Occurrences of insect outbreaks in Sweden in relation to climatic parameters since 1850

Abstract: This study was carried out in order to identify the main insect species responsible for forest damages (pests) in Sweden, and to explore the relationships between insect outbreaks and environmental variables (temperature, precipitation, and availability of insect host trees, including storm damaged trees). Special attention was given to changes in management practices through time, and their consequences for pest outbreaks. The occurrences of insect outbreaks were analyzed for the southern, central and northern Sweden during two periods, 1850-1950 and 1961-2014. A Principal Component Analysis was conducted for each of the three regions in order to assess which insect families and insect species caused the main forest damage. The ratio between broadleaved and coniferous trees was calculated to highlight the type of forests mostly attacked by insect pests. A Variation Partitioning Analysis was carried out to study the influence of the climatic variables (temperature and precipitation) and the volume of storm felled trees on the occurrences of insect species responsible for outbreaks. During the first period, insect outbreaks increased markedly after 1911, and remained at a high level throughout the period. The Variation Partitioning Analysis showed that this trend might have been strongly influenced by climate and storm events. However, for the second period of time an association between these variables and the insect species responsible for the main forest damage could not be distinguished, and a clear decrease in the occurrences of insect outbreaks could be noticed in comparison with the first period. Other studies clearly showed a relation between outbreaks and climate also for the second period. The inconsistency in this study between the two time periods is attributed to the different datasets used and to other processes, such as changes in forest management practice, the use of insecticides and other countermeasures.