Sanitation timing matters: Winter and spring felling effectiveness against Ips typographus in hemiboreal Norway spruce forests

The European spruce bark beetle ( Ips typographus L.) is one of the most important forest pests of Norway spruce ( Picea abies (L.) H. Karst) in Europe, and sanitation felling remains the standard method used to control ongoing infestations. In hemiboreal forest management, sanitation felling is often carried out in spring in combination with trap logs. We hypothesized that winter sanitation felling might also reduce bivoltine populations if a substantial proportion of beetles overwinter beneath the bark. However, field-based comparisons of sanitation timing remain scarce in the European hemiboreal region. We compared three treatments in five forest units in Estonia, focusing on small infestation spots in middle-aged managed spruce stands: winter sanitation felling, spring sanitation felling with trap logs, and untreated controls. Results revealed that late-season attacks represented only a small fraction of infestations and that overwintering survival of new-generation I. typographus adults beneath the bark was extremely low. Winter sanitation felling did not reduce subsequent infestation relative to untreated stands, whereas spring sanitation felling with trap logs reduced infestation probability locally, although the effect remained modest. Overall, stand and landscape context, together with regional beetle population dynamics, influenced infestation patterns more strongly than treatment.