Attraction and resistance in the Picea abies – Ips typographus system
Abstract: Decades of research have gathered detailed knowledge about the Eurasian spruce bark beetle (Ips typographus L.) and its interaction with its host tree Norway spruce (Picea abies (L.) H Karst). The strong pheromone attraction enabling this beetle to kill healthy host trees is a well known process, and pheromones are widely used to trap beetles in forest pest management. Yet, host finding and final host choice by the first attacking pioneer beetles is only vaguely understood. The aim of this thesis was to identify olfactory cues for host choice from habitat to final colonisation. The importance of non-host volatiles (NHV) and old-host volatiles (OHV) for habitat finding, indicating unsuitable habitats, was confirmed in a large scale field experiment at forest edges in Slovakia and Sweden. A blend of NHV/OHV dispensers created an artificial semiochemical diversity, and reduced attacks in treated versus untreated plots. An active inhibitory range of ≈15 m was determined for the protection. A detailed study of host bark chemistry in relation to natural bark beetle attacks showed the importance of induced host defence capacity for resistance in P. abies against I. typographus attacks. The only individual host compound that seemed to influence host choice in this study was 1,8-cineole. Volatiles that were collected from bark of felled and standing healthy trees were tested on I. typographus antennae by using combined gas chromatography-electroantennographic detection. The electrophysiological studies also including single sensillum recording technique revealed eight novel ligands of host compounds, of which six are oxygenated, eliciting strong responses in olfactory sensory neurons. Proportional increase of oxygenated host compounds in host chemistry were shown to be related to induced defence and decay of host bark, and are proposed as semiochemicals for host evaluation by I. typographus. Quantification of host volatiles released by felled, healthy, and naturally attacked standing trees indicate that the onset of beetle attack correlated with high amounts of released host volatiles. Field trapping experiments and laboratory no-choice feeding assays with a subset of the oxygenated compounds confirmed the inhibiting activity of 1,8-cineole, and indicated both negative and positive effects on pheromone attraction and feeding by other antennally active compounds.
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