Open access publications in the SLU publication database

Abstract

In Sweden, forest stands younger than 15 years constitute more than 10% of the total forest area. The aims of the research described in this thesis were to investigate how bees and wasps (Aculeata) use young boreal forests as nesting and feeding grounds. Aculeates are sun-loving creatures and do not thrive in closed forest stands. Life history traits of nest-provisioning bees and wasps are characterized by low fecundity and extensive parental care. Their lifestyle differs from most other insects and they exhibit many similarities with breeding songbirds. The offspring are produced in sheltered nests that are stocked with food. Bees collect nectar and pollen and wasps collect insects and spiders. Most of this thesis is about solitary wood-dwelling aculeates that nest in holes made by other wood-boring insects. About 5800 potential nesting holes in deadwood were surveyed and the insect species or genera that had made the hole identified. Less than 2% of these holes were occupied by any bee or wasp. The reason for the low occupancy was probably that the egg-laying females are demanding in their nest choice and rejected many “inferior” holes that would not protect their offspring. Standing deadwood objects had higher occupancy than stumps and tops from harvested trees. Among two bee species, the nest abundance of a pollen-specialist species was predicted only by this species’ specific pollen resource, whereas the nest abundance of a pollen-generalist species was predicted by both pollen (mainly flowers along roadsides) and nesting resources. Tree species, sun exposure and the colonization history of beetles and fungi all contribute to the species assemblage of wood-dwelling aculeates in high stumps. These factors may interact in complex ways. Some exposure to sun seems to be a common requirement for aculeates irrespective of which tree species that were used as nest substrate. The relationship between tree species and aculeate species assemblage seems to be highly dependent on prior colonization history. Bumblebees were more abundant along edges between young and older forest stands than in the middle of the young forest stands. The reason for this is probably associated with the fact that edges represent favorable nesting and mate-finding sites. Based on results in this thesis, it is concluded that young boreal forest stands are important habitats for bees and wasps. Retention and creation of standing deadwood at final fellings is recommended and special care should be taken along forest roadsides to preserve the flora.