Electrophysiological and behavioural responses of Ips typographus (L.) to trans-4-thujanol—a host tree volatile compound

Abstract

Key message: Spruce trees emit significant amounts of trans-4-thujanol, but the amount of this compound in bark decreases with tree age. Trans-4-thujanol acts as an efficient repellent for the bark beetle Ips typographus. Context: The spruce bark beetle, Ips typographus, is an economically significant pest of Norway spruce (Picea abies) that preferentially infests and kills old trees. Aims: We looked for spruce volatiles that were perceivable by I. typographus and that differed as the host tree aged. Methods: Bark beetles were collected in the wild. Bark samples from spruce trees of different age were hydrodistilled. Gas chromatography–electroantennogram detection (GC-EAD) was used to identify which compounds induced beetle olfactory responses. These were identified by gas chromatography–mass spectrometry (GC-MS). Behavioural responses of bark beetles were tested using a Y-tube olfactometer. Results: Several EAD peaks were recorded, of which trans-4-thujanol was consistently active and could be identified chemically unequivocally. The antennae of I. typographus females responded to a lower dosage of the compound than males, but both sexes were repelled by it. The bark of 10-year-old spruce trees contained 3 times more trans-4-thujanol than that of 35- 40-year-old trees, 27 times more than that of 70- to 80-year-old trees, and 200 times more than that of 120-year-old trees. Conclusions: Trans-4-thujanol is a bioactive compound in Norway spruce bark that varies in amount with tree age and affects spruce bark beetle behaviour under laboratory conditions. It is suggested that trans-4-thujanol might play a role in both spruce tree defence and tree choice by beetles.