Reproductive success of the spruce bark beetle Ips typographus (L.) and occurrence of associated species: a comparison between standing beetle-killed trees and cut trees.

Abstract: The spruce bark beetle Ips typographus is a severe insect pest on mature Norway Spruce. Under normal circumstances, the beetle reproduces in fallen trees, and also to some extent in standing living trees. Knowledge of its reproductive success in different sorts of breeding material is not only important in devising effective protective measures but also of ecological interest. Here, a direct comparison was made between standing beetle-killed trees and cut trees in a pairwise set-up. In 1998 in southern Sweden, stem sections were taken from both sorts of trees at six spruce stand edges and put in separate emergence traps to collect all emerging insects. Reproductive success, defined as number of daughters per mother beetle, was significantly higher in standing beetle-killed trees than in cut trees (on an average, 3.4 in standing trees and 1.7 in cut trees). Several possible reasons for this outcome are discussed, e.g. differences in egg gallery density, abundance of natural enemies and root contact. In addition to I. typographus, a large number of other beetles, as well as parasitic Hymenoptera and predacious flies, emerged from the stem sections. Several of these species are important enemies of I. typographus, e.g. Medetera flies which were nearly 10 times more abundant on standing than on cut trees. This study confirms that spruce trees colonised by I. typographus harbour a diverse and abundant set of species. It also appears that I. typographus in some situations can reproduce quite successfully in killed standing trees. [Copyright &y& Elsevier]

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