Drought-mediated host tree-bark beetle interactions

Extreme climatic events are main triggers of pest outbreaks, and an improved mechanistic understanding of drought effects on interactions of the Eurasian spruce bark beetle (Ips typographus) and its host Norway spruce is highly demanded. The proposed drought manipulation study deals with the problem of resource allocation in Norway spruce with specific regard to secondary metabolism. To what extent is constitutive and induced defence against bark beetle attack, such as resin flow and hypersensitive reaction on blue stain fungus inoculation, influenced by tree drought stress? In particular, patterns in the composition and concentration of monoterpenes in spruce bark as well as in volatile emissions are addressed. Response of bark beetles in terms of tree attack is tested in the field, in terms of direct repellent or attractive effects of different bouquets on male beetles in the lab. The in situ drought manipulation experiment will provide novel empirical evidence of effects of water deficiency on Norway spruce secondary metabolism important for risk assessment. Study results will be incorporated in a previously developed, comprehensive model framework addressing the complex interplay between drought stress intensity, induced and constitutive defence responses, and insect infestation. At the drought manipulation site, sample trees are embraced by rain-out shelters in the size of 5x5 m, further trees are chosen for control. Climate parameters and tree water status in terms of pre-dawn twig water potentials are recorded. Constitutive and induced defence is examined by resin flow, hypersensitive reaction on blue-stain fungus inoculation, and jasmonic acid concentrations in the bark. Volatile compounds emitted from the tree stems and bark extracts are analysed by Gas Chromatography and Mass Spectrometry. Using the novel attack box approach, bark beetle attack under controlled test conditions is regularly observed. Local bark beetle activity is checked by pheromone-baited flight-barrier traps and trap trees outside the forest stand. By use of an olfactometer, behavioural response is tested for various blends of monoterpenes identified in the course of the field experiment.