for insect rearing were cut from low ( n = 28) and high stumps ( n = 21) on five one-summer-old clearcuts in Sweden. High stumps were sampled at the base and at 1.5 m above ground. All emerging insects were determined to species or genus level. Proportion of stumps inhabited, density and Shannon's index for diversity and evenness were calculated. At least 17 phloem/fungivorus (predominately Scolytidae and Cerambycidae) and 8 predatory beetle taxa (mostly Cleridae and Nitidulidae) were recorded, as well as 9 parasitoid taxa (Pteromalidae, Eurytomidae and Braconidae) associated with bark beetle hosts. Most taxa occurred in both low and high stumps. High stumps were slightly better for parasitoids in terms of occupancy, density and index values. Phloem/fungivorus and predatory beetles occurred in nearly all low and high stumps (93–95%), whereas parasitoids were more often recorded at 1.5 m in high stumps (95%) than in the base (48%) or in low stumps (54%). Three parasitoid species occurred in significantly higher densities in high stumps than in low stumps ( Rhopalicus quadratus, Rhopalicus tutela and Dendrosoter middendorffi). The results show that above-ground parts of fresh spruce stumps to a high degree may serve as habitat for secondary and mostly harmless bark beetles. They also harbour generalist predators and parasitoids, including important enemies of forest pests such as the spruce bark beetle Ips typographus. The results support previous studies showing positive effects of high stumps on insect biodiversity, and suggest that also higher trophic levels such as parasitic Hymenoptera are favoured.