The spruce bark beetle, Ips typographus, was induced to attack Norway spruce by means of pheromone dispensers. The degree of attack on each tree was recorded and the trees were later categorized as surviving or dying, according to the degree of sapwood blue-staining caused by the attacks. A threshold of successful attack was observed; i.e. above a certain number of attacks the trees were successfully invaded by the beetles and their mutualistic blue-stain fungi. The height of this threshold increased with increasing tree vigour, measured as the relative increment of the sapwood cross-sectional area. © 1986.
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