science
Bark beetles find it easier to infest trees in dry seasons. Apparently because they specifically choose trees that are already suffering from stress, as a team from the University of Natural Resources and Life Sciences (BOKU) Vienna has now found out.
The dominating spruce forests in this country suffer the most from infestation by the bark beetle (Ips typographus), especially in high heat, drought and windthrow. “The observation that dry and warm periods promote its mass reproduction was written down in works that are more than a hundred years old,” says BOKU researcher Sigrid Netherer, according to a press release from the Austrian Science Fund FWF. However, the deeper connections between drought stress and the more frequent death of trees are far from fully understood, write Netherer and her team in the journal “Journal of Pest Science”.
Experiments with spruce
The spruces invest in various defenses against attackers such as the bark beetle. Thus, the tree makes resin with various compounds for defense. These processes and the composition of the resin are affected by drought, higher temperatures and other stressors.
In the teaching forest of the University of Natural Resources and Life Sciences (Boku) Vienna in the Rosaliengebirge on the border between Lower Austria and Burgenland, Netherer cut off ten spruce trees over two seasons with canopies as far as possible from the water supply. In the FWF-funded project, the effects of this were compared with ten trees that lived under normal conditions. For this purpose, the scientists exposed the various spruce groups to bark beetles from the Boku breeding facility in specially designed infestation boxes.
Beetle pioneers
The researchers also turned their attention to pioneer beetles. These are males who set out to explore new trees for their species to colonize. To do this, they use their sense of smell and then, to a certain extent, hijack the tree’s defenses by using components of the resin. This is how they signal to their peers that there is something to fetch. The Viennese scientists carried out the resin analyzes in cooperation with colleagues from the Max Planck Institute for Chemical Ecology in Jena (Germany) and the New University of Lisbon (Portugal).
The observation that the pests find it easier to cope with drought-stressed trees was confirmed again in the field test. Laboratory studies have also shown that the bark beetle pioneers show a preference for the bark and bast of spruce – as well as for trees that are no longer able to defend themselves so well against pests.