Effects of sanitation logging in winter on the Eurasian spruce bark beetle and predatory long-legged flies

Bark beetles may cause great damage to forests by killing trees over large areas during outbreaks. So-called sanitation logging is frequently used in both Europe and North America. Despite this, there are few evaluations of the efficiency of sanitation logging for reducing bark beetle numbers. The Eurasian spruce bark beetle Ips typographus (SBB) has caused unprecedented high tree mortality in Europe during recent years. Much of the sanitation logging is done during winter when SBB are in the adult stage. Sanitation logging may also affect natural enemies of SBB, especially larvae of long-legged flies, Medetera spp., that overwinter in the killed trees. The main goal of this study was to quantify how sanitation logging during the winter affects mortality of SBB and Medetera compared to leaving the trees in the forest. In one part of the study the proportion of the new generation of SBB that overwintered in 237 trees in two years across a climatic gradient was estimated. Within the area of the latest outbreak that started 2018 in Southern Sweden, the average proportion of SBB overwintering in the tree was 48%. Further north the proportion was significantly lower (mean 27%). In another part of the study, the amount of bark that was stripped off by harvesters during processing of 424 SBB-killed trees at six sites and the survival of SBB and Medetera until spring in stripped-off bark was estimated. On average 50% of the bark was stripped off during processing by harvesters. More bark was stripped off when processing trees with thawed (mean 61%) than with frozen bark (mean 37%). The estimated survival of SBB and Medetera in stripped-off bark was high (>70%), and consequently the relative reduction of both SBB and Medetera by sanitation logging was lower for trees harvested with thawed than with frozen bark. The mean reduction of Medetera by sanitation logging was greater than that of SBB (49% vs. 29%) mainly because all Medetera stay in the tree during winter while most SBB leave the tree before winter. The limited effect of sanitation logging on SBB reduction during winter, especially if bark is thawed, suggests that as much as possible of SBB-killed trees should be logged during summer. Logging during winter risks hitting Medetera proportionally harder than SBB, and this is a factor to consider in the control of SBB.