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1997

Sammendrag

Availability of defenceless or weakly defended breeding material, e.g. trees felled or damaged by major wind storms, is a necessary requirement for an outbreak of the spruce bark beetle. Water stress is also hypothesised to be an important predisposing factor, although this idea is mostly based on circumstantial observation of a coincidence between dry weather and beetle outbreaks. Whereas a causal relationship may well exist for certain scolytid beetles, experimental evidence is often scarce and in some cases contradictory. We examine information pertaining to population dynamics of the spruce bark beetle Ips typographus in Norway. This includesexperiments to assess effect of drought stress on host tree susceptibility to the beetle’s pathogenic fungal associate, Ceratocystis polonica, andresults from a population monitoring program which has been in operation since 1980. Because drought often coincides with hot weather, one can confuse a direct effect on beetle propagation by hot weather with an indirect effect on insect dynamics through predisposition of the host trees. We conclude that apart from extremely rare cases of severe droughts lasting for a long period of time, direct effects of spring weather on beetle propagation are more likely to affect I. typographus dynamics than are indirect physiological effects working through the host trees.