Rise and fall of a spruce bark beetle outbreak – Importance of colonisation density and reproductive success

Tree-killing bark beetles are an important disturbance factor in conifer forests. We studied if bark beetle performance in colonised trees may influence outbreak progression. Trees colonised by the Eurasian spruce bark beetle Ips typographus were sampled during an entire outbreak triggered by a storm-felling in southern Sweden. The study was conducted at nine storm gaps where no sanitation cuttings were done. Colonisation density (maternal gallery density) and reproductive success (daughters per mother) of I. typographus, and densities of competitors and enemies, were recorded from bark samples collected in the autumn from colonised storm-felled and standing trees during seven years. Yearly colonisation densities were much lower in storm-felled (68 – 103 galleries per m²) than in standing trees (218 – 717 galleries per m²). For standing trees the lowest colonisation densities occurred in the first two years of the outbreak and colonisation density was positively related to tree diameter. Beetle reproductive success was higher in storm-felled (9.4 – 9.6 daughters per mother) than in standing trees (0.4 – 5.7 daughters per mother). In standing trees the highest values occurred in the first two years of the outbreak. Reproductive success was significantly related to colonisation density (negatively), tree diameter (positively), parasitoid density (negatively at high densities) and attack year (negatively). Colonisation density had the largest effect. Our results demonstrate the importance of managing spruce forest in a way that increases tree vitality and minimises the risk for storm-fellings. Timely sanitation cuttings of colonised storm-felled and low-vitality standing trees may reduce the risk for outbreak initiations.