Population levels of bark beetles and associated insects in managed and unmanaged spruce stands

Relative population levels of the spruce bark beetle, Ips typographus (L.) (Coleoptera, Scolytidae) and associated insects were estimated in 12 spruce stands in central Sweden. Spruce bolts and window traps baited with semiochemicals were used for the monitoring. Six stands were unmanaged and had ongoing attacks on standing trees by I. typographus. This had led to an accumulation of dead spruce trees during several years. These six stands were compared pairwise with six old-managed stands with similar forest structure, but with no attacks during the previous years and with low amounts of dead trees. Catches of 17 species were included in a quantitative analysis. Four species, all known to be common predators in I. typographus galleries, were caught in significantly higher numbers in the unmanaged stands (two- to three-fold difference). In contrast, the number of I. typographus caught was almost identical for the two stand types. Our results indicate that predators of the spruce bark beetle may be more sensitive to certain forestry operations than their prey. Caging or baiting of bolts strongly influenced the colonization of predatory species and the number of I. typographus offspring that emerged. Compared to uncaged, unbaited bolts, offspring production was ca. 30% higher in bolts caged with a fine nylon netting and ca. 30% lower in uncaged bolts baited with ethanol and α-pinene. No difference between stand types was found in the production of offspring by I. typographus in the bolts. In a multiple-regression analysis, including the density of certain predators and of I. typographus galleries, one factor, namely ` Thanasimus larvae per bolt', could significantly explain some of the variation in I. typographus offspring production in the 36 bolts.