Terahertz scanner against bark beetle

Scanner makes burial mounds of woodchucks visible under the bark

Here the drills of the book printer (Ips typographus) are clearly visible, but in the forest the infestation is often recognized too late. A new Teraherzt scanner could help. © DavorLovincic / iStock
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Help in the fight against the wood pest: In the future, an infestation of trees by bark beetles could be detected easier and faster - using a portable terahertz scanner. Researchers have developed this technology in such a way that it is now also possible to screen stems and branches from the outside. As a result, the beetles' bores become visible as in a kind of sonar, without foragers having to manually search for the boreholes.

The increasing heat and drought is increasing our forests, many native tree species are fighting for their survival. To make matters worse, that the ever milder winter and dry summers benefit many wood pests. The bark beetle (Scolytinae) is currently benefiting from the situation and is spreading massively, especially in spruce forests. Whole forest pieces had already been cleared because of the infestation with these weevil species.

The problem: discovering a bark beetle infestation in time is not easy and very time-consuming. "Typically, this is done manually by searching the bark of the trees for the boreholes of these wood-destroying insects, " explains Kristi Krügener of the College of Applied Science and Art (HAWK). "On this basis, it is then estimated how large the affected tree population is."

Trial sample when examining with the new terahertz scanner. © HAWK

Terahertz scanner also for trunk pieces

But a new device could in future significantly simplify the detection of bark killer infestation. Kr gener and her team have further developed the principle of the terahertz scanner. This uses electromagnetic radiation from the frequency range between infrared and microwaves to illuminate clothing, plastic or other materials. Even in thin wood or bark terahertz rays can penetrate.

So far, terahertz scans of wood have only been possible with flat boards or slabs. Kr gener and her team have now developed a scanner that, thanks to a swiveling signal and receiver head, can also scan round and irregularly shaped objects with high resolution. The big advantage: "The depth resolution of such terahertz measurements is higher than the conventional X-ray techniques and comparable to that of micro-tomographs, " explains Kr gener, display

Reflections reveal drill holes

When searching for Borkenk fern, the terahertz scanner could be used like a portable transceiver. Because attached to the trunk, the rays penetrate into the upper layers of bark and wood and are reflected to different degrees depending on the nature of the interior. If there are drill holes in the bark sink, more radiation is thrown back.

"An infected sample reflects more terahertz radiation than a non-infected one, because there are more interfaces, " explains Kr gener. Because every air-filled drill hole breaks through the wood structure, it becomes visible as in the terahertz image. "The individual reflection patterns can then be used to map the internal structure of the affected wood, " says the researcher. In initial tests, she and her team were able to detect drill holes in the printer to within one inch of the wood surface.

The scientists still have to adapt their device in such a way that it becomes wearable and can thus be used locally in the forest. Then, however, terahertz technology could greatly facilitate the work of foresters and forest workers and make barking killer plagues more readily identifiable. (Applied Optics, 2019; doi: 10.1364 / AO.58.006063)

Source: The Optical Society

- Nadja Podbregar