Trapped in the forest: The longhorn beetle Tragosoma depsarium L. in south-east Sweden

Abstract:

The rare and elusive wood-living beetle Tragosoma depsarium, once widespread all over Sweden, is associated with large, sun-exposed pine logs required for breeding. Due to modern forestry and fire suppression, this type of substrate has become so rare in the landscape that the beetles’ existence in Sweden is threatened. Recently, the female sex pheromone of this species was synthesised, providing a new method to monitor beetle populations using pheromone traps. Such a method was used in the current study to relate the presence-absence of T. depsarium with different landscape variables at different scales; 100, 500, 1000 and 2000 meters, respectively. The occurrence of T. depsarium in Östergötland County was associated with the amount of protected areas and clear-cuts at 500 and 1000 meters, respectively. Additionally, the amount of pine forest within 2000 m from the trap showed a weak, positive effect on the number of beetles. In the whole south-east boreo-nemoral zone of Sweden, a positive correlation between beetle occurrence and protected areas were found at 2000 m together with a negative correlation for volume pine wood within 100 m. Also, emergence holes on pine logs around each trap were strongly associated with beetle occurrences. To conclude, using pheromone traps were an easy way of detecting beetles. Although the number of caught T. depsarium varied greatly over time, the sampling period matched the flight period well. For long-term survival of this beetle, the forests must contain breeding substrates and be opened up through prescribed burning, selective cutting and active forestry.