Predispositions and symptoms of Agrilus borer attack in declining oak trees

This paper presents results of a semi-quantitative study on the role of Agrilus biguttatus F. in oak decline in Belgium. Larvae of this insect breed in living subcortical tissues of European oak. Several factors favouring attacks are discussed, among which the overall health condition and the local physical and biochemical status of the host tree. Larvae, feeding galleries, pupae, imago and D-shaped emergence-holes of A. biguttatus were observed exclusively in declining and recently dead oaks. Attacks start in the south-facing, sun-exposed parts of the subcrown stem, with a preference for thicker-barked trees or similar areas within declining trees. The feeding of early larval stages induces subcortical necrosis and longitudinal bark cracking. The more destructive tunnelling of advanced larval stages cuts functional vessels and phloem elements, which enhances the decline. In conclusion, effects on wood quality and suitable control options are discussed.