In the past 20 years, several mass attacks by the eight-toothed spruce bark beetle (Ips typographus L.) occurred in Switzerland. Since 1984 data on compulsory fellings, numbers of infestation spots and numbers of beetles captured in pheromone traps have been collected in all Swiss forest districts. The annual data were published in short reports.
This paper gives an overview of the progress of bark beetle epidemics from 1984 to 1999. On the national level, two periods with epidemics of the spruce bark beetle can be distinguished. The first epidemic started after windthrows and drought in the years 1982 and 1983. It reached its height in 1984/1985 and faded at the end of the decade. The second epidemic started after the storm Vivian (February 1990). It reached its height in 1992/1993 and ended in 1997. Storm damage that produced high quantities of suitable breeding material was not the only factor for the bark beetle gradation; weather conditions were also exceptional, such as hot and dry vegetation periods.
Windthrows and exceptional weather conditions are mostly events on regional scales and do not affect the whole country with the same intensity. The gradation that appears to be an «epidemic of the spruce bark beetle in Switzerland after the storm Vivian» is in truth the accumulation of several local epidemics. This is shown by the number of compulsory fellings and infestation spots analysed at regional levels. Extreme weather conditions and storms not only directly influence the bark beetle population, they also stress standing trees, thus creating suitable breeding material for bark beetles. To analyse the influence of exceptional weather conditions on bark beetle epidemics, it is therefore necessary to evaluate regional data.