Early detection of bark beetle infestation using acoustic analysis

Uncontrolled proliferation of bark beetles poses a major threat to forests health. Ecological imbalances favor the spread of pests and lead to massive damage. To compensate for the consequences, the Free State of Thuringia alone is investing €500 million in forest conversion and reforestation by 2030. Early detection is crucial in order to be able to take effective countermeasures. Previous detection methods have relied mainly on visual inspections – such as identifying bore dust at the base of the trunk or assessing pathological changes in the tree – and on the analysis of satellite data, which is integrated into risk models. These methods are to be supplemented by the acoustic detection of bark beetles in order to identify infestation at an earlier stage and, if necessary, more easily.

As part of an internal research project at Fraunhofer IDMT, it has now been demonstrated in the laboratory that bark beetle activity can be detected at an early stage by analyzing airborne and structure-borne sound.

Acoustic detection as a promising approach

This innovative acoustic approach to detecting bark beetle infestation is based on analyzing acoustic signals generated by the beetles´acitvity. For this purpose, two freshly harvested spruce trunks from the Thuringian Forest that were free of infestation were examined; one of these was deliberately infected with bark beetles in the laboratory. The activity of the beetles and their larvae was monitored acoustically over a period of 20 weeks..

Results of the acoustic measurements

The evaluation of the acoustic signals showed revealed repeated phases of short, broadband pulses, that could be described as a series of “cracking noises.” Clear differences were observed between the infested and uninfested tree trunks with regard to these events. While an average of 2.6 acoustic events per minute were recorded in the uninfested trunk, this figure was 168.4 events per minute in the infested trunk. This high number of acoustic signals indicates intense bark beetle activity.

The sounds produced by bark beetles are concentrated within the audible frequency range, particularly between 2 kHz and 10 kHz. Interestingly, the acoustic events in the uninfested tree trunk were on average 2.5 dB louder than those in the infested trunk, which is attributed to the drying process of the wood.

Measurement setup

The measurements were taken in the anechoic chamber at Fraunhofer IDMT at a constant room temperature of 20 °C. A low-noise microphone system positioned at a distance of about 20 cm from the bark was used for recording. Data acquisition was carried out in two 15-hour measurement sessions to ensure continuous monitoring. During these measurements, vibrations were recorded and the bark beetle activity was visually monitored.

Outlook and further research

A key focus of future research activities will be the detailed analysis of the recorded sounds, with the aim of clearly distinguishing wood drying sounds from bark beetle activity. In addition, modern methods for improving the signal-to-noise ratio will be investigated to enhance the accuracy of acoustic detection further.

Another focus of research will be the combination of bark beetle sounds with field recordings from the forest in order to validate the feasibility in the relevant environment. These developments aim to provide forestry personnel with effective tools for the early detection of bark beetle infestation in individual trees. This will allow infested trees to be removed more effectively, containing new outbreaks at an early stage.