Open access publications in the SLU publication database

Abstract

Widespread defoliation of forests caused by insects or fungi cause economic losses throughout the world. Successful outbreak management involves cost/benefit estimation and requires knowledge of potential yield losses. Currently, such knowledge is scarce. This thesis evaluates the significance of single-year defoliation by the pine looper moth (Bupalus piniaria L.) (Lepidoptera: Geometridae) and secondary attack by the pine shoot beetle (Tomicus piniperda L.) (Coleoptera: Scolytinae) for yield losses in Scots pine (Pinus sylvestris L.). In a seven-year study, tree mortality and growth losses were quantified after a B. piniaria outbreak with a subsequent T. piniperda infestation at Hökensås in 1996. Secondary attack by T. piniperda was also studied in a Scots pine stand at Fredriksberg, infested by Gremmeniella abietina (Lagerberg) Morelet. in 2001. Tree mortality at Hökensås mainly occurred in areas with an average defoliation intensity of 90-100%. Tree mortality averaged 25%, and the “defoliation threshold” for tree survival was found to be 90% defoliation. Tree mortality peaked two years after the defoliation event, which coincided with high levels of pine shoot beetles. Involvement of T. piniperda in tree mortality increased with time since defoliation, and tree susceptibility to beetle attack increased with increasing defoliation intensity and decreasing dominance status of trees. As foliage of trees recovered, fewer trees were susceptible to beetle attack, and ultimately, beetle attacks ended as breeding substrate was depleted. Tree susceptibility to beetle attack was a function of tree vigour and beetle density. Growth losses at the Hökensås site were a combined effect of pine looper defoliation and shoot pruning by T. piniperda. Radial, height and volume growth losses were proportional to defoliation intensity. Whereas radial growth was little affected by beetle-induced shoot pruning, height growth was severely affected by beetle-induced damage to leading shoots. Growth losses alone, excluding tree mortality, were large enough to economically justify control of the pine looper outbreak had the outbreak been prevented in 1996. At Fredriksberg, T. piniperda colonised trees that would have survived the damage caused by G. abietina. However, trees with 90-100% foliage losses died because of the damage caused by G. abietina alone, and tree survival seemed to require that trees retained at least 20% of full foliage.