Degradation of white birch shelterbelts by attack of white-spotted longicorn beetle in central Hokkaido, northern Japan.
Widespread decline of white birch shelterbelts was observed in central Hokkaido, northern Japan. Many exit holes bored by adults of the white-spotted longicorn beetle have been found at the bases of the trunks of trees in these stands. The number of adult longicorn beetle exit holes (Nholes) of dead standing trees tended to be greater than that of living trees. Nholes tended to increase with increasing DBH, and there was a negative relationship between Nholes and tree vigor. We found a size-dependent lethal threshold in Nholes. A resonance-measurement device (RMD) for diagnosing the level of wood defection inside the trunk was also tested. The RMD examination together with the lethal threshold in Nholes can be a useful tool for the diagnosis of white birch trees. We estimated Nholes of dead standing trees with a DBH of 25 cm in each plot (ND25) to compare the severity of infestation among plots. Logistic regression analysis revealed that 50% of stands will be degraded if ND25 = 25.0. Thus, the degradation could also be evaluated by Nholes.