Monitoring spruce web-spinning sawflies Cephalcia spp.: the correlation between trap catches and soil sampling

The web-spinning sawflies of the genus Cephalcia Panzer (Hymenoptera, Pamph iliidae) are usually monitored by sampling prepupae in soil and by catching adults with visual traps, including yellow sticky traps. However, adult ca tches have never been compared with the actual density of prepupae. In this paper, a relationship is presented between the catches of Cephalcia arvens is Panzer adults (mostly males) and the number of prepupae ready to emerge (pronymphs) in ail outbreak area of spruce forest (Picea abies Karsten) in the Southern Alps (Asiago, Italy). A significant linear relationship betwee n the logarithmic estimates of the two variables was found. The model inclu des a spatial autoregressive parameter because adult catches were spatially correlated. The relationship allows to know whether or not the defoliation threshold of 20 pronymphs/m(2) in spring samples is being exceeded, based on the number of adults caught on yellow sticky traps (defoliation threshol d of 14.26 adults/trap). The variance not explained by the model is probabl y due to the flight behaviour of adult males, to the low precision of the e stimated pronymph density at low population levels, and to the attraction o f males by few females that are trapped and remain alive. The model was val idated in a large monitoring programme of undamaged alpine spruce forests a nd allowed detection of two newly infested stands out of six that were cons idered at risk of defoliation, because of adult catch exceeded the threshol d. Low adult catches were always associated with absence of defoliation. Ou r conclusion is that trapping of adult sawflies is a cheaper and easier mon itoring method than prepupae sampling in the soil. This makes earlier detec tion of outbreaks possible, and consequently more efficient control.