Doctoral thesis, 2015
Outbreak dynamics of the spruce bark beetle Ips typographus in time and spaceKärvemo, Simon
AbstractThe European spruce bark beetle Ips typographus (L.) is one of the most important pests of mature Norway spruce Picea abies (Karst.) in Europe. Outbreaks (periods with large-scale beetle-caused tree mortality) are often triggered by large-scale stormfellings or drought, which provide a large surplus of suitable breeding material. The overall aim of the studies in this thesis was to increase knowledge about how forest characteristics and local beetle populations influence tree mortality and bark beetle outbreak dynamics in time and space. We found that the size of infestation spots in general were small (<10 killed trees), that they had high extinction probabilities in the next beetle generation. The main factors increasing the probability of infestation spots in new locations (i.e. colonisations) and decreasing the probability of spot extinction were volume of spruce per ha and to some extent connectivity (i.e. distance and spot size) to neighbouring infestation spots from the previous year. The total number of killed trees during the outbreak in stands surrounding storm gaps increased with the size of the local population of I. typographus initially produced in the storm-felled trees in the storm gaps (measured as number of colonised stormfelled trees). The effect of the previous year’s local population declined as the outbreak progressed due to host tree depletion in the areas with the largest storm gaps. The reproductive success of beetles at the tree level scale was negatively influenced by the colonisation density (as a result of intraspecific competition), which in turn was affected by the trees’ diameter. The reproductive success and colonisation density differed strongly between the outbreak years. A low reproductive success in the final years may have contributed to the ultimate collapse of the outbreak. Increases in the density of natural enemies were lower than expected but may also have contributed somewhat to the outbreak collapse. The results from the different studies demonstrate a large complexity in the bark beetle-host tree interactions that influence the outbreak dynamics of I. typographus in time and space.
KeywordsBark samples; Colonisation density; Connectivity; Forest insect pest; Natural enemies; Norway spruce; Infestation spots; Reproductive success
Published inActa Universitatis Agriculturae Sueciae
2015, number: 2015:27
ISBN: 978-91-576-8252-9, eISBN: 978-91-576-8253-6
Publisher: Department of Ecology, Swedish University of Agricultural Sciences