Drought-mediated host tree-bark beetle interactions

Topic: European Norway spruce forests are highly prone to outbreaks of the Eurasian spruce bark beetle Ips typographus, often triggered by extreme climatic events such as storm throw and drought. Given the few alternative tree species which might in future replace Norway spruce in mountainous and alpine forest ecosystems, effective control and prophylactic measures rely on plausible risk models. It is therefore essential to understand how water deficiency of this important conifer species affects its susceptibility to bark beetle attack. To gain novel insights into the complex host/pest interrelationships, elaborate observations of tree defence parameters, emission of attractive compounds and the response of bark beetles are performed in a drought-stress experiment in the field and via laboratory experiments. Hypotheses: Drought-stress limits photosynthesis, therefore trees are provided with less energy for growth and the defence of invading organisms. Spring and summer drought is expected to influence the availability of carbon reserves for defence reactions of Norway spruce. The efficacy of tree resistance is indicated by the intensity of resin flow, control of fungal infections, and accumulation of repellent and/or toxic substances in the bark. Bark beetles are attracted or repelled by volatile compounds emitted from the bark, the composition and concentration of which differ with drought stress level of the trees. Methods: At the study site in the Rosalia Mountains, roofs are constructed around 12 mature Norway spruce trees in order to induce drought stress; 12 non-stressed trees are selected for control. Climate and tree physiological parameters such as temperature, precipitation, and twig water potential are recorded. To evaluate tree defence capability, resin flow and tree reaction on blue-stain fungus inoculation are examined. Samples are taken regularly from the trees in order to measure volatile emissions and compounds accumulated in the bark which are important for tree defence. An innovative attack box approach is used to test whether beetles try to bore into the trees and/or are defended by resin exudation. Reactions of bark beetles on different bouquets identified in the course of the field experiment are also observed under laboratory conditions, potentially including repellent and appealing effects. What is special about the project? This drought manipulation experiment addressing the complex relationship between drought stress intensity, tree resistance, and bark beetle infestation is unique for the important European tree species Norway spruce. Study outcomes will significantly improve a model framework designed to predict bark beetle attack based upon climatic and tree physiological thresholds and serving as specific example for tree-insect herbivore relationships under global change.

The Eurasian spruce bark beetle, the most important scolytid species on Norway spruce in Europe, benefits from precedent abiotic damage events such as storm throw and acute drought episodes. The Rosalia Roof Project is one of the few studies performed at mature Norway spruce trees researching the interactions between environmental parameters, host trees, bark beetles, and its important associated fungi under drought conditions. This study is the first to show a significant difference between stressed and control spruce trees in their defence response to a to a blue-stain fungus commonly transferred to the trees by bark beetle attack. Not only did the stressed trees show reduced resistance against fungal growth (larger hypersensitive wound reaction zones), but also did they receive a lower number of defended and a higher number of successful bark beetle attacks in field bioassays. Moreover, bark cores from stressed trees were preferred by male beetles in laboratory choice experiments. Together, these field and laboratory findings strongly support the hypothesis that pioneer beetles deliberately choose susceptible hosts and can easier overcome the defences of acutely stressed trees. The lower the concentration of defence compounds, such as mono- and diterpenes, in the bark, the more attractive were these bark cores in the laboratory choice experiments and the higher were total numbers of bark beetle attacks in the field. Hence, this is the first study to prove under both, field and laboratory conditions that strong chemical tree defence can inhibit bark beetle attack, at least in the beginning of an infestation. Interestingly, those trees that gained the highest number of borings in the field, most strongly induced their production of mono- and diterpenes in response to fungal inoculation in the end of the study season, which might be a sign of defence priming due to low level attack. At the same time, the fungi metabolized soluble sugars close to the inoculation hole, in this way reducing the trees' carbon supply for producing defence compounds. These observations align with the "tree exhaustion" hypothesis that virulent ophiostomatoid fungi might accelerate tree death by inducing strong defence responses and depleting carbon resources of the host trees.