Inter‐tree distribution of the spruce web‐spinning sawfly,Cephalcia abietis,at endemic density

1 The spatial distribution of forest defoliating insects at endemic density is a generally little known aspect of pest biology, which may have some importance in monitoring and prediction of outbreaks. In a natural spruce stand in Northern Italy it was possible to detect the presence of some species of web‐spinning sawflies (Cephalcia spp.) at endemic density. This was done over a 3‐year period by using trapping devices for both adults and larvae, including a trap which intercepted adult females climbing the trunk. Traps were operated on eight sample trees with signs of defoliation of the previous year (focal trees) and on 14 trees lacking signs of larval feeding (neighbour trees). 2 Trunk traps on focal trees caught a higher number of adult females than traps on neighbour trees in each of the 3 years considered. The distribution pattern of catches was not spatially autocorrelated at any of the distance bands considered. Focal trees produced more mature larvae per adult female of C. abietis than neighbour trees, indicating that insect performance was higher on focal trees. 3 The endemic population density of C. abietis was similar to that observed in other forests and the relationships between yellow trap catches and prepupal density fitted adequately with the predictive model validated for C. arvensis. The endemic density of the spruce web‐spinning sawflies seems to he higher than that of pine diprionids, and outbreak‐prone species of Cephalcia seem to be more abundant than non‐outbreak species even at very low population level.