Population trends and conservation of cavity-nesting bees and wasps

Juho Paukkunen and Ilkka Teräs, University of Helsinki

Reima Leinonen, Kainuu Regional Environment Centre

Nearly 600 bee and aculeate wasp species (Hymenoptera, Aculeata) are found in Finland, and about 150 of them nest in wood cavities, mostly made by beetle or wood-wasp larvae. Cavity-nesters are found in the wasp families Vespidae, Pompilidae and Crabronidae, and in the bee families Apidae, Megachilidae and Colletidae. Moreover, about 35 species live as cleptoparasites or parasitoids of cavity-nesting bees and wasps. These species consume the prey or nectar and pollen provisioned by the host females for their larvae, or eat the host larvae themselves. Parasites of cavity-nesters include wasps of the families Chrysididae and Sapygidae, and bees of the genera Stelis and Coelioxys.

In Finland, as in many other countries, cavity-nesting bees and wasps are suffering from the lack of suitable nesting places. Because of modern forestry, the amount of decaying and dead wood is diminishing in the forests. Also, log-walled barns and wooden fences are disappearing due to intensification of agriculture and depopulation of rural areas. Furthermore, the area of semi-natural grasslands, which provide important food sources for bees and wasps, has been decreasing rapidly. The aim of the present study is to collect information from the literature, insect collections and study materials of several field projects to estimate population trends of cavity-nesters and their parasites in Finland. New material has been collected from different habitats and different parts of the country using so called trap-nests.

During the project more than 70 000 specimens of bees and wasps have been determined in museums and private collections. Three new species have been found from Finland and the status of several other taxons has been revised. Some of the species, which have been thought to be endangered appear to be vigorous, but, on the other hand, in many species an alarming decrease in numbers and a clear reduction of the distribution area during the last decades has been detected. The collected data will allow a complete evaluation of the threatened status of cavity-nesters and their parasites in Finland. Also the usefulness of different conservation methods, including the use of trap-nests, can be assessed.