Escape in space from enemies: a comparison between stands with and without enhanced densities of the spruce bark beetle

Abstract

1 Populations of the spruce bark beetle, Ips typographus (L.), are known to grow rapidly in storm-disturbed stands as a result of relaxation from intraspecific competition. In the present study, it was tested whether a second mechanism, escape in space from natural enemies, also contributes to the rapid population increase.

2 The experiment was conducted during the initiation phase of five local outbreaks of I. typographus triggered by a storm-disturbance in November 1995 in southern Sweden.

3 The impact of natural enemies on the ratio of increase (number of daughters per mother) of I. typographus was compared pairwise between disturbed stands with high numbers of storm-felled trees and undisturbed stands without wind-felled trees.

4 Enemy impact was assessed by comparing the ratio of increase in uncaged (exposed to enemies) and caged (protected from enemies) bolts colonized by I. typographus prior to being placed in the stands. The experiment was conducted in the second and third summers after the storm-felling.

5 Enemy impact was about twice as high in stands without wind-felled trees compared with in stands with wind-felled trees in the second summer whereas there was no significant difference between the stand types in the third summer.

6 The result demonstrates that spatial escape from enemies contributes to the rapid population growth of I. typographus after storm-disturbances.